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ANIMAL AGGREGATIONS 


THE UNIVERSITY OF CHICAGO PRESS 
CHICAGO, ILLINOIS 


THE BAKER & TAYLOR COMPANY 
NEW YORK 


THE CAMBRIDGE UNIVERSITY PRESS 
LONDON 


THE MARUZEN-KABUSHIKI-KAISHA 
TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI 


THE COMMERCIAL PRESS, LIMITED 
SHANGHAI 


ANIMAL 
AGGREGATIONS 


A Study in General Soctolog y 


By W. C. ALLEE 


The University of Chicago 


THE UNIVERSITY OF CHICAGO PRESS 
CHICAGO - ILLINOIS 


COPYRIGHT 1931 BY THE UNIVERSITY OF CHICAGO 
ALL RIGHTS RESERVED, PUBLISHED MARCH 1931 


COMPOSED AND PRINTED BY THE UNIVERSITY OF CHICAGO PRESS 
CHICAGO, ILLINOIS, U.S.A. 


IN MEMORY OF MY SON 


WARDER ALLEE 
TOI G 28 
IN APPRECIATION OF HIS BOYISH ENTHUSIASM 
OVER THE PARTS OF THIS WORK 
WHICH HE UNDERSTOOD 


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PREFACE 


The attempt to summarize knowledge concerning the relations 
within and between different sorts of animal societies is not new. 
Espinas in 1878 undertook such an effort, and shows in his introduc- 
tion that Aristotle, Spinoza, Leibnitz, Montesquieu, Kant, Hegel, 
A. Comte, Herbert Spencer, and others had preceded him in the 
consideration of certain aspects of this problem. Since the time of 
Espinas, knowledge concerning the social insects and the develop- 
ment of insect societies has greatly increased. Wheeler, Forel, 
Buttel-Reepen, and many others have contributed both personal ob- 
servations and generalizing summaries of value; and I have no desire 
to enter into a field so ably covered. There does remain, however, a 
field of social, or perhaps subsocial, life almost entirely untouched by 
these students. They have been concerned with the fascinating prob- 
lems and intricate relationships presented by fairly well-developed 
societies. Here, I propose to investigate the relationships existing 
among the more loosely integrated collections of animals, which may 
rightly be designated as ‘‘animal aggregations,” with regard to their 
ecological and behavioristic physiology, as well as with regard to 
their strictly social implications. 

This book is built about a phenomenon or a series of phenomena, 
rather than about a philosophy. In the present form it may even be 
designated as notes on an unsolved problem; but since a presenta- 
tion of a problem is necessary for its ultimate solution, and since an 
inquiry into the universality of a given problem is imperative before 
undertaking laborious experimentation directed toward finding a 
solution, no apology is offered for summarizing our growing knowl- 
edge on the subject of animal aggregations at the present stage of 
inquiry into the problems involved. 

My own experimental work within the field covered by the pres- 
ent book began in 1911 and has continued intermittently to date. 
The investigation of animal aggregations has been at the center of 


vill 


viii PREFACE 


my research program for the last twelve years, during which time 
work has been actively carried on with the aid of a number of gradu- 
ate students, with facilities furnished by the University of Chicago, 
the Marine Biological Laboratory, and, more recently, with financial 
aid from a grant from the Rockefeller Foundation to aid investiga- 
tions in the biological sciences at this university. The preparation of 
the manuscript of this book has been, in part, supported by aid from 
this latter source. 

In addition to the loyal co-operation of students and colleagues in 
the accumulation of experimental data, of citations, and of criti- 
cisms, aid with the scattered literature has come from friends and 
acquaintances from five continents. The extended literature list is 
incomplete, but the labor of gathering and selecting the references 
used has been appreciably decreased by this cordial co-operation. 

Certain specific acknowledgments I have made in the text. I am 
also indebted to Drs. Marie A Hinrichs, A. M. Holmquist, Walburga 
A. Petersen, J. M. Shaver, and O. Park; to Messrs. M. R. Garner, 
J. R. Fowler, J. F. Schuett, W. A. Dreyer, Carl Welty, W. H. John- 
son, E. O. Deere, Ralph Buchsbaum, D. A. D. Boyer, J. F. W. Pear- 
son, and T. Park; to Mrs. Frances Church van Pelt and Mrs. Gret- 
chen Shaw Rudnick; and to Miss Edith Bowen, for citations to per- 
tinent literature, for permission to give results before publication, or 
for criticism of parts of the manuscript, or for all three; to Professor 
F. R. Lillie, who read critically chapters xvi and xvii; to Professor 
A. E. Emerson for similar service with chapters xix and xx; to Mr. K. 
Toda, who drew or copied the text figures; and to Marjorie Hill 
Allee for editorial assistance with the manuscript. Acknowledgment 
of courtesy in permitting reproduction of figures will be given else- 


where. 
W. C. ALLEE 
WHITMAN LABORATORY OF EXPERIMENTAL ZOOLOGY 
UNIVERSITY OF CHICAGO 
August, 1930 


CHAPTER 


CONTENTS 
INTRODUCTION 


I. THe GENERAL BACKGROUND 


VIII. 


XIX. 
XX. 


. CLASSIFICATION OF ANIMAL AGGREGATIONS 

. FORMATION OF ANIMAL AGGREGATIONS 

. GENERAL FACTORS CONDITIONING AGGREGATIONS 
. INTEGRATION OF AGGREGATIONS 


HARMFUL EFFECTS OF AGGREGATIONS 


. HARMFUL EFFECTS OF CROWDING UPON GROWTH , 
. RETARDING INFLUENCE OF CROWDING ON THE RATE OF REPRO- 


DUCTION a Coe ee 
CROWDING AND INCREASED DEATH-RATE . 


BENEFICIAL EFFECTS OF AGGREGATIONS 


. STIMULATION OF GROWTH BY CROWDING : 
. STIMULATING EFFECTS OF CROWDING ON THE RATE OF REPRO- 


DUCTION 


. EFFECT OF CROWDING ON SURVIVAL AND OXYGEN CONSUMPTION 
. PROTECTION FROM TOXIC REAGENTS 

. RESISTANCE TO HypoToNic SEA-WATER Ses os 

. RELATION BETWEEN DENSITY OF POPULATION AND INSECT SUR- 


VIVAL 


. COMMUNAL ACTIVITY OF BACTERIA . 
. Mass PHysIoLoGy OF SPERMATOZOA 


GENERAL EFFECTS OF AGGREGATIONS 


. INFLUENCE OF CROWDING UPON SEX DETERMINATION 
. MoRPHOLOGICAL EFFECTS: OF CROWDING 


CONCLUSION 
ANIMAL AGGREGATIONS AND SOCIAL LIFE . 
THE PRINCIPLE OF CO-OPERATION 


BIBLIOGRAPHY 


INDEX 


IOI 


INTRODUCTION 


CHAPTER I 


THE GENERAL BACKGROUND 


INTRODUCTION 


This study of animal aggregations is concerned with some of the 
physiological effects of crowding upon the individuals composing the 
crowd, and is offered as a contribution toward the development of 
general sociology upon a physiological basis. A few years ago it 
would have been possible to summarize the knowledge then existing 
on the subject with the statement that, except in hibernation or at 
breeding time, the physiological effects of crowding are uniformly 
harmful, whether attention is given to the effect upon rate of repro- 
duction, rate of individual growth, or longevity. Data on these 
harmful results will be presented later, but they are no longer ac- 
cepted as a complete picture; in this study they are needed to ob- 
tain a correct perspective for the recent discoveries of beneficial 
effects of relatively unorganized crowds of animals. 

Much attention has deservedly been given to the study of or- 
ganized societies, particularly those of mammals, birds, and insects— 
sometimes with relation to the light they may throw upon the social 
relations of man, but frequently on account of their own inherent 
interest. In the main, consideration of these highly organized social 
groups falls outside the interests of the present discussion, which will 
be limited, so far as possible, to the physiological effects of crowding 
upon organisms whose interrelations have not reached the level of 
development usually called “social.” 

The general physiologists contend with justice that one cannot 
understand the physiology of man without a knowledge of the gen- 
eral physiology of all animals and much of that of plants as well. The 
comparative psychologists conclude similarly that one cannot under- 
stand the working of the human nervous system without knowing 
how other nervous systems function. Similarly, an increasing num- 

3 


4 ANIMAL AGGREGATIONS 


ber of investigators are convinced that without a knowledge of gen- 
eral sociology we are likely to regard the social traits exhibited by 
man or by the ants as being peculiarly human or peculiarly formi- 
cine, when many of them are merely human or ant variations of 
social traits common to animals in general. Again, it is difficult to 
evaluate properly the origin and function of many of these general 
social traits without a proper understanding of their physiological 
antecedents among animals not usually regarded as having reached 
the social level. It is quite easy to consider certain bits of behavior 
as definitely social in origin and inherent in the social type of organi- 
zation which may be merely specialized developments of general be- 
havior common to most animals when crowded. 

One fallacy may be suggested at the beginning. All too frequently 
one gains the impression that sex forms the main, if not the only, 
physiological connecting link between the infrasocial and the social 
animals. I believe that a consideration of the facts to be presented 
will allow us to place this important social factor more nearly in its 
proper relation to other factors equally important. 

The problems dealt with in the present study are of interest also to 
the large group of students of animal ecology. It is generally known 
that ecology deals with the relations between the organism and its 
environment. This environment is roughly divided into two parts— 
the non-living and the living—which are commonly referred to as 
the “physical” and the “biotic” elements of the environment. Of 
these, the former has received particular attention at the hands of 
modern animal ecologists, since such factors as light, hydrogen-ion 
concentration, humidity, wind velocity, and temperature are more 
or less readily and definitely measured, and since others, such as soil 
type or the chemical composition of the waters of a lake or river, 
though less readily analyzed, are still capable of being studied on a 
quantitative basis. Meantime the analysis of the biotic relations of 
the environment has lagged, probably on account of the greater 
difficulties involved in the quantitative treatment of this exceedingly 
complex part of the environment. Even so, marked progress has 
been made in this analysis by recent students of the ecological rela- 
tions within animal and plant communities (Smith, 1928; Shackle- 


THE GENERAL BACKGROUND 5 


ford, 1929). In the present studies we shall find ourselves concerned 
with animal communities which, from their concentrated nature, 
necessarily make the biotic elements an important aspect in the en- 
vironment of any particular individual, while the physical elements 
of the environment act mainly through their influence on the entire 
aggregation or crowd. Such a situation must frequently obtain in 
assemblages of sea anemones, of Mytilus, of ascidians, or of crabs. 
In working at the aggregation level here considered, we find the 
ratio of importance of the physical and the biotic environment in a 
transition stage between that present in definitely social groups and 
that occurring in the more typical animal community, or biocoenose 
of the ecologist. 

We must emphasize the fact that all studies dealing with the 
biotic elements of the environment are likely to be less definitely 
quantitative than those dealing with the non-living environment. 
This is no reason for their neglect, but it is a reason why we may not 
expect their treatment to be precise and final. The present summary, 
gained from pioneering in this relatively new field, must be regarded 
as tentative in many respects. My own research program dealing 
with various aspects of the subject is only well under way; the pres- 
ent statements furnish a point of departure, rather than a gathering- 
in of conclusions. With the accumulation of evidence now being 
actively collected, the conclusions tentatively advanced here may 
be further confirmed, or they may soon be modified or entirely aban- 
doned. This must always be the case, even in the well-developed 
fields of physics and chemistry; and does not prevent summaries of 
knowledge to date having definite value, if they stimulate further 
research or give point to researches already in progress. 


TERMINOLOGY 


The general terminology causes unexpected difficulty. One usual- 
ly thinks that such words as “society,” “‘association,” and “com- 
munity” have a relatively stable meaning and that ‘‘biocoenosis,”’ 
for example, might be expected to be a quite exact term; but this is 
unfortunately not the case. 

According to writers on human sociology (Park and Burgess, 


DIGG 


6 ANIMAL AGGREGATIONS 


1921), “the terms society, community and social group are now used 
by students with a certain difference of emphasis, but with very 
little difference in meaning. Society is the more abstract and inclu- 
sive term, and society is made up of social groups, each possessing 
its own specific type of organization but having at the same time all 
the general characteristics of society in the abstract. Community is 
the term applied to societies and the social groups where they are 
considered from the point of view of the geographical distribution of 
the individuals and institutions of which they are composed. It fol- 
lows that every community is a society, but not every society is a 
community. An individual may belong to many social groups but 
he will not ordinarily belong to more than one community, except in 
so far as a smaller community of which he is a member is included in 
a larger of which he is also a member. However, an individual is not, 
at least from a sociological point of view, a member of a community 
because he lives in it but rather because, and to the extent that, he 
participates in the common life of the community.” 

The same authors evidently do not consider ‘‘association” as be- 
ing a sufficiently significant term to be given formal definition. In 
general sociology the contrast between what is ordinarily called an 
“association” and a “‘society” is important. Students differ concern- 
ing the proper criteria to use in making this distinction. Espinas 
(1878) recognized that there was a difference, and called ‘“‘associa- 
tions” accidental societies between animals of different species. Ac- 
cording to this pioneer in the field of general sociology, the charac- 
teristic trait of social life is to be found in habitual reciprocity be- 
tween activities which are more or less independent. He recognized 
certain similarities between associations and societies but regarded 
the former as less necessary for their constituent elements. Associa- 
tions, according to Espinas, are groups of convenience, not of neces- 
sity. Deegener sharpened this distinction (1918) on the basis of use- 
fulness of the animal group to the individual members. He designat- 
ed an “association” as a collection of similar or dissimilar animals, 
which does not have value for the individuals composing the group; 
and a “society” as one in which the collection does have distinct 
value for the individuals of which it is composed. 


THE GENERAL BACKGROUND 7 


Deegener’s criteria for the social value of his categories were far 
less sensitive than those which were shortly developed by other 
workers in this field, and which will be summarized in the body of 
the present discussion. Application of such distinctions, even in their 
present incomplete form, would necessitate a marked revision in 
Deegener’s scheme. 

Later, Deegener recognized that certain groups of animals are 
held together by a social force or instinct of which we know at present 
relatively little. The arrangement of such groups in his original sys- 
tem is obviously difficult. One may think of the satisfaction of the 
so-called “social force” or “instinct” as having definite value for the 
animal so satisfied. According to this reasoning, the group collected 
by social instinct would be a “society’’; although, since there is no 
other demonstrable advantage accruing to the members of the group, 
Deegener at first was inclined to regard such an aggregation of in- 
dividuals as an ‘‘association.”? Faced with this dilemma, he decided 
(1919) that associations whose occurrence depends upon a social in- 
stinct may be designated as “instinctive associations.” They are 
opposed to aggregations of purely accidental character which are 
formed not because of instinct but because of limited space or local- 
ized food. If the aggregation is formed from obvious mutual attrac- 
tion but without any recognizable objective benefit to the members, 
Deegener calls it an “instinctive association,” as with young spiders, 
young ticks, or groups of grasshoppers. 

Alverdes (1927) understands by “‘associations” the chance gather- 
ings produced solely by external factors, such as insects collected 
around a lamp, while “‘societies’”’ are genuine communities held to- 
gether by the force of a social instinct. ‘In short,” Alverdes says, 
“no social instinct, no society!’ According to this point of view, the 
individuals are collected into an association because of their re- 
sponses to environmental factors, but they collect into a society 
primarily because of the presence of other similar animals and only 

-secondarily because of the action of environmental forces. Alverdes 
would consider the lack of a social instinct all the evidence necessary 
for calling such a group an “association.” 

Wheeler (1928), commenting on these two classification schemes, 


8 ANIMAL AGGREGATIONS 


doubts the applicability of Deegener’s basic principle of benefit or 
no benefit, but commends Alverdes’ position as being essentially 
sound. 

The ecological use of these and related terms must needs be con- 
sidered. Modern ecological work has shown that each different kind 
of a habitat contains a more or less characteristic set of animals 
which are not mere accidental assemblages but are interrelated com- 
munities. When these are geographic in extent, they are usually 
spoken of as a “formation,” within which may be recognized smaller 
units or “associations,” which are composed of groups of habitat 
strata that are uniform over a considerable area but smaller than a 
formation. These associations are frequently composed, at least in 
part, of developmental stages, such that an orderly succession of 
communities can be recognized. Such a series, forming a unit of 
succession from initial to climax stages of an association, is some- 
times called a ‘‘sere”’; and the different developmental units of the 
whole association are called “‘associes.’”’ Thus, we have the animal 
communities or associes of the open sand, the foredune, the pines, 
the oaks, and finally the beech and maple forest, forming one de- 
velopmental sere arranged in the order given, within the beach and 
maple association near Chicago (Smith, 1928; Shackleford, 1929). 

In ecology the term “‘animal society,” according to the most re- 
cent usage (Smith, 1928), has been divided into two parts, one of 
which is called a “‘pre-society.”* This is a community of organisms 
living among the plants of an association and subordinate to the 
plant dominants. The “plant association”’ is named for one or more 
of the dominant plants, while one or more of the predominant ani- 
mals give the name for the “‘super-society.”” The super-society, like 
its accompanying plant association, generally covers an extensive 
area with an essentially uniform taxonomic composition. Within such 
a super-society one finds animal societies which are communities of 
lesser magnitude and which may be seasonal, or stratal, or confined 
to a given locality. When these societies, or recognizable subdivi-’ 
sions of them, are composed of animals closely bound together by 
biotic relationships such as have been described in general terms as 


« “Super-society”’ would appear to fit the meaning more exactly. 


THE GENERAL BACKGROUND 9 


composing a “‘web of life,” they are frequently called “biocoenoses.”’ 
Food and shelter relationships, climatic and edaphic factors, are im- 
portant determining conditions for a given biocoenosis. 

With the growing complexity of ecological terminology and the 
growing precision with which different terms are applied to various 
recognizable groupings of animals, a need has developed for some 
term which could be used in a general sense to cover any one of the 
named units from the largest to the smallest. The word ‘“‘communi- 
ty” has been reserved for this general purpose, and one may speak 
with equal propriety of the animal communities of the Amazon 
rain-forest or of a decaying tree within that forest. 

In the border-line field where general sociology meets and over- 
laps general physiology and ecology, the field which is being con- 
sidered in the present discussion, it seems desirable to have a term 
which may be applied loosely, but not incorrectly, to any of the 
recognized units lying below the groups accepted as definitely social, 
just as the term ‘‘community” is applied by the animal ecologists 
with equal propriety to strata, super-society, society, association, 
and what not. It is in this general sense, for this level of social or 
subsocial life, that I propose to use the term “aggregation.” I am 
not concerned with defining it closely in terms of the association or 
society of Deegener or Alverdes. It may be used with equal pro- 
priety in speaking of a group of frogs collected as a result of sexual 
attraction during the breeding season; or of a concentration of May 
flies about a light, where they have been collected by forced move- 
ment as a result of their strongly positive phototropism. There is in 
the term itself a strong suggestion that the groupings involved are 
not closely integrated, which is in keeping with the facts in the field 
to be covered. 

INSTINCTS 

In the course of this discussion we shall have reason to refer to 
‘““nstinct,” a term deservedly in disrepute among careful thinkers be- 
cause of the slipshod way in which it has been used. Early students 
of human sociology and recent zodlogical commentators on socio- 
logical phenomena have sadly overworked the word by referring any 
unanalyzed social behavior to the working-out of a social instinct. 


10 ANIMAL AGGREGATIONS 


That social instinct may be acting in given cases is not to be denied, 
but there has been an increasing and wholesome tendency to depre- 
cate the use of this term to cover ignorance. 

“Instinct” is hard to define. The most satisfactory definition 
known to the writer is that of Wheeler, who says (1913a): ‘An in- 
stinct is a more or less complicated activity of an organism which is 
acting (1) as a whole rather than as a part; (2) as a representative of 
a species rather than as an individual; (3) without previous experi- 
ence; and (4) with an end or purpose of which it has no knowledge.” 

It is obvious to one who has observed the reactions of animals 
that there are two types of behavior: the learned and the unlearned. 
Much of the latter is frequently called “instinctive,” with propriety, 
though in the case of many highly organized animals, including man, 
there has been an unfortunate tendency to regard, as instinctive or 
unlearned, behavior that is in reality based on very early training 
which has been entirely forgotten or overlooked. 

In man breathing, swallowing, gland secretion, and muscle con- 
traction are all unlearned; and some of these, for example the secre- 
tion of certain glands, cannot be effected by learning. These un- 
learned reflex actions of parts of organisms seem to be the simplest of 
a series of unlearned responses whose other categories are those re- 
flexes of an entire organism commonly called “tropisms,” and the 
more complex behavior usually called “instinctive.” 

It is becoming increasingly difficult to draw hard and fast lines 
between instincts and tropisms, or between either of these and the 
general functioning of living cells. It is further impossible to dis- 
sociate any of these three categories of behavior from the activities 
concerned with growth and development. If one considers in this 
connection the metamorphosis of a larva into an adult, which is 
usually regarded as the function of growth and development, one 
finds the processes concerned so inextricably bound with major and 
minor activities of the animal that the instinctive behavior cannot 
be clearly separated from the other processes going on at this time. 
Is the production of the silk cocoon of the moth an instinctive action, 
while the production of the thickened hypodermis to form the chrys- 


« Or without modification caused by experience (W. C. A.). 


THE GENERAL BACKGROUND . II 


alis of the butterfly is only a growth process? What is the essential 
difference between the two? 

In so far as is possible, we shall avoid dwelling upon the aspects of 
behavior usually called “instinctive,” except in reference to the 
literature. This is not due to a disbelief in the reality of instinctive 
social behavior, but rather to a conviction that progress lies in a 
field where the elements of behavior can be more exactly ascertained. 

The drive which leads an animal to exhibit such behavior as is 
usually classified as being due to the operation of social instinct I 
prefer to regard, as does Wheeler (1928), as an expression of appe- 
tite. Wheeler says in this connection: “It thus takes its place with 
the other appetites like hunger and sex, though it is feebler and more 
continuous, i.e., less spasmodic and, therefore, less obvious. It is 
most strikingly displayed, however, in the restless behavior of the 
higher social animal when isolated from the continuous, customary 
stimuli of its kind.” From this approach, the strength of the social 
appetite can become a subject for objective investigation, such as 
Warner (1928a) has recently made for the relative strength of the 
drives furnished by food or sex hunger; but such an objective 
investigation of the general social appetite has not yet been con- 
ducted. 

The scope of the discussion, some concepts, and a part of the ter- 
minology having now been considered, we may plunge directly into 
the mass of material awaiting analysis. 


CHAPTER II 
CLASSIFICATION OF ANIMAL. AGGREGATIONS 


It has long been known that animals not naturally bound together 
in organic union may aggregate into groups or clusters more or less 
closely associated, in which physical contact may or may not occur. 
Actual physical contact is normally found as part of the aggregation 
phenomenon among many Protozoa, as, for example, in Paramecium; 
in flatworms, such as the planarians; in earthworms; in echinoderms, 
such as starfish; in mollusks; in arthropods; and among many 
chordates, including ascidians, fish, frogs, reptiles, birds, and mam- 
mals. 

Among other animals similarly widely distributed through the 
animal kingdom, collections occur in which physical contact is not 
the rule. These may be illustrated by the jellyfish, ctenophores, or 
copepods that may discolor the ocean for miles; by collections of 
leeches, snails, or ostracods; by the swarms of gnats that dance to- 
gether like particles in brownian movement; by ants, bees, schools 
of fish, flocks of birds, herds of ungulates, and groups of various other 
mammals, including man. The highest development of aggregations 
not based on physical contact requires the possession of highly de- 
veloped sense organs. 

These two types of animal aggregations are not mutually exclu- 
sive, even when reactions associated with copulation are disregard- 
ed; for animals may be involved in first one and then the other-in 
different phases of their life-cycle or seasonal history. With many 
birds the loose flock of the daytime may be replaced by close physi- 
cal contact during the night roost. At times this may be due to the 
lack of adequate perching space, and show merely toleration of close 
proximity; but in other instances, as, for example, the Indian tree 
swift, there is a positive movement together even in the presence of 
abundant roosting space. Bats may show the same phenomenon 
during their daytime sleep. 


CLASSIFICATION OF ANIMAL AGGREGATIONS 13 


There are abundant examples of animals that lead wholly or par- 
tially solitary lives during part of their seasonal- or life-cycle but at 
another period come together into flocks or in actual physical con- 
tact. This is true of the cowbirds, reared singly from eggs surrepti- 
tiously laid singly in the nests of other species of birds. The young 
cowbirds develop quite out of touch with other members of their own 
kind and yet collect into definite flocks when adult. Another aspect 
of the same kind of behavior is shown by the grackles, which nest 
fairly separately but join in large flocks before the fall migration; by 
deer, which summer separately or in partial family groups but winter 
in herds; by frogs, which remain practically solitary during the year 
except for possible hibernation groups and then aggregate during 
the breeding season; by solitary bees or wasps, which for the greater 
part of the year are out of physical contact with their fellows and 
yet during the summer may form overnight aggregations in closest 
physical proximity; or, to give one more of many possible examples, 
by land isopods, which congregate into dense bunches when their 
habitat becomes dry. 

The aggregations of the physical-contact type are, of necessity, 
transitory in character in motile organisms; but in sessile animals, 
such as the ascidians, or the marine mussel Mytilus, this may well be 
the normal way of living. The physical-contact type of aggregation 
finds its most complete expression among the sessile colonial organ- 
isms that grow in dense stands of many individuals, which are physi- 
cally connected with each other throughout life. Obelia hydroids 
represent this growth form. 

Collections without physical contact, such as the flock or the 
herd, may be constant and normal for some species; and the animals 
in these are usually said to exhibit the social habit. This social habit 
finds its best development in the insects, such as the ants and ter- 
mites, among whom division of labor is carried out to its logical end, 
in that polymorphic forms have evolved of which some do not com- 
plete their sexual development while others specialize upon repro- 
duction. These have been well described by Wheeler and Forel. 

Animal aggregations may be classified on many other bases be- 
side that of the degree of physical contact. Deegener (1918) has 


14 ANIMAL AGGREGATIONS 


made an exhaustive classification of the different forms of animal 
groupings (Vergesellschaftung) in which he undertakes to arrange 
logically all such associations, ranging from the relatively simple 
colonies of the protozoans—Synura or Carchesium, where all the 
individuals are similar and all arise from the same parent-cell and 
are organically connected with each other—to colonies of ants with 
their complicated social structure, which may include, in addition 
to the ant castes themselves, their slaves, their commensals, their 
tolerated guests, parasites, parasites of the parasites, or parasites 
of other associated forms. 

A summary of the classification of animal aggregations as worked 
out by Deegener is given here at some length, not because I accept it 
entirely with all its implications, but because it is the most complete 
classification yet produced and because I am in hearty accord 
with the principle underlying this scheme of organization: that no 
hard and fast line can be drawn between well-integrated social or- 
ganizations and loosely integrated aggregations which are usually 
regarded as being definitely non-social. Further, experience with pre- 
senting this material to seminar students has shown the desirability 
of wading through a detailed outline, such as that of Deegener’s, in 
order to acquire a comprehensive view at one and the same time of 
the ramifications of the subject matter and of its inherent unity. 

It is the custom at present to ignore this work of Deegener or to 
fail to appreciate its essential value (Wheeler, 1928) because of ob- 
vious defects in its cumbersome terminology, in the criteria used to 
distinguish between major groupings, and because the categories are 
not clean cut and mutually exclusive. Many of these faults are in- 
herent in a pioneering classification of subject matter in any field, 
and others were caused by the lack of definite knowledge in 1918 of 
the relationships involved. On this latter count we are in a position 
to make improvements on Deegener’s classification at the present 
time, but we do not appear to be able to refine it sufficiently as yet 
to repay the trouble involved. 

The account given below is not a direct translation of Deegener’s 
1918 outline; but it follows that outline and gives his point of view, 
criticisms of which have been suggested and will later be elaborated. 


CLASSIFICATION OF ANIMAL AGGREGATIONS 15 


DEEGENER’S CLASSIFICATION OF AGGREGATIONS 


Part I. Accidental unions or associations are groups of animals 
without mutual benefit for individual members. ‘‘Accidental’’ is, to 
Deegener’s mind, a better term for these aggregations than ‘‘in- 
different,” because to him it plainly indicates the method of their 
formation, and also because the members of accidental aggregations 
are not always indifferent to each other. Accidental aggregations 
will be seen to be of various kinds, formed in various ways. They 
may consist of one or of a number of species. One cannot always be 
sure concerning the proper classification of a given association, which 
may as yet be merely a matter of opinion. Deegener recognizes that 
even the major distinctions are not always clean cut and that one of 
a pair of apparently closely similar groupings may be assigned to the 
accidental associations while the other is called an ‘essential so- 
ciety.” In the minor categories the methods of formation determine 
the classification to a considerable degree. 

A. Homotypical associations consist of members of the same spe- 
cies which have arisen either sexually or asexually, which may have 
remained together because they are the offspring of the same parent, 
or which may have become accidentally associated together although 
of different parentage. The former are called ‘“‘primary,” and the 
latter “secondary,” associations. 

Alpha. Kormogene associations’ are confined to invertebrates and 
do not occur in arthropods, echinoderms, and mollusks. They are 
those colonial forms in which the different individuals remain mor- 
phologically attached to each other. The advantages of the colony 
are not always clear. In Protozoa, relationships of individuals in the 
colony are not such as to guarantee nourishment for the entire 
colony; thus there is no advantage in this respect with this phylum. 
In the hydroid colonies, nourishment is better assured for the in- 
dividual by the colonial form. The colony does not appear to be 
formed necessarily because it is a more favorable adaptation to living 
conditions but because of the failure of the different elements to 
separate at fission. The tendency toward colony-building increases 


™ Budded colonial forms, as among the hydroids, cannot be regarded as “accidental” 
in the usual usage of that word. 


16 ANIMAL AGGREGATIONS 


as habits become sedentary, andis also more marked in relatively sim- 
ple animals having strongly developed skeletal parts, as the sponges, 
hydroids, bryozoans, and tunicates. 

I. Primary colonies arise as the result of division in which the 
smaller pieces remain together, or as a result of budding in similar 
fashion. 

t. Homomor phic colonies result when the divisions are equal and 
all members of the colony are similar, as in Synura, Carchesium, and 
Salpa chains. Such colonies as Zodthamnium may represent true 
societies, since all individuals may contract if one is stimulated, and 
so all may escape harm; while Carchesiwm does not, and so is placed 
in the present category. 

2. Heteromor phic colonies are formed when the divisions are un- 
equal, as is the case with the strobila of the Scyphozoa, or during the 
processes of asexual reproduction of certain worms, such as A utolytus. 

II. Secondary colonies, or concrescence colonies, arise by the sec- 
ondary union of individuals which are entirely separate for at least a 
brief period. 

1. Concrescence colonies having a genetic basis, in that the individ- 
uals composing the colonies originated from the same mother, are 
shown in Proteriodendron, Dinobryon, and secondary Salpa chains. 
The fact that identical or related forms have survived and can live as 
separate individuals indicates that these animals are able to live 
without the small and perhaps accidental benefit arising from their 
communal life. 

2. Concrescence colonies without a genetic basis are those in which 
the animals that later become attached together in one colony are 
not descendants of the same mother. These commonly occur in ses- 
sile animals, such as the ascidians, sea anemones, sponges, oysters, 
and Mytilus. If no organic union takes place, causing a real fusion 
between the different animals composing the colony, then the asso- 
ciation remains accidental. 

Beta. Associations of free individuals. 

I. Primary associations arise through asexual or sexual reproduc- 
tion when individuals descending from the same parent or parents 
remain near the place of origin and form an aggregation which varies 


CLASSIFICATION OF ANIMAL AGGREGATIONS 17 


from a loose to a firm integration. The primary cause of their being 
together lies in their common origin, but the cause of their remaining 
together is not of a genetic nature but may depend on the favorable 
character of the place or on the presence of food. In other cases one 
must assume the operation of a social instinct which holds the ani- 
mals together. 

1. Syngenia are primary associations which arise by means of 
asexual reproduction. This may be illustrated by Stentor coeruleus, 
which lives on decayed water plants and occurs frequently in such 
abundance as to give a blue color to the surface of the water. The 
aggregation is located in space by favorable food conditions. So long 
as there are only offspring from a single mother present, the aggre- 
gation would be called a monosyngenium; but when second and third 
generations appear from the same stem-mother, the group becomes a 
polysyngenium. Other unrelated individuals may wander into this 
favorable niche, forming a secondary association. Similar relations 
hold with Vorticella, but with both these aggregations there may be 
some social value accruing to the different individuals, since the 
combined vortex action of the cilia brings more food to each animal. 
This does not occur in hydroids, such as the common fresh-water 
Hydra, which reproduces asexually and remains in a purely acciden- 
tal aggregation in which there is no reciprocal relationship before 
sexual reproduction begins. Similar relations hold with various other 
simple coelenterates whose slight powers of locomotion tend to con- 
fine them close to the place in which they are budded free, providing 
it is a generally favorable location. 

2. Primary associations arising from sexual reproduction may form 
close unions which may rise to the widely extending reciprocity of 
the highest types of society found among animals. In the inverte- 
brates these are represented by the conditions obtaining in ant and 
termite colonies; in the vertebrates, by human societies. This part 
of Deegener’s outline undertakes to consider only the more primi- 
tive, purely accidental forms of this family union, in which the par- 
ents need not necessarily be concerned. Various combinations of sim- 
ple families where the young all originate from the stem-mother may 
be distinguished and divided as follows: 


18 ANIMAL AGGREGATIONS 


a) Sympaedium, in which the offspring of the same mother form 
the aggregation without the presence of either parent. This condi- 
tion is seen in some spiders and insects, where the young of the same 
mother remain together for a longer or shorter period. If the mother 
remains with the offspring, the group belongs to another category. 
Lophyrus caterpillars, which feed on pine needles, form an aggrega- 
tion due in the first place to the eggs being laid together. No obvious 
benefit accrues to the individuals. They are more conspicuous as a 
result of the grouping and cannot defend themselves better than if 
alone. The causal factors in such an aggregation are obscure. The 
fact that the eggs are laid together is not sufficient in itself, since 
other forms have their eggs laid similarly close together and yet 
separate immediately on hatching. It may be that the sluggishness © 
of the animals and the lack of disrupting stimuli explain a large part 
of the behavior; while, on the other hand, there may be a social 
appetite which holds the groups together. The problem becomes 
more difficult with those larvae which remain together during the 
early larval life and separate when partly grown. 

Many lepidopterous larvae that remain together during part 
or all of their larval life spin a common nest. The formation of 
such a nest may be due to the fact of living together rather than the 
living together being due to the need or use of a common nest. The 
ability to spin a common nest does not guarantee the actual building 
of one, for many spinning animals live alone. These larval colonies 
are common among animals in which the adults are winged, and 
hence are readily distributed during that phase of their life-history. 
Such a sympaedium occurs in solitary bees which lay eggs in cells. 
The resulting larvae and pupae form an accidental association, living 
together as offspring of a common mother. When adult, they fly 
away separately. 

b) A gynopaedium is composed of a mother and her offspring that 
remain together for a period. This grouping is not concerned with 
the relationship between mother and offspring beyond the fact that 
they remain together without obvious benefits accruing to the group 
from the association. The aphid stem-mother in the spring gives 


CLASSIFICATION OF ANIMAL AGGREGATIONS 19 


birth to young parthenogenetically. This gynopaedium, consisting 
of one female and her immediate offspring, may be designated a 
monogynopaedium. The young also reproduce parthenogenetically, 
and such a complex group may be called a polygynopaedium. These 
colonies are homomorphic; but as winged forms appear, heteromor- 
phic colonies are formed. In the autumn sexual generations appear 
and produce a resistant over-wintering egg, which carries the colony 
over the winter season. In this aggregation there are no benefits im- 
mediately apparent. The brood is not cared for by the older mem- 
bers or by each other. The individuals composing a crowd of aphids 
are more easily cared for by ants of the myrmecocolous species when 
together, but also are more easily preyed upon by their numerous 
enemies. The massed aphids also tend to destroy the food plant on 
which they cluster, to their own disadvantage. Deegener recognizes 
no social advantage, and therefore regards the aggregation as ac- 
cidental. 

c) Patrogynopaedia occur when both parents remain with their 
offspring in groups. Those with no social benefits for their members 
belong here, but this type of aggregation often carries with it some 
social advantage, and so usually belongs in a later category. Necro ph- 
orus beetles live with their young in decaying animal bodies. This 
association may confer social benefits under certain conditions, but 
they are not recognizable in all cases. In these scavenger beetles, the 
presence of a dead body seems to release a digging reaction whether 
the individual is solitary or in company with others. Each individual 
digs without reference to the others. The results may have no sig- 
nificance for the assisting beetles, but only for the pair leaving their 
eggs with the dead body. Obviously the whole has racial significance, 
although without significance for many of the participating in- 
dividuals. 

Combination family groups also occur in which the individuals 
composing the aggregation come from more than one stem-mother. 

d) Synchoropaedia are formed when eggs laid by different females 
in a favorable place hatch out and the larvae remain together from 
the very first, not as separate families, but freely mixed into a com- 


20 ANIMAL AGGREGATIONS 


mon aggregation. Mosquito (Culex) larvae in a rain barrel are an 
example of a synchoropaedium. When larvae of different species are 
present in the same rain barrel, we have a heterosynchoropaedium. 

e) Similarly, symphagopaedia may result from several groups of 
the same species laying eggs on the same food material except that 
here the favorable food rather than the favorable place becomes the 
integrating factor. This type of aggregation may be illustrated by 
flesh flies and, according to Deegener, by Drosophila. 

II. Secondary associations may be distinguished from primary as- 
sociations because they are the result of a coming-together of free 
individuals rather than their merely remaining together. The classi- 
fication is based on the integrating factor judged to be most impor- 
tant. 

1. Sysyngenia arise from the secondary fusion of two or more 
syngenia. 

2. Sysympaedia consist of fused ‘“‘children-families” and arise when 
one sympaedium meets with another. Deegener observed such in 
juvenile spiders of Epeira (1919b). The members of both groups 
mixed peaceably and gave no sign in their conduct that they were 
influenced by the foreign spiders; indeed, they did not seem to notice 
that their membership had been doubled, and new and old alike ag- 
gregated into one close mass. Another sympaedium was added to 
these two with similar results, although it was not ascertained 
whether or not the individuals of a given sympaedium remained for 
the most part together. 

Two sympaedia of caterpillars of Malacosoma castrense L. are not 
mixable when the larvae of one sympaedium are in the molting 
period; otherwise they mix without the caterpillars of the two broods 
appearing to sense the change in their association. Schulz (1926), 
in studying the reaction of caterpillars of Vanessa io L., V. urticae L., 
and Araschnia levana L., found, with the methods he used, no recog- 
nizable value to rest in the aggregations other than the satisfaction 
of a social instinct; and this value had lost much of its meaning, 
since the caterpillars are able to live if isolated, under which condi- 
ditions they spin small coverings in place of the usual communal 
nests. They will again take up communal life after an experimental 


CLASSIFICATION OF ANIMAL AGGREGATIONS 21 


isolation of four days. Marked sympaedia, some of which differed 
from each other in size of individuals, fused to a single sysympae- 
dium. When this divided late, the resultant groupings usually con- 
tained members derived from different original sympaedia. 

3. Sympolyandria are accidental polyandric associations formed 
on a synchoric basis, as that of Alcippe, a barnacle which dwells on 
the deserted snail shells occupied by hermit crabs, forming an ac- 
cidental heterotypical association; but the barnacles, considered 
alone, form a sympolyandria. Polyandria form a type of essential 
mating society to be discussed later in this outline. 

4. Synchoria are locality aggregations formed primarily because of 
a limited expanse of particularly favorable locations for living. Bar- 
nacles gathered together on available rocks are a good example. 

5. Syncheimadia are hibernating aggregations, such as those of 
snakes or salamanders. 

6. Synhesia are swarming aggregations under the influence of the 
breeding season, as illustrated by palolo worms. Factors concerned 
here include the simultaneous ripening of the sex cells, a limited 
favorable area, and the correct external conditions,’ which are fre- 
quently associated with lunar rhythms. Similarly, the swarms of 
May flies are due at least in part to simultaneous pupation rather 
than to sex attraction. 

7. Symphagia are aggregations about a favorable food supply, as 
flies collect about carrion or sugar. Here there is no obvious benefit 
from the association. 

8. Symporia are migration aggregations joined either because they 
originated in the same place or because they are going in the same 
direction, and may be illustrated by the migrating masses of fiddler 
crabs, of butterflies, or of salmon. 

9. Symphotia? occur when the aggregations collect about a source 
of light. Such a reaction is given by a great many insects, as well as 
by other animals (Mast, rorr). 


Considerations given later, particularly in chapters xvi and xvii, indicate that such 
swarms have a rather obvious survival value and hence should not be placed among the 
“accidental” groupings. 

2 Tf this type of category be included, it is necessary to include similar headings for 
tropistic collections due to the reaction to other environmental factors, such as heat, 


22 ANIMAL AGGREGATIONS 


10. Synaporia are collections due to unfavorable conditions, as 
when beetles are collected by the wind and deposited in beetle drifts 
in the same way that snow is drifted. 

Krizenecky (1923) recognizes two different types of synaporia, 
the passive and the active. The first are formed when the animals 
are passively carried together, as by wind or wave action. The latter 
are formed when animals faced with unusual disturbing con- 
ditions collect together. Such aggregations may be noted in the 
worm Enchytraeis humicolor, which ordinarily lives singly in the 
soil but which aggregates into symphagia about decaying food ma- 
terial. If the worms are placed in a dish of water, they aggregate 
into larger or smaller masses with the worms closely intertwined. 
Such clumped masses do not remain together; but after the group 
is closely formed, there comes a disintegrating movement which re- 
sults in the animals finally coming to rest scattered singly over the 
bottom of the dish. The animals remain thus scattered as long as the 
water is undisturbed. When subjected to renewed stimulation by 
adding chemicals or by mechanically disturbing the water, another 
aggregation cycle is set up. 

B. Heterotypical associations consist of collections of unlike species 
which may occur for the reasons given above, and which may be 
designated by adding the prefix /elero- to the proper term for the 
homotypical aggregation, as: helerosymphagopaedium, heterosyncho- 
rium, heterosyncheimadium, etc. Deegener recognizes also co-incuba- 
tia, which are breeding aggregations of different species of birds, for 
example, selecting a common, restricted nesting site. Finally he adds 
symphoria, which are formed when one or more species of animals 
settle upon another of different species, forming a heterotypical 
aggregation without obvious mutualism or parasitism, and are well 
illustrated by the barnacles, hydroids, snails, bryozoans, and others 
growing on the shell of an old horse-shoe crab (Limulus poly phemus). 


chemicals, touch, gravity, and the like. Rather, it seems preferable to replace this cate- 
gory by some such term as syvtropia, meaning those collections which are brought about 
by tropistic reactions to some environmental factor. Such collections occur, due to a 
combination of elements, including that of a limited space into which tropistic reactions 
lead animals to assemble and the incidental presence of numerous individuals in the 
region at one and the same time. In all these collections there is this time factor 
working; otherwise we could not recognize them as aggregations. 


CLASSIFICATION OF ANIMAL AGGREGATIONS 23 


Some of the overlapping inherent in this type of subject-matter 
classification appears when one considers a heterosynaporium, a col- 
lection of different species due to the action of unfavorable condi- 
tions, which Deegener illustrates by the growing concentration of 
water animals in a drying pond. Obviously, such a collection would 
be at the same time a heterosymphagium and a heterosynchorium. 
Apparently, Deegener would classify the animal community of 
modern ecology as a heterosynchorium, since it is composed of sev- 
eral species occupying the same place, although the individuals of the 
group are not of obvious advantage to each other. He does not ac- 
tually say that an ecological community should be so classified; he 
does use the term biocoenosis in connection with his discussion of a 
coral reef heterosynchorium. 

Part II. Essential aggregations or societies are communities of spe- 
cies of similar or dissimilar animals which have a real value for the 
individuals composing them, thereby differing from the ‘‘associa- 
tions” treated in the previous sections. 

A. Homotypical societies are composed of the same species. 

Alpha. Kormogene societies have the different individuals compos- 
ing them organically connected with each other. 

I. Primary colonies have arisen from the same mother. 

1. Reciprocal colonies are those in which all the individuals repre- 
sented stand in reciprocal relationship to each other. 

a) Homomor phic colonies have all the individuals morphologically 
similar and may be found among sponges and at certain times among 
hydroids and bryozoans. 

(1) Colonies formed by division may be illustrated by the colonies 
of Volvox so long as they remain free from specialized reproductive 
cells. 

(2) Colonies formed by budding occur commonly among the Hy- 
drozoa, the Bryozoa, and in many colonial chordates. 

b) Heteromorphic and polymorphic colonies are formed when there 
is a differentiation between the different members of the colony, as 
occurs in the hydroid Hydractinia, in which feeding, reproductive, 
and protective zodids may be recognized. Polymorphism is carried 
much farther in the Portuguese man-of-war, P/ysalia, and its allies. 
Here again we may recognize (1) colonies formed by division, as in 


24 ANIMAL AGGREGATIONS 


Volvox, when reproductive cells appear, and (2) colonies formed by 
budding, as in the hydroids. 

2. Irreciprocal colonies must be recognized in which all members 
do not contribute equally to the welfare of the whole. This is simply 
illustrated by the case of a budding fresh-water Hydra, where the 
new individual, the developing bud, has a parasitic relationship with 
the mother. 

IL. Secondary colonies develop by concrescence, as when the young 
fresh-water sponges developing from different gemmules coalesce, 
due to their proximity, and form one sponge body originating from 
several gemmules. 

Beta. Societies of free individuals may be classified as follows: 

I. Societies based on a sexual or genetic foundation. 

1. Primary societies: families in which the young are descended 
from a common father or a common mother or from common par- 
ents, and which remain together from the very first. 

a) Reciprocal families in which all members benefit from the social 
connection. 

(1) Sympaedia are composed of young of the same brood, but 
without either of the parents present. Such societies may be homo- 
morphic, as in the case of minnows or young birds, or heteromorphic, 
as in bee colonies after the queen’s swarm has departed. 

(2) Gynopaedia are composed of the mother and her immediate 
offspring, which may again be divided between homomorphic and 
heteromorphic groups. The former is represented by the mole crick- 
ets (Gryllotalpa), the earwigs (Forficula), and many birds and mam- 
mals; the latter group, by colonies of bees or ants. 

(3) Patrogynopaedia consist of a male and a female and their off- 
spring, and may be divided into monomorphic, dimorphic, and. poly- 
mor phic societies. Monogamous monomorphic societies of this sort 
are common among birds where both parents remain with the young. 

Polygamous monomorphic families are similarly common among 
many large animals, although monogamous families occur there, too, 
as among foxes. In dimorphic patrogynopaedia the offspring living 
with the parents are true larvae, as, for example, in the passalid 
beetles. The best example of a polymorphic colony of this type is 


CLASSIFICATION OF ANIMAL AGGREGATIONS 25 


given by the termites, where sexually mature males and females of 
one or more grades occur in the same nest with soldiers and 
workers. 

(4) In a patropaedium the male remains with his offspring for 
some time. Schulz (1926), in his analysis of the situation in the 
brooding stickleback fish (Gasterosteus aculeatus and G. pungitius), 
concludes that the value to the male is in the psychological realm, and 
quotes Deegener with approval as saying that the nest and young are 
of lively interest to the male stickleback, their loss is a misfortune, 
and the nesting and brooding phenomena are a source of inner peace. 
Obviously, such assertions are not susceptible of demonstration. To 
the eggs there is the benefit of added certainty of fertilization, of 
protection from other fishes, of aération, with resulting protection 
from fungus growth; while the young find a favorable place for 
development, passive protection by the nest, and active protection 
by the guarding male. The relation between eggs and young and the 
brooding male is essential rather than accidental, and therefore 
forms a true society. It is reciprocal, and the female is not concerned 
after the eggs are laid; therefore a patropaedium, which had its 
origin in a polygamous connubium existing merely as a mating rela- 
tionship, but this connubium is an association rather than a society. 
If the male dies, the society becomes a simple sympaedium, which 
would be accidental in nature, since the association of the young has 
no value for them. The relation of the young to the nest has a syn- 
chorium factor. The existence of the patropaedium is necessary for 
the well-being of the eggs but not of the young fishes. The relations 
between the males of the large- and small-mouthed black bass, the 
bullheads, and the fresh-water dogfish (A mia calva) and their nests 
and young give an opportunity for similar analyses. 

b) Irreciprocal families are those in which the social values rest 
only with the young. 

(1) Gynopaedia of this sort are to be found in the leeches (Glossi- 
phonia), according to Deegener; but Schulz detected evidence which 
led him to conclude that the female leech is somewhat interested in 
her eggs, and on this account he places these leech gynopaedia among 
the reciprocal societies. Similarly, careful observation might show 


26 ANIMAL AGGREGATIONS 


the same sort of value, if such it can correctly be considered, in the 
other cases cited by Deegener, such as the amphibians, Hylodes 
lineatus and Pipa pipa. 

(2) Patropaedia of this sort are thought by Deegener to be illus- 
trated by the relations in the obstetric toad Alytes, in which the 
male carries the strings of eggs twisted about his legs, and in Rhino- 
derma darwini, a small cricket-like frog of the moist beech forests of 
Chile. The male of the latter species takes the fertilized eggs and 
crams them into his singing pouch, which becomes greatly enlarged 
during the breeding season. Here they develop and transform, hop- 
ping forth from their father’s mouth as fully developed small frogs 
(Barbour, 1926). 

2. Secondary societies are those in which the individuals are not 
together from the very beginning, or at least those in which the 
primary social group becomes modified by secondary additions. 

a) Sexual societies of the Protozoa are such as are shown in ciliate 
conjugation. 

b) Connubium simplex of the Metazoa is a grouping in which mat- 
ing occurs between animals of the same species but of different sexes, 
or between hermaphroditic animals. 

(1) Polygamy includes polygyny, or the mating of one male with 
more than one female, as in polygynous birds, such as the domestic 
fowl, and in many mammals; and polyandry, in which several males 
mate with a single female without the female being free to all males. 
Among Deegener’s examples are the cases of double copulation in 
insects. In the case of Alcippe, a barnacle, the females as a rule live 
near each other, and from three to twelve dwarf males join each 
female and remain with her during their lives. Alverdes (1927) states 
that this sort of relationship is rare, but adds the case of Bonellia, a 
worm of which more will be said in a later section, and with which as 
many as eighteen males attach themselves to a given female and 
remain so for extended periods. Polyandry has also been observed 
among some spiders. 

(2) Monogamy is fairly widespread, at least in the form of seasonal 
pairings. It is found among beetles, as for example, the monogamous 


CLASSIFICATION OF ANIMAL AGGREGATIONS 27 


Passalidae, which remain with the larvae and the pupae. Alverdes 
lists also cases of at least seasonal monogamous mating among 
spiders, fishes, amphibians, reptiles, birds, and mammals. 

(3) Communal connubium, or promiscuity, occurs among many 
fishes at the spawning grounds, among certain lizards, and among 
gregarious bats. It is also reported for the American bison, for the 
American cowbird, and among various other birds (Alverdes). 
Miller (1928) summarizes evidence that this is a common state 
among anthropoid apes and certain monkeys; unlike most modern 
sociologists, he believes this represents the original mating relation- 
ship among Homo sapiens. 

(4) A conconnubium is formed when monogamous animals collect 
during the breeding period, forming small societies that continue 
during copulation. Deegener gives as examples the viper (probably 
Pelias) and birds, such as gulls, which move at mating time to a 
restricted location and there form seasonal pairs. 

c) Perversum simplex applies to those cases where males attempt 
to mate with each other, as has been observed for drones of the honey 
bee, after they are driven out of the nest in the autumn, and for 
various other insects, including certain beetles and house flies. 

d) Preconnubia occur when individuals of one sex collect at one 
place before the mating season, or both sexes may be present, but 
without mating. Such preconnubia occur among many frogs and 
birds. 

e) Synhesmia are swarming societies which collect under the in- 
fluence of reproductive drives. Androsynhesmia, male swarms; gyno- 
synhesnua, female swarms; and amphoterosynhesmia, or mixed 
swarms, are known to occur. 

Il. Societies that are not immediately based on a sexual or genetic 
basis are also known, as follows: 

1. Sysympaedia are combinations of sympaedia, such as occur in 
minnows. 

2. Syngynopaedia consist of two gynopaedia which have united as 
may happen with ants, or seals (Phoca gruenlandica), or wild hogs 
(Sus scrofa). 


28 ANIMAL AGGREGATIONS 


3. Sympatrogynopaedia are combinations of at least two patro- 
gynopaedia, and are known in monkeys, marmots, elephants, ante- 
lopes, and many other mammals. 

4. Adoption societies are those in which a female takes offspring 
from the same species. They are known for birds and mammals, for 
example among the wild hogs (Sus scrofa). 

5. Synandria are groups of males which herd together. Thus, male 
birds of several species are known to have this habit; and it is re- 
ported to be common also among mammals, as in seals and ante- 
lopes. 

6. Syngynia are similar groups of females, such as are formed by 
the stickleback fishes. 

7. Symphagia, again, are feeding societies formed of several in- 
dividuals, and illustrated by Necrophorus beetles during a portion of 
their life. 

8. Synchoria are societies united around a common place which 
has some peculiarly favorable quality or qualities. They are well 
illustrated by the common bird roosts, as of crows and robins, and, 
among insects, as wasps and Mellisodes bees. (See chap. iv.) 

g. Syncheimadia are combined over-wintering societies, and may 
be illustrated by solitary bees and coccinellid beetles. 

10. Symporia, again, are migration societies, such as swarms of 
bees or flocks of migrating birds or mammals. 

11. Synepileia are marauding societies or hunting bands, such as 
those of jackals and wolves." 

12. Sympaigma are groups of individuals brought together in or- 
der that they may engage in common play. Deegener cites the whirl- 
igig beetles (Gyrinus) as examples. Schulz (1926) has investigated 
this aggregation somewhat and concludes that play is not the prin- 
cipal integrating factor; he believes that the greater security fur- 
nished is the more important cause. Therefore he places them in the 
next category. Brown and Hatch (1929) think that the collection of 
gyrinid beetles is an example of a reaction to a general environ- 


«The American wolf pack apparently is usually a family affair, but may not always 
be so (Seton, 1929). 


CLASSIFICATION OF ANIMAL AGGREGATIONS 29 


mental pattern which they regard as more important than the bio- 
logical values involved. 

13. Symphylacia are societies that furnish protection for the in- 
dividuals composing them. 

B. Heterotypical societies are composed of individuals of different 
species. 

Alpha. Reciprocal societies. 

I. Integrated by sexual drives. 

1. Connubium confusa are societies of both sexes, but of different 
species, brought together ior the breeding season. Thus, male frogs 
will attempt to mate with females of other species, or with toads, 
or even with fish. Or another taxonomic level, coccinellid beetles of 
different species have been observed to attempt copulation. 

2. Perversum confusa are formed when individuals of the same 
sex congregate during the breeding season, although of different 
species, as for example, male frogs and toads, Rhagonycha melanura 
Oliv. with Luciola luistanica Charp. 

Il. Non-sexual combinations. 

t. Phagophilia are heterotypical reciprocal societies wherein each 
species benefits, although at least one of the two receives its food 
through its association with the other. Thus a passive species is 
freed of its parasites through the efforts of its active associates, 
showing one variety of mutualism. This is illustrated by cow- 
birds following cattle and feeding on the flies which infest the 
latter. 

2. Synsitia are also symbiotic societies in which one of the asso- 
ciates lives on the shell or the outer covering of the other, without 
being parasitic and without the type of relationship found in a_ 
phagophilium. Deegener regards the relationship between a hydro- 
zoan and a hermit crab, such as Hydractinia growing on the shell 
occupied by Eupagurus, as a synsitium. The former clearly receives 
transportation and fragments of food, while the latter may be 
protected by the nematocysts of the dactylozoids, as Deegener 
suggests. 

3. Phylacobia occur when two species live together in the same 
cavities, as Campanotus punctulatus termitarius Em., an ant, is said 


30 ANIMAL AGGREGATIONS 


to live (Wasmann, 1901~2) in the runways made by various ter- 
mites, receiving shelter and giving increased protection. Wheeler 
(t913a), Emerson, and other students of social insects are agreed that 
cases of reputed association in compound nests are in need of further 
careful investigation. Wasmann calls this relationship phylacobiosis. 

4. Trophobobia exist when one species feeds upon the excretions 
of or the waste of the other, and in turn provides protection for the 
weaker species. This relationship is found between certain species of 
ants and aphids. 

5. Symphilia are formed when one species receives food, protec- 
tion, and shelter from another, and in turn supplies excretions which 
are apparently narcotic in nature. This relationship exists between 
many ants and their myrmecocoles and between termites and ter- 
miticoles. 

6. Dulobia are illustrated by the slave-making ants which raid 
other colonies and carry off the young, which in time take over the 
routine work of the colony into which they are carried, receiving in 
return the advantages of being members of the given society. 

7. Adoption societies are formed by mutual adoption freely entered 
into by both species, and without recognizable advantages or notice- 
able harm for either. The ants, Formica consocians and F. incerta, 
are said to form such societies. 

8. Heterosymphylacia, as in the homotypical symphylacia, furnish 
increased protection for all individuals as a result of the social union. 
Thus zebras and ostriches, or giraffes and elephants, are reported to 
live together, thereby increasing the security of both constituent 
species. 

9. A heterosynepileium occurs when more than two species of ani- 
mals join forces and gain greater hunting efficiency for the group. 
Different species of storks over-wintering in East Africa have been 
observed to form common hunting bands and to conduct more or 
less organized drives for concentrating scattered grasshoppers. 

to. Confoederata are recognized by Deegener as being societies of 
unlike species united by mutual friendship or sympathy, and as 
having no other basis. Crows and jackdaws, alone or with starlings, 
golden-crowned kinglets and titmice, common creepers and wood- 


CLASSIFICATION OF ANIMAL AGGREGATIONS 31 


peckers, are given as examples. Obviously, such a category is with- 
out secure foundation, but is perhaps to be expected from a worker 
who believes that the future belongs, however the present resists, to 
the psychic and not to the mechanistic (Deegener, 19200). 

11. Heterosymporia are mixed migration societies, such as occur in 
birds and mammals, and are especially well marked on the plains of 
South Africa. 

Beta. Irreciprocal societies occur when the benefits extend mainly 
to one species, while the other may be decidedly harmed from the 
association. 

1. Synclopia, or thieving societies, are those in which one species 
feeds upon the stored food supplies of another, as thieving ants are 
known to prey upon stored termite food, or as thieving species of 
termites take the food of other termites. Wheeler calls this clepto- 
biosis; Forel designates it as lestobiosis. 

2. Syllestia are societies containing robber guests which prey upon 
the eggs or the young of the species with which they are associated. 
Thus staphylinid beetles may prey upon the brood of the ant colonies 
whose nests they inhabit, as Wheeler’s ‘“‘synechthren.”’ In somewhat 
similar relations are the hawks that prey upon flocks of migrating 
birds. The flocks of wandering grasshoppers, springbok, and the like 
are each set upon by its own particular set of predators which ac- 
companies the food flock on its migrations. 

3. Paraphagia are societies composed of harmless companions of 
their host feeding commensually on fragments neglected by the 
host. Alcippe, a boring barnacle, inhabits the snail shells which have 
been appropriated by hermit crabs, and feeds on fragments escaping 
from the feeding of the latter. Dermestid beetles occupy nests of 
other insects, feeding on waste material such as molted skins. The 
so-called synoektes of ants form paraphagia with the ants with which 
they live. 

4. Synoecium is the term given by Deegener to the association be- 
tween certain animals and the nests of other animals. This is known 
to be a widespread relationship. The crab Pinnixa lives in holes oc- 
cupied by marine mollusks. Birds’ nests have many animals regular- 
ly living in them; sparrows may build in storks’ nests. Fishes build in 


32 ANIMAL AGGREGATIONS 


the nests of other fishes (Reighard, 1920). Many similar examples 
could be given for other nests, such as those of ants and termites. 

5. Paroecia, or neighborly groups, are formed in which the less 
conspicuous animal species finds protection from the other without 
occupying a part of its nest. Thus, small fishes are frequently as- 
sociated with medusae or with the Portuguese man-of-war Physalia; 
while many animals, such as fish, worms, snails, and starfish, have 
similar relationships with coral colonies. 

6. Metrokoinia occurs in ants when the fertilized female of one 
species who has lost the ability to start a new colony joins herself 
with the fertilized female of another species that has retained this 
power, and is thus associated with a colony development which she 
would be unable to secure alone, and to which she contributes little 
or nothing. This relation has been described for Strangylognathus 
testaceus Sch., which has lost the power of colony formation, living 
in mixed colonies with Tetramorium caespitum L. 

7. Irreciprocal symporia occur when one animal species attaches 
itself to the surface of another without becoming parasitic and with- 
out contributing aid to the animal on whose back it grows. This 
relationship may exist between barnacles growing on whales, be- 
tween hydroids and crabs, and between stalked protozoans, such as 
peritrichs and suctorians, and the snails, crustaceans, or hydroids 
supporting them. 

8. Syncollesia are cemented societies in which one animal cements 
into its own covering the case or shell of another species of animal 
without killing off the original owner. Small mussels (Sphaeridae) 
and snails may be worked into the cases of caddis-fly larvae. 

9. Parachorium is the name given to the relationship that exists 
when one animal lives within the body of another without being 
parasitic upon it. Hydroids, sea anemones, polychaete worms, ophi- 
urids, and crustaceans live in the canal systems of sponges; and Pin- 
notheres, a crab, lives in the mantle cavity of Mytilus, the sea mussel. 

10. Parasitism is not easily separated from several of the preced- 
ing categories. A parasite, in the restricted sense used here, obtains 
its nourishment, at least, from the host with whose continued exist- 
ence the parasite is more or less closely bound. Frequently the nour- 


CLASSIFICATION OF ANIMAL AGGREGATIONS 33 


ishment of the parasite comes from the living substance of the host. 
Many categories of even such restricted parasitism are recognized, 
and may be found listed in reference works on the subject (Hegner, 
Root, and Augustine, 19209). 

We have given here an outline of Deegener’s classification of ani- 
mal groupings in detail, but it is not our intention to fit the different 
aggregations to be discussed later into their appropriate niches in 
this classification. In fact, certain of its more detailed aspects will 
not be referred to again. But it is upon the idea that there is an es- 
sential unity within the phenomena to be discussed that the present 
summarizing account has been prepared; this concept, although 
foreshadowed by Espinas, was first fully expressed in Deegener’s out- 
line. We shall return to it in the concluding chapters. 


CLASSIFICATION OF ALVERDES 


When we turn to the analysis of social phenomena by Alverdes, we 
find, as suggested in the introduction, that the relations composing 
the first part of Deegener’s outline are omitted as without social 
significance, since in them Alverdes cannot recognize the expression 
of a social instinct and since the entire discussion of these so-called 
associations is limited to a definition and slightly more than two 
pages of text. This omits consideration of much of the material to 
be presented in the present discussion, and limits markedly the 
field of general sociology. Even under these sharper limitations, the 
criterion of social life suggested by Alverdes, that of the possession 
of a social instinct, must necessarily be vague and easily capable of 
misinterpretation. 

The material which Alverdes believes to form the subject matter 
of general sociology is organized in the main about sexual relations, 
in which he recognizes such categories as monogamy, polygyny, 
father-families, mother-families, and other similar divisions which 
were also found in Deegener’s more inclusive outline. In addition, 
he recognizes that animal societies may be closed or open. In the 
former, new members are admitted only under special conditions, if 
at all; insect states are such. Within a closed community there is 
frequently an established hierarchy, as has been shown for birds 


34 ANIMAL AGGREGATIONS 


which, to be sure, are only partially closed communities (Schjel- 
derup-Ebbe, 1922, 1923). In the open societies membership is much 
less exclusive, and chance alone determines whether or not its mem- 
bers shall unite or separate. The open societies may be organized, 
like those of the saiga antelopes, which have guards and leaders; or 
unorganized, as in many groupings of what Alverdes regards as, 
strictly speaking, non-social insects, as when grasshoppers, butter- 
flies, caterpillars, and the like unite in migrating swarms. 


CLASSIFICATION OF ESPINAS AND WHEELER 


Wheeler, in his discussion of animal societies (1930), gives a sum- 
marized scheme of classification of social and subsocial groupings, 
based upon the work of Espinas, which is reproduced herewith in a 
somewhat modified form. The principal modifications made have 
been the placing of all distinctions between homotypic and hetero- 
typic groupings in the third and least important column, the re- 
arranging of the categories under associations, and the substitution 
of ‘“‘anthropoid”’ for “human” in the last category. Wheeler does not 
believe that the societies arose from associations, although he says 
that the ancient aggregative or associative proclivities may have 
been retained by many species and may serve to reinforce their 
specifically social behavior. This subject will receive more detailed 
attention in the last two chapters. 


CLASSIFICATION ON BASIS OF INTEGRATION 


It is illuminating to attempt a classification of social] grouping on 
the basis of the type or the degree of integration of the social group. 
Some of the available knowledge on this point will be set forth later. 
From many points of view this seems a most desirable basis of 
classification, but there is not at present sufficient exact knowledge 
to justify an elaborate attempt in this direction. When made, such a 
classification would follow the general outlines suggested by Deege- 
ner, at least to the extent that such a scheme would present the 
social organization of animals from the loosely organized, apparently 
chance aggregations due to collections around favorable locations or 
on account of physical limitations which prevent separation, through 


CLASSIFICATION OF ANIMAL AGGREGATIONS 35 


a series of small quantitatively, rather than qualitatively, different 
degrees of integration, up to the closely organized societies of ants 
and termites and the more extensive group societies of man. 


SIMPLIFIED SCHEMATIC ARRANGEMENT OF TYPES OF ASSOCIATIONS 


A. Associations: 


Loosely integrated, 
relatively unstable, 
and temporary sys- 
tems primarily de- 
pendent onthe reac- 
tions of individuals 
to environmental 
stimuli 


. Societies: 

More closely inte- 
grated, more sta- 
ble, and permanent 
systems primarily 
dependent on reac- 
tions of individuals 
to each other 


ns 


AND SOCIETIES 
(Modified from Wheeler, 1930) 


1. Passively collected aggregations or 


agglomerations. e.g., wind collected 


2. Actively collected aggregations or 
agglomerations, e.g., tropistically 
collected 


3. Food chain associations 
a) Predatory 
b) Parasitic 


4. Commensal associations 


5. Mimetic associations 


6. Symbiotic or mutualistic associa- 
tions 


7. Communities (biocoenoses) 

1. Persons (multicellular) 

2. Organically interconnected colonial 
organisms forming closed societies 


chiefly nutritive in function, e.g., 
sponges, colonial hydroids * 


3. Mainly reproductive societies closed, 


e.g., subsocial insects and social in- 
sects such as bees, ants, and termites 


4. Mainly protective societies, closed 


and open, e.g., flocks, herds, and 
schools 


5. Anthropoid societies; group societies 


Homotypic 
or 


Heterotypic 


Heterotypic 


Homotypic 


Homotypic or hetero- 
typic; i.e., may be 
pure or mixed colo- 
nies of dominant 
animals; dominants 
may be accompanied 
by social parasites or 
by various other 
sorts of associates 


The outlines of such a scheme of classification can be sketched. 


In doing so, its limitations in the present state of knowledge become 
the more evident. 
Alpha. Individuals organically connected. 
I: Individuals with true organic union, as in the hydroid Obelia. 
II. Individuals only superficially connected, as in the mollusks 


36 ANIMAL AGGREGATIONS 


Mytilus or Ostraea. 

Beta. Individuals not organically connected. 

I. Aggregations primarily due to reactions to environment. Ani- 
mals live at this level of group integration in a common habitat but 
without marked organization into groups. This category would in- 
clude the habitat communities of the ecologists. To some extent 
the plants share with the animals in the organization of this com- 
munity, usually, in fact, being the conspicuous factors in land com- 
munities; hence the modern emphasis by ecologists upon the biota. 
Further classification would depend on the physical or biotic factors 
in the environment which dominate the habitat. 

II. Aggregations primarily due to reactions to other organisms. 
These are generally recognized to be more closely integrated than are 
habitat communities, being bound together by biological relation- 
ships as well as by those of habitat. There is no sharply defined line 
to be drawn between the two. 

In addition to the subdivisions based upon the method of inte- 
gration, three fairly definite subdivisions can be recognized, based 
on degree of integration. 

t. Relatively slightly integrated groups in which the primary (in- 
dividual) reactions predominate, and whose survival value is ap- 
parent only after experimentation. The aggregations of isopods, 
Ophioderma, and Procerodes, to be discussed later, are examples. 
Further classification would depend on method of formation of the 
group and on the type of integration, as well as on the different sorts 
of animals of which it is composed. Many of Deegener’s groupings 
could be taken over here and in the next two categories. 

2. Moderately well-integrated groups in which the secondary 
(group) reactions predominate although primary reactions are still 
strongly in evidence. The survival value of the group is more ob- 
vious. Schools of fish, flocks of birds, and the like would frequently 
come under this category. 

3. Highly integrated groups in which the primary reactions are 
decidedly in the minority and the social value is strongly in evidence. 
Here would be classified the diéferent insect societies, together with 


CLASSIFICATION OF ANIMAL AGGREGATIONS 37 


the societies of man and those of the other vertebrates which ‘ap- 
proach these standard societies in their social organization. 

The difficulties inherent in the further elaboration of this scheme 
reveal at once the lack of natural divisions between the different 
levels of organization with which we are dealing. It is apparent that 
we must recognize that the whole field of interrelationships of organ- 
isms must be taken as the content of general sociology; we can only 
arbitrarily single out some particular level of social appetite, group 
reaction, community integration, social value, or exhibition of divi- 
sion of labor, as forming the beginning of social life. 


CHAPTERS ITIL 
FORMATION OF ANIMAL AGGREGATIONS 


The method of formation of animal aggregations differs with the 
degree of integration and with the different types of integrating 
factors. The discussion to be given here is not necessarily exhaustive, 
but the examples included may serve to illustrate the common meth- 
ods and some of the problems involved. 

One whole group of aggregations of individuals that are ordinarily 
solitary is caused by tropistic responses to environmental stimuli. 
Deegener recognizes one phase of this type of aggregation in his 
grouping called “symphotium” which occurs when individuals col- 
lect about a source of light. Aggregations of this general type may 
be called “‘syntropia,” as suggested earlier. The method of forma- 
tion of such aggregations attracted much attention in the three dec- 
ades and a half of J. Loeb’s work in this field, from about 1888 to 
1923. Loeb and his immediate followers were concerned chiefly with 
aggregations which result from environmentally forced orientations 
and movements. 

FORCED MOVEMENTS 

When exposed to certain stimuli, some animals react as if they 
were automatons forced by the interaction between their own organi- 
zation and their environment to move in a certain direction and to 
aggregate when available space is limited. The term “tropism” was 
at one time reserved for such reactions. These are well illustrated by 
the response of the larvae of the annelid worm Arenicola to light. 

These worms burrow as adults in the sandy tidal flats of the 
Atlantic Coast south of Cape Cod. The eggs are deposited in large 
numbers in a jelly-like mass which is attached at the opening of the 
burrow. The eggs develop into free-swimming ciliated larvae having 
two eye-spots symmetrically placed near the anterior end. Immedi- 
ately after hatching, the larvae are strongly positive to light and 
negative to gravity. Accordingly, they travel to the surface of the 

38 


FORMATION OF ANIMAL AGGREGATIONS 39 


water, where they may collect in great aggregations unless scattered 
by waves or by tidal currents. 

These larvae swim in a long spiral path orienting quite accurately 
to light. The orientation, Mast says (1911), is not entirely accurate 
but is subject to frequent muscular turnings which result in re- 
orientations. The general course is toward the light, as shown by 
the diagram (Fig. 1). The following account of the details of this 
reaction is taken from Mast’s description (1911), since he has been 
consistently critical of interpreting any animal reaction as approach- 
ing automatonism. 

“Tf the direction of the rays of light is changed after the larvae are 
oriented, they all appear to turn directly toward the source of light 
in its new position without preliminary trial movements.”’ Ordinari- 
ly, these larvae swim so rapidly that the exact details of their path 
are hard to follow. When caught under a sloping cover slip so that 
they can no longer swim spirally, if the larvae are caught lying on 
one side no definite movement is seen except a slight forward mo- 
tion; in those lying on either dorsal or ventral surface, the anterior 
end is seen to move constantly from side to side with a slight jerky 
motion, a movement undoubtedly due to muscular contractions. If 
light is thrown on such an organism at right angles, the lateral move- 
ment toward the illuminated side is at once increased, and the larva 
turns in that direction. “By using two sources of light so situated that 
the rays cross at right angles in the region where the specimen is lo- 
cated, and then alternately intercepting the light from each of the two 
sources, it can be seen clearly that the larva, by muscular move- 
ment, turns the anterior end toward the source of light directly. 
There is no trial reaction in this process. It is an asymmetrical re- 
sponse to an asymmetrical stimulation. The movement of these an- 
nelid larvae appear little more voluntary than the precise movement 
of algal swarm spores.”’ 

Galvanotropic reactions frequently produce aggregations in a 
diagrammatic fashion. Thus Paramecium, a protozoan well known 
to react usually by a reflex type of behavior which suggests Jennings’ 
designation of a “‘trial and error” reaction, exhibits a forced-move- 
ment type of behavior under the influence of a continuous electric 


40 ANIMAL AGGREGATIONS 


current. Jennings (1906), in his discussion of this reaction, says, 
“When a Paramecium is transverse or oblique to the direction of a 


1 ZZ 


Fig. 1.—(1) Arenicola larva in the free-swimming state proceeding on a spiral 
course. m,, Directions of light; a—-, positions in the spiral. Larvae react to changes 
in ray direction in positions b or d, but not in positions a and e. (2) Much enlarged 
sketch of larval head. The eye-spots are composed of a dark-brownish part y and a 
clear part «. Note the ciliary bands on (2) which are a part of the locomotor system. 
(From Mast, Light and the Behavior of Organisms; courtesy of Wiley & Sons.) 


current at the time when the circuit is closed (Figure 2) certain strik- 
ing effects are produced. If a current of medium strength is em- 


FORMATION OF ANIMAL AGGREGATIONS 41 


ployed, such as causes reversal of about half the cilia, the following 
results may be observed. On the anode side the cilia strike back- 
ward as usual. On the cathode side the cilia strike forward. As a 
result the animal, when in a transverse position, must turn directly 
toward the cathode side, the cilia of both sides of the body tending 


as, 


Fic. 2.—Effects of electric current on the cilia of Paramecia and the direction of 
turning in different positions (large arrows). The small internal arrows show the direc- 
tion in which the cilia of the corresponding quarter of the animal tend to turn the 
animal. At f the impulse to turn is equal in both directions and there is no result until 
the revolution on the long axis brings the aboral side to the cathode. (From Jennings, 
Behavior of Lower Organisms; courtesy of the Columbia Press.) 


to produce this effect, as indicated by the arrows in Figure 2. This 
happens even when the oral side is directed toward the cathode 
(Figure 2e). The animal turns toward the oral side—a result never 
produced by other stimuli, and due to the peculiar cathodic effect of 
the current.”’ 


42 ANIMAL AGGREGATIONS 


Once oriented so, the animals swim toward the cathode; if the 
current is reversed, a reversal is caused in the orientation and loco- 
motion of the animals. Many similar cases of forced orientation and 
locomotion under the influence of the galvanic current are to be 
found in the literature; in certain cases the animals move to the 
anode rather than to the cathode. A general summary of galvano- 
tropic reactions has been given by Loeb (1918). 

Similar forced movements which lead to aggregations under favor- 
able conditions are given in response to other stimuli, particularly 
those of chemicals and of gravity. They are not given by all members 
of the animal kingdom, and are more likely to be exhibited by those 
animals which, like the insects, have a disproportionate development 
of the sensory system in comparison with the central nervous sys- 
tem, so that the animal becomes the creature of its sensation (Ken- 
nedy, 1927). 

RANDOM MOVEMENTS 

On the other hand, animals may congregate as a result of a series 
of reactions, which suggest the method described by Jennings (1906) 
as “trial and error” or, as Holmes (1905) has put it, by ‘‘the selec- 
tion of random movements.” The classic case is that originally given 
by Jennings, of Paramecia collecting in the more-acid portion of the 
water they occupy. This reaction is in part, at least, a trap reaction, 
in that the animals do not react upon entering the more-acid region, 
but respond by the characteristic avoiding reflexes when they come 
in contact with less-acid water, and hence are caught in the region 
of higher acidity (Johnson, 1929). This reaction by Paramecia is so 
well known as to have been diagramed in all the current textbooks of 
zoology. It is worth emphasizing that such a method of formation 
of an aggregation, while less spectacular, is not necessarily less mech- 
anistic than is the type of reaction given by Arenicola larvae when 
they collect under the influence of directive light stimulus. It is also 
of interest to us that, as the Paramecia aggregate, the carbon dioxide 
given off as a result of their normal metabolic activities tends to keep 
the region more acid and thus the aggregation tends to perpetuate 
itself. 

When there is a limited space available, or a limited amount of 


FORMATION OF ANIMAL AGGREGATIONS 43 


optimum space, aggregations may form from either of these two 
reaction methods, the method used depending in part on the nature 
of the stimulus emanating from the favorable locality, but, in the 
main, on the reaction system of the animals involved. If the condi- 
tions are such that directive stimuli are absent, aggregations, if 
formed, will result only from the methcd of “trial.” This apparently 
happens many times in nature and in tue .aboratory. 

Land isopods (Allee, 1926) tend to collect in aggregations in the 
hot, dry summer and in the cold, and often physiologically dry, 
winter. These aggregations are frequently such as might result when 
shelter is limited, provided there is a tolerance for the presence of 
other similar animals; but at times these animals collect in much 
closer units than can be entirely explained on this basis. That is to 
say, the isopods do not occupy all the available and apparently 
equally desirable space, but clump together in one part of this. 

When the method of formation of the aggregations is studied in 
the laboratory, the grouping is found to be brought about by the 
“Selection of random movement” type of behavior. Usually the iso- 
pods wander over the surface of their container, preferably around 
the margin, and come to rest in the position in which they are ap- 
parently less stimulated. Downs (Allee, 1926) made a long series of 
observations in an attempt to find the method of formation of ag- 
_ gregations when conditions were as nearly uniform in all parts of the 
container as they could be made. Under these uniform environ- 
mental conditions the land isopods usually wandered about until 
one came to rest for some reason or other. Sometimes inequalities 
developed in an originally uniform environment; at other times the 
isopod apparently stopped for internal reasons. After one became 
quiet, there was a distinct tendency for others to come to rest near- 
by. These might or might not be in physical contact with the first; 
frequently they had crawled over it immediately before stopping. In 
their incipient stages these bunches were frequently quite loose. The 
isopods would then alternate periods of rest and of motion. During 
the latter, many, or perhaps all, might start up again; but often a 
nucleus remained, consisting of the original individual and one or 
more others. Around such a nucleus the isopods would again gather, 


44 ANIMAL AGGREGATIONS 


and the bunch would at last become consolidated by slight move- 
ments on the part of those on the periphery. Partially successful 
attempts were made to control the place of bunch formation on a 
uniform field by gluing a recently killed isopod to the substratum. 

When a drop of water was introduced on a dry background, the 
isopods tended to occupy all of that favorable location regardless of 
whether or not they were in contact. The bunching in close physical 
contact came later, and might take place as a thigmotropic reaction, 
perhaps modified by chemical stimuli, or might have been condi- 
tioned by the drying of the small moistened region. 

Similarly, detailed studies have been made on the bunching be- 
havior of the ophiurid starfish, Ophioderma brevispina Say (Allee, 
1927), which lives in the eelgrass along the Atlantic Coast of North 
America from Cape Cod southward. Individuals of this species have 
not been found in physical contact in nature during the summer and 
late autumn, but the collectors for the Marine Biological Laboratory 
report that large numbers may aggregate in late November and 
December. In the laboratory the tendency to collect in bunches dis- 
appears as conditions approach those obtaining in nature. Thus, 
bunches were absent or rare when eelgrass was present in approxi- 
mately natural condition. These relations held even under the tem- 
peratures of about 10° C. obtaining in laboratory aquaria in late 
December. 

When, however, the Ophioderma were placed in bare containers, 
bunches formed within a short time. The speed of formation was 
retarded by the slower movement accompanying low temperatures 
and dim illumination. The effect of changes in illumination are 
shown by the following example: With a constant temperature near 
20° C. one lot formed a compact aggregation in from 1 to ro minutes 
in different trials in direct sunlight; in from 14 to 25 minutes in 
diffuse light, and in from 27 to 56 minutes in complete darkness. 

Detailed observations of the method of formation of a large num- 
ber of these aggregations made under a variety of conditions show 
that the collections occur in the less illuminated part of the container 
when there is a difference in light intensity. When conditions are 
uniform, the starfish cluster about one of the least active individu- 


FORMATION OF ANIMAL AGGREGATIONS 45 


als of the lot. In both cases the aggregation forms after a large num- 
ber of apparently random movements in which the individuals react 
to the others present in much the same way that they do to pieces of 
glass rods or to eelgrass. Once formed, these aggregations tend to 
move together and so to form a more compact bunch. This may 
smack of a social tendency, although similar behavior is shown to 
occur when isolated individuals are adjusting themselves to the in- 
equalities found in a tuft of eelgrass or a loose pile of glass rods. 
These bunches of Ophioderma are formed in the same general manner 
already described for land isopods. 

Such behavior as that of the land isopods or of these starfish is 
obviously to a large extent conditioned by the reactions of the ani- 
mals to their physical surroundings. In the absence of elements usu- 
ally found in the normal physical environment, animals may so react 
to each other as partially to substitute for the normal environment; 
that is, other individuals may take the places usually occupied by 
non-living environmental items. Two types of explanation have been 
advanced for this kind of phenomenon, one of which implies some 
innate social tendency. The other explains such aggregations in more 
objective terms. 


THE FORMATION OF CELL AGGREGATES 


Roux (1894), a distinguished experimental embryologist, observed 
that when cells of the frog’s egg are shaken apart during early stages 
of cleavage and placed in water only a short distance apart, they 
slowly approach each other until they come in contact. He termed 
such cell behavior ‘“‘cytotropism.” In normal development this tend- 
ency acts to help keep the cells close together in a compact mass. 
Later Wilson (1910), Galtsoff (1925), and Child (1928), among 
others, have observed the behavior of dissociated tissue cells of 
sponges and hydroids. Some of these thoroughly dissociated cells 
move about and collect in cell aggregations which under proper con- 
ditions regulate into new organisms. Galtsoff for sponges and Child 
for the hydroid Corymorpha have concluded that these cells come 
together as a result of chance movements on the part of certain cells 
which incidentally collect other cells as they move and by chance 


46 ANIMAL AGGREGATIONS 


come together to form viable aggregations. Galtsoff’s statement con- 
cerning sponge cells is: ‘““The examination of the behavior of dis- 
sociated cells shows that the formation of aggregates is chiefly due 
to random movement of the archaeocytes which collect all the cells 
lying in their route.’ Child is more certain of the absence of definite 
cytotropism around dissociated Corymor pha cells and has observed 
cells when near together to move apart without aggregating ap- 
parently as often as he has found movement in the opposite direc- 
tion. 

The cytotropism observed by Roux can be interpreted as analo- 
gous with a very simple social appetite, or at least showing that mu- 
tual attraction between living units extends to dissociated embryo 
cells. From forces similar to those causing such simple mutual at- 
tractions of cells, we might expect social appetites to develop. Such 
a reaction may be regarded as a forerunner of the social instincts of 
many observers. On the other hand, in the formation of aggrega- 
tions of dissociated sponge and hydroid cells there is no evidence of 
such mutual attraction. The method is essentially the same as that 
just outlined for the formation of aggregations of land isopods and 
of starfishes. Yet under favorable conditions these cell aggregates 
formed without evidence of mutual attraction may develop into well 
integrated animals. 


PROTOTAXIS AND INSTINCT 


Wallin (1927) has postulated a factor or principle which he re- 
gards as of fundamental importance for many interrelations between 
cells or between whole organisms and which he has called the prin- 
ciple of “‘prototaxis.”’ This is defined as ‘“‘the innate tendency of one 
organism or cell to react in a definite manner to another organism or 
cell.” This reaction may be either positive or negative. The latter 
results in a mutual repulsion of organisms or cells, for, since organ- 
isms may be found separated for a number of reasons, Wallin recog- 
nized that negative prototaxis can be demonstrated only if the actual 
process is observed. On the other hand, positive prototaxis, which is 
“the affinity of one organism or cell for another organism or cell,” 
may result in such well-known phenomena as those of conjugation, 


FORMATION OF ANIMAL AGGREGATIONS 47 


symplasm, cell fusion, parasitism, and symbiosis. Obviously these 
are real phenomena; but, as detailed information concerning the proc- 
ess by which the cells or organisms come together is lacking, the fact of 
their being together is no more evidence for the existence of a posi- 
tive prototaxis than the separateness of other cells or organisms is 
proof of the existence of a negative prototaxis. 

If we waive this objection to accepting the principle of prototaxis 
as an all-inclusive explanation of all aggregations whether of cells or 
of organisms, and proceed to examine the nature of prototaxis, we 
find that, instead of a simple tropism which may be best understood 
as a reflex action of an entire organism, prototaxis is a compound or 
complex tropism which Wallin says cannot be analyzed. Certainly 
we can recognize different elements, such as chemotropism, thigmo- 
tropism, stereotropism, as well as reactions due to surface tension, 
temperature, light, moisture, and electrical potential. In fact, such 
an analysis indicates that Wallin’s conception of prototaxis is merely 
another name for the type of reactions referred to by many writers 
as being instinctive, except that no one would ordinarily regard the 
reaction of tissue cells as belonging in this category. Logically there 
is no real reason why they should not be so regarded, but the usage 
has been otherwise. 

Wallin’s conception of the formation of aggregates, whether of 
cells or of organisms, as being due to the expression of a fundamental 
biological tendency or principle, has two merits. In the first place 
it recognizes rightly that there is no logical line to be drawn between 
the behavior of tissue cells forming an animal body and that of plants 
or animals forming a close aggregation like those seen in symbiotic or 
parasitic relations. This is in line with the conclusions of Espinas 
and of Deegener, which I believe to be essentially correct, that 
there is no hard and fast line that can be drawn between the 
social and the infrasocial. Further, Wallin specifically recognizes 
that the ideas that have developed about symbiosis and parasitism 
have usually been based on the utility of the relationships and have 
also involved the idea of purpose. When such phenomena are con- 
sidered from the point of view of prototaxis, then parasitism and 
symbiosis and presumably all their related phenomena are merely 


48 ANIMAL AGGREGATIONS 


different end-responses in the expression of one and the same bio- 
logical principle, involving therefore only the vague type of utility 
necessary for the cumbersome working of natural selection and with 
no more suggestion of purpose than is inherent in scientific concep- 
tions generally. 

This analysis of prototaxis shows that it is in the main a renaming 
of the type of activities usually called “instinctive,” with the exten- 
sion of this sort of action to include the behavior of tissue cells and 
with a deprecation of the tendency to include a distinct teleological 
element which is usually present in discussions of instincts. The 
question immediately arises as to what social instincts or appetites 
may mean, and whether or not they are capable of analysis. 

Szymanski (1913) undertook to investigate this problem by com- 
paring the reactions of isolated caterpillars of Hyponomeuta and of 
Arye with those given by groups when placed under the same general 
conditions. Recognizing the fact that social reactions are not readily 
analyzed, Szymanski undertook to separate them into two cate- 
gories: (1) those peculiar to the individual, which, if fortunate, make 
possible the living-together of individuals as a social group, and 
which may be called ‘“‘primary reactions’; and (2) responses which 
arise as the result of the living-together of many individuals, and 
which may be called “secondary reactions.” 

In order to distinguish primary and secondary responses in a social 
group, Szymanski suggested and used the following procedure. The 
reactions of the individual are first studied with a view to finding the 
usual responses given to various stimuli; thereafter one studies the 
behavior of individuals as members of a group. In the latter study it 
is frequently possible to recognize elements of behavior which have 
been observed in the isolated individuals. If all the reactions given 
by the individuals of a colony can be recognized as primary re- 
sponses, such as would be given were all the animals isolated, the 
problem of group behavior is solved without the need for recognition 
of secondary or essentially social behavior; but if there is a residue of 
behavior which cannot be recognized as primary, then this is to be 
regarded as the secondary or true social behavior. 

Szymanski so analyzed the responses given by caterpillars of 


FORMATION OF ANIMAL AGGREGATIONS 49 


Hyponomeuta, which inhabit an irregularly rounded web usually 
placed between several branches of the food plant. The individual 
larvae exhibit no tropisms except a strong negative stereotropism. 
When placed singly on the ground, the larvae make a looplike path. 
They stop at almost every point of the loop and test out their en- 
vironment with their heads, selecting thus a place to lay their silk 
thread. Single caterpillars spinning their web behave similarly. The 
individual reaches out as far as possible from the place of beginning 
and lays down its thread. This action is repeated and results in a 
spreading of the web. Such a response to space Szymanski regards 
as a negative stereotropism. 

In one experiment eight caterpillars were observed in nest-build- 
ing. Six were placed together at one place, and one each at two 
slightly distant points. All began spinning webs as described above. 
The six spun a common web, which finally reached and fused with 
the webs of the isolated caterpillars, so that a joint web resulted. 
This probably happens in nature. So far, there are no reactions re- 
maining over and above the individual responses, and Szymanski 
concludes that in the formation of this common web there are no 
secondary or purely social reactions. 

Deegener (1922) disagrees with Szymanski’s observations and 
with his interpretations, particularly the latter. He, too, found that 
isolated Hyponomeuta larvae can spin webs, but concluded that they 
do not begin spinning as soon as if grouped together, and that the 
web spun by a group is smaller than that made by the union of webs 
spun by the same number of isolated larvae. In both respects he 
would recognize the working-out of a social instinct. Further, he 
believes that the caterpillars actively seek out the company of 
others, guided by sensing vibration waves, which may be merely 
refined touch perception. Back of all this he believes there is a need 
for association which leads the isolated larvae to seek their own kind. 
If they do not find their fellows, they build their own individual 
nests, which later they may abandon, wandering and seeking in 
order to associate themselves with other larvae. If none are found, 
they may remain solitary for days without losing their social in- 
stinct. 


50 ANIMAL AGGREGATIONS 


Szymanski (1913) further studied the formation of feeding aggre- 
gations of Arye caterpillars. Groups of these young caterpillars 
gather on their species of food plant and arrange themselves on the 
leaves so that they cling with their thoracic legs on the upper surface 
while the posterior end hangs down curled around the edge of the 
leaf. They arrange themselves so, side by side along the margin of 
the leaf. When the larva at the tip has eaten to the main vein, it 
may do one of three things: (1) turn around and go to the base of the 
leaf and begin feeding there; (2) leave the leaf entirely; or (3) cross 
over to the opposite side and begin feeding there. The older cater- 
pillars tend to lose this regular arrangement and behavior. 

By the usual type of analytical experiments the Arye caterpillars 
are shown to be positive to light, negative to gravity, and positive 
to certain touch stimuli. The method of locomotion consists in the 
extension of the anterior end and the drawing-up of the posterior. 
The posterior end shows a definite motor reflex upon stimulation. 
Thus, if touched at the posterior end, the posterior half of the body 
is raised. A similar reaction is given if the substratum is gently 
shaken. If one side is touched, the same response may occur, to- 
gether with a bending-away of the touched part. 

If one tests out the method of colony formation, one finds that 
when the larvae are placed at the base of the food plant they will 
crawl up on it, since they are positive to light and negative to grav- 
ity. When the first leaf petiole is encountered, they will turn aside 
onto that because it is narrower than the main stem, and for the 
same reason they will move along the edge of the leaf. On the leaf 
they move to the side most strongly illuminated, or to the side far- 
ther from the ground, as the case may be. 

The larvae crawl here and there over the leaf, passing over each 
other; or they may touch the larvae ahead and cause them to move 
forward. Finally one begins to eat, and gradually all settle to eating. 
The positions taken may be accidental, for wide spaces may occur 
between larvae, while others are closely crowded. The piece of leaf 
between two larvae becomes eaten away, so that eventually the 
head of the second larva touches the posterior end of the first. This 
causes the latter to raise its posterior end, as in the test experiments 


FORMATION OF ANIMAL AGGREGATIONS 51 


described above. The reaction will be repeated whenever the posteri- 
or end is stimulated, and only ceases when the abdomen curls over 
the edge of the leaf. In this way, and as a result of these reactions, 
the colony takes on its well-organized appearance, which depends on 
the interaction of the following factors: (1) the crowding of many 
individuals into a small space; (2) the tropic reactions of the larvae; 
(3) the character of the anterior and posterior end reflexes; and (4) 
the manner of locomotion and of feeding. 

Here, as in Szymanski’s analysis of the group formation in Hypo- 
nomeula, primary reactions play the principal rdle in the colony 
formation; but there are some elements of the behavior of the colony 
that Szymanski thinks may be due to secondary or social behavior. 
Thus, when the leaf is shaken, the posterior end of each larva is 
raised simultaneously. When we remember the great individual dif- 
ferences usual in behavior, the synchrony of this response suggests 
that there may be a social factor at work. However, it is possible 
that this, too, is merely an expression of primary or individual reac- 
tion, with the synchrony either more apparent than real or due to 
the proximity of the responding larvae. 

These investigations of Szymanski’s lead to the same conclusion 
as my own, formed independently, concerning the method of forma- 
tion of aggregations of land isopods and of Ophioderma. In these 
cases it is the primary, individual reactions that produce the group- 
ings, not the expression of a community spirit or of a social appetite. 
The only social trait necessarily present is that of toleration for the 
presence of numerous other similar animals within the same region. 
If this analysis be sound, as it appears to be, then one of the early 
stages of mutual interdependence is the appearance of toleration for 
the presence of other animals in a limited space, where they have 
collected as a result of tropistic reactions to environmental stimuli. 
Once formed, aggregations may persist for a considerable time, mere- 
ly because of the lack of disruptive stimuli. 

The conclusions of Szymanski are supported by Krizenecky (1923) 
in his work on the transitory aggregations of the enchytraeids al- 
ready mentioned in the chapter on classification. He thinks that 
individual reactions are important in the formation of these aggre- 


52 ANIMAL AGGREGATIONS 


gations, and that thigmotropic reactions are largely concerned. The 
observations of Essenberg and of Riley on water striders, of Clark on 
Notonecta, of the Severins on Belostoma, as well as the tremendous 
general literature on animal behavior (see Loeb, 1918, for a partial 
bibliography), show that aggregations do form in many cases with- 
out evidence of a positive social instinct or appetite, although this is 
not to be taken as proof that in other instances aggregations may 
not form as a result of social appetite. 


AGGREGATIONS OF ASELLUS IN NATURE 


The analysis of one other case is illuminating. For a number of 
years I had been seeking a favorable opportunity to apply in the 
field certain analytical methods worked out in studying aspects of 
the laboratory ecology of animal aggregations, and accordingly 
welcomed the information that a great aggregation of the common 
fresh-water isopod, Asellus communis Say, had been found in mid- 
winter in the Indiana dune country near-by. 

At the point where this collection occurred, a low sand ridge had 
been thrown up to serve as a roadway across an extensive cat-tail 
swamp, here about a quarter of a mile wide. To the east, the swamp 
stretched as far as could be seen from the low elevation of the road- 
way. To the west, there was also a very extensive continuation of 
the cat-tail swamp for at least a half-mile. The whole formed a major 
part of the headwaters of a small stream. 

The roadway was pierced at several places by culverts, introduced 
to relieve the water pressure above. These had proved inadequate, 
and at one place the water had washed away the ridge of sand and 
flowed over the roadway through an opening about 5 meters wide, 
with a current there sufficient to prevent complete freezing. When 
first seen, the ice was about 6 cm. thick and the effective stream was 
reduced to about 1.5 meters width. 

Here on the under side of the ice were tens of thousands of isopods, 
oriented to face upstream, and showing by their arrangement the 
definite lines of force of the current below. Thousands of other iso- 
pods were resting on the bottom in protected places, and many more 
were being swept downstream by the rapidly moving current. Some- 


FORMATION OF ANIMAL AGGREGATIONS 53 


times these collected into small balls of from 6 to 20 isopods, which 
rolled along the bottom until they found a lodging against some ob- 
struction or settled into a deeper pool where the current was less 
strong. There were many isopods on the sandy bottom of the stream, 
mostly facing against the current, but making very little progress 
against it. Chopping through the ice above or below the roadway 
revealed no comparable collection of isopods, although there was 
evidence of an increase in numbers as one neared the narrow chan- 
nels of the washout either from above or below. 

After the break-up of the winter ice, the majority of the isopods 
disappeared, although traces of the aggregation could still be seen, 
particularly in the sheltered places just below the opening of the 
stream into the lower swamp. There the isopods were mainly travel- 
ing downstream with the current, or were collected in sheltered 
places in deeper water or about lodged débris. As before, few were 
found in the open above the roadway. 

After the ice was entirely gone, and with the usual rise in water 
level, the aggregation re-formed. In early April a few were being 
carried downstream through the washout. Several more were to be 
seen along the margins, for the most part headed upstream, where 
some were able to make their way for a considerable distance. At 
the lower edge of the roadway, great masses of isopods had collected 
about willow shrubs, old cat-tails, or in deeper pools, wherever they 
might find a lodging. The largest of these masses was about 75 cm. 
across the current, 30 cm. up and down stream, and over Io cm. 
deep, a solid writhing mass of isopods. This was loosely joined with 
other similar units, each formed about some basis of support from 
the force of the current, the whole making an isopod barrier all along 
the lower margin of the washout, over 5 meters in length and about 1 
meter wide. The numbers concerned were unbelievable. They were 
to be measured by liters rather than by individuals. The mass can 
be imagined by thinking of the full swarms of some twenty or more 
beehives settling near each other. Conditions remained much the 
same for the next 3 weeks, with but a slight variation in the position 
of the largest mass, depending apparently on the strength of the 
current. 


54 ANIMAL AGGREGATIONS 


In late April the water level had again been raised by rain, and in 
the main current stood about 45 cm. deep, in place of the more usual 
15-18 cm. A new, smaller overflow had been formed near-by. The 
isopods were all gone from their place of aggregation; and although 
they were still plentiful all around the edge of the fan of sand washed 
down by the recent rains, they were not collected into the great 
masses found heretofore. In the slacker current just preceding the 
rains, isopods were no longer being carried downstream across the 
roadway; and one could not have collected more than 50 such drift- 
ers by watching all day. Now with the higher water level and the 
swifter current they were again being swept downstream from the 
upper swamp in numbers. 

With the higher water level of early April, smaller aggregations 
had appeared about the lower ends of the central iron culverts 
piercing the roadway, but now there, too, were dissipated. With the 
higher water of late April, the culverts situated at the edges of the 
swamp showed a marked current for the first time—not nearly so 
strong as that in the center culverts, but corresponding in strength 
to the latter when aggregation of isopods occurred near them. Now, 
for the first time, sizeable aggregations were present at the lower 
ends of these side culverts. At the upper end of one of these there 
was a log and much plant débris on and about which isopods borne 
down from the upper swamp might have lodged; but none were 
there, while they occurred in large numbers at the lower opening, 
particularly in eddies out of the main current. The current ceased to 
flow through the north marginal culvert within a few days, and the 
aggregations there disintegrated. Those at the opposite margin per- 
sisted for about two weeks. The aggregations below the main over- 
flow did not re-form, although many individuals could be seen at any 
time unsuccessfully attempting to make their way up over the shift- 
ing sandy bottom. 

The final breaking-up of the aggregations was not observed, 
though at any time small groups or single individuals might be seen 
becoming detached and borne away by the current. When the water 
rose, the increased velocity probably carried the whole lot off in a 
similar manner. At the end of the season some of the aggregated ani- 


FORMATION OF ANIMAL AGGREGATIONS 55 


mals died zm situ, especially if located at one side where the current 
became cut off. 

An increased flow following heavy rains in late May produced 
physical conditions similar to those of late April, but no aggregations 
were formed, although a few large isopods were carried downstream 
through the main spillway. 

The favorable localities were well watched the next winter and 
spring, but no large aggregations were found. In early April a small 
aggregation occurred below the culvert at the extreme south side, 
where the last one had formed the spring before. With the passing of 
time, the main washout had deepened so that a stronger current was 
running there than when the isopods had aggregated the preceding 
year. In general, there appeared to be fewer Aselli in the swamp, 
and one is led to suspect that there may have been an unusually 
large production of isopods preceding the formation of the monster 
aggregation observed in 1927. 


SEX RATIO 


In early spring one can usually determine the sex of Asellus by 
considering the size, shape of thorax, and presence or absence of the 
brood pouch. In the laboratory, sexes are easily and accurately de- 
termined. Careful observations showed that during the time of the 
great spring aggregations the ratio of the collected isopods ran as 
high as 25 males to 1 female, and never ranged below 9:1. 

November collections from the scattered isopods, both above and 
below the culverts, showed a 1:1 ratio. In early April of the next 
year, random collections from the relatively small aggregation at 
the lower end of a lateral culvert showed a sex ratio of 12 males to 
each female. At the same time, similar collections both above and 
below the aggregation showed a ratio of approximately 1:1. 

Five suggestions readily occur to account for the high ratio of 
males to females in the great bunches: 

1. The aggregation may be due to a mating or other social im- 
pulse acting more strongly in the males than in the females, which 
impels them to gather in these large groups. 

2. The males may tend to move about more and to come into 


56 ANIMAL AGGREGATIONS 


contact with the current and be swept off their feet, regaining a foot- 
hold only when the current slackens or when they reach a solid foot- 
ing. 

3. The males may possess less clinging power than the females. 

4. The females may be carried downstream as well as the males, 
but may escape from the bunches to the lower swamp. 

5. The aggregations may be formed from isopods that start up 
from the lower swamp and are unable to make progress when the 
swifter current is encountered. If this is a factor, it would imply that 
the males are more strongly positive in their rheotropic reaction than 
are the females. 

The last four possibilities would account for the formation of the 
aggregations through the operation of tropistic reactions of the iso- 
pods as individuals, the so-called “primary reactions” of Szymanski 
(1913); while the first would bring in a secondary or group reaction. 
The different possibilities may be considered in reverse order. 

The rheotropic reactions of both sexes were tested according to 
methods developed earlier (Allee, 1912). These tests indicated that, 
in the breeding season at least, the males are somewhat more strong- 
ly positive in their rheotropic reaction than are the females, and 
that they respond positively to stronger currents. In so far as the 
aggregations form as the trapping of positive isopods moving up- 
stream from the lower swamp, this helps to account for -the great 
discrepancy in the sex ratio. However, this is not the whole story. 

There is little evidence for the assumption that the females may 
be carried down from the upper swamp in the same numbers as the 
males but escape from the aggregation to the lower swamp. There 
were very few females found among the many isopods collected 
while being carried downstream. 

The supposition that the males have less clinging power than the 
females, at least in the breeding season, was subjected to direct ex- 
perimentation, using the method described by myself in 1914. The 
results indicated that there is little, if any, difference in the clinging 
ability of the males and females under the conditions of this test, 
with whatever advantage that may exist favoring the males. Such 
results are to be expected from a consideration of the mechanical 


FORMATION OF ANIMAL AGGREGATIONS 57 


difficulties of maintenance of position by females carrying a large 
brood pouch between their anterior thoracic legs. 

Of the tropistic non-social suggestions advanced as possible ex- 
planations of the greater proportion of males than females in the 
spring aggregations, one more remains for detailed consideration. 
This is the suggestion that the males move about more and so come 
into contact with the current more frequently than the more passive 
females. Such differential action would result in more males being 
swept off their feet and carried down from above, and also in more 
males coming in contact with a current strength which would call 
forth a positive rheotropic response and so bring them up from the 
lower swamp. This possibility is supported by the following kinds 
of evidence. The direction of the current impinging on a large bunch 
was artificially changed, and the current change resulted in a re- 
organization of the bunch of isopods in a new position. At a time 
when the main bunch showed a ratio of males to females of 25:0, 
25:3, 25:2, 25:2, with a total of 100:7, the reorganized bunch 
showed ratios of 50:1 and 45:4, with a total of 95:5, which is nearly 
twice the number of males per female as found in the bunch of 
longer standing. Again, I pulled from near a large aggregation a tuft 
of grass heavily covered with isopods. The sex ratio of those that ac- 
tively crawled from the grass onto my hand proved to be 4 males to 
each female. The sex ratio of all the isopods on a similar tuft was 
found to be 1 male to 3 females. In both cases the males showed a 
higher degree of activity. It is also true that the vast majority of 
animals taken while being carried downstream by the current were 
males, and that the sex ratio of the isopods on the water plants out- 
side the main current, but above the roadway, showed a higher 
number of females than males. 

Regarding the possibility that the males may be responding to a 
stronger internal séxual stimulant than the females, there is evi- 
dence from earlier work that in the breeding season the males do tend 
to cling to any passing isopod, and apparently have this tendency 
more strongly developed than do the females. The tendency to col- 
lect in bunches is so strong that spring isopods must frequently be 
tested singly for rheotropism or they will fail to respond to the cur- 


58 ANIMAL AGGREGATIONS 


rent at all. I have seen males which were responding definitely to a 
water current behave as if they perceived another isopod at a dis- 
tance of some 2-4 cm., discontinue their rheotropic reaction, and 
move directly to the nearby isopod and cling to it. I have no evi- 
dence of such reactions at distances greater than 5 cm., so that their 
effect would be operative in bunch formation only, after the isopods 
had been brought close together through the operation of some other 
factors. 

I have no knowledge of such isopod aggregations except in winter 
and spring, and unfortunately the sex ratios of the winter aggrega- 
tions were not taken. In this connection it must be remembered that 
the isopods do not start their breeding season in December in nature. 
Yet large aggregations were found at that time. The observations 
show clearly that the ratio of males to females is high in the spring 
aggregations, and suggest that this is due to the tendency of males to 
move about during the breeding season, which makes them more 
likely to be caught in the current and swept down from the upper 
swamp, and, on the other hand, more likely to come into contact 
with a current sufficiently strong to cause them to react positively, 
and so move upstream to the place of aggregation from the lower 
swamp. 

METHOD OF FORMATION OF THE AGGREGATIONS 

This subject obviously overlaps consideration of the preponder- 
ance of males, and the conclusions reached from much consideration 
of the problem are the same as those indicated there. As was to be 
expected, disturbances in the swamp just above the opening of one 
of the outlets caused a marked increase in the numbers of isopods 
carried down. These might lodge in slight depressions in the stream 
bed where the current was less strong; some 50 were observed to 
collect in a small depression less than 12 cm. in diameter within 5 
minutes following a disturbance in the upper swamp. 

Others were carried on by the current until they found physical 
support against rushes or other débris, or against other isopods which 
were in turn supported by the rushes. Thus, the bunch may be seen 
to grow on its upstream side, the newcomers using the other isopods 
as an extension of the support furnished by the lodged débris. 


FORMATION OF ANIMAL AGGREGATIONS 59 


But this is not the only method by which the aggregations are 
formed. Mention has been made already of the finding of a large 
aggregation at the lower end of a culvert whose upper opening was 
well protected by the presence of logs, grass, and other débris, 
through which the water ran easily, but upon which few isopods 
collected even at the sides where the current was certainly not of 
sufficient strength to tear them loose from available support. 

In laboratory experiments with artificial streams some isopods, 
mostly males, traveled against the current and collected in the more 
quiet water at the upper end of the trough. Similar behavior was 
repeatedly seen in nature. After the ice left, isopods from the great 
spring aggregations could be seen laboriously moving against the 
current over the sandy bed of the stream; while those located below 
the opening of one of the streams into the lower swamp, if not pres- 
ent in sufficient numbers to form an aggregation of three dimensions, 
were frequently spread thickly over the bottom, with all individuals 
headed upstream. 

Of all the isopods moving upstream, those near the margin were 
most successful. Usually, however, all were swept down sooner or 
later to the main group below. When a board was placed with one 
end resting in an aggregation so that it furnished a solid substratum 
on which the isopods might crawl, they immediately started up- 
stream as closely as they could stick on the board. On reaching the 
upper end, many were immediately washed down by the current, 
while others would continue over the precarious bottom for a short 
distance before they, too, lost their footing. If dikes were built so 
that the current impinging on an aggregation was slackened, the 
isopods started upstream in numbers, only to be swept down again 
when a stronger current was encountered. 

There is also a fatigue factor which causes the failure of these 
isopods to continue their journey upstream even in a fairly weak 
current. The length of time before reversal is roughly correlated 
with the physical condition of the isopods. In laboratory tests with 
isopods from these aggregations, reversal in a straight current oc- 
curred after an exposure of about an hour. 

If the impinging current is cut off completely by the construction 


60 ANIMAL AGGREGATIONS 


of a dam, the aggregated individuals begin a rearrangement which 
usually results in new aggregations being formed in depressions, or 
about some quiet individual or a quiet group, just as such aggrega- 
tions form in the quiet water of a laboratory tank. These groupings 
are usually less dense than those exposed to the drive of the current. 
The negative reaction to light is one of the factors conditioning this 
reaction; positive thigmotropism is another. If grass or other débris 
is present in abundance, the isopods usually collect in contact with 
the inanimate matter rather than piling up in great isopod aggrega- 
tions. There may be some collections due to positive chemotropism, 
for these aggregations cause measurable differences in their chemi- 
cal environment. 

I was much impressed, in all the observations made upon these 
groups, by the fact that so large a part of the formation of the aggre- 
gations could readily be explained on the basis of individual tropistic 
reactions to environmental stimuli largely produced independently 
of the massed isopods themselves. Relatively few of the causes of 
aggregation were left to be explained by the reactions due to social 
appetite. In this respect the situation is wholly similar to that found 
with land isopods, with Ophioderma, and with Szymanski’s cater- 
pillars. Again the main social trait exhibited appears to be that of 
tolerance for the presence of many other individuals in a limited 
space where they have collected, or—one might almost say—where 
they have been collected. The same idea can be expressed by saying 
that almost the sole social trait exhibited is immunity to injurious 
effects resulting from the presence of many others in a limited 
amount of space. It is interesting to note that there were also leeches, 
snails, and other animals collected in the same location and, to a 
large extent, by the same combination of physical forces and tropis- 
tic reactions that had brought the isopods together. 


GYRINID BEETLES 


The reactions concerned in the formation and maintenance of two 
more complex, more closely integrated types of aggregations have 
also been made. In the case of the gyrinid or whirligig beetles, giant 
aggregations may occur on the surface of streams or of still water, 


FORMATION OF ANIMAL AGGREGATIONS 61 


where the animals may be resting quietly or where they may exhibit 
what appears to be a perfect frenzy of erratic activity. As stated 
above, Deegener regarded these as forming play societies, while 
Schulz thought of them as having protective values. 

From the analysis of Brown and Hatch (1929) it appears that the 
aggregating behavior of these beetles is largely due to visual stimuli, 
since the aggregations break up in the dark. Further, the position 
they occupy in the laboratory tanks, though not necessarily in na- 
ture, may be determined by the lighting. These authors believe that 
the gyrinids are exhibiting a more complex type of behavior than 
that which is usually called “‘tropistic,” and refer it rather to some 
sort of configurationist behavior, in which orientation behavior con- 
sists of movements so co-ordinated that an invariant relationship is 
maintained between movements and variations of the visual field. 

They find evidence of two sorts of orientation: one in which the 
body axis is maintained in a relatively fixed position with respect to 
the base of orientation, and another in which the body is maintained 
in a relatively fixed region but without body orientation. The former 
is like the orientation called for by the tropistic theory. The latter 
bears at least a superficial resemblance to those cases where organ- 
isms move along a physical or chemical gradient in one direction 
without reaction to it but execute negative “avoiding” reactions 
when moving in the opposite direction, like the trapping of Parame- 
cia in weak acids, as described by Jennings. 

They believe that the location of an aggregation in nature is due 
to habituation to certain visual patterns, possibly of light and shade, 
to which the animals respond; these patterns are not significant in 
themselves but are a sign of the location of general environmental 
conditions which are of vital importance to the beetle and the spe- 
cies. If the patterns are slowly changed, the beetles may remain in a 
given position; and collections have been observed not to shift their 
position as much as a meter during a whole day, although the pat- 
tern of the field of vision changed radically in that time. If, however, 
the patterns are rapidly changed by a sudden increase in the com- 
plexity of the visual pattern, marked stimulation to activity results, 
which may cause a breaking-up of the aggregation. 


62 ANIMAL AGGREGATIONS 


Brown and Hatch, in their report, do not discuss the importance of 
the presence of other individual gyrinids in the immediate neighbor- 
hood in connection with the pattern complex. Rather, they give the 
impression that each beetle is reacting as an individual to a general 
environmental pattern, which traps it somewhat as Jennings regards 
individual Paramecia as trapped by a drop of acid, until a collection 
is formed. 

CATFISH AGGREGATIONS 

The young of the silurid fishes, the catfishes and the bullheads, 
exhibit a striking type of aggregation, which has been analyzed by 
Bowen (1929). In the species Ameiurus melas used in this work the 
young may be observed in the summer months swimming in close 
bunches near the surface of ditches or small ponds, packed together 
in a more or less spherical mass. A single fortunate dip has yielded 
over 500 of these minnows. 

If such a group is scattered, within a few minutes 2 or 3 individuals 
appear singly and come together somewhere near the original loca- 
tion of the entire group. Gradually they are joined by single fishes 
or by small groups which come into the same locality, apparently 
swimming at random. These show no reaction to the larger group 
until they are within 2 or 3 feet of it, when they swim directly toward 
the larger aggregation and join it. Within 30 minutes to 1 hour the 
original aggregation will have re-formed. 

-Appropriate tests showed that the individual fish were not react- 
ing to a gradient of chemical emanations from the group, for they 
do not respond to fish-conditioned water (i.e., water in which fish 
have stood until it exhibits various chemical and perhaps physical 
changes), even when the conditioning is greater than could be the 
case with an aggregation in nature. Cutting the olfactory nerves had 
no effect either on normal or on blinded fish. 

With these bullheads, as with the gyrinids, vision is the important 
factor in the formation of the aggregation and in its subsequent inte- 
gration. Neither blinded fish nor normal fish in the dark ever ag- 
gregate, and normal fish will follow a moving fish model in a way 
similar to that which results in aggregation when in the company of 
other normal fish. 


FORMATION OF ANIMAL AGGREGATIONS 63 


Ameiurus melas can sense through the skin the presence of another 
fish in motion, probably by detecting the vibrations set up by the 
tail of the nearby fish. Slight positive responses to others of a 
group are shown by blinded fish; this reaction is not affected by the 
destruction of the lateral-line organs but is almost eliminated when 
the skin is anesthetized with magnesium sulphate. 

When the bullheads come into actual contact with another object, 
a positive thigmotropic response is given. The barblets are dragged 
over the object; and by means of the sensations received, apparently 
chemical in nature, the fish is able to discriminate between paraffin 
models and live fishes, but it is apparently unable to distinguish be- 
tween fish of the same or of different species. 

Bowen sums up her observations in practically these words: “In 
the evening, as soon as it begins to grow dark, the aggregated young 
catfish separate and swim about, sweeping through the water or 
along the bottom with the barblets, giving a feeding reaction similar 
to that given by blinded fish at any time of the day. As soon as it 
begins to grow light the young fish come together into aggregations 
in which they remain for the entire day, re-forming in a short time if 
scattered by a disturbance. Some feeding may occur while the fish 
are aggregating, but it is doubtful if this occurs to any extent. Usu- 
ally the fish are in a close bunch actively pushing against each other, 
or resting at the surface in contact or close proximity. A thigmo- 
tactic reaction seems to be at the base of this behavior. Unless dis- 
turbed, older fish in the aquarium rest during the day in contact with 
the substratum, or more often in contact with one another. By 
means of aggregations the young fish can satisfy their positive thig- 
motaxis even while in motion. The pushing in a group suggests the 
importance of this. Catfish will also push against other species of 
fish, which, however, do not reciprocate. This contact reaction is 
largely one of pressure, but gustatory response apparently plays 
some part, as shown by the different responses to paraffin models and 
to fish. Whether this factor is instinctive or is influenced to any ex- 
tent by conditioning is yet to be determined. The reaction is ap- 
parently not species specific, since there is no evidence that young 
catfish show different behavior toward members of their own species 


64 ANIMAL AGGREGATIONS 


than toward other forms.” The aggregation arises from the tend- 
ency of the other catfish to respond by appropriate positive reac- 
tions, instead of making-off as fishes of other species do when ap- 
proached. 

With these fairly well-integrated aggregations of young bullheads 
analysis shows that the social appetite is diffuse rather than specific, 
and that in the normal fishes, aggregation involves a sight reflex, a 
touch reflex, and possibly a low-frequency vibration reflex, all of 
which may be given to other moving objects, non-living as well as 
living; a chemical reflex, the sign of which is reversed with non- 
living models; and, finally, reciprocal behavior on the part of the 
different individual members of the aggregation. The matter of re- 
ciprocal responses contributes the distinctly social element in this 
behavior. The extent to which the combination of these reflexes into 
a functioning whole depends upon the presence of an inherited social 
appetite, or upon early conditioned behavior, remains to be investi- 
gated. 


CHAPTER IV 
GENERAL FACTORS CONDITIONING AGGREGATIONS 


In many animal species the formation of an aggregation depends 
on the physiological state of the animal. This may be controlled by 
internal developments, such as the maturing of the sex products, or 
by external factors, as when land isopods are made to bunch by con- 
trolling the moisture of the substratum; but more commonly the in- 
ternal and external conditioning factors work together closely. Some 
of the more outstanding of these are discussed here. 


THE BREEDING SEASON 


W ater isopods.—My own attention was drawn to the general prob- 
lem of animal aggregations in 1911, while studying the factors con- 
trolling the rheotropic reaction in the common water-isopod A sellus 
communis. As spring came on, the stream isopods no longer gave 
highly regular, positive responses to the water current; but, as stated 
in the preceding chapter, one might strike across a strong current, 
guided apparently by sight, and seize another isopod, male or fe- 
male. From such a beginning one might soon have all the isopods 
under observation gathered into a compact rounded cluster, rolling 
over and over in the water. 

During the height of the breeding season stream isopods disregard 
the stimulus of a water current almost completely unless they are 
relatively isolated. On the other hand, I have repeatedly tried to 
induce half-grown Aselli to form such a cluster, even placing them 
in a watch glass with rounded, smooth bottom, where they were con- 
tinually brought in contact with each other, but no real aggregation 
resulted. Bunching may be induced in adults out of the breeding 
season, but many conditions that favor it in April during the height 
of the breeding season have little or no effect in late May (Allee, 
1923). 

65 


66 ANIMAL AGGREGATIONS 


Mosquitoes and midges.—Culicidae and Chironomidae form swarms 
of males which maintain position as groups, although the individuals 
within the swarm are continually darting from one part of the swarm 
to another. Such swarms have been known for years, although their 
significance has not been generally understood. Knab (1906) cites 
his own observations on the swarming of mosquitoes and reviews the 
literature to show that the swarms are composed of males which 
hover over or near prominent objects such as trees, corn shocks, 
house gables, or people. Enormous numbers of these dipterans may 
be present in the collections, which occur generally in the early 
evening of quiet and almost windless days. Straight-flying females 
dart into these irregularly gyrating swarms of males and emerge in 
copula with one male. One such newly mated pair was observed to 
emerge from one swarm only to enter accidentally another near-by. 
The copulating pair appeared to be greatly stimulated and flew into 
the open as soon as possible. The swarming males which were as- 
sociated for even so short a time with the mated pair also increased 
their rate of flying and “danced up and down at a furious pace for 
some time” before again quieting down to their normal rate of 
gyration. With growing darkness the activity of the swarms in- 
creased, but fewer successful matings took place; the entering female 
would be set upon by two or three males, and all would fall together 
to the ground, where they would separate. Later, females ceased 
entering the swarms, and the males gradually dispersed. Counted 
sex ratios of Culex were 897 males to 4 females (Knab), and of Chi- 
ronomus 4,300 males to 22 females (Taylor, 1900). Mosier and Sny- 
der (1919a) interpret the large morning swarms of tabanid flies which 
they observed in the Florida Everglades as aggregations of males to 
which females are attracted and into which they dart for the purpose 
of mating. 

Frogs.—With the approach of spring frogs desert their hibernation 
quarters for breeding places in the shallow ponds (Cummins, 1920). 
Many hibernate in the mud at the bottom of these same ponds; but 
others winter elsewhere, perhaps in nearby bodies of water or on 
land among masses of dead vegetation, or in localities similarly 
favorable. Cummins suggests that such frogs may migrate to open 


GENERAL FACTORS CONDITIONING AGGREGATIONS 67 


water caused by the early melting of ice in a pond with proper ex- 
posure. Banta (1914), Yerkes (1903, 1905), and Noble (1923) find 
evidence that frogs may respond to frog calls and splashings, par- 
ticularly during the spring breeding season. Studies on the breeding 
migration of toads indicate that with them the voice serves as a sex 
call (Courtis, 1907; Miller, 1909; Wellman, 1917). Boulenger (1912) 
concluded that the voices of frogs and toads do not control migra- 
tions toward breeding grounds or movements of individuals at the 
grounds. Cummins later came to the same conclusion as a result of 
his observations on a partially fenced pond, since he found that 
heavy migrations followed periods in which there was no croaking in 
or near the pond, and that, on the other hand, great vocal activity 
was not accompanied by increased migration. Certainly, vocal ac- 
tivity cannot account for the similar spring migration of the voice- 
less Ambystoma. 

The immediate inception of the migratory impulse must be in- 
trinsic and is probably associated with the conditions of the sexual 
glands. In frogs it is secondarily conditioned by weather, since waves 
of migration are coincident with high relative humidity and with a 
temperature of from 41° to 52° F. The migration is independent of 
daylight. All of Cummins’ illuminating observations still give no 
information as to why the frogs congregate in a given pond or how 
they learn of its existence. He does record that migration routes are 
not direct, so that we may assume that we are dealing, at least in 
part, with random movements, probably controlled largely by tem- 
perature. Blanchard (1930) concludes that the external control for 
the breeding migration is to be found in rainfall rather than in tem- 
perature relations. 

During the breeding season a gregariousness appears among frogs 
which does not exist under usual circumstances. This is not entirely 
accounted for by the tendency which the animals exhibit to seek a 
similar habitat for breeding, for if there are only a few pairs of frogs 
in a given place, they force themselves together as closely as possible 
and the eggs form a continuous mass. 

At the height of the breeding season several males will struggle for 
the possession of a single female (Banta, 1914); the struggles attract 


68 ANIMAL AGGREGATIONS 


other males, and one female may become the center of a struggling 
mass. One such group which Banta caught had 6 males fastened 
together about a single female and 5 others nearby but not yet at- 
tached. The actual egg-laying and fertilization of the eggs is ac- 
companied by the formation of a close aggregation (Fischer-Sigwart, 
1897). In addition to the male that has been i copulo for some time, 
these supernumerary males gather and, despite kicks from the first 
male, still manage to form a close clump. In Rana fusca one may 
find single pairs, but as a rule fertilization is a community matter. 
Supernumerary males also crawl over and among the egg masses and 
effect the fertilization of ova which may not have been reached by 
spermatozoa at the time of their discharge. 

At the close of the breeding season frogs scatter and resume a 
solitary, non-social existence. 

Fish.—Similar breeding clusters of fish have been described by 
Reeves (1907) with many identical details. With the rainbow darter 
supernumerary males crowd about the spawning pair and appear 
also to shed spermatozoa. Reighard (1903) has seen such behavior; 
but in the main his studies (1903, 1915, 1920) emphasize the orderly 
spacing of breeding holdings in fish, a phase of the aggregation 
phenomenon with which the present summary is not greatly con- 
cerned. The close contact between males and females of fresh-water 
animals with external fertilization is made necessary by the extreme- 
ly short life of the gametes shed into fresh water. Reighard has 
stated that fish sperm can remain functional for less than a minute 
under these conditions. 

Snakes.—Snakes are reported to form bunches in the breeding 
season similar to those described for frogs, except that they occur out 
of the water (Ditmars, 1907; Ellicott, 1880; Ruthven, 1908). Elli- 
cott records: “I first saw such a bunch of snakes on the stony banks 
of the Patapsco River, heaped together on a rock and between big 
stones. It was a warm and sunny location where a human being 
could scarcely disturb them. I reasoned that the warmth and the 
quiet of that secluded space had brought them together. Some hun- 
dreds could be counted, and all in a very lively state of humor, 
hissing at me with threatening glances and with such persistency 


GENERAL FACTORS CONDITIONING AGGREGATIONS 69 


that stones thrown at them could not stop them nor alter the posi- 
tion of a single animal. They would make the proper movements and 
the stone would roll off; all the snakes in this lump were common 
garter snakes (Eutaemia sirtalis L.). 

“The second time I noticed a ball of black snakes rolling slowly 
down a steep hillside on the bank of the same river. Some of the 
snakes were of considerable length and thickness and as I noticed 
clearly, kept together by procreative impulses.” 

Lunar pertodicities.—Such breeding aggregations are much more 
important in fresh-water and land forms, with whom the surround- 
ings are more injurious to shed sperm or eggs, but they do occur 
among marine animals. With marine organisms the most spectacular 
expression of breeding aggregations is to be found in the case of the 
large number of animals whose breeding rhythms coincide to some 
extent with lunar periodicities. The literature on this subject is ex- 
tensive (Woodworth, 1907; Fox, 1923; Legendre, 1925; B. H. Grave, 
1922, 1927); but while the facts are plain enough, the fundamental 
causal relations remain unknown. One illustration must suffice, 
based on the account given by Lillie and Just (1913) for the swarm- 
ing of the sea worm Nereis limbata in waters around Woods Hole. 

Nereis limbata has its swarming period only after twilight. Each 
run begins near the time of the full moon, increases to a maximum 
during successive nights, and sinks to a low point about the time of 
the third quarter, again rising and falling to extinction shortly after 
the new moon. They appear in four periods or cycles during the 
summer, corresponding to the lunar cycles in the months of June, 
July, August, and September. 

Only fully mature animals swarm. The swarming begins shortly 
after twilight and lasts for only an hour or so. The swarming animals 
are attracted by the light of a lantern. Males appear first, darting 
through the water in curved paths in and out of the circle of the 
light. Females are fewer in number and swim more slowly. The 
males outnumber the females hundreds to dozens. In the next few 
minutes the numbers increase, waning again after about three- 
quarters of an hour. 

New females appear each night, but some males may presumably 


70 ANIMAL AGGREGATIONS 


reappear on several successive nights. A swarming female is soon 
surrounded by several males. These swim rapidly in narrow circles 
about her. In a little while they begin to shed sperm, probably in 
reaction to some secretion from the female, rendering the water 
milky. Soon the female begins to shed her eggs, shrinking in bulk as 
she does so, until, a shadow of her former self, she sinks through the 
water to die. Lillie and Just, following a lead from Hempelmann 
(1911), assume that the maturing of the animals is dependent on 
some relation of the life-history to the phases of the moon, involving, 
probably, through lunar tidal variations, rhythmical alterations of 
conditions of nutrition. 


HIBERNATION 


Over-wintering aggregations of animals have long been known. 
This phenomenon in social bees has been noted in scientific litera- 
ture for almost two hundred years (Reaumur, 1734-42). Barkow 
(1846), in his monograph on hibernation written over three-quarters 
of a century ago, has a short chapter in which he calls attention to 
the winter aggregations of lepidopterous larvae, adult ants, bees, 
true bugs, beetles, including the frequently observed case of the 
coccinellid beetles, carp and the eel-like Muraena anguilla, snakes, 
frogs, and a few mammals, including marmots and bats. Barkow 
advances no theory to account for the congregation of these animals 
but does state that there is a suggestion current that the animals 
come together as a result of response to their sense of smell. 

This list of over-wintering aggregations has since been much ex- 
tended, especially by Holmquist (1926), who has made extensive 
studies on hibernating arthropods in the Chicago region. He reports 
that of 329 identified species taken during the winter season, nearly 
17 per cent were more or less closely aggregated. Omitting those 
known to be of a somewhat social habit at other times of the year, 
about 9 per cent of the species ordinarily solitary in the summer were 
aggregated in winter. 

In the social bees careful experiments have shown that tempera- 
ture-control results from such clusters (Phillips and Demuth, 1914; 
Phillips, 1917); and Holmquist (1928) has demonstrated that protec- 


GENERAL FACTORS CONDITIONING AGGREGATIONS 71 


tion from flooding, and other benefits, may accrue from the cluster 
formation of hibernating ants. 

In many cases these over-wintering groups are essentially shelter 
ageregations, apparently due to the small amount of serviceable 
shelter available. Often, however, all the apparently equally desir- 
able space is not occupied, so that the aggregation cannot be entirely 
explained on the basis of unavoidable crowding. In other cases 
Holmquist has been unable to find any environmental differences to 
account for the location of the hibernating aggregation. These 
groupings are partially under temperature control; but, as with other 
phenomena connected with hibernation, the temperature control is 
incomplete, and the problem of the exact nature of the causal factors 
remains open. 

AESTIVATION 

Aestivating aggregations have been less studied. Land isopods 
will form aestivating groups which may be either homotypic or 
heterotypic. Dr. C. H. Abbott has informed me personally that they 
collect in large numbers in protected places, and so pass the long, 
hot, dry summer of southern California. 


AGGREGATIONS CONTROLLED BY MOISTURE 


The chief controls of the aestivation reaction of these isopods are 
temperature and moisture. Of the two, laboratory experiments show 
the latter to be more important (Allee, 1926). When land isopods of 
various species are placed on air-dry filter paper, they collect in 
bunches within a few minutes, unless the substratum is too dry, 
when they will run about actively until at the point of death. If the 
substratum is moist, the same isopods will remain quietly scattered. 

These relations are shown in Figure 3. In the upper picture there 
are 25 isopods in a crystallization dish photographed 30 minutes 
after being introduced into the dish, which had the bottom covered 
with dry filter paper. In the meantime they were in a darkened 
room and the exposure was by flashlight. The lower photograph 
shows the effect of adding enough water to make the filter paper 
thoroughly moist without being sloppily wet. The same animals are 
shown as in the preceding photograph, but 15 minutes later, and 5 


72 ANIMAL AGGREGATIONS 


minutes after the background was moistened. The animals not 
shown in this photograph have crawled up the sides of the dish. 

A somewhat similar effect of drought in nature is reported for the 
California quail CE gemeey 1g01). Inan unusually dry season these 


Fic. 3.—(1) Land isopods in darkened 
room on dry background of filter paper. (2) 
Same animals and conditions as in (1) except 
that the filter paper has been moistened. 


quail do not breed but remain in 
flocks during the entire summer. 
The opposite type of moisture 
control is also observable. Too 
much moisture may produce 
well-defined aggregations. Thus 
Solenopsis geminata (von thering, 
1894), a species of ant which 
often nests in lowlands, will, if 
the nest is flooded, aggregate in 
a ball of some 15-20 cm. in di- 
ameter, with the larvae and 
pupae inside. By constant rota- 
tion they avoid too long sub- 
mergence, and at length may 
come against some solid object 
and so escape from the water. 
Wheeler (1913a) cites this case 
and mentions similar instances 
in this and other species of ants. 

The formation of the dancing 
bunches of midges already men- 
tioned, which one frequently 
sees aggregated in the space of 
a half-bushel basket, appear to 
be in part conditioned by the at- 
mospheric humidity, although 
the absence of wind is another 
obvious prerequisite. 


In both these cases the environmental conditions are uniform; and 
the animals, in grouping together, react to each other only. There 
are also the place aggregations controlled by moisture, when animals 


GENERAL FACTORS CONDITIONING AGGREGATIONS 73 


will collect in a limited area because it provides an oasis of moisture 
or of dryness in an otherwise overdry or overwet environment. Thus 
land isopods can be made to collect at will in a given spot by making 
it moist. Selous (1907) gives a striking picture of the congregating of 
large ungulates about an African drinking-hole in the dry season. 
The common fruit fly, Drosophila, struggling to escape too great 
moisture, aggregates in shifting masses at the top of a projection; 
these masses continually fall apart and re-form as the flies move up 
again. Under optimal conditions all of these move out of contact 
with their fellows. 


LACK OF NORMAL ENVIRONMENT 


The snake starfish, Ophioderma, lives in eelgrass in certain loca- 
tions along our eastern coast. Repeated attempts have failed to 
find this animal in contact with others of its own kind in nature 
during the summer (Allee, 1927). They are often found near together 
but never aggregated. 

Ten of these starfish were introduced into a laboratory aquarium 
made to approach normal living conditions by the introduction of 
eelgrass. Nineteen hours later 7 of the ro animals were sighted after 
a search lasting half an hour. One was found on the bottom at the 
side away from the strongest light; 6 animals were in the densest part 
of the vegetation in the same region; and, although not in immediate 
contact, all of them could probably have been inclosed in a 5-inch 
cube. The exact location of the other 3 animals could not be ob- 
served without disturbing them. These animals in the field may 
also be close together without actually touching. Only such loose 
collections were ever seen in this eelgrass aquarium. Extended ex- 
perience with these animals in the laboratory leads me to conclude 
that the tendency to bunch is greatly reduced in proportion as 
favorable natural conditions are approximated, and that the animals 
so congregated are usually found in regions to which they have been 
directed by their tropistic reactions. 

When, however, Ophioderma are placed as they are collected in a 
glass or similar container, they form dense mats of bunched animals 
with arms closely interwoven. The aggregations form in the shadiest 


74 ANIMAL AGGREGATIONS 


part of the dish and are to be explained in part by the fact that the 
lower animals are shaded by the upper ones, and so, having satisfied 
a negative phototropism and a positive thigmotropism, they remain 
quiet. 

The position of the arms shows the strong thigmotropic reaction 
of these animals. In the recently formed bunches there are a larger 
number of free arms than in older aggregations; at first the arms tend 
to extend out and up into the water. They may be entirely free, or 
they may touch another arm only where the two cross. Even in the 
early stages of the bunching some of the arms lie nearly parallel with 
each other. In bunches of longer duration there are practically no 
free arms. In one case I saw two starfish with four pairs of arms 
paralleling each other, and only two free. Larger bunches become 
ropelike masses composed of parallel arms or of arms intertwined 
like basketry. In these older aggregations the arms of the animals, at 
first extending freely, are turned back and interwoven with the 
others so that the outer edge presents a relatively regular line. When 
these starfish are isolated and left for a week or more in separate 
dishes exposed to light, frequently the arms are moved into contact 
until they present a sort of self-bunching. 

Laboratory aggregations occur in a large number of animals. 
May-fly nymphs, various isopods, earthworms, frogs, and others 
may readily be observed to form such bunches. The behavior ap- 
pears due to similar causes to that which results in the collection 
of foreigners into communities of their own nationality in our large 
cities; that is, a group of similar animals tend to minimize for each 
other the disturbing effects of unusual surroundings. 


“SLEEP”? AGGREGATIONS 


The sleep aggregations of insects have been relatively little written 
about, even in research journals; so it seems important to bring to- 
gether a more extended summary concerning such slumber aggrega- 
tions than is needed for the better-known overnight assemblies of 
birds. 

Fabre (1915) found some hundreds of the wasp Ammo phila (Sphex) 
hirsuta assembled under the shelter of a stone on the mountain side, 


GENERAL FACTORS CONDITIONING AGGREGATIONS 75 


and speculated much concerning this gregarious condition of a soli- 
tary wasp. The Raus (1916) found three related species sleeping in 
such assemblies, from which it would seem probable that Fabre was 
observing a slumber aggregation. With Chalybion caerulum both 
males and females may be found aggregated at night in about equal 
proportions. As many as a thousand have been found in one colony. 
Marked individuals will return to the same sleeping place for at least 
2 weeks. No one knows how the male of the species passes the day; 
the female labors about the nest. 

The solitary Spex wasps appear to choose their sleeping quarters 
independently; but since they select the same sort of place, they 
tend to form spaced aggregations. Prionyx sleeps sometimes singly; 
sometimes gregariously crowded close together on the top of a weed, 
with equal numbers of males and females present but without ob- 
served copulation. The males and females of the horse fly Tabanus 
sulcifrons are reported also to collect in favorable places to sleep 
(Hine, 1906). Similar observations are on record for various other 
insects. 

There is no evident protection from enemies in such assemblies. 
The sleep may be sound, and may extend so late that early birds 
could pick off the sleeping insects in numbers, as beetles are reported 
to kill off sleeping butterflies (Floerscheims, 1906). 

Schrittky (1922) observed in Paraguay an aggregation of from 20 
to 27 butterflies (genus Heliconus) that gathered nightly during 
August and September. The butterflies could be handled in the early 
mornings without waking them. The temperature then ranged 
around 5° C. The butterflies were quite restless in the evening long 
after dark, when the temperature was higher than in the morning. 
He also observed males of the genus Tetrapedia in aggregations at 
night; females are found in temporary aggregations until the time 
of fertilization, after which they separate. 

Banks (1902) gives observations on males of the solitary digger 
bees of the genus Melissodes. He saw these bees at dusk in his back 
yard clinging with mandibles and feet to grass blades. He records 
three or four returning for several nights. He cites the record of 
Schwarz (1896) to show that Melissodes pygmeus clasp twigs with 


76 ANIMAL AGGREGATIONS 


their mandibles. Bradley also records finding Melissodes agilis cling- 
ing on dried blades of wild oats alongside a newly cut grain field. In 
this same patch were large aggregations of a number of species of 
wasps—no two species on the same blade. He accounts for the aggre- 
gations of wasps as being caused by the cutting of nearby grain. 
Boyer and Buchsbaum of this laboratory, from their unpublished 
observations, think that the Melissodes which Bradley found ag- 
gregated were present because of the cutting of the flowers on which 
they ordinarily collect. 

Von Frisch (1918) gives observations on 6 solitary male bees, 
Halictus, which returned for 4 days to the same dry stem of a plant. 
He records that the bees would return to this plant if the weather 
grew bad or if the temperature became low, even during daylight. 
For 4 evenings after his original observation he observed 5 bees pres- 
ent on the same stem, although there were other similar stems near- 
by. One bee had been taken for identification. He cannot be sure 
that the same five returned each evening. 

The Raus have several notes on bees. Concerning Melissodes ob- 
liqua they say: “. .. . We found the twenty-eight bees clustered near 
the tops of a small clump of stalks. Since it was now almost dark my 
presence did not disturb them. They were huddled together in 
groups of two to five, with only three insects occupying their sites 
singly. » 

“The next evening twenty-nine bees, only one more, were asleep 
on these five stems, all clustered on the apical three inches of the 
dead plants. At the top of another plant ten feet away, two were at 
rest. If they had chosen this site for protection alone they would 
have rested singly on the plants, but since they huddled in groups 
they must have sought sociability also. They were so close together 
in some cases as to arouse suspicion about their mating, but a close 
examination proved the idea false. 

“The following night, July 21, twenty-four of these bees were here 
to spend the night in the same way. On the 22nd, thirty were pres- 
ent. On this evening I marked part of them with white paint..... 
As fate would have it, the next evening a cow had broken down their 
chosen stems, so none of the bees were there. However, fifteen were 
found on similar weeds nearby; seven of these bore the white mark- 


GENERAL FACTORS CONDITIONING AGGREGATIONS 77 


ings. This gave evidence sufficient to prove that the same bees re- 
turn to their chosen spot regularly . .. . all were males.”’ 

Here Boyer and Buchsbaum took up the problem, using the soli- 
tary digger bee, Melissodes agilis or aurigensia, which Professor 
Cockerell in a personal communication says are variants of the same 
species. They found Melissodes active in the field only on warm 
sunny days, with the temperature 16° C. or above, depending on the 
light. When it is cloudy or cool, the bees remain inactive on the sun- 
flowers Helianthus annuus and petiolaris, which they frequent. The 
male bees usually arrange themselves so that two are in contact when 
two or more become inactive on the same blossom. Several groups 
of two’s have been found on the same flower, isolated from each other. 

The bees are invariably inactive between twilight and sunrise. 
The beginning of activity depends on the amount of light and on the 
temperature. Controlled laboratory experiments showed that in 
bright sunlight activity started under stimulation at about 7° C., 
while in dim light the first activity came at 9° C. Similarly, spon- 
taneous activity began at 18° C. in the sunlight and at 21° in dim 
light. 

Aggregations of males at night were recorded as follows for one 
particular group of sunflowers: 


Year | Singly | Pairs Groups of 3 | Groups of 4 | Groups of 5 | Groups of 6 
EQ 2 ere teicisre ¥ 6, <18 s05y T0O 31 3 I I 3 
TQ QS ror criet sreireieieisveyers 3 39 5 I fe) 


Boyer and Buchsbaum marked some of these bees with paints of 
different colors so that they could follow individual reactions. There 
were some 500 flowers in this particular group. Each of these was 
plotted and followed night after night for its bee population. In all, 
201 bees were observed in 1927. Thirty-four of these were success- 
fully painted. Of the 20 painted bees which were seen again, to bees 
returned to the flower they occupied when painted. Eight others 
returned to the same plant but to a different flower. Fourteen re- 
turned at least twice to the same plant but to a different flower from 
that on which they had been painted. In all, 37 returns were noted 
to the same flower or to a flower within ro feet of the original one, 


78 ANIMAL AGGREGATIONS 


while 24 returns were noted to some flower more than 1o feet away 
from the original. 

In 1928, 14 bees were successfully painted. Of these only 4 were 
seen again; and of these, two returned to the same flower where they 
were painted and the other two returned to a nearby flower. A study 
of the details of the observations shows that males of Melissodes 
frequently return to the same flower night after night or in cool or 
cloudy weather. They are generally found in the same vicinity on 
successive nights, even if not on the same flower. They must neces- 
sarily return to a different flower if the one on which they have been 
staying is destroyed or dries up. No bees were observed on withered 
flowers. If they are blown to a distant part of the sunflower patch, 
they tend to remain there in a narrowly circumscribed area for the 
next several days. 

It must be noted that these overnight aggregations in Melissodes 
were composed of males only. They cannot have sexual significance. 
It seems entirely possible that we are concerned here with an in- 
cipient social habit which does not extend to many solitary species 
and is not found in all individuals of the species in which it occurs. 

Swarming locusts of several different species are known to pass the 
night in dense masses both as nymphs and later when they become 
adults. Much of this literature is reviewed by Uvarov (1928). Re- 
garding the overnight aggregations of these locusts, Uvarov says: 
“The night is passed on plants in dense bands, which are extremely 
conspicuous on the background of the vegetation owing to their 
blackish general color; during the night the hoppers are in a state of 
torpor caused by the cold.” If the day is cool, the slumber bands do 
not break up as they do on warm sunshiny days. Even when con- 
siderable numbers of the South African locust, Locustana pardalina, 
have become adult, they collect at night near the main nymph 
swarm, although they may range at considerable distance during the 
day. The night clusters of the flying adults are not so dense as those 
of the hoppers. 

Faure (1923), in describing the night collecting of nymphs, says 
they gather slowly together into fairly dense masses, forming clusters 


«J. F. W. Pearson has taken 3 female Melissodes and go males from early morn- 
ing collecting on Helianthus in this locality. 


GENERAL FACTORS CONDITIONING AGGREGATIONS 79 


that closely resemble close masses of bees. They swarm on the tall 
grass, or, if this is lacking, they pack together in the low grass, on 
stones or on the ground. At sunrise the swarm gradually breaks and 
continues its migration. These night clusters are conspicuous objects 
showing up as reddish-brown patches on the veldt. Man and pre- 
sumably other animals take advantage of these aggregations to de- 
stroy great numbers of the grasshoppers. The benefits accruing are 
not known. Nikolsky (1925, vide Uvarov, 1928) thinks that they 
conserve animal heat. Holmquist’s observations on mass collection 
of ants (1928a) suggest that they may at least slow down the rate of 
change of temperature. 

The congregation of birds for seen has been widely observed 
(Brewster, 1890; Davis, 1894; Bates, 1895; Widman, 1898, 1922; 
Allen, 1925), particularly for martins, robins, grackles, and crows. 
Many other birds are reported to gather in the roosts dominated by 
martins and robins. Extreme cases of close crowding in these roosts 
are reported by Baker (vide Allen, 1925) for the crested tree swift of 
India. 

“On arriving at their proposed meeting place,” Baker says, “‘they 
fly round and round, gradually lowering their flight until one bird 
makes a sweep and settles on some part of the tree near the top. This 
is the signal for the rest to perch, and in a few minutes they are all 
dotted about the higher branches. They then begin to close up with 
the bird which first alighted on the tree, finally collecting in a feath- 
ery ball, one on top of the other. Sometimes this happens again and 
again before they get settled, but at last the twittering stops and 
they are asleep for the night. It is wonderful how compactly these 
birds close up; a flock of eleven appeared not to take more than a 
foot long by half that breadth.” 

The Indian swallow shrike is said on the same authority to have a 
similar habit. Sharp records that the colonies of mouse birds of 
Africa, small birds resembling parrots, roost in small parties that 
cling together. 

It is well known that bats also gather into sleeping aggregations 
(Goldman, 1920; Howell, 1920; Allen, 1921). They may congregate 
in clusters comprising only a few individuals, or hundreds may hang 
with bodies touching. The groups may be homotypic or heterotypic. 


80 ANIMAL AGGREGATIONS 


To the human senses these bird and bat roosts are easily detected 
by their odor, and perhaps that is a factor in guiding the bats to the 
common sleeping place. 

Allen (1921) has banded clusters of these bats. He records re- 
covering three of a group of four from the same place where they 
were banded, after an interval of three years. 

These sleeping aggregations appear to be without mating signifi- 
cance. The Raus did not see copulation among the insects they ob- 
served; and, in fact, in many cases the sleeping groups were com- 
posed of males only. The robin roosts may contain both sexes and 
all ages of birds above the nestlings. With crows the common roost 
ends with the beginning of the breeding season, except for the bache- 
lors; and in general these roosts are not occupied by the breeding 
birds. After the breeding season the birds may return in family 
groups, a situation to be discussed later at some length. Among bats 
the sexes are segregated (Howell, 1920) during the time of gestation 
and of the care of the young, at a time when contact sleeping aggre- 
gations were observed. 

At the low level of integration of aggregations, with which we are 
especially concerned, the appearance of social appetite is an inter- 
mittent phenomenon. It may be awakened by gonadal activities 
that precede the breeding season or by the conditions which induce 
hibernation or aestivation. These varying exhibitions of a stronger 
social appetite are ordinarily part of an annual rhythm, but in many 
marine forms the rhythm may be a lunar one during the warmer 
season of the year. In the slumber aggregations, the periodic 
strengthening of the social appetite has a diurnal rhythm. Aggrega- 
tions may be induced or controlled by conditions of moisture or by 
the lack of normally favorable conditions; this phenomenon may or 
may not be rhythmicalin nature. At this low level of social integration 
the social appetite is not constant in appearance and in this regard 
becomes more like the sex and hunger appetites, in which rhyth- 
mical or spasmodic appearance is one of the usual characteristics. 
In more highly integrated social groups the action of the social 
appetite is steadier, and therefore less spectacular and less easily 
recognized. 


CHAPTER, V 
INTEGRATION OF AGGREGATIONS 


THE COMMUNITY LEVEL OF INTEGRATION 


It is instructive to regard an animal as a physiological system of 
physicochemical processes in dynamic equilibrium. When this is 
understood, one is prepared for the definition of an ‘“‘animal society” 
or an “ecological animal community” as a system of organisms which 
is in the process of dynamic equilibration. 

In the case of the animal considered as an organism, the different 
parts are integrated more or less perfectly into a unit, which has 
been receiving considerable attention in the last decade in studies on 
the organism as a whole as contrasted with the study of different 
parts of organisms. One can readily see that there are highly inte- 
grated organisms under close control of the nervous system or of 
hormones, the loss of any major part of which will strongly affect the 
whole system and frequently will cause death; but, on the other 
hand, there are the lower organisms much more loosely correlated, 
where the loss of even a major part of the body causes only tempo- 
rary inconvenience pending the regeneration of replacement tissues. 
Many of these more loosely organized animals are so poorly inte- 
grated that different parts may be in active opposition to each other. 
Thus, when an ordinary starfish is placed on its back, part of the 
arms may attempt to turn the animal in one direction, while others 
work to turn it in the opposite way. With sponges, the pores ad- 
mitting water to the canal system may be open and the flagella 
engaged in pumping water into the canals, while the ostia remain 
closed so that no water can be brought in (Parker, 1919). On ac- 
count of its loose integration, the sea anemone may move off and 
leave a portion of its foot clinging tightly to a rock, so that the ani- 
mal suffers serious rupture. 

It is to such relatively slack systems that an ecological animal 

81 


82 ANIMAL AGGREGATIONS 


community is to be compared, rather than to the highly integrated 
ant or bird or man. In human society we are accustomed to the idea 
of community integration. Thus a village is composed of a number 
of families which are connected as a unit not only because they oc- 
cupy a limited amount of contiguous space but also because they 
are bound together by social organizations such as church and 
school, by economic relationships of kinship or of marriage; all of 
these knit the community into a working unit. The organization 
is loose. Individuals may come and go. Whole families may depart 
and others move into the village, and yet the village retains a defi- 
nite unity, with a more or less marked individuality which may be 
quite distinct from that of neighboring communities. 

In such a community as the village, men are associated not only 
with each other but with other animals. There are the horses that 
supply part of the draft power; cattle that give meat and milk; 
dogs and cats that provide companionship and amusement to man, 
feed on his surplus food or on other associated animals, add to the 
dirt of his household and scatter bacteria and parasites; flies that 
feed on the refuse of man and breed in the excreta of his commen- 
sals; mosquitoes that breed in water reservoirs and feed on man 
and other animals; birds attracted by the nesting sites and food to 
be found near man; rats and mice similarly attracted, and snakes 
attracted by the birds and rats and mice; insects that prey on gar- 
dens and orchards, and insects that prey upon these; as well as 
other animals with little direct relationship to the community but 
occupying the same general space. 

If a progressive town board decides to instal a hydroelectric plant, 
the river is dammed and a breeding place is furnished for thousands 
of mosquitoes; if some of these are Anopheles, the malarial parasite 
may become prevalent. The breeding range of fish and of pond in- 
sects is extended at the same time that the human population is ad- 
justing to the use of cheap water power; the dam is a matter of con- 
cern to the whole animal community. 

The consequences of an unusually mild winter ramify also through 
the entire community. One result is that many insects live over 
which would ordinarily be winter-killed. These attack orchard, gar- 


INTEGRATION OF AGGREGATIONS 83 


den, and farm, affecting the food of grain- and fruit-eating birds and 
mammals and of man himself. These are finally checked by the sub- 
sequent increase in predaceous insects and birds that live on garden 
and orchard pests, and the rough biotic balance characteristic of 
animal and plant communities in nature thus tends to be restored. 

These instances are enough to illustrate the interdependence and 
general type of organization of an animal community of which man 
is a dominant element. The removal or introduction of animals, 
whether by accident or by purposive action by man, may upset the 
whole equilibrium, as has happened with the introduction of rabbits 
into Australia. 

A similar organization has long been recognized to exist in animal 
communities of which man is only a minor part, or perhaps no part 
at all. This type of organization has been called by J. Arthur Thomp- 
son the “web of life.”” Frequently the food relationships are the most 
easily demonstrated in a group of this kind. A partial idea of the 
complexity of such organization is given by a consideration of the 
food relations of the black bass as summarized from Forbes’ excel- 
lent essay on ‘The Lake as a Microcosm.” 


INTEGRATION OF THE BLACK-BASS COMMUNITY 


The organization of animal communities is more marked in the 
case of inhabitants of small bodies of water than of equal bodies of 
land, since conditions tend to isolate such aquatic animals and since, 
through long evolution, they have become closely integrated and 
highly independent of the newer societies of the land. The life in 
such a body of water represents an islet of older, lower life in the 
midst of the higher, more recent life of the surrounding region. It 
forms a microcosm, a little world in itself. The play of life is full, but 
on a smaller scale and less confusing to observe. 

In such a community one can see fully illustrated the degree of 
sensitivity characteristic of an organic complex, which has just been 
demonstrated for a man-dominated community. Whatever affects 
one species must have its influence on the whole assemblage. It thus 
becomes apparent that it is impossible to study any one animal 
completely if it be out of its relations to other animals and to plants, 


84 ANIMAL AGGREGATIONS 


even though the animal selected for study belongs to what is usually 
regarded as a non-social species. 

It is relatively easy, though sufficiently exciting to be called sport, 
given the right body of water and the proper season and bait, to lift a 
large-mouthed black bass from the water; but if one should under- 
take to trace out all the interrelations from which the black bass has 
suddenly been removed, he will have seen the whole complicated 
mechanism of the aquatic life of the locality, both plant and animal, 
of which the black bass forms a part. 

In the food of the black bass are to be found fishes of different 
species at different ages of the individual, representing all the im- 
portant orders of the fishes; insects in considerable number, especial- 
ly the various water bugs and larvae of the May flies; fresh-water 
crayfishes, shrimps, and a multitude of the small crustaceans called 
Entomostraca, of many genera and species. 

Looking at the food of the fishes upon which the black bass feeds, 
one finds that one of these eats mud, algae and Entomostraca, and 
another takes nearly every animal substance in the water, including 
mollusks and decomposing organic matter. The crayfishes are nearly 
omnivorous; of the other Crustacea, some eat Entomostraca and 
some algae and Protozoa. The insects eaten by the bass eat each 
other, other insects, and Entomostraca. At only the second step, 
therefore, do we find the black bass directly related to every class 
of animals, many plants, and the decaying vegetal matter of the 
water. 

Turning now to competitors, which are extremely numerous, we 
find that all the young fishes, except the suckers, feed at first almost 
wholly on Entomostraca, so that the young black bass finds himself 
at the very beginning engaged in a scramble with almost all the 
other fishes in the lake for food and, in fact, not only with the fishes 
but with the insects and mollusks and larger crustaceans that also 
live on these small entomostracans. The Mollusca are not in such 
direct competition; but they do compete, since they feed upon the 
microplankton which the Entomostraca themselves take as food. 

But the competitors of the bass are not limited to those which 
take the same food, for predaceous fishes, turtles, water snakes, 


INTEGRATION OF AGGREGATIONS 85 


wading and diving birds, and the large beetles, dragon-fly nymphs, 
and giant water bugs feed on the young bass at every opportunity. 

An illustration of remote and unsuspected rivalries is found in the 
relation of the black bass to the bladderwort (U¢ricularia), which fills 
many acres of the northern Illinois lakes. Upon the leaves of this 
plant are small bladders, several hundred to the plant, which are 
tiny traps for the capture of entomostracans and other minute ani- 
mals. The plant usually has no roots and lives largely on the animals 
taken through these bladders. Ten of these sacs, taken at random, 
upon examination gave 93 animals of 26 different species, of the 
Entomostraca and insect larvae. Hence, the bladderwort competes 
with the fishes for food and, by destroying large amounts, helps keep 
down the number of black bass in an otherwise favorable lake; and 
they have an especial advantage since, when the Entomostraca be- 
come scarce, they may grow roots and live as other plants. 

These simple instances suffice to illustrate the intimate way in 
which the living forms of a lake are united. 

A different phase of the story is shown by the study of fluviatile 
prairie lakes which are appendages of river systems and form in 
oxbow cut-offs or bayous, or in other regions where the usual deposi- 
tion of materials has been retarded. Normally they are connected 
with each other during the rainy period and for a longer or shorter 
time during the summer. The amount and variation of animal life 
in them is dependent chiefly upon the frequency, extent, and dura- 
tion of the overflows. In them we may see illustrated the method by 
which the flexible system of the animal community adjusts itself to 
widely and rapidly fluctuating conditions. 

Whenever the waters of a river remain for a long time outside its 
banks, the breeding grounds of the fishes and other animals are cor- 
respondingly extended. The slow and stagnant waters of such an 
overflow, frequently enriched by sewage to a limited extent, form 
the best possible place for the growth of myriads of algae and Pro- 
tozoa. This development allows a similarly great development of 
Entomostraca. These animals increase with tremendous rapidity due 
to the pace at which their life-circle is run and to their high rate of 
reproduction. The sudden development of food resources allows a 


86 ANIMAL AGGREGATIONS 


corresponding increase in the rapidly breeding, non-predaceous 
fishes; and at last the game fishes which derive their principal food 
from the non-predaceous fishes also increase in numbers. Evidently 
the multiplication of each of these classes acts as a check on the one 
preceding it. The development of Protozoa and algae is arrested and 
sent below normal by the swarm of entomostracans; the latter are 
met and checked by the vast swarm of minnows, which are in turn 
checked by the increase in predaceous fishes. In this way a gradual 
readjustment of the conditions will occur; but usually, long before 
this new equilibrium is reached, a new disturbance of the water level 
results in the recession of the water. As the lakes grow smaller and 
the teeming life they inclose is daily restricted within narrower and 
narrower bounds, a fearful slaughter ensues. The predaceous fishes 
thrive for a time, since their food is more easily caught; but finally 
they too are thinned out by the lack of food and of space. 

Year after year in such lakes and in other animal communities 
there is a fairly steady balance of organic life. The community re- 
mains in dynamic equilibrium. The rate of reproduction about 
equals the death-rate. Every species must fight its way from hatch- 
ing to maturity. Adults are as rare as human centenarians; yet no 
species is exterminated, and each is maintained at the average num- 
ber, for which we have reason to think there is sufficient food year 
after year. Two ideas explain the order that is evolved in such com- 
munities. First, there is the background of common interests among 
all elements of the community. New evidence concerning the nature 
of some of these common interests will be presented shortly. Second, 
there is the struggle for existence and the elimination not only of the 
less fortunate but, at times, of the less fit animals. 

Upon such a foundation as this, modern comparative sociology is 
built in part, and must be built in entirety if it is to be solidly ground- 
ed. With this conception of the type of integration existing in eco- 
logical animal communities, and with the realization that even such 
loosely knit communities can be regarded as constituting a unit, we 
are better prepared to search for integrations in animal aggregations 
and to evaluate those found. 


INTEGRATION OF AGGREGATIONS 87 


AGGREGATION INTEGRATION 


As has been said before, a decided advance toward social life is 
made by the appearance of tolerance for other animals in a limited 
space, where they have collected as a result of random movements or 
of tropistic reactions to their environment. This may occur in con- 
nection with some phase of breeding activity, but it may also be 
exhibited without sexual significance. Some of the less complex of 
these aggregations may exist because there is an absence of dis- 
sociating factors among a group of animals that have been hatched 
out in a restricted locality or that have been brought together by 
any other process. Thus, some of the aggregations resulting from 
tropistic responses may well owe whatever permanency they possess 
to the absence of disruptive factors rather than to any inherent gre- 
garious tendency or apparent advantage. 

Another advance in social life is made when these groupings con- 
fer especial survival values upon at least some of the individuals 
composing them. Such an advantage is illustrated by the slower 
rate of moisture change in an aggregation of land isopods out of 
water equilibrium with the surroundings. Under conditions of 
drought this results in a definite prolongation of life for the members 
of a group. Other examples will be discussed later. 

The land isopods and Ophioderma have gone little beyond such a 
stage in their social development. There is some slight evidence of 
mutual attraction, but the experiments to date do not indicate how 
much of this would also be exhibited toward similar inanimate ob- 
jects. There is also slight evidence of integrated group behavior, in 
that the bunch shows occasional periods of activity apparently 
originating in one individual and passed mechanically through the 
group. Such activity may be the beginning of disintegration of the 
group; but it frequently results in a closer aggregation, because the 
animals may move closer together during their brief period of ac- 
tivity. 

The state of development of integration by means of which the 
group, once it appears, acts as a unit is a very important criterion of 
the degree of social development it has attained. When there is no 


88 ANIMAL AGGREGATIONS 


integrative action, one is dealing with a crowd, a mere collection of 
individuals within a limited area. Apparently it was this aspect that 
Szymanski (1913) had in mind in distinguishing between primary 
reactions, the reactions of the individual, and secondary reactions, 
the reactions of the individuals as members of a group. 


TACTILE INTEGRATION 


The simplest form of group organization is found when animals in 
physical contact respond as a group to touch stimuli passed from one 
to another. Such organization may be sufficiently refined for the 
whole group to show definitely synchronous behavior. Collections of 
Liobunum, the harvestman, have been observed by Newman (1917), 
and later by myself, to give such reaction. One group was found by 
Newman resting on the under side of an overhanging shelf of rock on 
a steep hillside. The harvestmen were closely packed together within 
an area of about 5 feet in diameter. When first seen, they were hang- 
ing from the rock roof perfectly motionless. When the observer came 
nearer, they began a rhythmic stationary dance practically in uni- 
son. This died down shortly but could be started again by appro- 
priate mechanical stimulation. 

When the colony was first seen, the long legs of neighboring in- 
dividuals were interlocked, which would sufficiently account for the 
transmission of stimuli through the group. It should be noted, since 
we are interested in the state of integration of the aggregation, that 
the rhythm was not perfectly synchronous at the beginning but be- 
came practically so after a few seconds. 

Such integration, due to tactile transmission, must be present to 
some degree in all cases of aggregation in close physical contact. It 
is highly developed in the sleeping groups of bats (Allen, ro2r), 
which may hang in compact clusters, as already mentioned. If one is 
touched, the whole cluster may drop. Allen caught eighteen by hold- 
ing an insect net under the group and touching only one of the outer 
bats. 

The effect of physical contact in establishing synchrony in two 
reacting systems is illustrated by the observation of Fischer (1924), 
who found that two pieces of embryonic heart planted out in tissue- 


INTEGRATION OF AGGREGATIONS 89 


culture media beat at different rhythms even when taken from the 
same individual and kept as far as possible under identical cultural 
conditions. When two such pieces succeeded, by outgrowths from 
each, in establishing close organic union, the two beat in unison. 
Such a modification of behavior may involve factors of transmission 
distinct from those we usually regard as tactile. 


CONTACT AND ODOR INTEGRATION 


Sex recognition frequently causes animals to give characteristic 
group reactions; often there are only two animals forming a diminu- 
tive group. Sex recognition is frequently accomplished by contact 
relations alone. Such behavior is recorded for crayfishes (Pearse, 
1909; Andrews, 1910), spiders (Montgomery, roro), frogs (Banta, 
1914), amphipods (Holmes, 1903), as well as others. 

Among other methods of sex recognition, that due to chemical 
sense deserves prominent mention. This is well illustrated by the 
long distances certain male moths will fly to cluster about a female 
ready to copulate (Kennedy, 1927). Animals may aggregate at other 
times than the breeding season, due to the same sort of stimulus; 
and this stimulus is also frequently effective in maintaining the 
aggregations once formed. In fact, it is common for the principal 
stimulus causing animals to congregate to be the effective one in in- 
tegrating their aggregation. 

These two senses, odor and contact, are sufficient means of group 
integration to form the basis of well-unified societies. Much of the 
social organization of the ants and the termites appears to be based 
on them. The ants apparently live in a world of contact-odor shapes, 
as we live primarily in a world of color-shapes (Wheeler, 1913¢). 


VISUAL INTEGRATION 


Sight plays an important réle in the organization of many animal 
groups. When one vulture, soaring aloft, sees another swoop miles 
away, he moves over and also swoops; his action is seen by others, 
and thus these scavenger groups congregate rapidly, although they 
are practically lacking in a sense of smell. 

Aggregations of male frogs in the breeding season will follow and 


go ANIMAL AGGREGATIONS 


frequently tightly clasp any moving object, whether salamander, 
fish, or other males; and this reaction is based at least in part on 
sight. Aggregations of young catfishes are primarily integrated by 
sight and secondarily by water vibrations and chemical-touch sen- 
sations (Bowen, 1930). 

Other instances might be multiplied; but one spectacular one, that 
of the synchronous flashing of fireflies, must suffice. A considerable 
controversy has been waged over this subject, but the observation 
experiments of Hess (1920) seem to have established the fact of its 
occurrence. He found a valley of fireflies flashing in unison, with the 
flash apparently initiated on a hill at one side, from which it spread 
almost instantaneously over the valley. The next night in the same 
place the observer was able to obtain at least partial control of the 
flash and to alter to some extent the intervals between flashes. With 
a pocket flashlight he gave the initiating signal just before it would 
normally have occurred, and the insects followed the artificial lead 
until the interval was reduced to three-quarters its original duration, 
and then one-half. At the second trial at one-half the original period 
fewer insects followed the flashlight, and after that the flashing in 
unison was broken. 

Such synchronous flashings of fireflies are apparently more com- 
mon in the orient. Morrison (1929) has published a recent note upon 
their occurrence in Siam, based upon three years’ experience there. 
His account follows: 

“During the months of July, August, September, and until the 
heavy rains set in, on any dark night it is possible to see whole 
stretches of the river or canal banks lit up by the flashing of myriads 
of insects. These areas of synchronism may extend for several hun- 
dred yards at a stretch or may be confined to single trees, glowing 
and being extinguished with surprising regularity. Actual timing of 
this intermittence showed that luminescence occurs at the rate of 
approximately 120 times a minute. During the period between the 
flashes the light of the fireflies reached almost complete extinction, 
the intensity being so low that at a few feet from a tree of actively 
luminescing insects it is quite invisible. 

“Perhaps one of the first things which is called to the attention of 


INTEGRATION OF AGGREGATIONS QI 


the observer is the fact that this synchronism is confined to localities 
bordering on streams, or to low, water-saturated ground..... 
Around Bangkok it is commonly known that the synchronal flashing 
of fireflies is confined to one particular tree, the ‘ton lampoo’ of the 
Siamese—Sonneratia acida. In all of the observations which the writ- 
er has made, no exceptions to this have been found, but whether this 
particular tree is the gathering-place of the insects in cases of syn- 
chronism reported from other parts of the East is a question. 

“The fact that Sonneratia acida is the tree on which the insects 
congregate around Bangkok leads one to question the statement that 
has been frequently made to the effect that the synchronal flashing 
of the fireflies is a mating adaptation. S. acida is found both in man- 
grove associations, and also as a solitary tree growing along the 
banks of streams. In these latter cases the roots of the tree are often 
immersed in water, the tree at times standing several feet from the 
bank. If the females of the species are wingless, as is the case with 
the majority of the North American Lampyridae, there would be no 
opportunity for them to approach the tree. Furthermore, at no time 
have females been found on a tree of actively synchronizing insects, 
or within its vicinity. Observations on this point have been repeated- 
ly made and have been corroborated by local entomologists who 
have become interested in the problem. 

“Perhaps one of the most popular theories as to the cause of 
synchronism is that of ‘sympathy.’ According to this idea there is 
some particular insect which acts as a pace-maker for the rest, and 
they follow him, regulating their flashes by his. However, due to the 
fact that the insects are scattered quite generally over a tree and are 
not within sight of any one particular animal, this appears to be 
quite impossible. Furthermore, any follow-the-leader action on the 
part of the insects would result in a wave of light passing over the 
tree and originating from a definite point, a fact which is not the 
case once the synchronism has begun. 

“Tt is possible to inhibit the synchronism of a tree of insects by ex- 
posing them to a bright light for about a minute. When the light is 
turned off, the synchronism returns having its origin, apparently, in 
some individual or group generally located in the central part of the 


92 ANIMAL AGGREGATIONS 


tree. From this group, then, the synchronism extends over the entire 
tree in an irregular wave until all of the insects are flashing in 
unison. 

“Synchronism usually begins shortly after darkness has set in, the 
fireflies emerging from the nearby thickets and flying in an indirect 
course to the Sonneratia trees. During this flight to the trees there 
is no sign of a concerted flashing, the actions of the insects being 
similar to those found in our local forms during flight.” 

It seems probable that with these fireflies we are dealing with a 
phenomenon of two distinct aspects (Blair, 1915). One is a recovery 
response similar to recovery from fatigue. Such flashing would rarely 
be synchronous or near-synchronous. On the other hand, there ap- 
pears to be a releasing stimulus which, in the cases observed by Hess, 
might come either from the pace-setting flash of a firefly or of an 
electric torch. This brings up the problem of the leader in group inte- 
gration, for which we have not space here. It is discussed at some 
length by Child (1924). 


INTEGRATION BY SOUNDS 


Among many animals group organizations occur as the result of 
sound production. To be sure of this, one must have evidence that 
behavior is altered as a result of sounds. The fact that collections 
of animals, such as frogs or insects, are producing sounds which are 
loud to the human ear is not good evidence that they have group 
significance (Lutz, 1924). There is evidence that among some ani- 
mals sounds may be used in sex recognition. Perhaps they are more 
often of sexual significance in general sex stimulation which, while of 
advantage to the group, may yield no advantage to the producer of 
the sound; and may even result disastrously in the case of the young 
deserted by a nesting bird which had been stimulated to renewed 
sexual activity by an outburst of song. Such cases have been re- 
ported by creditable ornithologists (Sherman, 1924). 

Ohaus (1899-1900) and Wheeler (1923) report that the Passalus 
beetles, which have the habit of boring in logs, are kept together by 
auditory signals; and Professor Wheeler has more than once spoken 
of his observations, indicating that aérial sounds may play a part in 


INTEGRATION OF AGGREGATIONS 93 


the organization of ant colonies. But on this point there are other 
observations to the contrary (Fielde and Parker, 1905). 

Beebe (1916) thinks that there is a close correlation between habi- 
tat and habits of tropical birds and the development of their voices, 
which are popularly supposed to be one of the most striking attri- 
butes of tropical birds. He reports that solitary birds, living in the 
open country where the view is more or less uninterrupted, have a 
tendency to possess negligible voices. Inhabitants of dense jungle, 
if relatively solitary, have remarkable vocal powers, with loud 
staccato calls or with insistent rhythm, by means of which they com- 
municate with their unseen fellows. Such birds may be nocturnal in 
habit. Birds living in pairs or in families have, for the most part, 
vocal organs which they use to good effect; but they lack the super- 
lative voice development of solitary birds. Birds living in flocks have 
voices that are still less in evidence, though there are notable excep- 
tions to this rule, as, for example, the parakeets. 

In the matter of vocal performance, as with tactile and visual 
integrations, group unisons have been reported. The group singing 
of the western meadowlark is an example among birds. One of the 
most interesting cases is that of the snowy tree cricket, which has 
been much studied and which Fulton (1925) reported to effect 
changes of chirping rate in order to chirp in unison. 

Shull (1907), a careful and critical observer, concluded early in 
his studies that real synchrony does exist in the chirping of the tree 
cricket; but later he somewhat modified this opinion, saying that 
while he still believed that the singing insects do influence one an- 
other, he believed that cases of exact synchronism were usually 
accidental. Lutz (1924) was skeptical both concerning the fact of 
synchronism and concerning its importance with tree crickets. Ful- 
ton (1928, 1928a) in recent studies appears to have furnished con- 
clusive evidence that the Oecanthus song is both rhythmical and 
synchronous. After the usual listening tests, revealing almost per- 
fect synchronism, a number of the singing insects were placed in 
another cage at some distance, and the front tibiae containing the 
auditory organs were removed. This effectively broke up the syn- 
chrony except at those times when the individual rhythms appeared 


94 ANIMAL AGGREGATIONS 


to coincide for a brief period. Fulton records that “‘when three or 
more mutilated males were singing at once an utter confusion of 
notes resulted, so that the rhythmical quality of their songs was 
entirely obscured.” The removal of the tibiae did not seem to affect 
the general health of the insects. The loss of one or more legs ap- 
pears to be a matter of relatively small importance among these 
insects; they lived as long as did those with the ordinary quota of 
legs. Similar observations were made by Fulton on a katydid and on 
a grasshopper known as the ‘‘Nebraska conehead.”’ 

Synchronic behavior may, of course, merely mean that the group, 
while reacting as individuals, receive the stimulus at the same time 
and so react simultaneously. This is illustrated by the responses 
Minnich (1925) obtained when he exposed aggregations of caterpil- 
lars to various sounds. Such synchronism has no bearing upon the 
problem of group integration; but synchronism, such as described by 
Fulton, of responses by members of the group to each other may well 
have group significance. 

Buxton (1923) records an observation made some years before 
upon the production of rhythmical sounds by termites. ‘“T noticed,” 
he says, “small numbers of winged termites emerging at one p.m. 
from a subterranean nest under stones in a shady place by the road- 
side. The ground round the mouth of the nest over a radius of three 
feet was covered by thousands of small soldiers and a small number 
of large soldiers. All of these were making a rhythmical sound which 
resembled the noise made by sand falling on brown paper and which 
was caused by tapping their heads on the dead leaves on which they 
were standing. The sound was produced in perfect time at a rate of 
about 48 beats per minute, and in the intervals between the beats 
there was complete silence. This remarkable performance was not 
disturbed by my collecting a considerable series of the performers, 
but an hour later when I passed the spot, the emergence of 
winged adults had ceased and not a soldier was to be seen above 
ground.” 

The termites were determined by Silvestri to be Acanthotermes 
militaris Hag. Buxton does not believe that the rhythmical nature 
of the sound production could be explained by substratal vibrations, 


INTEGRATION OF AGGREGATIONS 95 


since the termites were standing on many different dead leaves scat- 
tered over a considerable radius. Gounelle (1900) had previously 
described the sound produced by termites by tapping their heads on 
plants as being like the sound produced by a pinch of sand hitting 
paper, but he did not record synchrony. Emerson (1928) found that, 
despite the possession of the so-called “‘auditory organs’ on the 
tibiae, Nasutitermes guayanae did not respond to a wide range of 
aérial sounds but did react to substratum vibrations. 

Much emphasis has been placed on the réle played by the human 
voice in the integration of human society; some social psychologists 
prefer to define man as a language animal. In this, man does not 
appear to be unique except in the degree to which language has been 
developed in his species. Craig (1908), in discussing voices of pi- 
geons as a means of social control, finds that in animals with so 
highly developed instincts as birds there is still much of the social 
life that cannot be explained on an instinctive basis. The reaction of 
the individual pigeon must be adjusted to meet the activities of other 
birds, its parents, its mate, its young, its neighbors, and chance 
strangers. The adjustment is very delicate and requires that each 
individual must be susceptible to the influence of others, an adjust- 
ment which is largely accomplished by vocal means. 

Perhaps more time has been spent on the vocal-auditory method 
‘of group integration than is justified by the conditions obtaining 
at the aggregation level with which this study is immediately con- 
cerned. Its interest by reason of its importance with the higher ani- 
mals must be the excuse. 


INTEGRATION BY LOW-FREQUENCY VIBRATIONS 


Much experimentation shows that animals that give little or no 
indication of perceiving sound vibrations coming through the atmos- 
phere respond definitely to vibrations of similar or lower frequency 
coming to them through water or through the substratum. With 
catfish, Bowen (1930) finds that blinded animals give definite reac- 
tions to the passing of another fish or of a model with a posterior 
part vibrating somewhat as does the tail of a fish. Such reactions are 
dependent on the presence of the sense organs of the skin. When 


96 ANIMAL AGGREGATIONS 


these are anesthetized, the blinded fish respond very little, if at all, 
to the passing of others. 

Various insects and other animals give no responses to aerial 
vibrations easily detected by the human ear, but readily respond to 
the same sounds when their receptacle is placed upon the piano pro- 
ducing the vibrations. Emerson (1929) has demonstrated that in 
the social termites mechanisms exist for producing substratal vibra- 
tions which can be detected at times by the unaided human ear, and 
easily when a microphone is used. He suggests that this may be one 
means of communication between these insects. Rabbits have long 
been known to signal by ground thumpings. The extent to which 
this kind of vibration is used in the aggregations with which we are 
specifically concerned awaits investigation. 

Buxton (1923) records an instance of co-ordinated movement 
among arctiid moth larvae which illustrates some of the possibilities 
of this type of integration. These caterpillars live in webs on herb- 
age in groups numbering several scores. If the web is disturbed, 
the larvae jerk the anterior ends of their bodies sidewise with a sharp 
flicking movement. All jerk together and maintain the reaction at a 
rate of about twice a second for as much as 20 to 30 seconds. Then 
they cease this movement and resume feeding. If they wander even 
an inch or so from the web, they do not take part in this movement. 
If an elongated web is chosen for the experiment (for example, a web 
4X12 inches), the movement of the larvae is not simultaneous, but 
waves of movement may be seen to pass through the mass of larvae 
from the point of disturbance so that the movement is organized 
but not synchronous. Obviously, the stimulus is conducted along 
the web. More mature larvae that have left the web do not generally 
give this movement when disturbed, although they may so respond 
when another crawls over them. 


POSSIBILITY OF BIOPHYSICAL INTEGRATION 


It is probably too early as yet to speculate with profit concerning 
the possibility of other, more subtle methods of group integration, 
such as the observations of Gurwitsch (1926), Borodin (1930), 
and others suggest may result from exploration of the field of bio- 


INTEGRATION OF AGGREGATIONS 97 


physics. These workers believe they have demonstrated that rapidly 
growing plant and animal tissues give off radiations which are able to 
stimulate other tissues completely separated from the so-called 
“senders” by being inclosed in quartz tubes so that these “‘detectors”’ 
show a decided increase in mitoses on the radiated half as compared 
with the non-radiated part of the same stem. 

A favorite experiment consists in placing a moist onion root, 
attached to part or all of the bulb from which it grew, into a small 
tube made of quartz. One or more onion roots from different bulbs 
are introduced into open-ended glass tubes and are also kept moist. 
The former is to be the detector; the latter, the sender. The sender 
is carefully centered so that its growing root tip points directly to- 
ward and at right angles to the detector root, and is allowed to re- 
main so for from 1 to 25 hours. The detector is then marked with 
India ink on the side away from the sender and is killed in an ap- 
propriate fixing-solution and sectioned. In fixed and stained sections 
the number of mitoses on the exposed and non-exposed sides are 
compared. Most of the work reported to date shows uniformly a 
greater number of mitoses on the exposed side; the work of Rossman 
(1928) is an exception. These are believed to have been induced by 
the action of mitogenetic rays. Definite but conflicting wave-lengths 
have been announced for these rays. 

This field needs further clarification before we can begin building, 
with a sense of security, upon the suggestions opened by this work. 
If the presence and importance of mitogenetic rays are finally es- 
tablished, we shall then have to inquire carefully whether or not we 
have similar subtle means of group integration in the field of bio- 
physics which may help us resolve the problems in social and sub- 
social behavior that are epitomized by Maeterlinck’s phrase “the 
spirit of the hive.” 


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HARMFUL EFFECTS OF AGGREGATIONS 


CHAPTER VI 
HARMFUL EFFECTS OF CROWDING UPON GROWTH 


Our knowledge concerning the methods of aggregating and the 
factors conditioning the formation of aggregations has grown steadi- 
ly and gradually, as has our information concerning their integra- 
tion. On the other hand, marked advance has been made since 1920 in 
the investigations of the physiological effects which such aggregations 
produce upon the individuals of which they are composed. The type 
and extent of such effects make one of the crucial tests of the impor- 
tance of the phenomena. If these aggregations are merely forced 
reactions resulting from limited space or from blind tropistic be- 
havior, or if they result only as an expression of a social appetite or 
instinct, their significance is more remote and the problem of their 
origin is more difficult of solution than if they can be shown to have 
group value even in their poorly integrated stages. Failure to ob- 
serve such values for many aggregations led Deegener to conclude 
that their formation must be due to some inexplicable instinct. 

In the investigation of this problem we must first inquire whether 
or not the aggregations with which we are dealing have positive or 
negative survival value which can be recognized. Even if positive 
survival value is found in a number of cases, the problem is by no 
means solved; but the methods to be used in its solution will be more 
clearly indicated than if we are forced to rely upon the postulate of a 
former survival value, of which the only remaining evidence is a 
weak social appetite persisting frequently in the face of present 
negative survival values. 

Even with the recently devised methods of analysis, the harmful 
effects of such aggregations are frequently more easily apparent than 
are the benefits. To the eye of the naturalist depending on field ob- 
servation for his data, benefits do not become obvious until the ag- 
gregation is sufficiently well integrated so that members may be 
warned of the approach of danger by some group attribute, such as 


Iol 


102 ANIMAL AGGREGATIONS 


the multiplicity of eyes in the group, or can attack or defend them- 
selves more effectively by the multiplicity of claws or of teeth. Most 
of the experimental approaches to this subject have been similarly 
limited. 

The impressive array of facts, accumulated by observation and 
experiment, which indicates that loosely integrated aggregations 
have harmful effects will be summarized in the present chapter so far 
as the rate of growth is concerned. The next following chapters will 
give other facts concerning harmful effects upon the rate of reproduc- 
tion and upon longevity. 

Dermestes beetles feeding on a limited amount of carrion exhaust 
their food sooner when more than one is present. This is also true of 
leaf-eating caterpillars, sap-sucking aphids, or tissue-filling parasites. 
It is only with well integrated groups of predators catching lively 
creatures as food that the feeding aggregation becomes of value. A 
school of young minnows is much more likely to catch a given Daph- 
nia than is a single individual, and each member of the group is more 
likely to feed upon the Daphnia stirred up than if he swam alone. 
This type of group advantage increases with group organization, as 
shown by the grasshopper drives of African storks. 

The same number of relatively defenseless individuals are more 
easily gobbled down by an enemy when aggregated than when 
scattered. One of the insect sleeping-clubs described by the Raus 
would provide a substantial breakfast for the proverbial early bird, 
and a hungry centipede would have easy picking in a group of aesti- 
vating land isopods. In locust control measures, men take advantage 
of the tendency of locusts to collect in dense overnight aggregations. 

There is a general ecological assumption that the accumulation of 
the waste products of a given species in their habitat tends, with 
most animals, to limit the time of their occupancy, at the same time 
preparing the way for another species to come in. This is sometimes 
considered one of the major biological factors causing ecological suc- 
cession, a process well illustrated by the sequence of fauna in a pro- 
tozoan infusion. 

PLANT TOXINS 

It has long been thought that one of the major causes of such suc- 

cession among plants is the accumulation of more or less specifically 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 103 


toxic root secretions. Almost a century ago De Candolle, the French 
botanist, suggested that the reason for the decrease in yield following 
the continued growth of the same crop on the same soil is due to the 
accumulation in the soil of harmful material given off by the growing 
plants. Liebig apparently adopted this view for a time but aban- 
doned it later, thinking that the observed benefits of crop rotation 
were due to the different nutrient requirements of the crops rather 
than to the accumulation of poisons in the soil. Pickering (1917) 
gives conclusive evidence that root excretions may have a toxic effect 
upon growing plants. In his work he used mustard plants growing in 
earth, on the surface of which rested a tray with a porous bottom, 
with a large central walled opening through which the plants grew. 
This tray held 5 inches of earth. The presence of such a tray made 
practically no difference in the growth of the plants in the pot below, 
even when the tray itself contained a growth of mustard plants, 
providing their roots were kept out of contact with the soil of the 
lower pot and that water from around the roots of the upper plants 
was not allowed to reach the lower soil. When washings from the 
upper growth were allowed to drain into the lower pot, carrying 
leachings from the plants grown in the upper tray, growth of the 
experimental seedlings was reduced to o.o1 of that given in control 
pots. Pickering found such results common and widespread, and 
especially well shown by the effect of grasses on the growth of apple 
trees. In summarizing all the evidence on the subject, Russell (1927) 
concludes that, while a toxin can be shown to be present, the toxin 
concerned’js not stable and is non-specific. 


CESSATION OF GROWTH IN BACTERIAL CULTURES 


The long-recognized failure of cultures of micro-organisms, such 
as bacteria and molds, to continue growing indefinitely has been 
attributed to three main causes: the exhaustion of foodstuffs, the 
accumulation of metabolic wastes or specific ‘““autotoxins,” or the 
limitations imposed by actual physical crowding. Henrici (1928) 
gives a good summary of the present state of knowledge in this 
field. 

The possible effect of physical crowding in limiting growth must 
be excluded because much more dense growths can be obtained with 


104 ANIMAL AGGREGATIONS 


organisms on filter paper or on agar than when they are grown free 
in liquid broth; and when the organisms are repeatedly filtered off so 
that the physical effects of crowding are periodically eliminated, the 
growth period is not thereby prolonged. There can be no doubt but 
that the exhaustion of food materials does play an important role 
in the limitation of cultures, but the question as to how important 
this is in comparison with the accumulation of waste products or 
“autotoxins” has not been decided. 

Henrici says, ‘““The idea that growth is limited by the accumula- 
tion of some toxic substance is the one that seems to be most general- 
ly accepted, though the evidence for it is far from being convincing.” 
The evidence supporting the idea that the toxic substances are im- 
portant is as follows: Eijkman (1904, 1906, 1907), working with a 
number of species of bacteria, grew them in gelatine until the culture 
was densely crowded. He found then that if he took a part of this, 
heated it to boiling and, after cooling, reinoculated it, it would then 
support growth; but that another part, heated only slightly and then 
allowed to resolidify, would not produce growth in a new surface 
inoculation. Since heating to boiling-point would add no new food 
material, Eijkman concluded that he was dealing with some ther- 
molabile product of metabolism or a more specific growth-inhibiting 
substance. 

Further experiments showed that the toxic material would not 
pass through a porcelain filter, that heating which killed the living 
organisms destroyed the toxicity of the medium, and that treatment 
with such volatile agents as ether and ammonium sulphide not only 
killed the bacteria but rendered the medium again capable of sup- 
porting growth after the volatile material was driven off. They also 
showed that if gelatine in which Bacillus coli had grown was resolidi- 
fied into a plate and reinoculated with more of the same organisms 
and covered with a layer of fresh gelatine, there would be no growth; 
or, if fresh gelatine was inoculated with B. coli and then had one part 
covered with fresh gelatine and another with the so-called “coli- 
gelatine,’ growth would take place in the former only. Inoculation 
of paper dipped in agar and then placed over the coli-gelatine did not 
yield a growth unless the coli-gelatine had first been heated. 


HARMFUL EFFECTS OF CROWDING UPON GROWTH. 105 


Rahn (1906), with B. fluorescens liquefaciens and three other 
species of bacteria, obtained essentially similar results except that in 
his experiments treatment with ether killed the bacteria without 
removing the unstable toxic material. The ether could be evaporated 
off but no growth would occur on reinoculation. Appropriate con- 
trols made by treating sterile broth with ether, which was then 
evaporated, showed growth on inoculation. Heated cultures gave 
growth when similar cultures treated with ether gave none, show- 
ing apparently that the food value of the medium was not ex- 
hausted. 

Chesney (1916), working with pneumococci, found that if the 
organisms are removed by centrifuging a rapidly growing culture, 
those remaining will continue to grow at the same rate; but that if 
the culture be similarly treated after the period of maximum growth 
is past, growth is delayed and some of the cells may die off. Filtrates 
from 24-hour cultures inhibit growth of new inoculations of similar 
organisms, but lose this property if the filtrates are allowed to stand 
for a time in the incubator. Chesney concludes that the cells do 
produce an unstable, toxic, growth-inhibiting material. Some inves- 
tigators have believed this substance inhibiting growth in pneumo- 
cocci is fairly specific in its action; but Henrici cites later work show- 
ing that the limitation of growth in pneumococcus cultures is due to 
three factors: the accumulation of acid, the production of peroxide, 
and the exhaustion of the nutrients. The first two come under the 
general heading of ‘toxic products of metabolism,” which are here 
shown to’ limit growth of this organism. Hajos (1922) also demon- 
strated growth-inhibition due to the accumulation of products of 
metabolism among the colon-typhoid type of bacteria. 

Curran (1925), again using B. coli, found a thermolabile growth- 
arresting material readily adsorbed by bacterial filters. The first 
50 cc. of filtrate from a 200 cc. solution which had supported bac- 
terial growth for 3 days was found to support growth on reinocula- 
tion much better than did the last 50 cc. of the same solution. It 
would appear that the filter became loaded with the growth-inhibit- 
ing material and so allowed material that was stopped at the begin- 
ning to pass at the end of the filtering. Using a different sort of or- 


106 ANIMAL AGGREGATIONS 


ganism, Kuester (1908) finds that molds grown in a nutrient solution 
produce conditions which check the growth of further inoculations 
before the nutrient supply is exhausted. 

In the face of this evidence, Henrici is still unconvinced of the 
general applicability of the theory of the production of a toxic ma- 
terial serving to limit growth, and holds, rather, to the idea that the 
exhaustion of the nutrient material is the crucial point. He suggests 
rightly that the Eijkman type of experiment may only show that 
media may contain sufficient material to support a heavy population 
without growth and still be able to support growth in a smaller 
population; that heating the medium to kill the bacteria may cause 
a release of nutrient material, making it available for the reinoculat- 
ed organism; and finally cites the work of Graham-Smith (1920) in 
which he was able to revive staphylococci by adding concentrated 
meat extract and thus inducing new growth after the period of maxi- 
mum growth had passed, and could postpone the death phase in- 
definitely by small daily additions of meat extract. Henrici is prob- 
ably correct in concluding that different factors may limit growth in 
different cases and that there is no sound basis for believing in the 
production of specific autotoxins. 


EVIDENCE FROM TISSUE CULTURE 


In tissue-culture work Carrel and Ebeling (1923) and Mottram 
(1925) report a substance which inhibits growth of explants present 
in extracts of all adult tissues, in serum and even in extracts of em- 
bryos. The latter have usually been found to favor growth in such 
cultures. Heaton (1926) has found such a substance in yeast extracts 
and in a number of adult-animal tissues, especially the liver. He 
thinks that the failure of adult tissues to grow easily in vitro, and the 
stoppage of growth of connective tissue 7m vivo, as contrasted with 
the continued growth of epithelia, is to be attributed to this growth- 
inhibiting substance. Heaton finds it to be thermostabile, though 
destroyed by heating up to 125° C. It is soluble in water and alcohol 
up to 75 per cent strength, but is insoluble in 97 per cent alcohol. It 
seems to be destroyed by autolysis. Its action is greater on older 
than on younger embryonic tissues. 


HARMFUL EFFECTS OF CROWDING UPON GROWTH. 107 


GROWTH INHIBITION IN ANIMAL CULTURES 


The work on animal cultures most closely connected with these 
investigations on bacteria and on tissue culture is that dealing with 
the growth in a protozoan infusion. In two studies (1911 and 1914) 
Woodruff demonstrated that Paramecia excrete substances that are 
toxic to themselves when present in their environment and that 
probably play an appreciable rdle in determining the time of maxi- 
mum number, rate of decline, and other characters. Similar conclu- 
sions were reached as a result of work with the hypotrich as regards 
their own excreta, but they are immune to the effects of Paramecium 


A B & D 


Fic. 4.—A record of the rate of division of Paramecium aurelia in a series of four 
experiments (A, B, C, D) to determine the effect of different volumes of culture medium, 
changed every 24 hours, on the rate of reproduction. The ordinates represent the 
average daily rate of division of the four lines of organisms in the respective volumes 
of medium, averaged for 4-day periods. Rate of division in 2 drops, - - --- - ; 5 drops, 
 2OlCU TODS; ceencvese«s ; 40 drops, -+-+-+-+ (From Woodruff, 1911.) 


excreta. In a protozoan infusion the appearance of dominant Pro- 
tozoa at the surface runs in this order: Monad, Colpoda, Hypo- 
trichida, Paramecium, Vorticella, and Amoeba. ‘This ecological se- 
quence is due in part to accumulation of toxic material and in part 
to the supply of available food. 

This problem is closely related to the consideration of the effect 
of the size of the effective environment, whether lake, pool, or labo- 
ratory container, upon the contained organisms, which in turn is 
closely related to the whole problem of crowding. 


108 ANIMAL AGGREGATIONS 


In 1854 Jabez Hogg with some right apologized to the London 
Microscopical Society for taking their time with observations on the 
subject of the pond snail Limnaeus stagnalis, which had already been 
well studied; but in the midst of his tedious record he states that a 
snail kept in a “‘small narrow cell will grow only to such a size as will 
enable it to move freely.”’ This is the first recorded observation that 
has come to my attention of the limiting effect of volume on growth. 

Semper (1874, 1881) took up the problem twenty years after 
Hogg’s observations, using the fresh-water isopod Asellus and the 
pond snail Lymnaea stagnalis. With the former he found that when 
animals living in a balanced aquarium were sealed into glass dishes, 
they might be left for nearly 2 years, with an adequate food supply 
and, he believed, an adequate oxygen supply for the three or four 
generations that would be produced; but under these conditions the 
last generation was abnormally small. With the snails Semper divid- 
ed the same mass of eggs into different lots, which were placed in 
variously sized containers ranging from 100 to 5,000 cc. Food was 
kept at an optimum, but the snails placed in the containers of smaller 
volume grew more slowly than did their fellows placed in the larger 
vessels. Similar results were obtained regardless of whether the 
snails were isolated into a given volume or were put in groups, so 
long as the volume per snail was the same in both cases. Semper 
found that optimum growth lay between 4oo and 500 cc. per snail 
and that increases beyond this point gave no further increase in 
growth. The effect of increase in volume was much more marked in 
the smaller volumes. Later workers are agreed that relatively large 
volumes of water per snail are necessary for optimum growth. 

Semper recognized the complex nature of the problem and at- 
tempted, by chemical analyses made by a trained chemist, to find a 
chemical cause. Failing in this attempt, he advanced the hypothesis 
that some substance unknown to him was present in the water, prob- 
ably in a very minute quantity, ‘“‘which, by its relations to the water 
which holds it in solution, and by its osmotic affinity to the skin of 
the animal, can be absorbed only in a determined and extremely 
small quantity..... Since, according to this hypothesis, the 
amount of the substance absorbable in a given time depends on the 


HARMFUL EFFECTS OF CROWDING UPON GROWTH  1o9 


volume of the water .. . . the attainment of full size within a definite 
period would only be possible if the volume of water were so great 
that the Lymnaea could at all times absorb this unknown stimulant 
from the water.”’ This hypothesis, in some form or other, has been 
proposed, apparently independently, by a number of workers since 
Semper’s time. 

Semper seems to have been certain of the evolutionary significance 
of the limitation of growth by volume. He found it impossible to 
obtain full-sized individuals from snails stunted during the first year 
of their lives; and if the causes checking growth were repeated regu- 
larly through the succeeding generations, he felt that a dwarfed race 
must arise. Whitefield (1882) came to the same conclusion, using 
Lymnaea megasoma from Vermont. Whitefield continued the crowd- 
ing for four successive generations, during which time the snails be- 
came successively smaller and more slender, so that an experienced 
conchologist did not recognize their relation to the shells of the 
parent stock. 

Yung (1878, 1885) concluded from his experience in raising tad- 
poles in containers of various sizes and shapes that dwarfing is due to 
a lack of aération. De Varigny (1894) took up the problem with 
Lymnaea again and in general obtained the same sort of results 
reported by both Semper and Yung. A snail kept in a liter of water 
with a surface of 257 sq. cm. for 5 months was nearly twice the 
length of one kept in the same volume of water but with a surface 
area of 3.14 sq. cm. In order to facilitate the analysis, De Varigny 
suspended a glass tube 2-3 cm. in diameter in containers of various 
sizes. The glass tubes were closed over the bottom with muslin, and 
each contained a single snail. Each day these tubes were lifted from 
the water and replaced two or three times in order to secure complete 
mixing of water. Even so, the contained snails grew approximately 
the same regardless of the volume of water with which they were in 
contact through the muslin screen. In one instance the growth was 
the same in such a muslin-bottomed tube as compared with that of a 
snail in a corked tube which prevented all exchange between the 
inner and the surrounding water. From these experiences he con- 
cluded that Semper’s explanation would not hold and that the size 


110 ANIMAL AGGREGATIONS 


to which snails grow depends in some way on the actual volume to 
which they are exposed and on the surface area of such water. His 
explanation was that in the small tubes the snails needed to move 
about less to obtain their food and that, with this decrease in exercis- 
ing, there came a decreased rate of growth. According to De Va- 
rigny, dwarfing from crowding is not so much due to the actual 
numbers in the vessel as to the ‘‘psychological” influence of num- 
bers, which inhibit exercise, just as a man is less likely to walk a 
considerable distance on a crowded street than on a deserted one. 
He also believed dwarfing to be-affected by the accumulation of 
faeces. 

Willem (1896) bubbled air through his snail cultures and found 
growth of the contained snails greatly increased. He concluded that 
aération is important because, even in lung-breathing pond snails, 
he believed cutaneous respiration to be more important than lung- 
breathing and alone sufficient for the animal. 

Davenport (1899) reviewed much of the evidence on the relation 
between crowding and rate of growth, and concluded with Hogg that 
in respect to the size attained, as in other qualities, the snail has the 
power of adapting itself to the necessities of its existence. 

Vernon (1895, 1899, 1903), working with echinoderm larvae, con- 
cluded that dwarfing is due to a concentration of the excretory prod- 
ucts in the media. He found that if eggs of echinoderms were allowed 
to develop in water which had previously contained other eggs for a 
considerable period of time, the larvae of the second batch were dim- 
inished in size as compared with the control. The growth of the 
larvae appeared to be reduced by their own excretory products, or 
especially by those of adult echinoderms, the more so if these belonged 
to the same species. On the other hand, he found that the excretory 
products of two species not closely related were favorable to 
growth. 

Warren (1900), working with the common entomostracan, Daph- 
nia, found that continued breeding in small aquaria with the medium 
unchanged caused dwarfing. This result he attributed to the action 
of the excretory products, which he found to be somewhat specific, 
since ostracods and copepods flourished in cultures of Daphnia in 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 111 


which the latter were dying out. Such results are similar to those 
reported by Woodruff and others for protozoan infusions. 

Legendre (1907) returned to the problem of the effect of crowding 
on the growth of snails, using Lymnaea stagnalis and Planorbis 
corneus, raised in one series of experiments In stagnant water and, in 
the other, in water changed periodically. As in the case of previous 
workers, he found the smaller shells in the stagnant water, and at- 
tributed the cause to the accumulation of excretions. In further work 
reported the following year, using another species of Lymnaea, 
Legendre changed the water every 2 hours in order to avoid the 
accumulation of excreta, and varied the factors of volume of water, 
surface area, and number of individuals. After 51 days he obtained 
the same shell size in all such experiments. He recognized that a 
number of factors might bring about retardation in crowded ani- 
mals, but laid particular emphasis upon the retarding effect of the 
excretions. 

Colton (1908) continued work on the effect of crowding on growth 
in Lymnaea. Food was recognized as an important element, but just 
how important Colton’s work does not reveal. He did find that snails 
need a certain amount of sediment to aid in grinding their food, and 
that certain salts, for example calcium sulphate, aid growth. Colton 
found that washed and filtered snail faeces placed in aquaria has- 
tened the growth of the snail, probably due to the increase in algae 
caused thereby. His aération experiments support the conclusions of 
Willem that these pulmonate snails have a large proportion of cutic- 
ular respiration. Concentrated excretory products caused dwarfing; 
accordingly decreases in volume of water per individual present, 
whether in isolations or in crowded cultures, caused a decrease in 
growth rate. Popovici-Baznosanu (1921) minimized the effect of ex- 
crement, thinking the amount of food more important. 

Crabb (1929) has recently reinvestigated this entire problem with 
the pond snail Lymnaea stagnalis appressa, taking care that his snails 
were free from trematode parasites, and supplying them with food 
known to be favorable for growth in laboratory conditions. He used 
eggs from the same egg mass for experiments run simultaneously; 
since this snail reproduces by self-fertilization, individuals obtained 


Tena ANIMAL AGGREGATIONS 


from the same egg mass would be expected to have similar genetic 
constitution. He concludes that food insufficiency and foul media 
are the most common growth-inhibiting factors in snails reared in 
otherwise favorable media. Extreme crowding markedly retards 
growth, but the individuals rapidly reach normal size after transfer 
to standard conditions, unless they are too old. The volume of me- 
dium has little effect on the growth of isolated snails providing foul- 
ness is not permitted. Aération promoted growth through reducing 
foulness rather than by increasing the respiration of the snails. 
Daphnia introduced into the culture are beneficial to snail growth, 
since they retard fouling of the medium. He found no evidence that 
environmentally induced dwarfing is transmitted, though on this 
the experiments were not continued through enough generations to 
be conclusive. 

Crabb, in his work, continued the general methods of study of this 
problem which have been used since the time of Hogg, adding re- 
finements which make his conclusions the more trustworthy. Un- 
fortunately, he did not take advantage of the method originated by 
De Varigny (1894) and used extensively by Goetsch (1924), which 
allows a separation of the factor of available space from that of 
available volume. In this procedure Goetsch placed animals in the 
experimental aquaria in separate tubes thrust through corks to keep 
them afloat and covered at the lower end with gauze, which allowed 
diffusion connection with the entire aquarium while limiting the 
amount of available space. 

Goetsch was led to this method by the experience of Bilski (1921), 
who found that the relatively active tadpoles of Bufo and of Rana 
esculenta grew less rapidly when subjected to frequent changes of 
water than they did when metabolic wastes were allowed to accumu- 
late. Bilski also found that an increase in numbers slowed down the 
rate of growth more than would be expected by the change in vol- 
ume relations involved, when the rate of growth was compared with 
that given by an equal number of animals placed in different aquaria. 

Goetsch experimented upon sessile Hydra, upon the relatively 
slow-moving flatworms, and upon amphibian larvae which are capa- 
ble of rapid locomotion. As might be expected, he finds different 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 113 


factors important for different animals. Thus, with Hydra, volume 
per animal is the controlling factor because of the restriction of food 
which it conditions. There is no stimulation or depression caused by 
the crowding of Hydra into a narrow space; and, within reasonable 
limits, concentration of excretory products are not effective. With 
Planaria food is again the most important factor, but growth is in- 
hibited by the concentration of excretion products or of stale food. 
With the active amphibian larvae, if food is controlled, the major 
limiting factor is furnished by the more frequent collisions in a dense 
population or in a restricted area, and the concentration of excretory 
products plays a wholly secondary rdle. 

Church (1927) extended these experiments to include the rate of 
growth of the tropical fish Platypoecilus maculatus rubra in connec- 
tion with other experiments upon the effect of crowding upon the 
rate of growth of fishes. Eight liters of water were used in glass 
aquaria, each of which contained 2, 8, or 16 fish. In each series of 
experiments, one set of aquaria contained small fish 8-10 mm. long, 
another set held fish 12-14 mm. long, and the third set was supplied 
with fish 20-23 mm. Adult Platypoecilus range from 30 to 35 mm. 
The amount of oxygen and the pH of the different aquaria did not dif- 
fer significantly. The water was left unchanged during the entire ex- 
periment, which ran in some cases as long as 70 days, except that 
there were slight additions to replace the small amount lost by evap- 
oration. The fish were fed the same number of Daphnia per fish 
per day. 

Under these conditions the large fish always grew less rapidly the 
more fish there were present in a given container. With the small 
and medium fish there was some indication of more rapid growth 
early in the experimental periods among the fish grouped 8 to the 
aquaria; but as the experiment progressed, the rate of growth was 
always greatest when the fewest fish were present. Shaw (1929) has 
repeated these experiments, with similar results. The experience of 
these two workers demonstrates that when there is sufficient con- 
centration of waste products the rate of growth is retarded. 

In following out the Goetsch type of experiment, Church placed 
transparent celluloid containers in the center of each aquarium. 


114 ANIMAL AGGREGATIONS 


These were 4.5 cm. in diameter and were covered with coarse scrim 
at the bottom. They were suspended by wires so that each extended 
1.5 cm. below the surface of the water, thus giving to the contained 
fish a volume of 24 cc. in which to move about, as contrasted with 
the 4,000, 1,000, or 500 cc. volume per fish to which the 2, 8, or 16 
fish were exposed in the surrounding aquaria. A single medium-sized 
fish was transferred to each of these tubes, regardless of whether 
those in the surrounding aquaria were large-, small-, or medium- 
sized. 

Under these conditions the fish within the small tubes grew less 
than did those in the aquaria. At the end of the first ro days the 
average length of the 34 fish in the tubes showed 1.35 per cent in- 
crease, while the medium-sized fish in the surrounding aquaria grew 
6.51 per cent. At the end of 20 days the difference was still more 
striking. The 25 fish in the tubes had grown in this period on the 
average 2.78 per cent, while those of the same original size in the 
larger volume of water had grown 12.83 per cent. So far as known, 
the size of the container was the only variable. The meshes of the 
scrim cloth were open throughout the experiment; but to guard 
against the possibility of lack of adequate diffusion, the tubes were 
raised once daily to insure a complete change of water. Such results 
are similar to those Goetsch secured for the relatively swiftly moving 
tadpoles, and are probably due to the effect of overstimulation 
caused by frequent contact with the walls of the small tube. 

As stated above, Bilski (1921) points out that when limitation of 
growth rate is caused primarily by stimulation from repeated con- 
tacts, and when the number of individuals present is proportional to 
the different sizes of the vessels, the rate of growth is not the same. 
If we take two vessels of different sizes, a and 6, and populate them 
with a and b number of animals respectively, so that each animal has 
the same amount of space available, in the simplest case the stimu- 
lation will come from the contact, or near approach, of two animals. 
The relation of the size of the two containers will be a:b, which in a 
simple case might be 2:3. The stimulation possibilities from group 
interference would be a(a—1):6(b—1). Substituting the values sug- 
gested above, we get a stimulation possibility of 2:6. Under such 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 115 . 


conditions one would expect to find growth retardation with increase 
in numbers to be much greater than if volume relations alone were 
the responsible factor. 

Inspection of his experimental results in comparison with a simple 
formula built on the assumption that the growth would be inversely 
proportional to the group stimulation, 


i een 


in which y represents size, x stands for the number of animals in a 
given space, and & is a constant, shows that the influence of the 
stimulation is not on this order but is approximated by taking an 
exponential value of «, namely «3/7, The equation then becomes 


Values calculated from this formula fit fairly well with Bilski’s ob- 
servations on the effect upon the growth of differing numbers of 
tadpoles in jars of equal size; the observations of Semper on the 
growth of snails in relatively small vessels; and, according to Bilski, 
with the observations of Hoffbauer on growth in carp. Another for- 
mula derived by a continuation of the same reasoning better fits 
Semper’s results with snails in larger volumes. 

Bilski recognizes the general significance of his results and believes 
that such diverse phenomena as the reported dependence of size of 
mammals upon available land, and other similar relationships, in- 
cluding even a correlation between the size of children and available 
space, may depend upon an application of this principle. Farr (1843, 
1875) worked out an equation essentially similar to that of Bilski to 
describe the relation between death-rate and the density of human 
populations. Brownlee (1915, 1920) finds that Farr’s law fits a wide 


116 ANIMAL AGGREGATIONS 


range of biological and biochemical relationships, including even the 
relation worked out by Kennealy (1906) between the racing record 
for a particular distance and the length of the race. Pearl (1925) 
finds that essentially the same equation describes the effect of crowd- 
ing upon the rate of reproduction in Drosophila. 

The problem is obviously complicated by many factors, but it is 
interesting and probably significant that the relationship can be ex- 
pressed mathematically in a similar way for such a wide range of 
phenomena. It is almost an anticlimax to have to record that physi- 
cal disturbance due to numbers is not the only factor controlling 
growth in rapidly moving animals, such as fish, under crowded con- 
ditions. The careful work of Church and of Shaw, already summa- 
rized, demonstrates that the accumulation of waste products is also 
effective with fish, just as a long line of evidence culminating in that 
given by Goetsch proves that it is effective in the slower-moving 
planarian worms. 

More recently Willer and Schnigenberg (1927) and Kawajiri 
(1928) have independently tested the effect of crowding on the rate 
of growth of young trout in running water. Both report essentially 
similar results; the work of the former will be reviewed here, since it 
is the more comprehensive. These workers used young of the brook 
trout during their prehatching, yolk-sac, and early feeding stages. 
In their experiment they tested a wide range of conditions. They 
used the same number of eggs or of young in different volumes and 
_ with different surface areas, and in other tests used different num- 
bers in the same volumes. All experiments were carried on with 
water running at a rate of from 3.3 to 65 cc. per second. 

Their results show that moderate crowding after hatching has no 
adverse effect upon fish whose prehatching development has been in 
equally crowded conditions. In fact, under these conditions, one set 
of experiments show an apparently beneficial effect. On the other 
hand, crowding the eggs produces definite retardation in length and 
perhaps also in weight at hatching time. Such retardation Js corre- 
lated with the volume of water rather than with the area of the 
screen on which the eggs rest. 

Exposure of uncrowded eggs to water that has flowed over a mass 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 117 


of developing eggs is found to produce about the same degree of re- 
tardation as is furnished by crowding. Under these conditions the 
dwarfing effect must be a result of toxic materials accumulated in 
the water. The general importance of these results is enhanced be- 
cause of the fact that they have been obtained from animals grown in 
running rather than in stagnant water. There was an indication of a 
condition of optimum crowding" in the early experiments which was 
not sustained by later work, although specific experiments designed 
to test this point were not attempted. 

Peebles (1929) has taken up the problem of effect of numbers 
present upon the rate of cleavage of echinoderm eggs, and upon the 
rate of growth of arms of plutei, in the light of developments in 
tissue-culture work. She finds, as did Vernon (1895) and Springer 
(1922), that embryo-water contains substances which check growth, 
but adds the observation that some of the inhibiting effect is counter- 
acted when living larvae are present. She produces evidence that the 
growth-inhibiting substances are associated with the lipoids and 
that, after their removal, growth-promoting substances can be dem- 
onstrated to be present. These latter will be discussed in chapter ix. 

The relation between the size of the effective environment and 
that attained by the animals living therein has more than laboratory 
interest. The belief is widespread that fish grow larger in large lakes 
than in small ones. Pearse and Achtenberg (1920) report such a 
correlation between size of lake and size of contained yellow perch. 
This correlation is not uniform, for numerous exceptions could be 
cited; for example, Jewell and Brown (1929) find no such relation- 
ship holding between size of fish and the size of the small Michigan 
lakes in which the fish live. 

Hesse (1924) states that the same relation holds for mammals with 
regard to the size of available range: those living on small islands 
attain a smaller adult size than related forms on larger bodies of 
land. In many cases the reduced amount of available food in the 
smaller habitats has been recognized as being sufficient to explain 
the observed phenomena. Semper (1879) critically discussed this 


* Kawajiri reports that the survival-rate increases as the number of fry in a box 
increases. 


118 ANIMAL AGGREGATIONS 


general situation long ago and left the impression that the suggest- 
ed relationship was either not proved or only indirectly related 
to the suggested space factor. The idea that there is a direct con- 
nection between available space, and size attained in land animals, 
still has, however, considerable vitality, as is shown by Bilski’s 
suggestion (1921), following his careful statistical analysis of the re- 
lations between available space and growth in tadpoles, that the 
smaller size of children reared in slums, as compared with that of the 
children of more fortunate parents, is to be accounted for by the 
smaller space available per child for the former and the resulting 
greater degree of stimulation by repeated contacts, such as have been 
shown to result in decreased growth in tadpoles, fish, and other 
rapidly moving animals. 

There can be no doubt that crowding decreases the rate of growth 
in many instances, and any interpretation of the facts to be present- 
ed later concerning beneficial effects of crowding up to an optimum 
population must take this fact into consideration. When one at- 
tempts to summarize the evidence concerning the factors causing the 
retarded growth in crowded conditions, he finds a decided lack of 
unanimity among the different investigators, indicating that in all 
probability there are many factors which may produce the same 
result. - 

It is instructive to review the retarding factors suggested to date. 
They are of two kinds: the vague and the definite. In the former 
category one must put the suggestion of Hogg, working with snails 
in 1854, that they adapt themselves to the necessities of their exist- 
ence, which Davenport, 45 years later, said still summarized the 
state of knowledge on the subject at that time. There is also Sem- 
per’s postulated X-substance necessary for growth in snails and 
water isopods (1874, 1881); the autotoxins of the bacteriologists; and 
the growth-inhibiting substances of the tissue culturists (Heaton, 
1926) and of Peebles (1929) for echinoderm larvae; as well as a 
“space factor’ seriously discussed by many observers (cf. Willer and 
Schnigenberg, 1927). As commonly used, this space factor is about 
equivalent to Hogg’s conception. 

Regarding this group of suggested retarding factors, the best we 


HARMFUL EFFECTS OF CROWDING UPON GROWTH 119 


can say at present is that they are unproved. We shall find the sug- 
gestion of an X-substance made in many different connections be- 
fore we have finished this discussion. It is useful as a hypothesis but 
is not to be confused with concrete fact. However, the recent de- 
velopments concerning the importance of small traces of vitamins, 
and the work upon “bios” and upon tissue-culture inhibitions, will 
keep us from dismissing this hypothesis too hastily. 

Of the definite factors suggested, we have lack of sufficient aéra- 
tion, in addition to undernutrition, reported as operating in crowded 
tadpoles (Yung) and among snails (Willem, Colton, Crabb). There 
can be little doubt but that insufficient aération is an effective factor 
under many conditions. The suggested harmful effects of lack of ex- 
ercise in snails (De Varigny) now appear groundless. The accumu- 
lation of excretory products reported as an effective agent by many 
workers appears to have undoubted and marked influence, whether 
in echinoderm larvae (Vernon, Peebles), in Daphnia (Warren), in 
snails (Legendre, Colton, Crabb), in planarians (Goetsch), or in fish 
(Church, Willer and Schnigenberg, Shaw). Evidence in favor of this 
conclusion will accumulate as we proceed. 

The reduction of available food correlated with crowding, whether 
caused by increase in numbers or decrease in volume, is another un- 
doubted factor in the situation, as shown for snails by Colton and 
Popovici-Baznasanu and for Hydra by Goetsch. With some animals, 
such as Hydra, it may be that this is the only factor operating. With 
rapidly moving animals, the effect of frequent contacts resulting in 
overstimulation of some sort also contributes to the retardation of 
growth in crowded animals, as in tadpoles (Bilski, Goetsch) and in 
fish (Church). 


CHAPTER Vil 


RETARDING INFLUENCE OF CROWDING ON 
THE RATE OF REPRODUCTION 


In the preceding chapter we have assembled evidence to demon- 
strate that among many animals overcrowding tends to produce 
dwarfed individuals, and have discussed the factors that have been 
suggested as operating to produce this effect. As might be expected, 
there is frequently a retardation of the rate of reproduction as well 
as of the growth-rate of the individual. In many respects the two 
phenomena overlap. The evidence for the slowing-down of repro- 
ductive rate under crowded conditions will be examined in part in 
the present chapter. At another place consideration will be given to 
the data brought forth by Robertson and others which indicate that 
under certain conditions the rate of reproduction is increased in early 
stages of protozoan or other cultures when more than one animal 
is present in a limited amount of medium. 


7 


REDUCED DIVISION RATE IN INFUSORIA 


Balbiani (1860) reported from a single experiment on Parame- 
cium that this infusorian must be in not less than 2—3 cc. of medium 
for the greatest productivity to be realized. Kulagin (1899) sug- 
gested that this is due to the accumulation within the medium of 
excretions analogous to toxins, which gradually accumulate until the 
nucleus is affected. 

Woodruff took up this problem in 1911 in an effort to find the 
effect of excretion products of Paramecium on its rate of reproduc- 
tion. Since the experiments of Woodruff usually form the starting- 
point for present-day citations on this subject, they deserve to be 
given in some detail. 

The reproduction of P. aurelia was followed for from 16 to 20 days ° 
in four volumes of hay infusion: 2, 5, 20, and 4o drops, which were 
changed at 24- and 48-hour intervals in different series of experi- 


120 


RETARDING INFLUENCE OF CROWDING I2I 


ments. The results are given graphically in Figure 4. For the ex- 
periments in which the medium was changed every 24 hours the 
Paramecia in 5, 20, and 4o drops are shown to have divided 2.4, 
6.4, and 7.4 per cent more rapidly, respectively, than did those in 2 
drops. When the medium was changed every 48 hours, the per- 
centages for the same volumes were 5.3, 9.3, and 9.25. The results 
are given throughout as averages for 4-day periods. 

From these experiments Woodruff concludes, ““The rate of repro- 
duction of specimens from pure lines of Paramecia when bred under 
identical conditions of temperature and culture medium is influenced 
by the volume of the culture medium (within the limits tested in the 
experiments) and the greater the volume, the more rapid is the rate 
of division.” The slight discrepancy with the 4o-drop cultures 
changed every 48 hours is unexplained, but the suggestion is offered 
that the bacteria always found in such cultures, and which are used 
as food by the Paramecia, develop so rapidly under these conditions 
that they may exhaust their own food or produce sufficient excretion 
products to be injurious to the associated Paramecia. Otherwise, 
Woodruff believes that by his culture methods, which included cross- 
inoculations between the different cultures, he has eliminated the 
bacteria as agents causing the observed difference in rate of Para- 
mecium division. 

The conclusion that the recorded effects are due to the accumula- 
tion of Paramecium waste products rests on three lines of evidence. 
In the first place, as we have just seen, the rate of division is higher, 
for the periods and amounts tested, the larger the amount of avail- 
able medium. Second, the rate averaged 8 per cent greater in the 2- 
drop cultures changed daily than in similar cultures changed every 48 
hours. The other cultures similarly showed a 6 per cent increase if 
changed daily. Finally, culture media in which Paramecia had flour- 
ished for to days before removal were shown to have a depressing 
effect upon the reproductive rate of Paramecia replaced in it, as 
compared with the effect of an infusion which had contained no 
Paramecia but which otherwise was as nearly comparable as the two 
could be made. 

Woodruff (1913), as a result of further experience, concluded that 


22 ANIMAL AGGREGATIONS 


the substances which Paramecia excrete into their medium are es- 
sentially species-specific, at least to the extent that they do not uni- 
formly influence the rate of reproduction of the hypotrich Stylony- 
chia. This hypotrich produces conditions within its own culture me- 
dium which are definitely depressing for hypotrich reproduction and 
without necessarily affecting the rate of reproduction of Paramecium. 

The question of species specificity has not attracted the work it 
deserves; but, stimulated by the researches of Robertson, to be re- 
ported in a later chapter, several workers have retested the effect of 
crowding upon the rate of reproduction of Paramecia and of other 
protozoans. Without exception, all the workers reporting so far, 
Robertson included, have confirmed the conclusions reached by 
Woodruff for cultures running the length of time for which his 
averages were taken. A detailed discussion of this later work is post- 
poned for the present. 

From general considerations it appears highly probable that the 
relationships outlined above and in the preceding chapter, if properly 
adjusted, could so affect an animal (for example, Paramecium) that, 
while it might be able to continue to live, its powers of reproduction 
would be lost. Crampton (1912) tried this experiment. He found 
that a single Paramecium confined in a capillary tube could be kept 
from fission for as long as 32 days, while controls relatively un- 
restricted as to space were dividing at a rate that would produce 
4,300,000,000 animals in the same time. He recognized three factors 
as working to produce this effect: lack of sufficient nutrition, ac- 
cumulation of waste products, and stimulation from the narrow 
limits. That lack of sufficient or proper food is not the sole cause is 
shown by his experience that the confined animals could be released 
to swim about in a culture of Bacterium termo daily for as long as 12 
hours out of the 24, without division, if the remainder of the day 
were spent in the confinement of the tube; and that they could be 
held so without division for a week, while controls were dividing on 
an average of once a day. Such Paramecia remained plump and well 
nourished in appearance; those left in the tubes for long periods with- 
out changing became transparent and emaciated. Stylonychia gave 
similar results. It is significant that Crampton centrifuged his ani- 


RETARDING INFLUENCE OF CROWDING 123 


mals, which must have brought them into violent contact with the 
walls of the capillary tubes. 

Crampton’s work was in many ways an extension of Conklin’s 
earlier observations concerning the size attained by the gasteropod 
Crepidula plana with relation to the amount of available space. The 
dwarfing of these snails when crowded, Conklin thought, should be 
interpreted as due to space inhibition of cell division. 

These facts were reported by Conklin in 1898. Crepidula plana 
lives within the shells harboring hermit crabs. If the shells are small, 
the contained Crepidula are few in number and are dwarfed; if 
large, the Crepidula may be present in numbers and be large. Since 
there may be but r small individual in the small shells, while there 
may be 4 ‘“‘giants”’ in a large one, Conklin believed that the difference 
in size is not due to differences in available food; nor is it due to the 
presence of accumulations of excreta, since both shells are equally 
open to the surrounding ocean. Neither is the result due to the lack 
of room to move about in, since both large and small Crepidula are 
relatively firmly attached to their substratum. Rather, there is a 
space retardation of cell division, since the cell sizes of the one are no 
larger than the other. If the small Crepidula are transferred to a 
larger space, they will increase in size. The stimulus acting to retard 
cell division in these dwarfed Crepidula is more obscure than in the 
case of the rapidly darting Paramecium confined in a capillary tube. 

Kalmus (1929) has added two other factors to Crampton’s three, 
in reporting his own studies on the effect of inclosing Paramecium 
caudatum, Stylonychia, and S pirostomum in capillary tubes. He finds 
that the age of the culture and the solubility of glass in the culture 
medium have decided effects. 

Capillary tubes made of two kinds of glass were used: “Schot- 
schem, nr. 20’ and the Bohemian glass made by Cavalier. The tubes 
measured 100-200 pw and the length of the contained column of liquid 
was from 8-304. Some of the observations are summarized in 
Table I. These results and others show that the type of glass in 
which small amounts of culture medium are held may affect the con- 
dition of the contained animals. 

Kalmus concludes that his observations show that the retardation 


124 ANIMAL AGGREGATIONS 


of division in small volumes is approximately proportional to the 
ratio of total surface to the volume of medium. Animals from young 
cultures are more sensitive to limited volumes than are those from 
old cultures. A fully bacterized medium retards the poisonous effect 
of small volumes by furnishing more food and by tending to keep 
the Paramecia out of the most toxic region next the glass, and by 
binding the toxins present, thereby rendering them relatively harm- 
less. These toxins may be of two sorts: there are the poisons which 
may leach out of the glass into the limited amount of medium in 


TABLE I 


Schotschem Glass Cavalier Glass 


24 hours: 


IDINAROIN OH A PMS. ocnecasseonces II animals 1 animal 

Conjugating of 25 pairs.......--..- 2 pairs 4 pairs 

Deadior 25 palrseas see eis sens 4 animals 9 animals 
48 hours: 

DivisionvoteosspallSmee eerie eee 17 animals 9 animals 

Conjugating of 25 pairs............ I pair 3 pairs 

Deadkotwocnpallseeeeieer meee eee 5 animals 15 animals 
72 hours: 

IDYiynisitern, i AG foe NES. jonagacpon0dee 19g animals 16 animals 

Conjugating O1e5 AES. yer.nere feel coe etree (orl eee eran ree 

IDERGl Oi OS ORNS. -bsaoacenunsocuder 14 animals 29 animals 


sufficient quantity to have decided effects, and there are the meta- 
bolic products of the Protozoa themselves. The question of the fixing 
of toxins takes us somewhat afield from our present considerations 
and will be left to be taken up later in detail. 

Unlike Crampton, Kalmus found that divisions of protozoans are 
possible even when they are contained in small capillary tubes. Ap- 
parently he did not subject his animals to the action of the centri- 
fuge, which may partly account for the difference in results. How- 
ever, when there are so many different factors operating, such as 
composition of glass, age of culture, and bacterial flora, one cannot 
be sure of the precise factor or factors causing the differences in 
observed results. From his observations, Kalmus challenges the en- 
tire conception that a small amount of available space, acting direct- 
ly, may limit the rate of cell division and thereby the size of meta- 
zoans. In this he overlooks the important results obtained by 


RETARDING INFLUENCE OF CROWDING 125 


Goetsch? on the effect of stimulation by contact with the walls of a 
small container in limiting the growth of active animals, even though 
the contained liquid be effectively connected with that of a much 
larger vessel. 

Undoubtedly there may be a limiting toxic effect of materials 
leached out of glass, particularly from soft glass. The dangers result- 
ing from the use of such glassware have been known for years. In ad- 
dition there may be a physical as well as a chemical effect from the 
glass walls of an inclosing vessel. Such effects are shown in the recent 
work of Drzewina and Bohn (1927). They base their experiments on 
the report of Norrish (1924, vide Taylor), who found that bromine 
combines with ethylene about twice as fast in contact with a surface 
of stearic acid as with one of glass, and that, on the other hand, the 
reaction within a paraffin-lined dish is about one-thirtieth of that 
given when the exposed surface is one of stearic acid. Using the 
marine flatworm Convoluta, Drzewina and Bohn found that these 
small worms survive only about half an hour when placed in sea 
water in a glass dish coated with stearic acid. In this instance the 
worms are not affected by dissolved chemicals, since stearic acid is 
insoluble in water. There is no change in the pH of the water, and 
water which has stood in such dishes is non-toxic when removed. A 
glass dish coated with paraffin becomes less toxic than is a plain glass 
dish. If the Convoluta in a glass dish on a white background are ex- 
posed to sunlight, they do not maintain their normal activity so 
long as when they are in a paraffined dish. There is also greater pro- 
tection in the latter against the toxic action of metallic silver. Para- 
mecia behave similarly. They die more rapidly in a dish covered with 
stearic acid, whether in light or in shade. Paraffin protects them 
against the action of metallic silver and of neutral red, even though 
they take up as much neutral red in a paraffined dish as in a plain 
glass dish. Drzewina and Bohn conclude that stearic acid catalyzes 
reactions of living animals but that paraffin inhibits them. They sug- 
gest that the action is similar to the action of paraffined glass in 
preventing the coagulation of blood, and advance the theory that 


«It is not yet definitely proven by chemical tests that the tubes with one end covered 
by cloth do allow free diffusion of excretory products. 


126 ANIMAL AGGREGATIONS 


both effects may be attributed to the electrical charge carried by the 
paraffin. 

Warren (1900), in his work on the effect of crowding on Daphnia, 
had previously found that media in which excretory products are 
allowed to accumulate cause a decrease in the number of genera- 
tions and the number of offspring in a brood, and that reproduction 
ceases long before the animals die. Such water is injurious, though 
not usually fatal to fresh Daphnia; and the reproductive power of 
the newly introduced Daphnia is soon reduced. The injurious nature 
of the water seems to pass off after a sufficiently long period. 

Our experience in growing Daphnia in quantity for fish food in a 
considerable volume of water, of perhaps 10-100 liters, accords with 
the experimental results of Warren. Events run as follows: A month 
or 6 weeks after having stocked such an aquarium with a few Daph- 
nia, conditions being favorable, several hundreds of animals may 
be living in good condition and reproducing. Then suddenly a change 
begins. The greater number die, young and old alike. Perhaps from 
1 to 3 per liter survive, and these will live for months without pro- 
ducing eggs. After a very considerable time eggs are formed and 
Daphnia may become fairly plentiful again, but the second swarm 
is never as numerous as the first. During the time when the Daphnia 
have ceased to reproduce and have, for the most part, died off, the 
water may be teeming with other entomostracans, ostracods or cope- 
pods. This indicates a certain specificity in the effect of the Daphnia 
metabolic wastes. The duration of the period of depression of repro- 
duction is greatly shortened by keeping the food value of the medium 
at a high level. 


EFFECT OF CROWDING ON RATE OF EGG-LAYING OF HENS 


The effect of density of population upon rate of reproduction in a 
different medium and with animals far removed in habits and in the 
evolutionary scale from Protozoa or Entomostraca was reported by 
Pearl and Surface (1909) from the experiments of Professor Gowell 
of the Maine Agriculture Station. These men report the result of 
investigations concerning egg production extending over several 
years. The chickens studied were kept in pens containing 50, 100, 


RETARDING INFLUENCE OF CROWDING 127 


and 150 hens each. The pens with the smaller flocks provided 4.8 sq. 
ft. of floor space per hen. In the largest flock this was reduced to 
3.2 sq. ft. per individual. The number of pens is shown in Table IT. 

In all there were 700 pullets placed in the 5o0-bird pens, 500 in the 
100-bird pens, and 750 in the 150-bird pens. Conditions varied some- 
what from year to year, so that Pearl and Surface warn that ‘‘wher- 
ever comparisons between years are instituted, great caution must 
be exercised in drawing conclusions.” 

Due care was taken to select the members of the different pens 
with hereditary constitutions equally disposed to egg-laying, so far 
as this factor could be regulated. All were from the same breed, and 


TABLE II 


50-Bird Pens | 100-Bird Pens | 150-Bird Pens 


IGG no. Goigu hon c | 6 I I 
O05 ORME eer | 4 2 2 
INO 7s bacogoune | 4 2 2 


individuals were distributed among the different pens so that the 
percentage from ancestors of different productivity were the same 
throughout. The experiment with which we are concerned ran three 
seasons. Results are graphically given in Figure 5, which shows the 
mean annual egg production per hen. An inspection of this figure 
shows that each year there is a trend toward reduced egg production 
in the pens with the greatest number of birds. During two of the 
three years the decrease in rate of laying is practically the same 
between the 50- and the 1oo-chicken pen as it is between the roo and- 
150. The results obtained the first season, 1904-5, are different and 
affect the mean differences, as is seen from the fact that the pens 
with 50 birds produced on the average 129.69 eggs per season; 
those with 100 produced 123.21, while those with 150 gave 111.68: 
The mean difference between the pens with 50 birds and those with 
150 amounted to 18.01 eggs per year. 

The difference between the 100-bird pens and the 150-bird pens, 
where there were two factors acting—increase of numbers and de- 
crease of floor space—is approximately twice as great during these 


128 ANIMAL AGGREGATIONS 


three seasons as is the difference between the 50- and the 1o0o-bird 
pens, where numbers only were varied. The experiments on the 
whole indicate, as Pearl and Surface conclude, that the mean an- 
nual egg production is much influenced by the differences in environ- 
mental factors present in the experiment. 


140 e 


/20 


/1/0 


LVI EAN OGM PRODUCTION 


/00 | 


J5O /00 150 
BIRD PENS BIRD PENS BIRD PENS 


Fic. 5.—A graphic summary of the relation between size of flock and mean annual 
egg production in the domestic fowl. (From Pearl and Surface, 1909.) 


In an attempt to get at the underlying factors they suggest that 
there is another element involved besides the physical density of the 
population, which they are inclined to place on the psychological 
level, and which works even when the amount of floor space per in- 
dividual is equal. The conditioning of the surrounding medium is of 
a different type from that of crowded aquatic animals, where the 


RETARDING INFLUENCE OF CROWDING 129 


excreta and glandular secretions are dissolved in the surrounding 
liquid medium and come of necessity into intimate contact with each 
of the contained animals. Presumably, with chickens we are free 
from inequalities in food, although in the larger pens some may 
have fared better and others more poorly, especially in view of the 
flock organization which Schjelderup-Ebbe (1922) has described. 
Availability of equal floor space does not insure equality of use, and 
crowding was probably greater the greater the numbers present. 
Even so, the significance of these observations is not lessened, and 
the conclusion of Pearl and Surface may be justified that we are here 
dealing with physiological effects on the reproductive system pro- 
duced by physiological effects on the nervous system of the order 
usually spoken of as ‘‘psychological.”’ 

It becomes important to follow the differences in egg production 
during the course of the year with these pullets housed with different 
degrees of crowding. The results of such analyses are published by 
Pearl and Surface (t911) and are summarized in Figure 6. The 
months from November to July are based on the averages of records 
for 4 years; from July through September on the records of 3 years. 
October is not included because records for only 2 years were avail- 
able. 

The data summarized in these graphs show that there is no harm- 
ful effect from keeping pullets in large and crowded flocks during 
early winter egg production near the beginning of the laying period. 
In fact there appears to be a significant advantage accruing from the 
crowding in the first really cold winter month, December. On the 
other hand, the s5o0-bird pens show a distinctly better production 
than do the other lots in late winter and early spring, about the time 
of heaviest egg production. This difference does not obtain between 
the birds kept in lots of 100 and those in lots of 150. The harmful 
effects of summer crowding on egg production shows plainly when 
the most crowded pullets are compared with less crowded lots. 
Overcrowding affects summer egg production in a distinctly ad- 
verse manner. 

There would thus seem to be three distinct aspects of the effects 
of crowding on egg production in the domestic fowl. First, in early 


AVERAGE EXCESS 


130 ANIMAL AGGREGATIONS 


winter at a time of relatively low egg production, when the nights 
are becoming increasingly cold, the large crowded flocks apparently 


NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT 


Fic. 6.—Diagram showing the average excess in mean egg production of different 
sized flocks of barred Plymouth Rock pullets in the first 11 months of their first year 
of laying. 

, 50-bird pens compared with 1oo-bird pens. 

------ , 50-bird pens compared with 150-bird pens. 

duecesevecks , 100-bird pens compared with 150-bird pens. 

The broken lines running parallel with the zero line approximate the mean probable 
error. Points below the zero line indicate that the larger number per pen gave a higher 
average egg production. Points above the zero line show that the pen with the smaller 
numbers gave the higher average. (From Pearl and Surface, 1911.) 


conserve animal heat, so that greater egg-laying occurs in the more 
crowded pens. Second, as the period of maximum egg production is 


RETARDING INFLUENCE OF CROWDING 131 


reached, crowding has the opposite effect, for reasons not yet clear. 
The nights are still cold, frequently colder than in December, when 
the opposite results were obtained. Probably the differential effect 
of crowding is associated with acclimatization to the cold; the gen- 
eral physiological condition of the hens must be different at the 
height of the laying season from that at its beginning, and this shift 
in physiological state may account for the reversed effect of crowd- 
ing; still, perhaps the psychological factor invoked by Pearl and Sur- 
face to cover admitted ignorance may be the only feasible suggestion 
as yet. Third, following the approach of warm weather and the com- 
ing of the hot summer months, the birds of the crowded pens prob- 
ably have difficulty in maintaining comfortable temperatures, par- 
ticularly while roosting. 

In concluding this discussion of the effect of crowding on the rate 
of egg production in chickens, it is of interest to note that the varia- 
bility in the rate of egg production increases with crowding when the 
annual egg production is taken as the unit. When this is broken into 
monthly periods, it is seen that the greatest effect of crowding is to 
be found at the beginning and the end of the laying-year, at a time 
of low production. From February to July, at the time of heaviest 
laying, the environmental differences implied by flock size as used in 
these experiments do not affect the relative variability of produc- 
tion. Unfortunately there are no data concerning egg production of 
chickens isolated in pens with the floor areas per individual used in 
these experiments. 


EFFECT OF CROWDING ON RATE OF REPRODUCTION IN DROSOPHILA 


Pearl and Parker (1922) have contributed another bit of signifi- | 
cant evidence to our problem by their work upon the influence of the 
density of population upon the rate of reproduction in Drosophila. 
In this work mass matings were made from a given line. The off- 
spring from this mass mating were used in making up the matings in 
the experiments to be described. Half-pint milk bottles were used as 
containers. The procedure was definitely standardized throughout. 
Sets of four bottles were started, each containing 1, 2, 3,....9, 
mated pairs of flies. Sets of three bottles contained, respectively, to, 


132 ANIMAL AGGREGATIONS 


12,15, 20, and 30 mated pairs; two bottles held 50 mated pairs each; 
and one bottle had 25 mated pairs. At the end of 8 days at 25° C. the 
surviving parent flies were transferred to fresh bottles for a second 
breeding period of 8 days. The only variable known to be significant 
throughout this series was the density of the population. All the 
offspring from the two breeding periods were counted and sexed. 
The results tabulated as the rate of reproduction per female per day 
during the first 16 days of life are shown in Figure 7. 

In this figure the circles give the observations, and the curve is the 
graph of the following equation fitted by the method of least squares: 


Y=34..53 €7 0-018 y-—0.658 , 
which in logarithmic form becomes: 
log y=1.54—0.008%—0. 658 log x, 


when y signifies the flies per mated female per day and x is the num- 
ber of mated flies per bottle, taken over the whole 16-day period the 
experiment ran. 

The observations include a total of 23,922 progeny flies, which is 
a large enough number to cause the results to be treated with re- 
spect. Further, it is apparent that the curve fits the observed facts 
closely. In the preceding chapter, I have called attention to the 
fact that this formula is related to that which Bilski developed to 
describe the effect of crowding on the rate of growth in tadpoles, to 
that of Kennealy for the relation between length of race and the 
record established for that distance, and to that of Farr for the rela- 
tion between density of human population and the death-rate. These 
phenomena must be based on a common fundamental biological 
relationship. 

When these results are analyzed further, we find that the greatest 
drop in rate of reproduction of adult flies per female per day comes as 
the number of original mated pairs per bottle increases from 1 to 2, 
and the next greatest drop comes between the bottles having an 
initial population of 2 and 3 mated pairs. This result cannot be due 
to larval crowding, since the bottles containing 9 mated pairs of flies 


RETARDING INFLUENCE OF CROWDING 133 


22 


20 


PROGENY FER #2 DAY 


(OOS ON 4 ONmSON G0) 7080) 90 


WIEAN PLIES FER BOTTLE 


Fic. 7.—Pearl and Parker’s (1922) curve showing the decrease in rate of reproduc- 
tion in Drosophila as cultures become more crowded. 


134 ANIMAL AGGREGATIONS 


produced 2,117 adult offspring. The 80 cc. of banana-agar food with 
an exposed surface of 23.76 sq. cm. per bottle must therefore have 
been capable of supporting this number of larvae in the time avail- 
able. The bottles with 1, 2, and 3 original mated pairs produced, re- 
spectively, 1,348, 1,124, and 1,877 total imagoes in 16 days. The food 
available would have allowed at least 2,117 larvae to pupate and 
produce adults. 

The exposed area, as well as the amount of food present, has been 
shown to have a distinct effect upon the numbers of Drosophila pro- 
duced. Harnly (1929) varied the area of standard food with the 


250 
40 SQ.C/. AREA 


=o 
DS SSS 


=~ 
(-) Co} 
24 52 
150 
O77 
100 
S44 
50 


Fic. 8.—Showing the relation between productivity and the area of food with 
Drosophila. The vertical column of figures gives density; figures on the graph show the 
area of food surface in square centimeters. The corresponding total volume capacities of 
the containers are: , 1181, 2365, 473, and 250. (Data and figure from Harnly.) 


depth kept constant at 25 mm. The five areas tried were those fur- 
nished by culturing the flies in vials, 4-ounce bottles, half-pint, and 
pint milk bottles, and in 250 cc. Erlenmeyer flasks. These different 
culture-containers gave food surface areas of 4.4, 11, 24, 40, and 52 
sq. cm., respectively. Summarized results are given in Figure 8, 
which shows graphically the effect of surface food area upon the total 
yield from a single pair mating for a period of 1o days. The largest 
yield under these conditions was given by a surface area of 4o sq. 
cm.; 52 sq. cm. had about the same productivity as 24 sq. cm. The 
viability was greatest in the flies reared with the largest amount of 
space. 


RETARDING INFLUENCE OF CROWDING 135 


The explanation of Harnly’s results is not necessarily obvious or 
easy. It may be that there is actually a surface-population optimum 
which stands below the largest surface and volume of food available. 
A possible factor may be that with greater area and volume wild 
yeasts or molds grow too rapidly for the Drosophila to control. Be- 
fore coming to this conclusion, it is well to note the sizes of the dif- 
ferent containers, which were: vials, size not stated, 118, 236.5, 473, 
and 250 ml. The population curve may be a result of the available. 
space rather than of the food surface acting alone. Such an interpre- 
tation would be in line with the data of Pearl and Parker shown in 
Figure 7. More work is needed, however, before one can draw as- 
sured conclusions. 

The tendency toward universality of the effect of crowding upon 
the rate of reproduction is shown by the fact that Hill (1926) and 
Sarles (1929), working with hookworms, have reported counts on 
population density of these parasites in relation to egg production 
which show that as the number of worms in a given host increases, 
the egg output per worm decreases. 

Pearl and Parker conclude the account of their work upon crowd- 
ing and the rate of reproduction in Drosophila with the following 
statement, which Pearl repeats in a later book: ‘In general there 
can be no question that this whole matter of influence of density of 
population in all senses, upon biological phenomena, deserves a great 
deal more attention than it has had. The indications all are that it 
is the most important and significant element in the biological, as 
distinguished from the physical, environment of organisms.” With 
this position I am in complete accord. 


CHAPTER VIII 
CROWDING AND INCREASED DEATH-RATE 


Measurements of growth, reproduction and length of life, sum up 
many of the physiological processes that may be affected by crowd- 
ing; the first two of these have already been considered in some de- 
tail. These three functions are closely connected, and it has been 
impossible to keep their treatment entirely separate. Thus, in the 
preceding chapter, the discussion of the effect upon the rate of re- 
production of confining Paramecia within a small capillary tube was 
extended to include a partial discussion of such treatment upon the 
longevity of the animals in order to get sufficient control of the avail- 
able evidence to be able properly to evaluate factors affecting the 
decrease in rate of reproduction brought about by crowding. 

Inspection of the material previously presented demonstrates that 
the ability of adult organisms to live is not necessarily the same as 
their ability to reproduce. Kuczynski (1928), in studying the bal- 
ance of birth and deaths among the human population of Western 
Europe, describes the differential effect of changing conditions upon 
fertility and upon the death-rate, and concludes that human fertility 
has become a problem in itself largely divorced from the problem of 
mortality. 

The experience of Warren that Daphnia lose their reproductive 
capacity long before they die, and that in such a condition they may 
be able to live through adverse conditions produced by overcrowding 
and again take up reproduction when conditions become more favor- 
able, is a case in point. Kalmus adds observations upon Paramecia 
along the same line. The usual interaction between fertility and mor- 
tality is such that in a given amount of liquid medium the popula- 
tion increases to a maximum whose size depends on the volume of the 
medium and the concentration of the food material, and then gradu- 
ally falls to complete or nearly complete extinction. This course of 

136 


CROWDING AND INCREASED DEATH-RATE 137 


events is shown in Figure 9, which is taken from the work of Myers 
with Paramecia. 

If the initial volume is relatively large (16 drops, or about 0.8 ml.), 
Myers finds that fission begins at about the same rate, so far as 12- 
hour periods of observation show, regardless of whether the seeding 
be with 1, 2, or 4 individuals. The population of such cultures rises 
rapidly to a peak which is essentially the same for all the seedings 
just mentioned; at its peak it ranges from 123 to 126 individuals and 


2 4 6 8 70 72 14 76 (SeeTSO 


Fic. 9.—Showing the rise and decline of populations of Paramecia in 0.8 cc. of 
culture fluid by, respectively, 1, 2, 4, and 8 individuals. The horizontal axis shows 
successive periods of 12 hours each; the vertical axis gives numbers of individuals in 
the populations. (From Myers, 1927.) 


then falls off at about the same rate. The cultures seeded with a 
single individual take longer to reach the maximum than do those 
seeded with 2 or 4, but otherwise the course of the population history 
is similar. 

When 8 individuals are introduced in place of 1, 2, or 4, the maxi- 
mum in Myers’ cultures showed a population of only half that given 
with the smaller seedings. The reason for this difference is not clear. 
On the surface it appears that the larger initial seeding either ex- 
hausts the food supply more speedily or poisons the culture medium 


138 ANIMAL AGGREGATIONS 


more rapidly than do the other seedings or acts in both ways at the 
same time. 

The point of especial interest to us in these observations is the 
fact that with certain initial densities of populations, even in an un- 
changed medium, the maximum reached is practically identical and 
independent of the numbers originally introduced. 

The effect of the exhaustion of food supply has been eliminated by 
Chapman (1928) in his work with the confused flour beetle, 77iboli- 
um confusum. Chapman introduced varying numbers of these beetles 
into a definite amount of whole-wheat flour and found for this insect, 
as Robertson, Cutler and Crump, Myers, and others had previously 
found with various Protozoa, that there is a definite limit to the 
number of organisms that will develop in a unit volume of culture 
medium. 

Chapman’s work does, however, introduce one new fact into the 
situation. His choice of experimental material is particularly for- 
tunate in that the beetles can be screened out of their floury environ- 
ment and the eggs, larvae, and pupae, as well as imagoes, can be 
counted and the flour renewed at each observation. Hence, in place 
of the usual more or less symmetrical population curve found by 
other workers dealing with a population composed of all age groups, 
in which the population, after rising to the maximum determined by 
the nature of the culture medium and the amount of space available, 
falls away to approximate or total extinction on account of the ex- 
haustion of food or the addition of excretory products, Chapman is 
able, by periodically renewing the environment, to carry his beetle 
population along for extended periods, perhaps indefinitely, with 
approximately the same number of individuals present per gram of 
flour. In his terminology, ‘‘a condition of equilibrium is attained in 
which the biotic potential is equalled by the environmental resist- 
ance and the population remains relatively constant.” 

Appropriate tests showed that the stationary character of the 
population when in equilibrium was not due to absence of eggs or to 
their lack of fertility. Rather, the lack of increase in population be- 
yond a certain point was due to the eggs, pupae, and, to some extent, 
the larvae, being eaten by the adult beetles. When eggs were placed 


CROWDING AND INCREASED DEATH-RATE 139 


in flour cultures containing only male beetles, the percentage of 
eggs eaten varied directly with the population of adults per gram of 
flour. 

Chapman’s experience with Tribolium can be shown in a number 
of ways; perhaps as significant as any is the result of carrying to 
equilibrium a series of beetle environments of different sizes, of 
4-128 grams of whole-wheat flour, seeded with 1, 2, 4, 8, 16, and 32 
pairs of beetles each, making one pair of introduced beetles per 4 
grams of flour. This experiment may be followed in Table HI, which 


TABLE III 


BEETLES (Tribolium confusum) PER GRAM OF WHOLE-WHEAT FLOUR 
(Data from Chapman, 1928) 


Days 4G 8G 16 G. 32G 64G 128G 

ORM tisai ce: 0.5 0.5 0.5 0.5 OBS 0.5 
TG hep asks tee 15 a9) 20 07) 21 19 
ROS tsfayeeave 30 25 26 22 24 23 
5 Omeeretrcra te 35 33 32 35 32 34 
64.......-. 39 39 34 39 40 oii 
Ghote cL CRON 35 41 20 36 37 39 
TiO Tees tats cchais = 40 46 38 44 49 39 
THUAY Peeps ticis 48 45 36 43 40 Fike) 
TRA R ioteteioie shat Qi 50 41 41 48 45 
TiS Ol nets siete -< 38 49 46 44 45 47 
TA Tey syctescuere 3 46 49 46 43 42 40 


gives the total number of the beetles present per gram of flour at 
different times in the history of the cultures, regardless of the de- 
velopmental stage of the beetles. 

In all the conditions tested by Chapman, the mean number of in- 
dividuals per gram of flour after equilibrium was established was 
43.97, with a standard deviation of 4.27, and a probable error of 2.88. 
Chapman found, as will be shown in detail later (chap. x), that the 
time taken to reach this equilibrium differed with the initial seeding 
per gram of available flour, while the equilibrium population re- 
mained constant per gram of flour; and that, with the same initial 
seeding throughout, the equilibrium population per gram of flour re- 
mained constant but the time taken to reach equilibrium varied 
directly with the quantity of flour available. This equilibrium is 
primarily a food relation, or a food and space relation, since the 


140 ANIMAL AGGREGATIONS 


metabolic products are removed by the periodic changes of the flour. 
The equilibrium is apparently based upon a competition between 
adults and larvae, as is shown by the fact that in one 16-gram en- 
vironment the number of adults was accidentally reduced on the 
seventy-eighth day of the experiment and was never returned to its 
place in the geometric series, while the number in its total popula- 
tion—eggs, larvae, pupae, and adults—did so return. 

Following current tendencies, Chapman interprets his findings in 
terms of a mathematical formula, C=Bp(R), when C is the concen- 
tration of insects, R is the environmental resistance, and Bp is the 
biotic potential, which he defines as the mean maximum rate of re- 
production in a given period under given conditions. Substituting 
and solving, we find: 


pa '43:97%8.4)-25 
43.97 


Soi o 


The concentration of insects per gram of flour is 43.97. The aver- 
age number of eggs laid per day in these experiments is 8.4, and one- 
fourth of the population are egg-producing females. The formula so 
given represents the state of equilibrium only. 

The work we have been discussing summarizes the effect of the 
environmental factors associated with crowding upon the total popu- 
lation. The work which deals most directly with the harmful effects 
of crowding on length of life is that of Drzewina and Bohn. In con- 
nection with their studies on the relation existing between mass of 
toxic liquids and the contained mass of animals, Drzewina and Bohn 
have found that many cases of protection are furnished by increasing 
the numbers of animals present in the same solution. These will be 
reported later. In some instances they record the opposite results 
(1921d, 1922). 

When KCl was used as a toxic agent with cultures of Convoluta, a 
small marine planarian, other things being equal, those in the solu- 
tion containing the larger number died first. Similar relations hold 
when the same number of individuals are placed in differing amounts 
of the same strength of KCl solution: those in the smaller amount of 
liquid die more rapidly. The fresh-water planarian Polycelis nigra 


CROWDING AND INCREASED DEATH-RATE 141 


reacts similarly. These investigators believe that the planarians give 
off a substance which causes autodestruction, and that, if this be 
true, such destruction is hastened by increasing the mass of indi- 
viduals in proportion to the amount of liquid. 

Their interpretation is supported by the observation that if fresh 
Planaria are introduced into a solution of KCl which has already 
contained others, their death is hastened. If, after some time in such 
a solution, a part are removed to a new solution of similar strength 
of KCl, these die more slowly than do their fellows left in the original 
solution, which contained not only KCl but also some substance or 
substances given off by the worms themselves. 

Later (1928) they observed that around cytolyzing flatworms the 
H-ion concentration (acidity) of the solution is greatly increased, and 
they considered this to be the factor which causes the increased 
mortality of the groups. The larger the number of cytolyzing in- 
dividuals, the more rapid and the greater the increase of the H-ion 
concentration, and consequently the more rapid and pronounced are 
the lethal effects involved. Such a process accelerates itself in the 
presence of many individuals, or, on the other hand, is not effective 
in the case of a few scattered animals. If the latter die, they do so 
because of the lethal effect of the KCI alone. 

Fowler (1927, 1931) undertook to test the effect of a large number 
of electrolytes upon the rate of survival in certain crustaceans, using 
mainly a species of Daphnia. His results show that there is a distinct 
correlation between the survival value of the group and the degree 
of toxicity of the salt solutions employed. Tests upon the rate of 
oxygen consumption have shown that crowded Daphnia, in the con- 
centrations tested, use less oxygen per individual than do isolated 
Daphnia under similar conditions. When the toxicity is sufficiently 
great so that death occurs within a relatively short time, this group 
depression tends to favor group survival. On the other hand, when 
the concentration is low and the effect of the chemicals is de- 
ferred, the isolated individuals live longer than the group. Such re- 
sults are in accord with Child’s (1915) differential susceptibility find- 
ings when planarians are subjected to relatively strong or relatively 
weak concentrations of various toxic agents, particularly KCN. 


142 ANIMAL AGGREGATIONS 


Under conditions of high toxicity the animals, or, in the case of 
Child’s worms, certain regions, which have the lowest rate of general 
metabolism are least affected by the toxic agent and survive longest. 
With weaker solutions, on the other hand, the most vigorous indi- 
viduals, or, in the case of the worms, the most vigorous regions, can 
acclimate most readily and hence survive longer. Fowler’s results 
show that depression due to crowding may have definite survival 
value under certain conditions but that with weaker concentrations 
of even the same salts crowding decreases the chance of survival. In 
the latter aspect his results support the facts reported by Drzewina 
and Bohn in their experiments with KCl. Fowler’s results fail to 
support the hypothesis that a specific autodestructive material is 
produced. They extend the later explanation of Drzewina and Bohn 
by indicating that the unknown autodestructive substance is the 
carbon dioxide produced by the animals, which does raise the H-ion 
concentration as Drzewina and Bohn determined. 

For further consideration of the relation between density of popu- 
lation and the death-rate it seems best to take up the case with re- 
spect to man, since with human populations this relationship has 
attracted particular attention for a considerable period of time. We 
have already referred to the generalization known as Farr’s law; this 
states that if the death-rate be represented by FR and the density of 
population per unit area by D, then R=cD”, where c and m are con- 
stants. 

Brownlee (1915) rehabilitated this law by showing that the statis- 
tics used by Farr, which came from the decade 1861-70, compared 
favorably, so far as the relation between population density and 
death-rate was concerned, with those of the decade 1891-1900, as 
given by Tatham. Brownlee’s republication of these tables and his 
calculations are given herewith; see Table IV. 

Brownlee calls attention to the fact that the values of m corre- 
spond roughly for each type of analysis in the two periods but that 
in the case of the life-table death-rates they correspond to the third 
decimal place, which is as much as could be statistically expected. 
He concludes that Farr’s law is thus shown to be a definite law oper- 
ating independently of the changes due to sanitary progress. Re- 


CROWDING AND INCREASED DEATH-RATE 143 


gardless of improvements in sanitation and in medicine, the ex- 
ponent does not vary, but only the multiplying constant. Therefore 
m represents the law, while c represents rather the coefficient of 


TABLE IV] 
Corrected | Same Fitted Crude ° Life-Table | Same Fitted 
No. of _ |Persons per Death- by Least Death- Same Fitted Death- by Least 


Districts Sq. Mi. y Farr 


Rate Squares Rate Rate Squares 


A. Showing Figures Relating to Density and Death-Rate, 1861-70 


: t t 
IG acieie ears 166 15.50 16.70 16.75 18.90 19.90 20.73 
RY Maas ere 186 17.02 17.00 19.16 19.10 Pit ey 20.96 
LQ Fevers cate 379 20.52 18.99 20.87 20.87 23.97 225% 
Actes 1,718 24.35 24.03 25.02 25.02 26.09 20.19 
OWE ac 4,499 27.94 27.92 28.08 28.08 28.54 28.84 
Lita sctets 1D er) 33.98 32.67 32.70 82270 BORO 31.92 
ggeaoe 65,823 | 40.55 42.39 38.74 38.74 Sj esui 37-74 
E%=3.708 E%=2.70 E%=2.01 
A=1.17 = .go = .61 


B. The Same for 1891-1900 


is tt tt 

DG basrctetys 136 11.63 13.06 14.20 14.16 17.38 17.18 
si ae 161 12.54 13543 15.05 14.51 18.01 18.12 
Oia epee 181 13.44 £3670 15.44 14.68 18.62 18.33 
O2R aan © 201 452 = T4250 15.40 15.38 19.30 19.02 
Ie eee 407 15.53 15.68 16.08 16.28 20.05 19.90 
OR cis 457 16.53 I5.99 16.67 16.52 20.24 20.13 
Bileiele 734 17.58 Ff. BE 17.64 17.56 21.45 21.02 
GOs o6. 5.06 T3208 18.53 1Q.05 18.04 18.88 22TO 22). 37 
iS iLlensvepe tor 1,705 19.42 19.93 18.61 19.54 DP opr 22.99 
Diya) 2,339 20.37 21.00 19.50 20.35 23.30 DBZ 
TiO w erskaisks 4,42 21.56 22 eai 20.21 22.08 24.18 PN 24 
Ti ssa. 4,884 22.36 23.76 20.69 22.35 24.72 25.50 

Onepee a's 4,194 23.48 23.16 2205 PH OB 25.49 25.10 

ae encore 2,925 24.33 21.80 23.29 20.94 26.07 24.21 

iS crsetrae 7,480 26.54 Phy iat 24.74 23.60 27.58 26.68 

Aber R ere 555503 34.82 32.67 23.67 30.49 3325 32.58 

E%=4.3 E%=3.8 E% = 2.03 
A=1.05 A=1.14 A= .63 

4 From Brownlee, Journal of Hygiene, XV, 16. 

* R=7.534D-157" { R=10.234D-11998 t R=12.419D-118 

§ E%=mean experimental error; /\ =?¥ of the mean of the squares of the errors. 

¥** R=12.40D).-16715 tt R=13.57D-7ss tt R=10.83D-2078 


sanitary and general conditions of health. The coefficient c decreased 
from 12.42 in the earlier period to 10.83 for the later. In other words, 
in the same general area, conditions of living have so improved in 


144 ANIMAL AGGREGATIONS 


the thirty years’ interval shown in Table IV that density of popula- 
tion had only 0.875 the effect during the 1890’s that it had in the 
1860’s. Here we have evidence that relatively mild crowding affects 
longevity in men. 

This is to be expected when we eoucidler the relatively greater 
ease of transmission of contagious diseases in the more crowded 
areas. Such dangers from the crowd are illustrated in a simplified 
form by one of the Ophioderma experiments to be reported in full in 
another connection. In these experiments the survival of 8 isolated 
brittle starfish, each in a 1-liter Erlenmeyer flask, was compared with 
a group of 8 similar starfish in an 8-liter bottle. Usually the group 
outlived the isolated individuals; but on one occasion one of the 
members of the group died soon after the daily inspection and change 
of water, and so polluted the whole 8 liters that all the remainder 
were dead on the following morning. When an isolated animal died 
similarly, the effects of its death could not extend beyond the limits 
of its single flask. 

When we pass in review the materials presented in the chapters of 
which this is the third, we find much evidence supporting the gener- 
ally accepted dictum that crowding is harmful for poorly integrated 
groups of animals, breeding and hibernation seasons excepted. We 
have seen that crowding may slow down the rate of growth and may 
result in dwarfed individuals, that the rate of reproduction may be 
decreased, and that the death-rate may be greater. These effects 
have been reported for so many different animals from such a wide 
range of the animal kingdom that there can be no doubt of their 
general significance. But this is not the whole story. In many of the 
experiments to be reported in our next section, we shall find that 
crowding does not always produce harmful results; and that under 
many conditions there are distinctly beneficial results, providing the 
crowding be not too great. When considering these beneficial re- 
sults, we must, however, always keep in mind the harmful effects of 
overcrowding. 


BENEFICIAL EFFECTS OF AGGREGATIONS 


CHAPTER IX 
STIMULATION OF GROWTH BY CROWDING 


Having sketched in some detail the harmful effects resulting from 
the crowding of many animals into a relatively small space, it is now 
possible, with a better perspective, to look into the more recently ac- 
cumulated evidence that harmful results do not necessarily follow 
the formation of such aggregations and that they are often useful and 
even necessary to the welfare of the individual. 

The extent to which the phenomenon of aggregation affects the 
rate of growth in a positive manner has been relatively little in- 
vestigated. In the work of Colton (1908) upon Lymnaea it will be re- 
called that he found crowding generally decreased the rate of growth 
in snails. He found, however, that the snail faeces, if washed free of 
easily soluble material and placed in weak solutions with snails, 
tended to increase the rate of growth. With concentrated solutions 
of faeces the results were reversed. Similarly, weak solutions of urea 
favored snail growth, though stronger solutions retarded it. 

Popovici-Baznosanu (1921) also found that under certain condi- 
tions snails grew more rapidly in stagnant water, conditioned by the 
snails, than in fresh water. In short experiments (1914) Io young 
Lymnaea attained a length of 9.5 mm. in fresh water while those 
living in stagnant water grew to 10 mm. Later he tested this effect 
for a longer period. The young Lymnaea from three egg masses were 
placed in three culture dishes of identical dimensions as regards vol- 
ume and surface of water; after a long sojourn, when the water was 
thoroughly snail-conditioned, Popovici-Baznosanu took half of the 
individuals and placed them in better conditions of existence, in 
culture jars with a large volume and a relatively large surface, and 
containing fresh pure water. Elodea was used as food both for those 
in the stale and those in the fresh water, and in the same quantity 
for both. After 106 days the results were as given in Table V. 


147 


148 ANIMAL AGGREGATIONS 


In only one of the three cases was there a clearly significant dif- 
ference; yet Popovici-Baznosanu interpreted these results as mean- 
ing that in the stagnant, snail-conditioned water, the higher plants 
present, as well as the walls of the jar, are covered by growths of 
microflora, which he regards as forming the chief food of the snails; 
and that the snails therefore grew more rapidly in cultures contain- 
ing a rich microflora than in those with only a scanty supply. Colton 
had interpreted his results similarly. 

The observations of Eigenbrodt (1925) that Drosophila grow larg- 
er in small culture vials when present in numbers of from 8 to 16 than 


TABLE V* 
Brawn Surface of Water | Volume of Water Condition of | |Length of Largest 
(Sq. Cm.) (Cc.) Water Shell (Mm.) 
I 1 113 1,300 Conditioned 19 
Set eoete nape SETS PEN eee: nee = bS5 Rae 6.5 

II 113 I, 300 Conditioned 19 
©), 0. @) 0B i0/ 1s), 018 Siete) e158) @ ee) 6 a lo 208 4 , 5 Io Raw I 8 
113 I, 300 Conditioned 15 

1 Gy ata ne tS tae aaa 4 
116 1,640 Raw 15 


* Data from Popovici-Baznosanu. 


at other population densities may be explained on the assumption 
that too few Drosophila larvae per culture fail to control the growth 
of harmful elements of the yeast or bacterial flora as well as optimal 
numbers do, while overcrowding overcontrols the growth of the food 
plant. This would result in a growth optimum occurring, as sug- 
gested, at a relatively low population density but distinctly above 
the minimum populations studied. These results should be compared 
with the relation between numbers present and Drosophila survival 
given in chapter xiv. 

Bilski (1926) tested the effect of crowding upon the rate of re- 
generation of the tails of Rana esculenta tadpoles. Five of these tad- 
poles were kept isolated, and five similar ones were placed together 
in the same sort of dish and with the same amount of water which 
was given to each of the singles. Although in most cases there was a 
decrease in length of body from tip of head to the root of the tail, 
there was growth both of the tail stump and of regenerated material. 


STIMULATION OF GROWTH BY CROWDING 149 


The proportions of decrease and of growth or regeneration differ be- 
tween isolated and grouped animals. The results for the 7 days the 
animals were observed are given in Table VI. The results indicate, as 
much as a single experiment is likely to, that there is a greater re- 
generation with the decreased volume per animal, which is compen- 
sated by the greater growth of the tail stump when the animals are 
isolated. Bilski states that this experiment is supported by his gen- 
eral experience in many similar experiments in other phases of the 
work, but cites no direct support of these results. 


TABLE VI 


SHOWING THE EFFECT OF CROWDING ON THE RATE OF REGENERATION OF 
TAILS OF FROG TADPOLES IN 7 Days. (IN MILLIMETERS) 


(Data from Bilski) 


DIFFERENCE PERCENTAGE OF DIFFERENCE* 
ConDITIONS 
| Rody Length | Tail Stump | Body Length | Tail Stump Regenerated 
(B) (S) (B S (R) 
lsolatedaememeneaer —1I.4 2.5 —11.8 15.1 Sear 
Grewal, -scoddoor OMY 2.0 — 6.2 12.6 18.3 


B, length from tip of head to root of tail. 
S, length of tail stump after cutting. R, length of regenerated material. 


* Percentage B is calculated on the basis of the original body length; percentages S and R are in terms 
of the original tail length before operation. 


CROWDING IN TISSUE CULTURES 


Work with tissue cultures has yielded pertinent evidence concern- 
ing the beneficial effects of crowding on growth. The literature in 
this field is enormous; and no attempt will be made to cover the 
different ramifications of the subject, with most of which we are not 
immediately concerned. It has been known for some years (Carrel, 
1924) that tissues to be grown in vitro must have a proper back- 
ground on which to creep. One of the most used backgrounds is of 
fibrin network. In many recent studies this is placed as blood-plasma 
in a thin layer over the bottom of a special culture flask. The tissue 
to be cultured is introduced aseptically into this sterile medium, 
which is then covered with a sterile fluid that has Tyrode solution as 
its main ingredient but which contains other materials such as serum 
or a Saline extract of embryonic tissues. The latter, or some fraction 
thereof, appears to be necessary for real growth of such cells as 


150 ANIMAL AGGREGATIONS 


fibroblasts or epithelial cells. Extracts of sarcomas are superficially 
similar to extracts of embryos in their growth-producing qualities. 
Leucocytes (macrophages) behave in reverse fashion, growing per- 
manently in pure serum and being inhibited by the presence of em- 
bryonic extracts. 

Inorganic substances, oxygen excepted, apparently do not affect 
growth-rate of cells 7m vitro when present in approximately the same 
concentrations as in the blood of the animal furnishing the tissues 
under cultivation. Any departure from such concentrations yields 
adverse results. Only approximately isotonic media allow indefinite 
survival. The exact nature of the growth-promoting substance found 
in embryonic extracts is still unknown. It appears to be associated 
with the protein fraction and is particularly associated with pro- 
teoses which result from a brief digestion of the protein with peptone 
(Carrel and Baker, 1926). Prolonged digestion destroys the effec- 
tiveness of this material. Willmer (1928) has been unable to confirm 
this work, but concludes from the evidence furnished by Carrel, 
Baker, and others that tissues can get some energy from amino- 
acids but that their nitrogen supply is chiefly obtained from pro- 
teoses (embryo extract contains both elements). These are heat- 
stable substances; but most workers find that there is present in 
embryonic juice a thermolabile growth-promoting substance which 
is easily destroyed by heat or is adsorbed when heated, which does 
not pass through a Chamberlain filter, and which is destroyed by 
prolonged shakings. Carrel (1924) has called such substances “‘tre- 
phones”; Fischer (1925a) calls supposedly similar substances ‘‘des- 
mones”; and Burrows and Johnson (1925) named them the 
“archusia.”’ 

Tissue-culture workers appear to be agreed upon the necessity of 
keeping the cells from normal tissues in numbers, for successful cul- 
tivation in vitro. Harrison (1928) says in this connection: “It is a 
very interesting and at present inexplicable fact that single somatic 
cells isolated in culture media do not proliferate. Experiments to 
this end made in my own laboratory some years ago but not pub- 
lished did not succeed and other workers have reported similar ex- 
perience. As Fischer puts it, a colony of fibroblasts cannot arise 


STIMULATION OF GROWTH BY CROWDING I51 


from a single cell even when the nutrient conditions are most favor- 
able. Likewise small groups of cells if isolated do not undergo divi- 
sion and their growth remains at a standstill. On the other hand 
certain tumor cells (Rous chicken sarcoma) are capable of multiply- 
ing and producing colonies when isolated singly.” Similarly, we 
know that in nature single egg cells will grow. The germinal area of 
the hen’s egg is an excellent example of an isolated bit of protoplasm 
which, under favorable conditions, will grow. It is of interest to us 
to note that Wright (1926) has found by dialysis a growth-stimulant 
in the incubated yolk of hen eggs which is not shown when such 
yolk is added directly to tissue-culture medium without dialysis. 

Haberlandt (vide Fischer), in his work with plant cells, could se- 
cure increase in size from certain isolated cells but did not find cell 
division in such cultures. He (Haberlandt, tg19—22) reports a direct 
relation between the size of the piece of plant tissue transplanted, or 
the number of cells within it, and the number of cell divisions. From 
these studies this investigator has concluded that the inciting to cell 
division comes from substance given off by injured cells, which he 
terms ‘‘wound hormones” or ‘‘division hormones.” 

A dramatic instance of the effect of heterotypic crowding upon 
growth of tissue cells im vitro is furnished by Carrel and Ebeling 
(1923). Cultures of leucocytes and of fibroblasts were made together 
in the same flask of plasma. As usual under these conditions, the 
fibroblasts did not grow, while the leucocytes grew well. In time they 
spread until they came in contact with the languishing fibroblasts, 
when a marked revival and initiation of growth took place in the 
latter cells. This agrees with the generally known fact that in the 
tissues of early embryos, when growth is taking place most rapidly, 
there is a mass of growing tissue tightly packed together which is 
supplied with a relatively small amount of blood. In tissue cultures 
growth takes place best when the cells are present in relatively large 
numbers in a small amount of medium which is stagnant but proper- 
ly supplied with oxygen. Both kinds of observations suggest that 
the cells forming metazoan tissues are dependent greatly upon one 
another for their growth. 

Fischer (1925) has suggested that this dependence is due to the 


152 ANIMAL AGGREGATIONS 


slow diffusion of products of metabolism or secretions from one cell 
to another. He thinks these travel by protoplasmic bridges and are 
independent of Carrel’s ‘“‘trephones,”’ since fibroblasts that cease to 
grow in the presence of an abundance of these trephones may be 
restored to rapid growth by the presence of active healthy cells. 

Burrows and his co-workers (1925, 1926) have put forward an in- 
teresting and ingenious suggestion which explains many aspects of 
the interrelations between cells and the fact that they must be pres- 
ent in numbers before growth will occur and at the same time ex- 
plains other characteristic activities of cells in tissue cultures. These 
workers suggest that in the presence of a sufficient amount of oxygen, 
about one-third of an atmosphere, the cells secrete a hypothetical 
substance or group of related substances which as stated above are 
called ‘‘archusia,”’ which are supposed to function somewhat like the 
desmones of Fischer except that the function of archusia is profound- 
ly modified by their concentration. If present in high concentration, 
they display an enzyme-like action which causes self-digestion of the 
tissues; if the concentration is somewhat lower, the presence of ar- 
chusia allows the cells to digest fats and proteins and to grow, pro- 
viding the medium is otherwise suitable. In more dilute solutions, 
tissue growth ceases; but the cells display their characteristic mi- 
grating ability, which is frequently shown in cultures, or parts of 
cultures, in which no growth is going forward. In yet more dilute 
concentrations, the cells lose their power of carrying on their ordi- 
nary activities, round up, and become dormant. 

Archusia are water soluble, are secreted by cells, and can diffuse 
through cell membranes to the outside medium. They tend to collect 
in quantity when many cells are together in a minimum volume 
under stagnant conditions, which are known to favor growth of 
tissue cultures. When too great a volume of medium is present in 
proportion to the number of cells, or if such cells as fibroblasts are 
isolated, archusia escape into the surrounding medium and growth 
ceases. Cells isolated into sufficiently small volume should grow, 
according to the implications of this hypothesis; but the needed 
volume may be so small that other complicating factors arise. Such 
a substance would also be carried away by repeated washings, which 


STIMULATION OF GROWTH BY CROWDING 153 


are known to be harmful to cells whether grown in vitro or in vivo. 
Archusia have properties resembling bios and vitamin B and have 
been thought to be identical with the latter. 

The whole concept of archusia is in the hypothetical stage at pres- 
ent, and more evidence is needed before coming to a definite con- 
clusion concerning its validity. 

Heaton (1926) has worked upon the effect of vitamin B upon the 
growth of cells 77 vitro. He finds two elements present in extracts of 
yeast and of liver—one which stimulates growth and another which 
depresses it. The two can be separated by their different solubility in 
alcohol. Burrows and Jorstad (1926) think that vitamin A is neces- 
sary for the functioning of cells and is produced when cells are digest- 
ing fats and growing under the stimulus of relatively high concentra- 
tions of archusia (vitamin B?). They regard vitamins A and B as 
antagonistic, and balanced in cells that are functioning. In fact, 
most observers are agreed that fats and lipoids are associated with 
substances which inhibit the growth of cells, while some portion of 
the protein molecule is associated with the promotion of growth. 


EFFECT OF CROWDING ON GROWTH OF SEA-URCHIN PLUTEI 


Certain of these results of the tissue culturists have been applied 
to the problem of the effect of crowding upon the rate of cleavage and 
of the growth of the arms of sea-urchin plutei by Peebles (1929). By 
treating extracts of sea-urchin eggs and larvae with alcohol or with 
acetone, a growth-inhibiting substance was obtained which definitely 
retarded the rate of growth of eggs or of plutei. When this fraction 
containing lipoids was partially removed, growth acceleration was 
observed, as shown in Figure ro. Further experiments showed that 
there was a decided difference in growth, depending on whether the 
alcohol-soluble or alcohol-insoluble fractions of extracts of echino- 
derm plutei were used. These results are shown graphically in Fig. rr. 
Peebles was also able to remove growth-inhibiting substances from 
such extracts by adsorption, as shown in Fig. 12, but has not been 
able as yet to isolate either the growth-inhibiting or the growth- 
promoting principle. 

Peebles, in summarizing her work, says: ‘The eggs and larvae of 


154 ANIMAL AGGREGATIONS 


Cleavage 


0 
50 60 70 80 90 100 10 120 
Jime in Minutes 


Fic. 1o.—Showing the rate of cleavage in eggs of the sea urchin treated with acetone 
extract from which the fat has been partially removed. Acetone (extract) minus fats, O. 


Control, A. (From Peebles, 1929.) 


Cleavage 


eS 


0 
HO  §©120 =: 130 


40 50 60 70 80 90 100 
7ime in /4inultes 

Fic. 11.—A comparison of the results obtained by Peebles (1929) by using the filtered 
alcoholic extracts of plutei (4A) with that of the precipitate (x, v). Ninety-seven per 
cent ethyl alcohol (filtrate), &; 75 per cent ethyl alcohol (filtrate), A; control, O; 75 
per cent ethyl alcohol (precipitate), x; 97 per cent ethyl alcohol (precipitate), v. 


STIMULATION OF GROWTH BY CROWDING 155 


the sea urchin and starfish contain growth promoting substances. 
These substances pass out into the surrounding water during seg- 
mentation, and later stages of larval development..... There is 


59 


aN 
a 


aS 
oO 


Divisions of -Ocular Micrometer 


20 30 40 50 60 70 
7ime in Hours 


Fic. 12.—Figure showing growth in length of plutei in the presence of animal 


charcoal (A) and fuller’s earth (x) compared with those growing in sea-water (CO). 
(From Peebles, 1929.) 


some experimental evidence in favor of the conclusion that the in- 
hibiting substances are associated with the lipoid constituents and 
the accelerating factor is contained in the protein molecule 


156 ANIMAL AGGREGATIONS 


The retarding effects of secretions of growing embryos are removed in 
the presence of animal charcoal and fuller’s earth. The percentage of 
normal larvae resulting from eggs grown in the presence of these 
adsorbents is greatly increased, while mortality is decreased.” 

By using hanging-drop cultures, a part of which contained isolated 
sea-urchin eggs while the others held small groups of eggs, Frank and 
Kurepina (1930) report accelerated growth in the grouped eggs. 
These results are particularly noticeable if the temperature is al- 
lowed to rise slightly above the normal. A résumé of two of their 


TABLE VII 
A 
Number of eggs per drop. ....... Te? 16-20 
Number of such drops......... €.~) 810 3 
83 hours after fertilization........ 75% have 8 blasto- o % at 8 blastomeres 
meres 12% past 8 blasto- 
25% past 8 blasto- meres 
meres 88% at 16 blastomeres 
B 
Number of eggs per drop.......... I-2 3-6 - 10-20 
Number of suchvdropss)--)-..a.. 10 I 5 
421 hours after fertiliza- 
LOTR aye eter hcferelsectes 75% nomovement o%nomovement 0o%no movement 
25% slight move- 26% slight move- 4% slight move- 
ment ment ment 
10% plainly moy- 40% plainly moy- 12% plainly mov- 
ing ing ing 
o% gastrulae* 34% vigorously 60% vigorously 
moving moving 
o% gastrulae 24% gastrulae 


* Percentages as reported in original work. 


experiments, showing the type of results obtained under these con- 
ditions, is given in Table VII. These results are interpreted by the 
experimenters to indicate a stimulating effect of self-radiation as 
suggested by Gurwitsch’s mitogenetic rays. It is clear that such an 
interpretation is far-fetched at present; but the results indicate, 
despite careless reporting, that there is an optimum number of eggs 
which lies well above the minimum at which, under certain condi- 
tions at least, growth is favored as compared with that shown by 
eggs isolated into similar amounts of sea-water. 


STIMULATION OF GROWTH BY CROWDING 157 


HETEROTYPIC CROWDING IN TISSUE CULTURES 


We have noted above the case of fibroblasts growing in plasma 
only when under the close influence of leucocytes, as an instance of 
the direct effect of different kinds of tissues grown together upon the 
ability of the one to grow at all; there is also evidence that differen- 
tiation is stimulated or accelerated by the presence of two sorts of 
cells in close association. Thus Ebeling and Fischer (1922) combined 
a ten-year-old strain of fibroblasts grown in pure culture with a two 
months’ strain of epithelium which had been similarly grown in pure 
culture. After the two had been grown together for some time, the 
epithelium became rounded into a sort of epithelial glandular tissue 
lying within a supporting network of fibroblast cells. Champy (1914) 
and Drew (1923) have reported somewhat similar results from com- 
bining these two kinds of tissue cells into one culture. 


GROWTH-PROMOTING SUBSTANCES 


The possibility of growth being promoted by small amounts of 
obscure chemical substances is indicated by the well-known work 
upon vitamins in connection with the growth and well-being of man 
and certain other mammals and of birds. The exact application of 
the facts developed in connection with work on vitamins with ani- 
mals at the level of group life with which we have been dealing is at 
present unknown, since practically no work has been done upon the 
vitamin relations of the invertebrates and little upon those of the 
lower vertebrates. From the work upon the higher vertebrates we 
know that vitamins A and B are both growth-promoting substances 
whose absence from the diet leads to serious disturbances and finally 
to death, and whose presence even in minute amounts promotes the 
normal metabolic processes which result in growth. 

The possibility of growth-promoting substances being concerned 
with the physiological effects of groups of animals upon the individ- 
uals composing the group is further indicated by the work on “bios.” 
The literature on this subject is voluminous and confused. Tanner 
(1925) presents an exhaustive review and bibliography of the re- 
searches from 1860 to 1924. “Bios” is the name provisionally given 


158 ANIMAL AGGREGATIONS 


by Wildiers (1901) to a mysterious organic substance which he be- 
lieved to be necessary for the proliferation of yeast cells. After ap- 
proximately a quarter of a century of work upon the subject, Tanner 
summarizes the situation regarding bios as follows: 

“One group of investigators denies the existence or need on the 
part of the yeast plant, of a substance like ‘bios.’ They feel that 
yeasts will grow without this accessory substance. 

‘Another group believes that ‘bios’ is necessary for the growth of 
yeasts. They are unable to secure growth of yeasts in pure solutions 
without it. Certain of these investigators have reported fractiona- 
tion of ‘bios’ into components which are necessary to one another. 

“A third group of investigators believe that yeast will grow in 
pure nutrient solutions without ‘bios’ but that the addition of a 
‘bios’ containing substance may cause increased growth. Whether 
this acceleration in growth following the addition of a ‘bios’ con- 
taining substance is due to ‘bios’ or to some other factor in the pre- 
parate has not been satisfactorily established. In this connection it 
is well to point out that even a medium such as beer-wort which is 
rich in ‘bios’ may be improved by the addition of other ‘bios’ con- 
taining substances. 

‘A fourth group may also be recognized including those who have 
isolated ‘bios’ or substances having ‘bios’ properties.” 

Throughout his review Tanner’s attitude is satisfactorily critical; 
and from a study of it, supplemented with certain of the original 
research reports, it seems to me that the evidence favoring the view 
that there is a growth-promoting substance which markedly stimu- 
lates yeast growth is too strong to be disregarded at the present 
time. Concerning whether the yeast cells are able to synthesize this 
substance from nutrient solutions lacking it, as has been claimed, the 
evidence is not yet so clear. 

The same problem in a somewhat different guise is met with in the 
studies concerning whether inorganic substances taken alone are ade- 
quate for the growth of green plants. This question was most re- 
cently raised by Bottomley (1915 and subsequent papers) and 
Mockeridge (1920, 1927), who showed that certain complex organic 
substances, when partially broken down by bacterial action, stimu- 


STIMULATION OF GROWTH BY CROWDING 159 


lated the growth of Lemna and other water plants to a marked de- 
gree. Bottomley gave the name “auximones” to these substances 
which were effective in promoting growth for green plants. It soon 
became apparent that the green plant Lemna can grow and multiply 
for indefinite periods in a purely inorganic medium (Clark, 1924, 
1926; Ashby, 1929), and Wolfe (1926) was led to the point of view 
that Bottomley’s theory of the need of growth-promoting substances 
by green plants was completely refuted. Ashby made a more com- 
plete analysis of the problem (1929a) and has demonstrated that 
small amounts of organic substance obtained from fresh horse dung 
will increase the rate of growth of Lemna if present in only o.2 parts 
per million and that the growth-rate is little affected by additions of 
this material beyond 2.0 parts per million. 

The duckweed which Ashby used in these experiments had been 
growing for 6 months on a purely inorganic medium made up in 
glass-distilled water. Environmental conditions such as light, tem- 
perature, and pH were adequately controlled. The mean frond 
weight remained the same in control and experimental solutions; 
the mean frond number increased 42 per cent; the area of the fronds, 
33 per cent; and there were roughly 80 per cent more chloroplasts in 
the cells of fronds treated with 0.002-0.02 grams per liter of dry ex- 
tract, as compared with untreated fronds. 

The addition of 0.2 parts per million of organic matter to a solu- 
tion containing already 1,210 parts per million of mineral matter 
will significantly increase the growth of Lemna. The power of the 
extract is not affected by autoclaving; hence the effect is not due to 
an enzyme. The ash constituent of the extract does not increase the 
growth-rate, and increasing the nitrogen content by adding 0.003 
grams per liter of KNO, did not affect the growth, while adding one- 
hundredth of this amount of nitrogen as organic matter did signifi- 
cantly increase the rate of growth. It seems clearly established by 
this work that, while Bottomley’s “‘auximones”’ are not essential for 
plant growth, the addition of extremely minute amounts of organic 
material produces this effect by acting as a catalyzer. 

The work with vitamins, tissue extracts, bios, and auximones in- 
dicates clearly that the presence of very small amounts of organic 


160 ANIMAL AGGREGATIONS 


material may strongly affect the growth processes of organisms pres- 
ent. Relatively slight conditioning by the products of plant or ani- 
mal metabolism produces marked results, which are not necessarily 
increased by increasing the amount of material present. One reason 
for the failure to recognize the presence and effectiveness of such 
compounds lies in the exceedingly minute minimal quantities neces- 
sary to produce maximal effects. Unless especial care is taken, traces 
of such materials will be present as contamination and will produce 
as great an effect as if more were added. 


CHAPTER xX 


STIMULATING EFFECTS OF CROWDING ON 
THE RATE OF REPRODUCTION 


In a preceding chapter we have seen that there is much support for 
the conclusion that crowding decreases the rate of reproduction 
among animals generally, with specific instances among the 
Protozoa; the Crustacea, of which Daphnia is an example; in the 
insect Drosophila; and among birds. We have also seen that there is 
at times an optimum crowding for the growth-rate, which does not 
necessarily coincide with the minimum population density. It is now 
necessary to examine whether the evidence that has been advanced 
demonstrates a similar optimum, at least for certain animals at some 
time in their life-cycle, in so far as their rate of reproduction is con- 
cerned. 

The phenomenon which we are to discuss may deservedly be 
called ‘“Robertson’s phenomenon,” since Robertson was most active 
in collecting evidence of its existence. He himself gave it the name of 
“allelocatalysis,” which he defined (1924a) as meaning ‘‘the acceler- 
ation of multiplication by the contiguity of a second organism in a 
restricted volume of nutrient medium.”’ 

His announcement of the existence of this phenomenon naturally 
awakened an immediate interest among biologists generally and 
among students of the Protozoa in particular, many of whom have 
been unable to confirm its existence. Hence it becomes necessary to 
examine the development of the problem in an effort to evaluate the 
results of work centered on it. 

Robertson (19214a, 1923, 1924a, 19240) found that when two in- 
fusorians, Enchelys and Colpidium (later identified as Colpoda), are 
introduced into the same restricted amount of fresh culture medium, 
the early rate of reproduction following a period of readjustment, 
called the “lag period,” is not merely double that shown when a 
single infusorian of the same species is similarly treated, but is some 

161 


162 ANIMAL AGGREGATIONS 


multiple in excess of this. He reports that he has obtained a rate of 
reproduction from two and a half to ten times that which might 
otherwise be expected. In his later work Robertson found this effect 
to be more marked when the transplants are freed from contamina- 
tion with the parent culture medium by repeated washings. The in- 
creased rate of reproduction, which Robertson calls the “‘allelocata- 
lytic effect,’ does not depend upon conjugation, because this does 
not occur within the conditions of the experiment. Robertson at- 
tributes the stimulation to the diffusion of some agent from the 
organisms into the culture medium, which accelerates their repro- 
ductive rate. When more than one organism is initially present, the 
concentration of this substance within the organism is higher; and 
the rate of multiplication is increased as a direct consequence. 

Table VIII gives Robertson’s allelocatalytic data from his 1921a 
paper, showing the effect of placing 2 Enchelys farcimen together in 
a single drop (0.08 cc.) of culture medium as compared with isola- 
tions of single individuals into the same amount. The figures given 
include all cases recorded in this paper, except those which Robert- 
son says were run under conditions unfavorable for allelocatalysis 
but which were run and described to test out the conditions under 
which allelocatalysis might occur. Omitted cases include those in 
which the parent culture was over 3 days old; those in which one of 
the two was purposely killed; and those introduced into bacteria- 
free media. 

Jahn (1929) calls attention to the fact that the averages of the 
generation time, during the first observation after isolation, in four 
much cited experiments of Robertson’s, Nos. 238A, 240A, 237A, and 
242A, show a variation of + 31 per cent, about a median of 9.1 hours; 
and that one pair of experiments, Nos. 310A and 311A, with 1 iso- 
lated individual and one culture of 2 individuals, shows a similar 
variation of +20 per cent. He reasons that since Robertson has 
listed these experiments in several publications they must be typical, 
but that the acceleration in experiment No. 311A is less than the 
variation in 1-animal cultures and that therefore all Robertson’s 
results must be questioned. 

However, enough data has been cited in Table VIII to show that 


STIMULATING EFFECTS OF CROWDING 163 


TABLE VIII 


SHOWING ROBERTSON’S ORIGINAL ALLELOCATALYTIC DATA 


(The Data in the Last Column Are Based on Those of the Preceding Column, 
Restated To Show Directly the Effect of the Presence of a Second 
Organism in the Same Limited Amount of Medium) 


No. after 24 


No. of Animals 
Culture No. Hoss 


Ratio per Single 
Introduced 


Ratio Original Animal 


Normal Hay Infusion; Parent Culture 24 Hours Old 


oe go} 1:5 1:2.5 
Ae ae sh 1:5.6 ta 
eee NS a 14.3 oe 
noe ‘et rg. ape 
ee | gee a) 3.4 ig 
2 i} oe, 11.35 
eA ns a 138.5 ria 25 
See Me lee 2 


ee ee a B75 ae 

a ee. S| a4 nee: 

7 pee ne ss 2.5 we 

ee a.) 2 y 12.9 ene 
| 

ees |) is 4 4 - 

cone ee, Ree =} 135.5 12.75 


164 ANIMAL AGGREGATIONS 


‘ TABLE VIII—Continued 
5 No. of Animals No. after 24 . Ratio per Single 
Culture No. Introduced Hours | Ratio Original Animal 


Normal Hay Infusion; Parent Culture 48 Hours Old—Continued 


BADIA Ras eter srete civ’ I 8\ ie ES: 
DAN Nnapgae avecvtedehes 2 arf 9 7.95 
DA Dee een rete pets I 10\ on caret 
PYG Y Bsa ten SAG Otro 2 34f “we asi 


Normal Hay Infusion; Parent Culture 72 Hours Old 


DUN a daeauslerelemnseere I I , p 
DANG. Marios cieny- 2 24f use aes 
Zia Blea ne eR I 2 
7M Oa Seca or 2 6 230 be 
TODA ts susvesets eh ever I 2 k 
LO Qa setencbvsc ccten 2 6 es pier 
LOUD ie, <h.s otto chek I I : 
TO 2B Le sisters clays crass 2 6 ual 1:3 
DOQIN seabins micterafencite = I I 
208 Wes acetate 2 38 gio roo 
Normal Hay Infusion; Parent Culture Age not Given 
AIA... scucn'e cue sles ete I 8\ : ha 
DIGIAY i god cepcnoheseclees 2 24 ro | ge 


they cannot be so easily dismissed. As will appear shortly, I hold no 
brief for the allelocatalysis theory as stated by Robertson; but it is 
only fair to note, as Robertson himself states (1927), that in order to 
be closely comparable, experiments such as these must have the 
same history and be run at the same time, a fact well known to 
experimental workers in this and other related fields of physiological 
zoology. It is therefore hardly fair criticism to compare a single 
allelocatalysis experiment run at one time with another run later. 
Fortunately, we do have a statistical method of analysis which takes 
into consideration the biological sensitiveness of paired experiments, 
that is, ‘“Student’s” method (Student, 1925; Fisher, 1925). Applying 


STIMULATING EFFECTS OF CROWDING 165 


this method to the results listed in Table VIII, we find that there 
are 128 chances in 10,000 of random sampling giving as large dif- 
ferences in the ratios per individual animals as those given in the 
first eight comparisons listed, which is well within the range of 
statistical significance. It will be noted that these include all the 
experiments cited by Robertson (1921a) that were run in normal hay 
infusion with the parent culture 24 hours old. If one extends the 
inquiry with Student’s method to include all the cases listed in 
Table VIII, one finds that the results are almost statistically per- 
fect; that under no conditions would random sampling be expected 
to give the results shown in that table. 

But the fact that the results are statistically significant does not 
necessarily mean that they support a given hypothesis. It merely 
means that in the case of Robertson’s experiments here cited, 
Enchelys divided significantly faster when 2 individuals were placed 
together in a drop of culture medium, as compared with r individual 
of similar history similarly treated. 

Robertson thought at first (1922) that old Enchelys culture fluid 
contained no substances that were toxic to the Infusoria isolated 
into it from young cultures; but as evidence accumulated, he 
changed his opinion to conclude (1924) that “‘the ultimate cessation 
of reproduction in old cultures is attributable also to the accumula- 
tion of a product of growth, and possibly of the same product that 
was originally responsible for the acceleration.” The presence of an 
accelerator Robertson finds indicated by: (a) the accelerative effect 
due to the addition to the culture medium of a small proportion of 
culture fluid which was previously inhabited by the infusorian En- 
chelys; (b) the increase of reproductive rate when the volume of 
fluid is reduced so that less of the accelerator passes into the fluid and 
more remains within the organism; and (c) the fall of the reproduc- 
tive rate when one of two recently divided individuals is isolated into 
fresh culture medium, in comparison with the reproductive rate of 
the other left in the original medium. 

Originally Robertson did not believe this phenomenon to be due 
to a heavier growth of bacteria, although the results are said to de- 
pend on the presence of an optimum amount of bacteria in the cul- 


166 ANIMAL AGGREGATIONS 


ture medium. Later, after the evidence furnished by ‘Cutler and 
Crump showed that the growth of Colpidium is much more facili- 
tated by the “contaminating bacteria”? which they found ordinarily 
associated with this infusorian than it is by Sarcina, which they 
employed as a food organism, Robertson (1927) considered the pos- 
sibility that Colpidium and other ciliates are restricted in their food 
to certain species of bacteria for which they modify the nutrients 
available, thus existing in a sort of symbiosis with their food species. 
However, he concludes that whether the allelocatalytic effect origi- 
nates with the ciliates or with associated food organisms, it remains a 
mutually accelerative effect of contiguous organisms upon their 
reproductive rates. 

Robertson shows that his results are not due to the carrying-over _ 
of twice the amount of parent culture medium when 2 organisms, in 
place of 1, are transplanted; for in his experiments infusorians 
washed with medium similar to that into which they are subse- 
quently subcultured show the effect even better than unwashed ani- 
mals. Neither is the effect due to mere numbers, for recently divided 
or dividing individuals, if transferred together, act as single animals. 

To explain the observed results, Robertson advances the following 
hypothesis (1923). During nuclear division each nucleus retains the 
charge of autocatalyst with which it was provided, and adds to it 
during the course of nuclear synthesis. At each division the auto- 
catalyst is shared between the nuclear substance and the surround- 
ing medium in a proportion determined by its relative solubility and 
by its affinity for chemical substances within the nucleus. The mu- 
tually accelerative or allelocatalytic effect of contiguous cells is due 
to each cell’s losing less of the autocatalyst to the medium because of 
the presence of the other. According to this hypothesis, the auto- 
catalytic effect of growing colonies, whether attached or detached, 
is due to the same cause. 

Fischer’s work (1923), in which he found that fibroblasts grow in 
vitro only when tissue cells are numerous and close together, can be in- 
terpreted as giving supporting evidence to the allelocatalytic hypoth- 
esis. Burrows (1924), growing cancer cells im vitro, found a similar 
stimulation; this and other similar evidence from tissue culture has 


STIMULATING EFFECTS OF CROWDING 167 


been reviewed at some length in the preceding chapter. Similar evi- 
dence from the field of general bacteriology will be presented in 
chapter xv under the heading proposed by Churchman (1920), 
“Communal Activity of Bacteria.” 

On the other hand, Cutler and Crump (1923), in cultures of Col- 
pidium, failed to find the allelocatalytic effect when the volume of 
the medium was reduced; and Greenleaf (1924), using Paramecia 
and Pleurotricha, in a brief note records his failure to confirm Robert- 
son’s work. Peskett (1924, 1924@) observed such stimulation to divi- 
sion in but 3 cultures of yeast out of 128 examined. 

Robertson (1924a, 1924b) re-examined the problem in the light of 
results published up to that date, and explained the lack of success 
of other workers as being principally due to their failure to wash the 
organisms before transfer. His reasoning here (1927) is as follows: 
Presumably, if contiguous animals can affect each other’s growth 
without the occurrence of conjugation, they must do so through the 
agency of some soluble substance which they emit and which is 
transferred from one organism to another through the medium which 
they inhabit. Presumably also, this soluble substance must be very 
abundant in the thickly inhabited cultures from which subcultures 
are usually prepared. This leads, of necessity, if Robertson’s reason- 
ing is sound, to the removal of the parent culture medium from the 
animals by washing before transfer if allelocatalysis is to result. 
Robertson reports (1924a) that allelocatalysis increases with pro- 
gressive removal of preformed catalyst until a maximal effect is 
reached just before its total removal. In a preceding paper (1924) he 
presented evidence that washing not only removed adherent auto- 
catalyst but also washed out the accumulation of this hypothetical 
substance from the living cells themselves. 

Robertson. also emphasizes (19246) an earlier statement that in 
comparing the reproductive rate care must be taken to estimate the 
population some time before it has attained maximal density. The 
end result of introducing a second individual is to reduce the rate of 
reproduction, since the final maximum is the same in all cases and is 
independent of the size of the seeding. 

Peskett (1925, 1925a) returns to the problem of possible alleloca- 


168 ANIMAL AGGREGATIONS 


talysis in yeast. Apparently he used very careful technique, washing 
his transplants and plotting a growth curve based on a number of 
examinations at different phases of the culture. He finds that in 46 
cultures with synthetic media whose volumes ranged from 2 to 
146 cu. mm. he cannot attribute any differences observed to changes 
in volumes, as would be required if Robertson’s hypothesis of allelo- 
catalysis holds for yeast. With 57 cultures, containing bios and vary- 
ing from o.5 to 362 cu. mm., he observed a tendency for more rapid 
growth in the cultures with the larger volumes. In only 2 cases out 
of 8 near the minimal volume did he find evidence of acceleration in 
the cultures. The statistics on mortality of the cultures show that 
those started with 2 cells usually have a higher viability but that 
the percentage of difference is not great. He concludes that allelo- 
catalysis does not occur in the yeast studied. 

Cutler and Crump (1925) also repeated their experiments with 
Colpidium, thoroughly washing their animals before transfer. They 
again failed to obtain evidence of the stimulation demanded by 
Robertson’s hypothesis. It may be that their technique differs suffi- 
ciently from that of Robertson to account for some difference in 
results, since the infusorians which Robertson washed were fre- 
quently injured in the process while Cutler and Crump found no 
deleterious effect from their washings. 

Calkins (1926) gives the results of 60-day experiments with 
Uroleptus, in which 1, 2, 3, and 4 animals were introduced into 1 
drop of fresh medium, respectively. The division rate was found to 
be inversely proportional to the initial number of individuals inocu- 
lated. 

Returning to this problem in 1927, Robertson states that all of 
the data and conclusions concerning allelocatalysis in Infusoria 
which have been issued from his laboratory in recent years remain 
valid, save that they may apply to the associated food organism and 
not to the Infusoria themselves. 

Robertson, in further considering the possible reasons for the 
discrepancy between his own work and that of his critics, stresses the 
need of being sure that the animals isolated to test for allelocatalysis 
should have the same division rate and that this effect is shown 


STIMULATING EFFECTS OF CROWDING 169 


only when more than one living cell is present and that the presence 
of a second cell which has died does not affect it. He particularly 
criticizes the work of Cutler and Crump and of Peskett because of 
the high death-rate in their subcultures, not only among the inocu- 
lum but also, as Cutler and Crump have shown, among the animals 
produced in the subculture itself. He also criticizes the artificial me- 
dium used by the latter investigators, and presents evidence to show 
that their synthetic medium is not well buffered above pH 7.8, which 
is the optimum region for the growth of Enchelys. Data are also 
presented showing the great difference in viability of different In- 
fusoria from a crowded culture, and he suggests that a part of the 
discrepancy between his own work and that of Cutler and Crump is 
due to differences in the protozoan-bacteria ratio on account of the 
different techniques employed. 

Greenleaf (1926) reported in full his experiments along this line, 
using Paramecium aurelia, P. caudatum, and Pleurotricha lanceolata, 
inoculated in from 2 to 40 drops and Stylonychia pustulata in 2 and 
5 drops of culture medium, to determine the influence of volume of 
culture medium on the rate of cell division, and the influence of cell 
proximity on this rate, and to investigate the factors concerned with 
the lag period. In the experiments covering the first point, Greenleaf 
reports the results by means of giving the average per diem rate car- 
ried through a period of 5 days. Under these conditions, in 4 experi- 
ments only, out of 27, were 2 drops found to be more favorable for 
reproduction than 5 drops. Similarly, in only 4 experiments out of 
27, did the 5-drop cultures give a higher rate of reproduction than 
the 20-drop cultures. In the comparisons between the 20- and the 
4o-drop cultures, the former gave a higher rate in 8 experiments; the 
latter in 9 cases; and 1 was a tie. 

Again, in his investigation of the effect of 2 animals present in a 
given volume, 2 or 5 drops, he used the average per diem division 
rate based on 5-day periods and found that the single animals divid- 
ed the more frequently in both cases, although he did find that 
“there was less difference in the division rates of the animal carried 
alone and the two carried together in the 5-drop series than in the 
2-drop series. This indicates that in the larger volume the depressing 


170 ANIMAL AGGREGATIONS 


effect of the presence of 2 animals is less marked than in the smaller 
volume.” He did not attempt to carry this further to find the rela- 
tive effect of a further increase in volume. 

Greenleaf’s work is chiefly to be criticized because of his taking 
average division rates for a period of 5 days instead of following the 
actual division rate per day—the more so since Robertson had pre- 
viously called attention to the necessity of such care and Peskett 
(1925) had used particular care to examine his cultures at fairly 
frequent intervals in his later studies upon the rate of growth in 
yeast cells. 

In keeping with his other results, Greenleaf found that isolating 
infusorians into 5 drops of culture medium causes a higher rate of 
reproduction if the cells are re-isolated after 24 hours than if they are 
allowed to stand for 48 hours in the same medium. ~ 

Myers (1927) worked with washed Paramecium caudatum and 
found that “increasing the numbers of individuals present in a given 
volume does not increase the rate of reproduction either from the 
beginning of the cultures or after the first fission.” In his work he 
used 2-16 drops of culture medium, measuring from o.1 to 0.8 cc. 
His observations were usually made three times per 24 hours, at 
intervals of 6, 6, and 12 hours. 

Petersen (1929) justly comments on a part of this work of Myers, 
showing that his system of taking observations at 6-, 6-, and 12-hour 
intervals may mask significant results since an indicated interpreta- 
tion of his data shows that he does not regard changes of from 6.0 
to 8.4 hours before the first division time as being significant, al- 
though the higher number represents an increase of 4o per cent. Of 
course, the technique used may require greater differences before 
significance is reached.’ 


«In the light of this criticism supported by tabular evidence, it is hard to understand 
Jahn’s (1929) difficulty which causes him to assert that Petersen has selected certain 
data from two sets of Myers’ experiments. Actually she used all the data published by 
Myers concerning 16-drop cultures which bore on this point, and lists all three sets of 
these experiments both in her summarizing table of Myers’ work and in the accompany- 
ing discussion. From other remarks concerning Petersen’s work, Jahn seems to have 
overlooked the fact that she was interested in contrasting the effects obtained by 
testing for allelocatalysis in small and in relatively large volumes of medium, from 
which came her particular attention to the results reported by Myers from his 16-drop 


STIMULATING EFFECTS OF CROWDING 171 


Yocum (1928) reported results resembling allelocatalysis from his 
work with Oxytricha. He isolated unwashed individuals to 4 and to 
10 drops of sterile medium when too drops equal a cubic centimeter. 
After 24 hours he found that when all cases were averaged, the divi- 
sion rate in the smaller volume exceeded that in the larger by 14 per 
cent, and that the average in the smaller volume in each of his series 
was greater than in the larger volume by over ro per cent. He inter- 
preted this as meaning that the smaller volumes reach a high con- 
centration of an autocatalyst earlier than do the larger volumes. 

The work of Petersen (1929) chronologically belongs at this point; 
but it will be examined in more detail shortly, after considering the 
work of Jahn (1929), who investigated the relation between density 
of population to the growth rate in Euglena sp. Euglena was chosen 
for its ability to grow in autotropic media. Single isolations were 
abandoned on account of excessive evaporation. Of 22 cultures 
started with washed sister-cells isolated into 3 or into 6 drops of 
medium, 5 cultures were counted at the end of to days. Three of 
these showed larger numbers in the smaller volume, one showed the 
reverse, and the other had about equal numbers. No data are given 
for the intervening period. 

Thereafter mass cultures were run in which the ratio of the initial 
volume of individuals to medium ranged from 1: 383,300 to 1:3,833,- 
ooo. Results are recorded as obtained by a series of dilutions of from 
0.005 to 0.55, 0.006 to 0.66, 0.089 to 0.0894, 0.081 to 0.73, 0.14 to 
0.555, and 0.42 to g.5 thousands per cubic centimeter of culture 
fluid. Initial counts were made in from 1 to 4 days in the different 
series. In Series 2, 3, 5, and 6' the initial count in the more concen- 
trated medium was made approximately 1-3 days before that in the 
least crowded series; hence it is hard to judge initial conditions. 

Only in Series 1 do the recorded data show the initial count made 
at the same period in the different experimental lines; and here the 


cultures. It may be, as Petersen suggests, that the method of recording data chosen by 
Myers does not allow his results to be used as evidence for allelocatalysis. If that be 
true, there are similar reasons for thinking that, in so far as the 16-drop cultures are 
concerned, they cannot be used as evidence against this interpretation. 


* The time of making the initial count in Series 4 is not given. 


172 ANIMAL AGGREGATIONS 


results at the end of the first day’s exposure clearly bear out the 
author’s conclusions that his mass experiments with Euglena offer 
no evidence of any allelocatalytic effect. As would be expected, from 
all of the work to date, Robertson’s included, after the first few days 
Jahn found a significant difference in rate of increase, with a more 
rapid rate in the less dense populations, which he is inclined to 
ascribe to the combined effect of relatively more food and relatively 
less wastes. The effect of mass of Euglena, which is capable of 
growing in autotropic media, may well differ from that of holozoic 
organisms, particularly since their CO, relations diverge so de- 
cidedly. 

The work which throws most light on this tangled problem is that 
of Petersen (1929). Petersen used Paramecium caudatum in her ex- 
periments. They were grown in a boiled-hay medium which was 
bacterized 24 hours before using. Animals were isolated by means of 
a drawn-out pipette. Any attempted washing was done by re-isola- 
tion. Throughout her experiments 24 drops equaled 1 cc. 

The usual method of comparing individuals of the same clone was 
not followed in this work because it was found early in the investiga- 
tion that there may be a marked difference in the division rate of 
animals from the same clone. Even the division rate of sister-cells 
showed a difference. Her method, as finally worked out, was to iso- 
late 2 animals with approximately the same rate and time of divi- 
sion. These were allowed to divide until 4, or 8, or 16 cells were pro- 
duced, according to the needs of the experiments, at which time all 
the descendants of one animal were subjected to one set of condi- 
tions while all from the other were placed under the conditions with 
which comparisons were to be made. Because of the re-isolation so 
early in the life of the subculture, the lag phenomenon was avoided. 

Under these conditions, four sets of experiments were run, as 
follows: (1) cell proximity in small volumes of medium; (2) variation 
in volume of medium; (3) cell proximity in large volumes of medium; 
and (4) conditioning of medium, both large and small volumes. 
Petersen’s results will be given in some detail because of their im- 
portance. 

Preliminary experiments in 2 drops of medium gave some slight 


STIMULATING EFFECTS OF CROWDING 173 


indication of a stimulating effect upon the rate of reproduction when 
2 organisms, instead of 1, were present; and two series of experiments 
were run to investigate this tendency. In the first series the animals 
were washed once; in the second they were isolated unwashed. Ex- 
aminations at 12, 18, 24, 36, and 48 hours failed to reveal any indica- 
tion of acceleration due to the presence of 2, as compared with 1 
animal, in the original subcultures, either with the washed or with 
the unwashed cultures. Again, contrary to the report of Robertson, 
there was no speeding up of the division rate after the first fission. 
Further experiments, made with small volumes after Petersen had 
found that a different relation exists when animals are subcultured 
by one’s, by two’s, and by four’s in larger volumes, still gave the 
same negative effects. With these small volumes the work of Peter- 
sen is in agreement with that of Myers and of Cutler and Crump 
rather than with that of Robertson. 

Because of the evidence of retardation in the early experiments, 
Petersen thought that the volumes used might be entirely too small. 
Acting on this lead and using 20 drops of culture medium (24 drops 
equal 1 cc.), Petersen transferred 1, 2, and 4 Paramecia, which had 
been washed three times, into this volume of bacterized medium, 
with the results shown in Table [X. In these experiments there is 
evidently an acceleration in the early division rate of the subcultures, 
associated with an increase in the number of animals introduced. 
Further experiments shown in Tables X and XI demonstrate that 
this is not an accidental result. 

The question as to whether the accelerative effect is species- 
specific was apparently answered for this particular organism by the 
fact that in 5 sets of experiments similar to those reported above 
which were found to be noticeably infested with a small ciliate after 
the first 24 hours of isolation, the rate of division was nearly or quite 
independent of the initial seeding. Similar results were obtained with 
the only other set of contaminated cultures entering into this series 
of experiments. 

So far, these experiments of Petersen’s have shown that with 
small volumes, 3’; cc. or less, there is no acceleration with the intro- 
duction of a single individual in the initial subculture, but that with 


174 ANIMAL AGGREGATIONS 


TABLE IX 


SHOWING Divison RATE OF SETS OF 4 PARAMECIA TRANSFERRED SINGLY, IN Two’s, 
AND IN Four’s, TO 20 Drops OF BACTERIZED MeEpiumM. WASHED 
TWICE IN BACTERIZED AND ONCE IN ALKALINIZED MEDIUM 
(Data from Petersen) 


PETANSlEDs meeteceiciotso vei ONE Two Four 

TOUTS miss iciiole efoieleicie s ° 24 48 ° 24 48 fo) 24 48 
4 5 9 4 5 16 4 8 18 
4 4 9 4 5 10 4 5 12 
4 5 9 4 6 13 4 6 16 
4 4 II 4 5 20 4 7 16 
4 4 9 4 6 $f) 4 6 14 
4 8 16 4 8 9) 4 8 24 
4 6 12 4 8 24 4 8 24 
4 ai 14 4 8 16 4 8 16 
4 7 16 4 14 24 4 14 24 
4 9 IO 4 8 12 4 site) 14 

Wotal Sess 40 59 II5 40 73 162 40 80 | 178 

Mean division rate|...... 1G || Datskh lloocacc Hae || ZO |lowooos 2.0 | 4.45 

TABLE X 
SHOWING THE EFFECT OF DOUBLE WASHING AND TRANSFER OF PARAMECIA 
TO 20 Drops OF ALKALINIZED BACTERIZED MEDIUM 
(From Petersen) 

SP Tansleriste ce csfereieioiers ONE Two Four 

FOUTS! ac ceeereicen eter ° 24 48 ° 24 48 ° 24 48 
4 6 21 4 8 30 4 8 38 
4 7 12 4 8 16 4 10 36 
4 6 25 4 7 34 4 8 37 
4 8 23 4 8 30 4 8 52 
4 6 22 4 8 34 4 8 40 
4 5 28 4 5 34 4 7 42 
4 7 25 4 8 47 4 9 56 
4 6 9 4 8 22 4 8 33 
4 7 18 4 8 BD 4 8 38 
4 6 16 4 Gy 18 4 8 20 
4 6 14 4 8 16 4 8 22: 
4 5 22 4 8 17 4 8 24 
4 5 14 4 6 18 4 8 20 
4 6 28 4 7 34 4 7 40 

Motal ee vc cece 56 86 277 56 104 | 382 56 113 | 499 


STIMULATING EFFECTS OF CROWDING 175 


large volumes, $$ cc., such acceleration does occur; with still larger 
volumes we begin to see that the effect is in reality a relationship be- 
tween the mass of animals and the mass of culture media. Inocula- 
tions similar to those made above, but made into 4o drops of me- 
dium, showed in 48 hours some evidence of acceleration of division 
rate with an initial seeding of 4, but with this volume 2 individuals 
reacted as did 1. Whatever the process of conditioning the medium, 


TABLE XI 


SHOWING THE EFFECT OF DOUBLE WASHING AND TRANSFER OF 
PARAMECIA TO 20 Drops OF UNALKALIZED BACTERIZED MEDIUM 
(From Petersen) 


eLranSierccrat rink Acer ONE Two Four 

lao an oGn co doaBoend Gone fo) 24 ° 24 ° 24 
4 14 4 16 4 16 
4 16 4 16 4 16 
4 15 4 16 4 16 
4 20 4 28 4 30 
4 18 4 16 4 28 
4 14 4 24 4 30 
4 19 4 22 4 20 
4 2 4 20 4 20 
4 16 4 27 4 30 
4 18 4 23 4 31 
4 19 4 24 4 26 
4 21 4 26 4 30 

sO tal meses exeaa cee 48 211 48 264 48 305 

Mean division rate..]....... Ae 3 Otel tee ese PNG Wee lhe ayerats, oe 6.35 


2 were no more effective in 48 hours with 1.67 cc. of medium than 
was a Single individual, while 4 were significantly more effective than 
either. These results are exhibited in detail as Table XII. 

When the numbers produced in 20-drop cultures initially contain- 
ing 1 and 4 individuals are examined by Student’s method for de- 
termining the statistical value of paired experiments, one finds, if all 
of Petersen’s comparable data are considered, that there are 2 
chances in 10,000 of obtaining so great a deviation from the mean by 
random sampling. When the similar data for the 40-drop cultures 
are similarly considered, the probability, even from this short series 


176 ANIMAL AGGREGATIONS 


of experiments, is shown to be slightly less than 5 chances in a 100, 
which is usually considered to suggest statistical significance. 
Petersen approached this problem from another angle. Experi- 
ments were carried on to determine whether or not the division of an 
animal in a limited amount of medium would accelerate the rate of 
division of individuals isolated into the same volume. Both washed 
and unwashed Paramecia were so tested. First, washed animals were 


TABLE XII 


PARAMECIA WASHED THREE TIMES AND TRANSFERRED SINGLY, IN Two’s, AND IN 
Four’s, TO 40 Drops oF ALKALINIZED, BACTERIZED MEDIUM 


(From Petersen) 


AbEMB cogsgeaGooces One Two Four 
ELOUTSfreleversnreraeiasvoccs ° 24 48 ° 24 48 ° 24 48 
4 8 16 4 8 16 4 8 16 
4 8 16 4 8 16 4 8 16 
4 8 16 4 8 16 4 9 19 
4 4 4 4 4 4 4 4 4 
4 4 4 4 4 4 4 4 4 
4 4 4 4 4 4 4 4 8 
4 8 16 4 8 16 4 12 18 
4 8 16 4 8 16 4 Io 16 
4 8 16 4 9 16 4 ie) 16 
4 8 32 4 8 Be 4 16 40 
4 8 22 4 8 22 4 8 24 
Motallaeyaee eraser 44 76 162 44 77 172 44 03 181 
Mean division rate|...... oFS || BAGO) llasaace WEI S00) Hows soc ere |) clas 


isolated into 2 drops of bacterized medium. At the end of 12 hours 
the animals were removed, and the slides in which fission had oc- 
curred were noted. Five sets, consisting of 8 animals each, each set 
being the fission products of 1 animal, were isolated singly into the 
drops in which fission had recently occurred; and comparable isola- 
tions were made into those drops where animals had lived for the 
same length of time but without division. There was no significant 
difference in division rate at the beginning or at the end of the ex- 
periments, on washed or unwashed individuals. 

Petersen says: “This is not what would be expected on the basis 
of Robertson’s work. His reports indicate that acceleration of divi- 


STIMULATING EFFECTS OF CROWDING 177 


sion rate should appear after the first division in a limited volume, 
due to the liberation of an autocatalyst from the nucleus to the 
pericellular medium at the time of division. 

“‘Another attempt to accelerate division rate by the use of condi- 
tioned medium was made with cultures of large volumes. Cultures of 
20 drops in which an animal had lived and divided were inoculated 
with isolated unwashed animals and their division compared with 
that of other comparable animals also unwashed, which were isolated 
to 20 drops each of fresh bacterized medium. 

‘Animals isolated into the 20 drops in which one individual had 
divided and lived (d) and animals isolated into 20 drops of fresh 
bacterized medium (n.d.) at the end of 18 hours gave the following 
numbers respectively. 


Gs sciecicoct iy 4, 2,4,4, 2, 2,3,3, 2,3, 2,2, 2,2,2,2 41 
Tee Menerenr. He ity Bey OH Tayi Diy Ge ORs aegis gta ea a CO) 


“Thus indications are that an animal living and dividing in 20 drops 
of medium conditioned it in such a way that an acceleration in the 
division rate of other individuals isolated into it is marked. 

“Apparently in the small volumes, one individual is able to condi- 
tion the medium sufficiently to cause rapid multiplication; and there- 
fore when one animal is transferred to a small volume of conditioned 
medium, its rate of division is not faster than that of a similar in- 
dividual transferred to unconditioned medium. But in large vol- 
umes, one individual is not able to condition the medium immediate- 
ly; and when, therefore, one animal is introduced into a large volume 
of conditioned medium, division takes place faster than in the case 
of a similar individual introduced into unconditioned medium. 

“Greenleaf and Myers in work with conditioned medium used 
only small volumes, 2-5 drops. Both report a depressed division 
rate in the conditioned medium, which they interpret as due to the 
increased toxicity of the old conditioned medium.” 

The importance of Petersen’s work lies in the fact that for the 
first time in the development of this phase of the problem of the 
effect of initial numbers on the rate of reproduction in relatively 
small subcultures, she has been able to obtain both positive and 


178 ANIMAL AGGREGATIONS 


negative results at will, depending on her manipulation of volume 
relations. Her work and discussion make it impossible for one to 
lump together all volumes in testing for the possible stimulating 
effect of the presence of more than 1 cell in a limited amount of 
medium. 

Here we shall have to let the discussion of Robertson’s phenome- 
non rest for the time being, pending the accumulation of further 
data. So far, the evidence demonstrates that reproductive rate does 
not always depend on the number of the original transplants; that, 
in fact, under many conditions and with many organisms it has not 
yet been shown to have any positive correlation. All are agreed that 
in the end the rate of reproduction falls sooner in small subcultures 
seeded with more than one organism and more rapidly than it does 
when a single individual is isolated. However, the repeated experi- 
ence of Robertson, supported now by Yocum’s work in so far as it 
covers the same ground, and by the work of Petersen with larger 
volumes of culture medium, demonstrates that true acceleration of 
the rate of reproduction may occur associated with the introduction 
of more than one organism into a limited amount of medium. 

When the very establishment of the phenomenon is a matter of 
such great difficulty, it is to be expected that the explanation of the 
phenomenon, when it does occur, is still uncertain. At present we 
cannot accept Robertson’s hypothesis of the action of an autocata- 
lyst; and while there is evidence for the production of some condi- 
tioning agent, some X-substance, which renders the culture more 
favorable to growth, more light is needed, particularly concerning 
the réle of bacteria in the phenomenon among other possible factors, 
before much progress can be made toward a solution of this aspect 
of the problem. Perhaps when we have examined the relations of 
masses of larger animals to survival under adverse conditions, a sub- 
ject to be taken up soon, we can better understand the complexities 
of the present problem. 

An effect strikingly similar to Robertson’s phenomenon has been 
described by Chapman (1928), though not with that in mind. In his 
work upon the effect of a limited environment, in this case whole 
wheat flour, upon the number of confused flour beetles, Tribolium 


STIMULATING EFFECTS OF CROWDING 179 


confusum, that will develop, Chapman found, as was stated earlier 
(chap. vii), that with these animals and with this medium it is pos- 
sible to demonstrate that the total population per gram of flour 
arrives at a constant level of 43.97 + 2.88. Chapman’s table record- 
ing the population level attained at successive intervals when 2, 4, 
8, 16, 32, and 64 beetles are placed in 32 grams of flour for each lot is 
repeated here as Table XIII. 


TABLE XIII 


SHOWING THE NUMBER OF BEETLES PER GRAM OF FLOUR WITH INITIAL POPULATION 
VARYING FROM 2 TO 64 PER 32 GRAMS. THE RESULTS ARE GIVEN 
IN TERMS OF BEETLES PER GRAM OF FLOUR 
(From Chapman) 


GRAMS 
Days 

32 32 | 32 32 | 32 32 
Os sisietecsnaiave x 0.062 0.125 0.25 0.5 I 2 
TsDnshc cree iciee- es Sos 9 16 27 42 
Dn coogoonoe 2a 12 1g 20 30 38 
Ly eee ret 27 32 30 43 55 
OOrcceie jerseys, 28 31 30 29 46 47 
OTe theses | 30 44 42 43 53 53 
LOAB errata cke 2 | 40 43 38 2 52 44 
1O22%S, Sie a tach ne 45 43 30 50 50 46 
T3O srispane erste a0 2 47 42 44 50 45 


Chapman is interested in showing by these figures, as by those 
cited in chapter vu, that, regardless of the initial seeding, a point of 
equilibrium is reached after which the population remains relatively 
constant. His medium is distinctly advantageous in this respect, for 
with the Protozoa which we have just been considering, after the 
similarly constant point of equilibrium is reached, the population 
declines on account of the changes induced in the culture medium. 
In flour beetles, as we have said before, this decline can be controlled 
by changing the culture medium. 

Our own interest is in examining the effect of numbers in the ini- 
tial population upon the early rate of reproduction. The relation- 
ships existing for the 11- and 25-day periods shown above are given 
in Figure 13, which has been drawn from the data just given. The 
figure shows immediately that for these first periods in the develop- 


180 ANIMAL AGGREGATIONS 


ment of the population the greatest increase comes with an initial 
population of 4, while the least rapid rate of reproduction comes with 
the smallest number of individuals present. 

This indicates, as did the data of Petersen, that there is an opti- 
mum initial population-medium relationship, which, for certain 


fos) 


~ 


fon) 


oO 


b 


* // days 


Rate per female day 
Ww 


ine) 


"*+.,25 days 


2 4 8 16 32 64 
Initial population per 32 8ms. flour 


Fic. 13.—Showing the relation of initial density of Tribolium population in 32 gm. 
of flour to rate of reproduction. Recalculated from data reported by Chapman. 


volumes of medium, is different from that given by the smallest 
population present. The fact here seems as clear as could be expect- 
ed from an experiment not designed to test this particular point. 
More recently T. Park, working in this laboratory, has confirmed 
Chapman’s results (unpublished data). The explanation is even less 
apparent than in the case of Robertson’s phenomenon in the Pro- 
tozoa. 


CHAPTER XI 


EFFECT OF CROWDING ON SURVIVAL 
AND OXYGEN CONSUMPTION 


Studies on the causes and effects of animal aggregations begun in 
tgt2 and carried on intermittently thereafter were summarized 
(Allee, 1920) for the water isopod A sellus communis, three species of 
land isopods, and the ophiurid starfish Ophioderma brevispina. The 
results showed that, in general, bunching in these species is most 
prevalent under adverse conditions and when there is no means of 
satisfying the normally positive thigmotactic reaction. Of the single 
factors tested, the tendency to collect in bunches is most strongly 
encouraged in Asellus by the breeding reaction, in the land isopods 
- by the amount of moisture present, and in Ophioderma by the 
amount of light. 

All of these animals had shown in preliminary experiments a 
lowered rate of metabolism immediately following the formation of 
the aggregation, as measured by their oxygen consumption or car- 
bon-dioxide production. When isolated and bunched animals stand 
for long periods of time, the effect on the metabolic rate is reversed. 
Both come to have lower rates than at the beginning of the experi- 
ment, but the decrease is much greater with isolated than with 
bunched animals. In the land isopods this decrease is accompanied 
by a greater loss of water by the isolated individuals. The conclu- 
sion was drawn that ‘“‘under laboratory conditions the formation of 
aggregations serves to make these animals more quiet and in the 
long run proves to be what is usually called an adaptive reaction.” 
This early statement remains a fair summary of the evidence since 
collected in this laboratory. 

In an earlier chapter attention was called to the fact that land 
isopods aggregate into fairly compact masses in the absence of water 
and separate when water is added. Experiments on two species, 
Oniscus asellus and Cylisticus convexus, were run to test the effect of 


181 


182 ANIMAL AGGREGATIONS 


bunching on changes in water content as indicated by weight of the 
animals (Allee, 1926). When the isopods were placed on a moistened 
filter paper with maximum moisture present with which the animals 
would remain bunched, the aggregated isopods increased in weight 
3.8 per cent from water intake while their isolated fellows were in- 
creasing 9.7 per cent. 

Similar experiments with bunches and single individuals placed 
under desiccating conditions showed that in a typical experiment 
isolated isopods lost water three times as fast as did a group of Io 
isopods gathered into a close bunch. Further, all 10 of the bunched 
animals in this typical experiment were alive with a loss of weight of 
less than 16 per cent after 7.45 hours, when the last of the isolated 
individuals was found dead. The last 2 isopods to die showed a 
water loss at this time of about 44 per cent. Sixty per cent of the 
isolated isopods were found dead after 4.38 hours’ exposure. 

These observations show a definite survival value of a group of 
animals even at the low level of social integration existing among 
land isopods. It may be remarked again that, outside the breeding 
season, as these were, isopods collect in bunches due primarily to 
non-social, individual tropistic reactions, and that almost their only 
social attribute is tolerance for other animals and their products 
within a limited space. 


STARFISH AUTOTOMY 


The essentials of the situation among the ophiurid starfish are 
similar, but the mode of expression differs (Allee, 1927). Ophiurids 
have the practice of fragmenting their arms under certain condi- 
tions. Early experiments showed that there is a greater tendency to 
practice autotomy with isolated than with bunched Ophioderma. 
The tests to be considered were run in connection with respiration 
studies to be described later. Two groups of 8 each were placed in 
two large bottles each containing 8 liters of sea-water. Eight similar 
starfishes were isolated into flasks of about 1 liter each. The water 
was usually changed daily and never left longer than 48 hours. No 
attempt was made to supply food. 

The following schedule of numerical symbols was adopted: o, per- 


EFFECT OF CROWDING ON SURVIVAL 183 


fect arms; 1, tip gone; 2, end gone; 3, one-third arm gone; 4, one- 
half arm gone; 5, two-thirds arm gone; 6, mere stump left. Indi- 
viduals were removed from the bottles when they showed frag- 
mented arms yielding on the foregoing scale a total value for the five 
arms of a single starfish of 15-20, regardless of the distribution of 
effects between the different arms. Table XIV gives the mean time 
in days before each lot was discarded. The data at hand are faulty 


TABLE XIV 


SHOWING THE TIME IN Days IN LONG RESPIRATION EXPERIMENTS BEFORE 
THE DIFFERENT Lots WERE DISCONTINUED BECAUSE OF DEATH 
OR THE FRAGMENTATION OF ARMS 


Experiment No. ee pecone Mean Isolated | Difference! Glass* ane 
Tet eheiste eis, 21 19.6 20.6 20er 16.6 Sie lattelecenensks 21 
Diya sveishereyi asks es Tus 33 Tee 25 ORG llaahontcs 14 
Breese es tetayevai'e: ayeiett 5.0 a0) 5.0 Gaus sy llaeoooonc 5 
ADAM SEY s (avanoustere ct Qg.0 9.25 O-13 6.13 SIRO A Mh coir 8 
Cr rseteienccs Cina 9.63 9.56 9.6 6.2 Goel 9.75 II 
Omenctorssstectiers ciswts Rous 75 7.63 6.5 Te FS) 8 

Wiis eg oon. TOMS 10.87 10.81 Siateley, latte claoool ea sO mada ace hoo 

Mean of last 
two only. . 8.69 8.53 8.62 OR 50) [Perera SJoee= nano 


= * The Ophioderma in this column were isolated into liter flasks containing glass rods bent into various 
shapes. 


in that, after respiration experiments were stopped, there were fre- 
quently some individuals that had not yet reached the degree of 
fragmentation necessary for removal. In some cases these were ob- 
served, though under altered conditions, until autotomy had pro- 
gressed past the arbitrary dead line; but in others this was not done, 
and the last day of respiration tests was recorded for use in this 
table. This procedure markedly favors the isolated individuals since 
there were fewer of them so treated. The length of time the experi- 
ment ran depended mainly on the temperature, since fragmentation 
proceeds more rapidly at high temperatures. 

In every instance except one, the mean survival time was greater 
for the bunched animals than for those isolated under similar condi- 
tions and at the same time. In four of the six comparisons the dif- 


184 ANIMAL AGGREGATIONS 


ference is large enough to be of significance. Student’s method of 
statistical evaluation shows that for all six experiments there are 49 
chances in 1,000 of random sampling yielding so great a variation 
from the mean in either direction. 

The one exception where the isolated individuals showed a greater 
survival time is instructive. For some reason one or two of the ani- 
mals in each of the bunches of this experiment died soon after one of 
the daily inspections, disintegrated and polluted the whole liquid, 
and caused the death of the remainder. Such an extreme catastro- 
phe could not happen with the isolated individuals. 

The survival of the starfishes isolated into liter flasks containing 
small heaps of variously bent glass rods is also instructive. Here the 
starfishes came to rest on or among these glass rods just as in nature 
they rest among eelgrass blades. It will be noted that for the two 
tests made, the survival is comparable with that of grouped animals 
rather than with the starfishes isolated into bare containers. It ap- 
pears as though the satisfaction of the thigmotropic appetite has 
approximately the same survival value whether the satisfaction 
comes from contact with the piles of glass rods or the group of in- 
dividuals of the same species. 


RESPIRATION STUDIES WITH ISOPODS 


The effect of aggregation upon the aggregants can be followed 
more closely by studying the effect upon the rate of respiration. 
Several such studies have been made with different animals, some of 
which will be summarized. 

Preliminary experiments upon the two species of land isopods 
mentioned above, indicated that soon after these isopods aggregate 
their rate of respiration is decreased, as compared with similar ani- 
mals isolated for a similar time (Allee, 1926). This tendency appears 
after the isopods have been bunched for 5 minutes, and extends at 
least through the first hour of bunching. When the bunches and 
solitary individuals are compared after standing in the laboratory 
for a longer period of time, the isopods taken at random from the 
bunches are giving off carbon dioxide the more rapidly. 

Oxygen consumption of two other species of land isopods was 


EFFECT OF CROWDING ON SURVIVAL 185 


tested in manometer respirometers such as were described by Kraj- 
nik (1922). The results obtained support the conclusions reached in 
earlier studies with carbon-dioxide production, that is, that with 
land isopods the recently bunched individuals are carrying on respi- 
ration at a less rapid rate than when recently isolated. In exact 
ratios, the rate of oxygen consumption with recently bunched and 
recently isolated Armadillidium in two tests were 1:1.486 and 
1:1.368, while with Tracheoniscus the same ratios were 1:1.214 and 
Tha 4A. 

The Armadillidium tests allow other comparisons. Since groups of 
isolated or bunched individuals were set away in the dark for ap- 
proximately 24 hours, one can compare their physiological condition 
near the beginning of this period with that at the end and also make 
cross-comparisons. 

In general, the determinations show that, under the conditions of 
the experiments, there is a very marked decrease in oxygen consump- 
tion after 24 hours’ isolation and starvation—7o and 65 per cent, 
respectively, in two sets of experiments. There is a similar but less 
pronounced reduction when the animals are bunched and starved— 
in that case 31 and 29 per cent, respectively. 

At the end of 24 hours the bunched isopods are uniformly using 
more oxygen per unit weight than are the isolated individuals. The 
observed ratios were 1:1.64 and 1:1.48. All the differences men- 
tioned are statistically significant, since the least difference in means 
is still over eleven and a half times their combined probable error. 

Experiments extending over 50 hours showed that the bunched 
isopods kept their higher rate of respiration during this period as 
compared with similar isolated individuals. The aggregated individ- 
uals exhibited a greater range in rate of oxygen consumption during 
this time, probably due to the greater difference between repose and 
occasional activity. They also were much less likely to move about 
than were the isolated individuals. In all these experiments neither 
set of isopods was fed, but the water content was manipulated so 
that there was no essential difference in weight, as shown by random 
weighings. 

Calibrations show that oxygen consumption for the Armadillidium 


186 ANIMAL AGGREGATIONS 


isolated for about 1 hour is at the rate of 219.6 cc. per hour and kilo- 
gram; for those isolated for about 24 hours it is 77 cc. per hour and 
kilogram; for those bunched approximately an hour, 160.5 and for 
those bunched for about 24 hours, 130.2 cc. per hour and kilogram. 
Similar calculations for the Tracheoniscus show a mean oxygen con- 
sumption of 263.5 cc. per hour and kilogram when isolated less than 2 
hours, and of 196.7 cc. when bunched about the same time.’ 


STARFISH RESPIRATION 


The marine ophiurid starfish, phylum Echinoderma, stands well 
removed from the land isopod, phylum Arthropoda, in the evolution- 
ary scale and in habitat. The oxygen consumption of isolated and of 
bunched Ophioderma was tested by Winkler’s method. 

A typical experiment was made on three groups of 8 animals each, 
selected so that there would be the same size relations in each group. 
The members of each lot were momentarily dried on filter paper and 
weighed in a known amount of sea-water. The lot weighing most 
was placed together in a bottle holding approximately 8 liters (ac- 
tually 8,500 cc.); the lot weighing next most was separated, and 
each individual was placed in an Erlenmeyer flask of about 1 liter 
capacity (actual average 1,143 cc.). The third and lightest group of 
8 animals were placed together in a second bottle like the first. Ap- 
propriate blanks were run on both sizes. All bottles and flasks were 
fitted with rubber stoppers, which carried tubes for drawing titra- 
tion samples without contact with air and with a minimum disturb- 
ance. The containers were gently reversed twice just before sam- 
pling to insure an even mixing of the sample, and other due experi- 
mental precautions were taken. Determinations were made 4 hours 
after the start of the experiment to determine initial relations. Later, 
tests were made at 24-hour intervals, except that during the middle 
course of some experiments they were made at 48-hour intervals. 


* Hyman (1923) found the oxygen consumption of the anterior pieces of Planaria 
dorotocephala to be at the rate of 260 cc. per hour and kilogram. Krogh (1916) lists a 
rate in calories equivalent to 393.6 cc. per hour and kilogram for A pis mellifica, and 
of 72 and again of 115.2 for Musca. The highest rate listed in Krogh’s tables for 
crustaceans, all gillbreathing, is of 6.528 cc. per hour and kilogram. The values given 
above for land isopods are subject to a calibration error of about 5 per cent. 


EFFECT OF CROWDING ON SURVIVAL 187 


The longer tests were terminated piecemeal, as different individ- 
uals fragmented their arms to such an extent that comparisons were 
no longer fair. The records show the rate of oxygen consumption, as 
well as the total oxygen consumption, of twelve groups of aggregated 
Ophioderma during their entire starvation period, and of six lots of 
similar size and number which had been isolated until they, too, died 
of starvation and confinement. In addition to the corrections for 
the slight difference in amount of water available for each animal, 
the crude data were further corrected for differences in weight, since 
tests had shown that, within the size limits used, oxygen consump- 
tion is directly related to the size of the animal, although the greatest 
observed spread between the total weight of 8 isolated animals and 
either of the accompanying bunches was only 0.4 gm. 

Six tests were run to determine the initial relations in oxygen con- 
sumption of isolated and bunched individuals. When these were 
examined at the end of the first 4 hours, it was found that on the 
average the groups of larger individuals, totaling 48 in all, had con- 
sumed 208 cu. mm. of oxygen per individual of 1.25 gm. moist 
weight. The 48 smaller animals making up the second bunch had 
consumed 197 cu. mm. when reduced to the same standard weight; 
while the 48 isolated starfishes had consumed 379 cu. mm. per stand- 
ard individual of 1.25 gm. Here we see again that at the start of the 
experiment the isolated individuals are consuming oxygen at a much 
higher rate than are their aggregated fellows when the only known 
difference between them is that the latter have been allowed to col- 
lect in bunches. When these results are examined statistically by 
Student’s method, we find that there are 66 chances in 10,000 of 
random sampling yielding such a wide deviation from the mean. 
Needless to say, such results are statistically significant. This initial 
relationship continued in most cases past the first 12 hours, and 
sometimes past the 24-hour sampling, but had usually disappeared 
before 48 hours. 

Two of these initial sets of experiments and four other similar 
ones were followed through with starving animals until terminated 
by death or by fragmentation of the arms. The mean results in 
terms of total oxygen consumption of the surviving members of each 


188 ANIMAL AGGREGATIONS 


lot are summarized in Figure 14, which in A plots the mean rate per 
hour per living individual in the isolated (solid) and aggregated state 
(broken line). The broken line is drawn as the mean of the two 


75 


70 


315) 
50 Sse 


45 “Se 


7 
, 
‘ 

, fy 

00) 


J 2 5 4 is) 


Fic. 14.—Showing, A, the mean rate of oxygen consumption per hour at the be- 
ginning and during successive fifths of the starvation period; each space on the ordi- 
nates represents 1 cu. mm. oxygen. B, The mean oxygen consumption; each space on 
the ordinates represents roo cu. mm. of oxygen. The broken line gives the respiration 
of the bunched individuals. 


EFFECT OF CROWDING ON SURVIVAL 189 ° 


series of bunches which were run simultaneously. The means for 
each of these are indicated for each point. The graph starts with 
the rate given during the first 4 hours and proceeds with that given 
during the different fifths of the experiments. This means of chart- 
ing is used because of the varying length of the experiments to be 
summarized, all of which were physiologically the same length, in 
that they were terminated when the starving individuals being 
tested had reached approximately the same degree of depression as 
measured by death or by fragmentation of the arms. 

Graph B shows similarly the total mean oxygen consumption for 
each group. The initial amount indicated is that which would have 
been given had the animals continued to respire for a period equaling 
the others in length but at the rate given in the first 4 hours of 
experimentation. Again the solid line represents the isolated in- 
dividuals and the broken line the bunched animals, and again the 
spread of the means for each series of bunches is indicated. This 
form of presentation shows at a glance the more rapid consumption 
of oxygen in initial stages, particularly on the part of the isolated 
individuals, in contrast with the reduced rate later. The increase in 
the rate of respiration near the close of the experiment is due to 
the greater consumption of oxygen in the process of autotomy and 
to the inclusion of some cases in which decay of fragmented parts of 
arms may have occurred. 

The isolated animals have a rate of oxygen consumption 186 per 
cent above that of the groups during the first 4 hours. Later this 
mean rate of consumption falls for both, but more rapidly for the 
isolated than for the bunched individuals, so that, when the entire 
course of the experiments is considered, the rate of use of oxygen 
by the isolated animals is only 83 per cent of that of the accompany- 
ing groups. The differences of these means have about the same 
statistical value as that noted for the initial 4 hours of respiration. 

The foregoing statement and graph does not show the extent of 
the difference between the oxygen consumption of the isolated and 
the bunched animals, since, on the average, the former died off more 
rapidly than did the bunched individuals. The results reported here 
are in terms of the mean oxygen consumed by the animals still liv- 


190 ANIMAL AGGREGATIONS 


40 


1 2 3 4 S 


Fic. 15.—Showing the total oxygen con- 
sumption of isolated (solid line) and bunched 
starfish (broken lines) in the different fifths 
of the starvation experiments. The heavy 
broken line gives the mean of the two 
bunches shown by the lighter lines. Each 
space on the ordinates represents 1 cc. of 
oxygen. 


ing; hence, in the later stages 
of the tests we are comparing 
the mean rates of the most 
hardy of the isolated individu- 
als with that of a large bunched 
group that has been subjected 
to less rigid selection. 

When the differential autot- 
omy is considered in connec- 
tion with oxygen consumption, 
it is found that the effect of 
isolation is much more marked 
than when only means of sur- 
vivors are compared. These re- 
lations are shown in Figure 15, 
which gives the total oxygen 
consumed, corrected for weight 
of the different animal groups. 
Again the initial value is that 
which would have been con- 
sumed had the initial rate held 
for a period equaling the others 
shown. It is added only for the 
purpose of graphic comparison. 
The preceding types of analysis 
showed that after the initial 
period the rate of oxygen con- 
sumption remained approxi- 
mately constant at their re- 
spective levels for both bunched 
and isolated individuals. On the 
other hand, the complete data 
show a decided falling-off in 
oxygen consumption near the 
end of the experiments, due to 
the decrease in numbers of ani- 
mals present. The effect of the 


EFFECT OF CROWDING ON SURVIVAL IgI 


differential length of life under experimental conditions is shown in 
the earlier and greater reduction in oxygen consumption in final 
stages of the isolated rather than of the bunched individuals. 

The possibility of consuming a similar amount of oxygen in the 
same time was the same for the lot of isolated as for the bunched 
Ophioderma, had they been given equal treatment. The conclusion 
is obvious that when similar Ophioderma are isolated without food 
in clean glass receptacles of approximately equal volume per number 
of individuals present, their rate of oxygen consumption, following 
a period of initial stimulation, is significantly depressed, and their 
expectation of remaining intact, or of living, is less than if they are 
allowed to aggregate. 


FACTORS CONTRIBUTING TO GROUP PROTECTION 


The question immediately arises as to the type of beneficial in- 
fluence exerted by the grouped individuals upon each other. There 
has not been opportunity to investigate this aspect of the problem 
thoroughly. Certain data have been collected that are of decided 
interest, and these will be presented not as a final answer but as a 
possible guide to the ultimate solution of the problem. 

The earlier students of the effects of crowding, who uniformly 
found only deleterious effects, supposed that there is some harmful 
secretion given off by the crowded animals. Similarly, workers who 
have reported beneficial effects of increased numbers of individuals 
in relation to volume of the containing water have postulated some 
beneficial secretion which is preservative in action. Drzewina and 
Bohn, in their series of studies upon this subject, in which they used 
a wide range of aquatic organisms, suggested that the observed 
beneficial effects are due to a hypothetical chemical which is secreted 
by the mass of animals in sufficient amounts to have an autopro- 
tective effect. When the crowded conditions produce harmful 
results, they postulated another chemical substance which is auto- 
destructive. 

Inasmuch as these Ophioderma live naturally among eelgrass 
where they crawl over the blades, and since the formation of dense 
aggregations is greatly retarded if not entirely prevented by the 
presence of eelgrass in the stock aquaria, it seemed possible that 


192 ANIMAL AGGREGATIONS 


the observed effects might be due to physical rather to chemical 
factors, at least in this case. In other words, it seemed possible 
that in the absence of suitable non-ophiurid materials the animals 
might themselves serve as substitutes for certain elements of the 
normal physical environment. 

This possibility was tested by placing glass rods of different 
lengths, bent into different shapes, in some of the Erlenmeyer res- 
piration flasks and using these for isolation chambers. The glass 


TABLE XV 


SHOWING THE INITIAL RELATIONS IN OXYGEN CONSUMPTION OF Ophioderma Iso- 
LATED INTO FLASKS CONTAINING IRREGULAR HEAPS OF GLASS Rops, IN CoM- 
PARISON WITH BUNCHED INDIVIDUALS AND WITH THOSE ISOLATED INTO PLAIN 
FLASKS 

(The Results Are in Terms of Cu. Mm. of Oxygen Consumed 
per Mean Individual of 1.25 Gm. Weight) 


Experiment No. Bunches Glass Difference Isolated Difference ee 

LS pe eere anes once 192 280 88 421 I4I 22-24 

Dis Pe. coycttaiahay cigs tat 120 155 35 176 21 23-24 

Bie fas.a te cist ves 226 220 3 22 203 DOD 

AN tavay Noh ccvers 220 410 190 397 = b3 DAD Aw 

Lecce aon s 290 269 =D 507 238 24-24 

ON Phe Sra 166 201 125 253 = 2! Bs 
Mean... 202 272 70 379 LO sea (oat ice es 


rods formed a loose irregular pile, over and through which the 
starfish could crawl or against which it could come to rest. Four 
such flasks, with their accompanying controls, were added to the 
series already described. Such experiments were begun only after 
the conclusions outlined above were clearly indicated; they com- 
prised six sets of 4-hour tests of initial respiration and two of the 
long-time respiration experiments. The tests were made similarly in 
every respect to those already described; the flasks containing glass 
rods were placed alongside those with isolated and bunched indi- 
viduals with the behavior of which they were to be compared. 
The results of exposure for 4 hours under these conditions are 
shown in Table XV. Here the respiratory rate is calculated in 
terms of 1.25 gm. per individual and for the actual amount of water 


EFFECT OF CROWDING ON SURVIVAL 193 


present after the glass rods were added. The table gives a compari- 
son of the initial respiration of 24 Ophioderma isolated into flasks 
containing glass rods, with that of 48 isolated into plain flasks, and 
with 96 divided among the accompanying bunches. 

The Ophioderma associated with the clean glass rods consumed 
more oxygen per individual in 5 out of 6 cases than did their asso- 
ciated bunches. The mean difference of 70 cu. mm. is beyond the 
statistical border of significance, since there are 85 chances in a 
1,000 in this case and 112 in the next, that random sampling would 
give so great a deviation from the mean. On the other hand, the 


TABLE XVI 
SHOWING FINAL MEAN RELATIONS IN OXYGEN CONSUMPTION OF Ophioderma 
SEPARATED UNDER THE CONDITIONS SHOWN FOR THE 
HEADING FOR TABLE XV 


(The Results Are in Cu. Cm. of Oxygen Calculated to 
Equal Weight for Each Series) 


: = . Second Isolated with | yx;_- Temperature 
Experiment No. | First Bunch Banh Isolated GincaeRods Weight (Gr.) (Canieeade) 
Set rere 14.050 14.103 II.QI4 17.926 1.34 21-25 
(le cic eee 7.820 7.809 5.220 10.811 Tee 21-24 


individuals associated with the glass rods consumed less oxygen than 
the accompanying animals isolated into plain flasks, in 4 out of the 
6 cases, with a mean difference of 107 cu. mm. Thus the initial 
effect of the presence of the heap of glass rods with the isolated in- 
dividuals was to produce a rate of oxygen consumption intermedi- 
ate between that of the bunched and the other isolated individuals. 
The position of the mean, nearer to that given by the bunched indi- 
viduals than to those isolated in plain flasks, while suggestive, is not 
significant statistically. 

The results in the two total respiration experiments which con- 
tained this feature are given in Table XVI. Here the presence of the 
glass rods was accompanied by an even greater total oxygen con- 
sumption than that of the bunched individuals and, as was shown 
in Table XIV, the autotomy effects are more closely related to those 
of the bunched than to those of the other isolated animals. 


194 ANIMAL AGGREGATIONS 


We have, then, indications that the provision of an opportunity 
for physical contact with the lifeless glass rods produces effects 
similar to those given by the bunching of live starfishes. Obviously, 
more work is needed at this point before we can come to a definite 
conclusion. Other preliminary experiments indicate that the pres- 
ence of irregular heaps of paraffined glass rods or of paraffined 
rubber tubing tends to prevent autotomy of arms of individuals as 
compared with other starfishes isolated into plain dishes. 


NATURAL AGGREGATIONS OF ASELLUS 


The fortunate discovery of a group of gigantic aggregations of 
water isopods, Asellus communis, gave opportunity for further res- 
piration experiments on these under natural conditions. An account 
of some aspects of these aggregations has already been given, de- 
scribing the methods of formation of such groups. For our present 
purposes it is necessary to remember that these aggregations oc- 
curred at the downstream end of culverts and on the downstream 
side of an overflow across a sand roadway which divided an exten- 
sive cat-tail swamp at the head waters of Dune Creek, in the Indiana 
dunes region. At the lower edge of this overflow, just before it 
widened out into the lower swamp, great masses of A sellus collected 
in winter and early spring about willow shrubs, old cat-tails, or in 
depressions where they might find a lodging. 

As was shown in the analysis in a preceding chapter, the forma- 
tion of these aggregations was conditioned to a large extent, perhaps 
completely, by the interaction of the tropisms of the individuals 
with environmental factors. In turn, the accumulation of animals 
was sufficient to affect decidedly the water surrounding and pene- 
trating them. This effect was shown by the loss of 60 per cent of 
the oxygen normally present in the stream. In one observation the 
stream above the main bunch had 6.37 cc. of oxygen per liter, while 
a collection from the midst of this large aggregation had but 2.61 
cc., a loss of 3.76 cc. of oxygen. Similarly the pH of the stream was 
lowered as much as 0.2 of a pH unit in extreme cases, and averaged 
0.09 of a unit less than normal when all the twenty-three tests were 
considered. These changes were brought about when the clusters 


EFFECT OF CROWDING ON SURVIVAL 195 


were located in a stream 15-18 cm. deep where the surface water 
was moving at the rate of 25 cm. a second. 

Field tests were run to determine the effect of aggregation upon 
the rate of oxygen consumption of the aggregated isopods. For this 
purpose, a respiration chamber was made by firmly attaching a 
short piece of snugly fitting rubber tubing to a small wide-mouthed 
bottle of about 14 cc. capacity. The rubber tubing was long enough 
to extend beyond the bottle mouth about the length of the bottle 
neck. A second similar bottle inserted into this rubber collar until 
the mouths of the two met tightly made a respiration chamber of 
27.3-30 cc. capacity. The rubber tubing served as a collar to hold 
the two bottles firmly bound together, and came in contact with 
the water only to a very slight extent. At the end of the respiration 
period the contained isopods were all shaken down into the first 
bottle. The other was removed, closed with its glass stopper, and 
used as an isopod-free collection sample. The agitation necessary 
to shake the isopods all into one bottle must have mixed the con- 
tained water fairly thoroughly. 

A sample collected in this manner was treated according to the 
usual Winkler’s method, except that only one-fourth of the usual 
amount of the reagents was added. Titration was carried on in the 
field, using a standardized sodium-thiosulphate solution of about 
0.002 normality. Blanks were run with each set of five respiration 
tests. These tests were always run in parallel series—one composed 
of animals taken from the aggregations; the other containing the 
same number of isopods of similar size that were scattered about 
singly when collected, and which had not been aggregated recently 
if at all. Ten male isopods, or 20 of the smaller females, were the 
standard number used at one time in each respiration chamber. 
The respiration tests of lots from the aggregation and those that 
before the tests had been scattered singly were, with one exception, 
run simultaneously. In this exceptional case, the results obtained 
may be affected by the greater amount of oxygen present in the 
water when the isolated animals were tested. 

Care was taken to collect the water for the tests from the same 
depth at the same point in the stream, and in as nearly the same 


196 ANIMAL AGGREGATIONS 


manner as possible. After the respiration chambers were closed, 
they were placed side by side in running water and were thus exposed 
to identical conditions during the respiration period, which usually 
lasted for 1 hour. The isopods from each series were brought into 
the laboratory, where their moist weights were determined. The 
results obtained are summarized in Table XVII. The figures on 
oxygen consumption and weight are mean results from the number 
of groups indicated in the first column, or from the same number of 
isolated individuals as were tested in the groups. 
TABLE XVII 
SHOWING THE AMOUNT OF OXYGEN CONSUMED BY GROUPS OF 10 MALE ISOPODS DURING 


TuHerR Frrst Hour AFTER REMOVAL FROM A LARGE BUNCH, AS COMPARED WITH 
A SIMILAR NUMBER OF ANIMALS THAT WERE ISOLATED WHEN COLLECTED 


Qo 
5 a BuncHep Isopops SCATTERED Isopops 
Number ag - rar 
Groups A 7° O. Calculated vs O. Calculate 
Ea Cone O: Ween foLbatial Cu. ay O: weet ianaual 
FalS) se (Grams) Weight Use (Grams) Weight 
Oba ke 5 Bey Ubeeieyer | Roleco aie mane 19.9 Ta396" Nic Agee epee 
(esr iene 12 28.9 I.1055 30.6 42.9 igo ye eka clos, gre © 
5 9 31.4 TGQQOr. Blicenre ache c 41.9 1.187 46.9 
AN eeveys x 15 BB Wash lpeab aes doc 48.0 Tee 52.8 
Geers 5 ae ng} 64.0 Toa Ste eater cree 80.3 1.16 86.2 


* Last test ran 2 hours. 


Twenty-five comparable tests were run with or without correcting 
for weight. The set of 10 isopods that had previously been scattered, 
in all cases but two, used more oxygen during the respiration period 
than did the set of the same number taken from the aggregation and 
started at almost the same time. Always the total amount of oxygen 
consumed by all the members of one series of animals that had been 
scattered was greater than that used by the series from the aggre- 
gated isopods run at the same time. 

Without correcting for weight, the mean difference in oxygen con- 
sumption in these five different series shows that those that were 
solitary when collected used 12.33 cu. mm. more oxygen during the 
first hour’s exposure in a respiration chamber than those that came 
from the aggregation. Application of Student’s statistical method 
shows that there is less than 1 chance in too of getting so great a 
deviation in either direction from the mean in random sampling. 


EFFECT OF CROWDING ON SURVIVAL 197 


Such results are statistically significant. If observed differences in 
weight are considered, the calculated difference in oxygen consump- 
tion between the bunched and the isolated isopods is still greater. 

A group of tests were run to find the respiration rate of the 
bunched isopods as compared with those scattered among the vege- 
tation in the upper swamp. These latter animals had not aggre- 
gated; nor had they been exposed recently to the stimulus of a strong 
current. The tests were made as were those preceding, except that 
females were used because they were so much more numerous than 
males in the swamp. Since the females are much smaller than the 
males, 20 were placed in each respiration chamber, where to males 
had been used in the other tests. The rate of oxygen consumption 
was slightly higher in the isopods taken from the aggregations than 
that of the similar females from the vegetation of the upper swamp, 
even when the observed figures are corrected for differences in 
weight; but the results as observed are without statistical signifi- 
cance. Before calculation for weight differences there are 8 chances 
in 10 of securing so great a difference by random sampling. More 
work is needed at this point, but the results indicate that the fe- 
males of the aggregations are in more nearly the same physiologi- 
cal state as those scattered in the upper swamp than were the males 
from the aggregations like those scattered in the current below the 
swamp but above the aggregations. 

The rate of oxygen consumption of these females taken from the 
aggregations approaches that of the males run at approximately the 
same temperatures. The males tested on the afternoon of April 15, 
at a temperature of 15° C., consumed, on the average, 33.2 cu. mm. 
of oxygen per 1o males. The 20 females tested 2 days later at 17° C. 
used, on the average, 30.2 cu. mm. of oxygen. They weighed 1.1853 
against 1.3765 grams for the: males. If they had continued to con- 
sume oxygen at the same rate per unit weight and had weighed as 
much as the males, they would have used 35.1 cu. mm. of oxygen 
in the same time. 

If the comparison ‘is carried a step farther, we find that these 
females do not have a rate of oxygen consumption comparable with 
that of males scattered through the stream just above the aggrega- 
tions. Using values calculated from equal weights, we find that the 


198 ANIMAL AGGREGATIONS 


latter consumed 52.8 cu. mm. of oxygen as against 35.1 for the ag- 
gregated females. If the male isopods in the upper swamp have 
about the same rate of oxygen consumption as the females there, 
a point unfortunately not directly tested, we can reconstruct their 
respiration history as follows: 

The isopods in the upper swamp, on being swept off their feet, 
struggle against the current and are borne along clutching at straws 
as they pass. Sometimes an isopod clasps another and both are 
carried along head over heels until they are deposited in some de- 
pression with a sandy bottom, from which they start upstream only 
to be carried down again. Such isopods have their rate of respira- 
tion greatly increased as a result of their exertions. If they are 
carried’ on down so that they lodge among the grasses below or 
among their fellows who have already lodged there, they again take 
on about the same rate of respiration they had shown in the upper 
swamp. 

COMPARISONS AND CONCLUSIONS 

Like the land isopods and the brittle starfish, A sellus is not usually 
regarded as a social species. These isopods are frequently found in 
greater abundance in one place than another; but outside the breed- 
ing season, environmental analyses usually reveal significant differ- 
ences which may condition differential distribution; and, as we have 
seen, these natural aggregations originate largely through the reac- 
tions of individuals to their environment, rather than through social 
impulses. Great aggregations of these animals in nature have hith- 
erto escaped notice; and, so far as I am aware, no other great natural 
aggregation at the low level of group organization here existing has 
been analyzed to find the physiological effect of aggregation upon 
the aggregants. 

The laboratory experience outlined*above had previously shown 
that, conditions being otherwise favorable, land isopods and the 
brittle starfish, Ophioderma brevispina, have their rate of oxygen 
consumption increased as a first effect of aggregation. Later the 
rate of oxygen consumption is lowered as compared with individuals 
isolated under similar conditions. The respiration relations were 
not obtained at the beginning of the aggregations in the case of the 


EFFECT OF CROWDING ON SURVIVAL 199 


isopods in the field. Neither was it possible to arrange to make 
respiration tests without disturbing the aggregations as was done in 
the laboratory. One must use care in interpreting results based 
upon the oxygen consumption given during the first hour following 
the considerable disturbance occasioned by the extraction of 10 
isopods from the midst of an aggregation as compared with that of 
to isopods for the first hour that they are placed together in a small 
respiration chamber after having been previously scattered. 

It must also be noted that the isopods from the aggregations and 
those collected while still separate did not behave similarly in the 
respiration chamber. The former tended to gather in the restricted 
space furnished by the necks of the bottles forming the respiration 
chamber, where they were well shaded by the rubber tubing that 
held the two bottles together. Those collected while scattered tend- 
ed to remain so, and were more likely to be found exposed to the 
brighter light at the ends of the bottles. Since all lots tried behaved 
uniformly in this regard, and since previous location also made a 
difference in the oxygen consumption, the differential behavior with- 
in the respiratory chamber may be taken as one of the factors in 
determining the respiration relations observed. The exact weight 
that should be given this factor has not been determined. Such 
behavior might result in the shaded, more aggregated isopods using 
less oxygen during the respiration period than would the unshaded 
scattered individuals, as was found to occur. 

As a result of experience with respiration of laboratory aggrega- 
tions, it seems probable that the differential respiration observed is 
due to a difference in muscular tonus between the aggregated and 
the isolated animals. If this idea is applied, we have the reasonable 
assumption that isopods taken from the upper swamp far enough 
away so that they are undisturbed by the currents set up by the 
overflow are under approximately the same muscular tension as are 
those taken from the dense aggregations, and that both are under 
less muscular tension than are the isolated iospods that have re- 
cently been exposed to the full impact of a current which in many 
instances has been carrying them hurtling along over a sandy stream 
bed. The evidence at hand allows no suggestion as to whether the 


200 ANIMAL AGGREGATIONS 


respiration results observed are due to muscular tension acting as 
a direct result of the previous position of the isopods or whether 
they are due to the difference in muscular tension resulting from 
the differential behavior, which is in turn conditioned by their previ- 
ous position. 

The possibility that the increased carbon-dioxide tension and the 
decreased oxygen supply within the aggregation may have reduced 
the oxygen requirements of these animals is negatived by the fact 
that the rate of oxygen consumption of isopods from the center of 
the aggregations was slightly, but not significantly, higher than that 
of those taken from the upper swamp where the oxygen tension and 
pH were approximately that of the main current above the large 
clusters. 

The results obtained from the study of these enormous aggrega- 
tions of isopods in nature demonstrate that such masses can affect 
environmental conditions markedly, even in flowing water. Obvi- 
ously, such groupings would have still greater effect in quiet water 
oronland. As we shall soon see, such aggregations may have survi- 
val value in the laboratory, due to the modification they produce 
in their environment. No tests were made of the possible survival 
value of the isopod aggregations in nature, but the fact that they 
do alter their environment to a measurable degree extends the possi- 
bility of the application of the laboratory studies on the survival 
value of masses of animals. As in the case of the laboratory groups 
of land isopods and of Ophioderma, these water isopods in nature 
under adverse conditions tend to use other isopods in place of in- 
animate elements of their physical environment, when the former are 
present in considerable numbers and the latter are lacking in their 
usual ratio of abundance in proportion to the numbers of animals. 

In conclusion we may recall the statement at the beginning of 
this chapter, which these later studies have served to confirm, that 
under laboratory conditions the formation of aggregations serves 
to make these animals more quiet, and in the long run proves to be 
what is usually called an ‘‘adaptive reaction”’; and, so far as we have 
information, the same results come also from aggregations under 
natural conditions. 


CHAPTER XII 
PROTECTION FROM TOXIC REAGENTS 


Shortly after the publication of the preliminary announcement 
which the preceding chapter restates, Bohn and Drzewina (1920) 
published the first of a brilliant series of studies, apparently not yet 
concluded, which opened for investigation another phase of the 
subject that has yielded much evidence concerning the possible 
survival value of groups of animals for the individuals composing 
the groups. The general results obtained may be summarized in 
their own words (1926): 

“Nous avons recherché l’intervention du facteur masse dans les 
réponses de divers organismes vis-a-vis de multiples agents nocifs 
du milieu extérieur. Dans une masse M d’eau, on introduit une 
masse m d’un étre vivant (m étant égal ou inférieur 4 M/r1oo), 
cette masse résiste 4 un agent nocif déterminé (substance chimique, 
par exemple); mais une masse plus petite, m/10 ou m/1oo, ne 
résiste pas; tout se passe comme si la masse de matiére vivante 
exercait, vis-a-vis d’elle-méme, un effet protecteur (auto-protec- 
Lion) 5” 

MASS PROTECTION FROM COLLOIDAL SILVER 

Drzewina and Bohn worked with colloids of the heavy metals, 
particularly with colloidal silver. We have spent much time in this 
laboratory, also; studying mass relations of animals when exposed 
to colloidal silver;t and if the following report and discussion is 


t The stock suspension of colloidal silver was made as follows, from directions fur- 
nished by Dr. Terry-McCoy of the Department of Chemistry of the University of 
Chicago: Dissolve 4 grams of commercial dextrine in 100 cc. of water and then 4 grams 
of pure caustic soda. Dissolve 3 grams of silver nitrate in 20 cc. of water and add to the 
dextrine-soda solution. The precipitate of silver oxide that is formed is gradually re- 
duced by the dextrine, the color changing to reddish brown. After 20-30 minutes, add 
100 cc. of go per cent alcohol and stir. Allow the mixture to settle for 15-20 minutes, 
and then pour off the liquid from the particles of silver at the bottom. Add about 200 
cc. of water and the silver particles will generally disperse; however, if this is not the 
case, shake or stir until an even suspension of silver results. 


201 


202 ANIMAL AGGREGATIONS 


based largely upon our own work (Allee and Schuett, 1927), it is only 
because I am more familiar with this, and by no means because it is 
more important than work in this field done by Drzewina and Bohn 
in following out their original discovery. 

It is easy to demonstrate, as Drzewina and Bohn state, that 
within limits, the greater the mass of animals, the better the pro- 
tection. Thus with Planaria dorotocephala, twelve sets of 2 each, 
exposed at room temperature to tro cc. of water containing to drops 
of the standard suspension of colloidal silver, showed the beginning 
of head disintegration in from 4.5 to 10 hours; while ten similar lots, 
each containing from 10 to 75 worms, all lasted over 36 hours in the 
same volume and concentration. A species of Cladocera and the 
isopod Asellus communis showed similar, but less marked, group 
protection. 

Similar tests, in which the animals were exposed to the action of 
dilute suspensions of colloidal silver for considerable time and were 
then washed and transferred to water to which they were accus- 
tomed, yielded such results as are listed in Table XVIII. With the 
worms, the results are summarized in terms of survival after ex- 
posure, with added information concerning the number of worms 
that were able to crawl either with or without stimulation with a 
camel’s hair brush. Other data at hand show that the protective 
effect of the group was greater than is indicated by this summary, 
but the summarized account is sufficiently conclusive. With the 
brittle starfish, Ophioderma, tests were made concerning the righting 
ability of the animals after exposure to colloidal silver. In these 
tests, after exposure, the washed animals were transferred to sepa- 
rate finger bowls each containing 250 cc. of well-aérated sea-water. 
It will be noted that the mean righting time of the isolated animals 
is given in minutes while that of the animals tested in groups of 
5 is given in seconds. be 

More cases with different concentrations could be given, but 
the result is the same in all. This agrees entirely with the experience 
of Drzewina and Bohn, who used a wide range of animals; and with 
that of Bresslau, using different toxic reagents with ciliates. Yet the 
possible menace of crowding, so frequently given as the only result 


PROTECTION FROM TOXIC REAGENTS 203 


of confining many animals within a small space, may easily be 
demonstrated even here. With the Ophioderma, for example, if the 
glass finger bowls were covered with glass plates during the exposure 
to colloidal silver, thus stopping free gaseous exchange, the grouped 
animals were in much worse condition than were their isolated 
fellows. | 

A priori, one would expect exactly such a result, since in each 
case there is much less of the toxic substance present per individual 


TABLE XVIII 


SHOWING THE RESULTS OF EXPOSURE TO SUSPENSIONS OF COLLOIDAL SILVER 
FOLLOWED BY TRANSFER TO WATER SIMILAR TO THAT TO 
WHICH THE ANIMALS WERE ACCUSTOMED 


All were run at room temperature. 


Time 
Number 
Drops Exposed to} Number Number 
< Cc. of : Number Serie That 
Animal r Colloidal * Colloidal That r hat 
Water Silver Animals Silver Survived we ee Died 
(Hours) NALS 
Planaria maculata 15 12 6(3) 2 II 2 7 
15 12 1(75) 2 72 66 8 
5 I 5(3) 26 9 ° 6 
5 I 1(25) 26 25 23 ° 
P. dorotocephala. . 10 2 40(1) 7 15 I 25 
10 2 3(25) 7 75 63 o 
Dendrocoelum lac- 

UOT Diktom acter 5 I 5(3) 26 3 I 12 
5 I 1(24) 26 18 18 6 

ee Mean 

Ophioderma bre- Righiedy) time 
US Mboccccoos 50 2 48(1) 14.5-16| 41 30 min. ° 
50 2 12(5) | 14.5-16| 59 38 sec. re) 


composing the larger groups than when from 1 to 3 animals are 
placed in the same amount of the same concentration of toxic sub- 
stance (Pieron, 1921). Drzewina and Bohn consider this possibility 
(1921¢, 1928) and dismiss it as inadequate because of their experi- 
ments upon the effect of reconditioned toxic solutions and upon the 
relation of volume of the toxic material to its effect upon exposed 
animals. They summarize their conclusions repeatedly in some 
such words as the following (1921d): 


204 ANIMAL AGGREGATIONS 


“Bref, si chez diverses espéces que nous avons examinées jusqu’icl 
(Paramécies, Colpodes, Stylonichies, Stentors, Hydres, Convoluta, 
Glossiphonies, tétrads de Grenouille) tout se passe comme s'il y 
avait émission rapide d’une substance ow de substances assurant une 
défense; chez Polycelis c’est le contraire:* on assiste non pas a4 une 
auto-protection mais 4 une auto-destruction.” 

They do not believe that the greater resistance of the group is 
due to a more rapid using-up of the toxic substance, and cite two 
types of experiments to support their contention (1921). In the 
first type they expose some hundred of Convoluta in one suspension 
and 2 each in the accompanying dishes. Even if the former receive 
ro drops of colloidal silver to 25 cc. of water and the isolated animals 
have only 2 drops for the same volume, they find that the large 
group survives after the others are disintegrated. This is a fairer 
test than if the same amount of colloidal silver were added in both 
cases, but still it must be remarked that the ratio of colloidal silver 
per individual is about ten times greater in the case of the 2 individ- 
uals, when compared with the larger group. 

We have made some exact tests to cover this point, and I present 
the summary of our results in Table XIX. The tests reported 
show clearly that if a group of 25 Planaria are placed in the 
same volume and the same concentration of colloidal silver as 
are isolated individuals, the former survive the experience in 
good condition while the majority of the latter succumb, and 
almost all the others are severely affected. Further, if similar pla- 
narians are placed at the same time in the same volume of water, 
to which has been added sufficient colloidal silver to make the 
concentration per individual equal to that given the isolated worms, 
the bunched animals all succumb to a treatment that left 37.5 per 
cent of the isolated individuals alive, even if most of them were 
strongly affected. And finally, if the groups are placed in propor- 
tionate volume and proportionate concentration of colloidal silver, 
their condition approaches that of the isolated individuals. This 
evidence shows that the greater protection which the mass furnished 
when exposed to colloidal silver was due to the smaller amount of the 


t Against KC] (W. C. A.). 


PROTECTION FROM TOXIC REAGENTS 205 


toxic substance which each individual had to remove from suspen- 
sion in order to lower the strength below the threshold of immediate 
toxicity. The mechanism for this removal will be discussed later. 
Another type of experiment was run with Ophioderma to deter- 
mine whether the protective effect of the bunch was due to the fact 


TABLE XIX 


Animats UseD: Planaria dorotocephala, 15-25 Mm. Lonc, Exposrep To ACTION OF ° 
COLLOIDAL SILVER FOR 7 Hours, THEN WASHED AND TRANSFERRED TO WATER 
To WuicH THEY WERE ACCLIMATED, AND EXAMINED ON THE DAY FOLLOWING 
THE EXPOSURE 


I. 40 planarians isolated into ro cc. of water plus 2 drops colloidal silver 


No. Percentage 

Normal, crawling when stimulated........... I 2n5 
esstthanthalt disinteprated|;.. so. 945.5. 0- 45 2 5.0 
Half and less than two-thirds disintegrated... 4 10.0 
Two-thirds but not all disintegrated. ........ 8 20.0 
Wihollysdisintepratedeas 1.4. -ceacise fee +. 25 G@2n5 

II. 150 planarians in 6 lots each in 250 cc. water plus 50 drops colloidal silver 
Normal, crawling when stimulated........... ° O 
esstthnan naltedisimteprated oe. oye ste s ° fe) 
Half, but less than two-thirds disintegrated... 18 12 
Two-thirds but not all disintegrated.......... 52 35 
Wihollyzdisinteprated® q.40. es... ossste = aaa: 80 53 

III. 71 planarians in three lots each in ro cc. of water plus 2 drops of colloidal 

silver 

Normal, crawling when stimulated........... 63 88.8 | 
Normal, not crawling when stimulated....... I Tee 
Head started toydisimtiegrate. Wa )..2. fs wa ss 7 9.8 
Head completely disintegrated.............. ° 0.0 

IV. 75 planarians in three lots each in ro cc. of water plus 50 drops of colloidal 

silver 


All wholly disintegrated. 


that the bunched animals were exposed to a smaller concentration 
of colloidal silver than were the isolated animals. Since the area of 
the surface would affect the aération and so affect survival values, 
the larger numbers were placed in crystallization dishes with a diam- 
eter of 24 cm., while the smaller bunches and the isolated animals 
were in the usual finger bowls with a diameter of 10 cm. The essen- 
tials of the set-up were: 


206 ANIMAL AGGREGATIONS 


One bunch of ro animals with 25 cc. of sea-water and 1.5 drops of 
colloidal silver each, and with a surface exposure of 45 sq. cm. per 
animal. 

One bunch of 5 animals with 5 cc. of sea-water and o.3 drops of 
colloidal silver each, and with a surface exposure of 15.6 sq. cm. 
per animal. 

Five isolated animals with 25 cc. of sea-water and 1.5 drops of 
colloidal silver each, and with 78 sq. cm. surface exposure. 

A second set was exactly similar, except that the animals were 
exposed to 2, 0.4, and 2 drops of colloidal silver per individual. 

The exposure in a typical experiment lasted 15.5 hours, after 
which the animals were washed as usual and transferred to fresh 
sea-water for observation for the following 24 hours. At intervals 
records of numbers that righted after having been turned over, and 
other evidences of activity, such as spontaneous motion of tube feet, 
were recorded. 

All of the animals bunched in the to cm. finger bowls recovered 
sufficiently to right themselves within 24 hours, despite the fact 
that 4 of the 10 were much corroded by the action of silver that had 
settled to the bottom of one of the finger bowls. In both experi- 
ments, the isolated animals fared next best, although they made a 
much poorer showing than did the bunched individuals that were 
exposed to a much lower amount of colloidal silver per animal; 
only 6 of this set of 10 righted. The bunched animals, exposed to 
the same volume and amount of colloidal silver as those isolated, 
ranked a poor third, which may be due to the decreased surface 
exposure or to some of the better-known ill effects of crowding. 

These experiments indicate either that the decreased amount of 
colloidal silver or the reduced volume of sea-water present per 
individual, or both, markedly favored the survival of the animals 
bunched in the small amount of water present in the finger bowls. 
Drzewina and Bohn emphasize the latter factor, and accordingly 
experiments were run to test out this point. Contrary to the expe- 
rience of Drzewina and Bohn (1921a), the toxicity with Ophioderma 
was found to depend on the concentration of the colloidal silver 
rather than on the volume to which the animals are exposed. 


PROTECTION FROM TOXIC REAGENTS 207 


It is worth noting that the protective action of the bunch appears 
only in the more toxic suspensions, where apparently the toxic 
strength per individual was more nearly reached even with the 
bunched animals. Drzewina and Bohn apparently would interpret 
such an observation as meaning that the autoprotective substance 
is secreted more rapidly under more toxic conditions. 


RECONDITIONED SOLUTIONS 


The second and more convincing experiment used by Drzewina 
and Bohn as evidence in favor of their view that the group pro- 
tection is furnished by some sort of autoprotective secretion, rather 
than by exhaustion of the toxic substance, is as follows (1921a): 

“Nous décantons la solution ot! depuis 24 heures séjournent une 
cinquantaine d’embryons (tadpoles of Rana fusca) et dont la teinte 
révéle la presence du colloide; nous y ajoutons le méme nombre de 
gouttes que la veille, 1 par exemple, et nous y placons deux em- 
bryons neufs du méme age. Ceux-ci survivent, alors que des in- 
dividus témoins, placés dans une solution neuve a 1 goutte de 
collargol, succombent, comme c’est la régle pour les isolés. I] semble 
ainsi que, attaquées par le colloide, les larves émettent, rapidement, 
une substance (ou des substances) qui a pour effet de les protéger.”’ 

Wholly similar experiments were run with the brittle starfish, 
Ophioderma, to test for the presence of the postulated autopro- 
tective secretion. Colloidal suspensions in which Ophioderma had 
been exposed were reconditioned by 3 hours aération with room 
air in order that the new lot might not suffer from low oxygen 
tension, and were filled to the original volume with distilled water. 
The original suspension had been made with 1 drop of colloidal 
silver for each 25 cc., and the same amount was added to this re- 
conditioned water. Tests showed that the aération of freshly pre- 
pared suspensions for this length of time caused a color change 
but did not markedly affect the toxicity. 

Three lots, each consisting of one group of 5 bunched animals and 
4 isolated individuals, were placed severally in 50 cc. of such a re- 
conditioned suspension, g hours after the previous experiment had 
closed. These were run simultaneously with three similar lots in 


208 ANIMAL AGGREGATIONS 


freshly prepared suspensions of 2 drops of colloidal silver in 50 cc. 
of sea-water. All conditions were similar. The temperature was 
18.5° C. The results are summarized in Table XX. 

If Drzewina and Bohn were correct in thinking that the colloidal 
silver is not removed from solution on exposure to the animals, 
there should have been an excess accumulation of this substance 
in the twice-used water; but after again conditioning the used sub- 
stance, it was necessary to add the same amount, 1 drop of colloidal 


TABLE XX 


SHOWING RiIGHTING TIME OF Ophioderma AFTER AN EXPOSURE OF 17 HOURS IN 
RECONDITIONED AND. FRESH SUSPENSIONS OF COLLOIDAL SILVER 


All gave the righting reaction. 


BuNncH ISOLATED 
Number | Mean Time | Spread Number | Mean Time Spread 
Reconditioned Suspension 
15 | 73 sec. | on aoe 12 | 25 sec. 5-118 sec. 
New Suspension 
15 | 20 sec. | 7715 SCG: 12 | 25 sec. | 10-65 sec. 


silver per 25 cc. to this suspension, in order that it might be as 
‘deeply colored as was a fresh suspension made up with that amount 
of the colloidal silver. There was only sufficient colloidal silver left 
in this twice-used suspension to discolor the liquid. Either the 
animals or the aération, or both, had removed the greater part of 
the silver; and it can be readily demonstrated that the effect was 
not wholly due to aération. The water thus treated was distinctly 
sirupy, and evidently held organic matter, received from the two 
lots of Ophioderma which it had contained. 

The effect of this twice-reconditioned suspension was tested as 
before. Comparison tests were run with newly prepared suspen- 
sions, and the results are summarized in Table XXI. 

There is no evidence here of the presence of an autoprotective 


PROTECTION FROM TOXIC REAGENTS 209 


secretion in the sea-water, protecting animals from the action of 
the colloidal silver; nor is there evidence for the presence of an 
active autodestructive agent, which Drzewina and Bohn also postu- 
lated to explain certain cases in which the greater mass of animals 
exposed to KCl die more rapidly than do the solitary individuals. 
In the presence of an active autodestructive secretion one would 
expect the mass to die more rapidly than do the isolated individuals, 


which is not the case. 
RAB ICH Oxexa 


SHOWING RIGHTING TIME OF Ophioderma AFTER AN EXPOSURE OF 19 HOURS IN 
TwIcE-RECONDITIONED AND IN FRESH SUSPENSIONS 


BuncH IsOLATED 


Number | Mean Time Spread Number | Mean Time Spread 


Reconditioned Suspension 


I5 | 46 sec. IO—-221 Sec. rae | 3.45 rs. | \77/sec-—22 hrs. 
New Suspension 
15 34 SEC. 10-175 sec. 12 | 1.5 hrs. rosec.—14 hrs. 


* One failed to give the righting reaction. 


There is good evidence that the secretion of slime or other or- 
ganic matter into the suspension does remove the colloidal silver 
and so render the solution less toxic. It is particularly significant 
that, after having been twice used, the suspension required as much 
colloidal silver as was used in a fresh suspension to bring it to the 
same color. Similar results were obtained from other experiments. 

More recent experiments in this laboratory, the results of which 
are unpublished as yet, show clearly that groups of goldfish live 
longer in a given volume of a given concentration of colloidal silver 
than do isolated goldfishes similarly isolated into the same amount 
of the same concentration of colloidal silver suspension; and that, 
under these conditions, the group removes significantly more silver 
from suspension than do the isolated individuals. The experiments 
do not yet show conclusively whether this will account for all of 
the observed group protection. 


210 ANIMAL AGGREGATIONS 


SPECIFICITY OF MASS PROTECTION AGAINST COLLOIDAL SILVER 


There are two types of specificity possible: the protective secre- 
tion may have no other function, and so be specific in that sense; 
or it may be limited in protection to the species producing it. The 
first type of specificity will be discussed later. The latter aspect was 
investigated by placing one animal in a restricted volume of water 
containing a number of animals of a different species. All our ob- 
servations show that the protective action of the mass is not limited 


TABLE XXII 


SHOWING THE NON-SPECIFICITY OF THE PROTECTION AGAINST COLLOIDAL SILVER 


Animals Water (Ce.) |giver (Dreps)| (Hours) 
Mianya Clad ocer ayer, Alsellats sas. 2-1 -1teletee 179) 5 55 8 Over 36 
Tt HIRAIIGE G nate ob Spin otc DE SCOOT OOD U Opis T2n5 8 AGS 
Many Cladocera, 1 Planaria.............- TOG 8 Over 36 
t JAM sn nko oarous ae OO DO On CUR OI One 1D 5 8 og 
rte) AIS AUS, ae IAT, sce pons nuabouadoous 1255 8 Over 36 
Teg LG IULG ED sae ie ere nage ete ab ues aasl sve he 25 8 505 
Desiccated parotid gland, 2 Planaria...... 10 10 Over 36 
DELO MOTUG Ae este oo ee Ores aE 10 Io ok 
Sel ahi, 4% I7oh, can acs veccuasbaee 10 5 Over 36 
ON JEM OW HIE Lahey cee Od OD Ole TT COC REE 10 5 Less than 18 
Snarllslimewoeelaardiaennc peta cite 10 5 Over 36 
Phd UN Uh. 66-6 aoe SAO OUT OCS Ie Orne ite) 5 Less than 16 
it JERR, IO O MBs 6 soon bab one EoOuODOE ime) 5 Over 36 
OI) Hi HON this btvortects bate. 0 AON AE oe ie) 5 II 


to a given species. This is what would be expected if the fixing of 
the colloidal silver in some manner is the principal element in the 
protective action. The results of typical experiments are summa- 
rized in Tables XXII and XXIII. 

As these tables show, such diverse organisms as Cladocera, A sellus, 
pond snails, pond leeches, Dendrocelum, and even pond moss, if 
present in quantity, markedly protect planarians from the toxic 
action of colloidal silver. Even the actual presence of living organ- 
isms is unnecessary; snail slime without the snails protects efficiently 
apparently by adsorbing the colloidal silver. The slime becomes 
densely colored as the water becomes lighter. Suspensions in 
water of desiccated parotid glands of sheep exhibit similar adsorptive 
phenomena and have similar protective value. 


PROTECTION FROM TOXIC REAGENTS 21a 


A still more severe test of the specificity of the protection was 
made by placing recently killed Asellus in glass dishes containing 
to cc. of water plus 2 drops of colloidal silver, and introducing 
with these dead isopods one Planaria dorotocephala. At the end of 
7 hours’ exposure, 20 planarians isolated into similar volumes of 
the same concentration showed, 6, one-third disintegrated; 13, one- 
half disintegrated; and one, wholly so. The 5 worms isolated into 
suspensions containing dead isopods showed 1 intact though bloated; 
3, heads disintegrated; and 1 with both head and posterior end 


TABLE XXIII 


SHOWING COMMUNITY PROTECTION AGAINST COLLOIDAL SILVER 
Two drops of colloidal silver in 5 cc. pond water; exposure 13 hours, then washed and 
placed in pond water. Temperature 21° C. 


Animals Crawling Lived Died 


DS 


HOH OH OW 


AB JUG AOU GRUCHUD : 6 50600008 b0a000055 
TOMSOLVtEU RE WOCUICIOMEE eens eestor 
25 Dendrocelum, 1 P. maculata........... 
30 Glossophonia, 1 P. maculata........... 
Ad) AlSAN MS, 1 IES (QU CHT. nb ooaabaon eae 
2S SNAG AM, 1 12 CHUGH, 5 G50 00655 e 
[Poel iss, me IP. amelie, 5 on 6anandoave 


disintegrated. Thus the presence of the recently killed isopods de- 
cidedly protected the otherwise isolated worms from the toxic ac- 
tion of the colloidal silver. This, taken with all the other evidence 
at hand, furnishes convincing proof that the protective action of 
the mass against colloidal silver extends beyond the species limits. 

We have shown in the preceding pages that, within limits and 
other conditions being equal, there is greater protection the greater 
the mass of the animals present when exposed to the same amount 
of colloidal silver in the same volume of water. Further, the protec- 
tion is largely, and perhaps completely, furnished by the fixation 
of the toxic substance by the mass of animals, so that each escapes 
receiving a lethal dose; while, with the same concentration, isolated 
individuals receive a stronger dose of the toxic substance. The 
colloidal silver may be differently fixed in different animals; but 
with those that secrete slime, like planarians, the colloidal silver is 


Ee ANIMAL AGGREGATIONS 


adsorbed on the slime. With other animals observed, it may be 
removed by adsorption on the surfaces of the animals themselves. 
Finally, as would be expected from this mechanism, we have dem- 
onstrated that the protection furnished by the mass, is, at least to a 
considerable extent, independent of the species present. 

Our experiments do not support the hypothesis that group pro- 
tection among these aquatic animals is furnished by the rapid pro- 
duction in the presence of a toxic agent, such as colloidal silver, of a 
more or less mysterious autoprotective secretion. It is true that 
the production of slime by planarians actually serves as an auto- 
protective agent, not only in fixing colloidal silver in these experi- 
ments, but very probably in protecting the planarians from sudden 
changes in culture or habitat water. But slime production cannot 
be regarded as a specific autoprotective secretion, either in the sense 
that it is used for no other purposes, for obviously it plays many 
other roles in the economy of slime-producing organisms, or in the 
sense that slime is limited in its protecting power to the one 
species producing it, since the protection furnished by aggregations 
of mixed species is easily demonstrated. 


OTHER CASES OF GROUP PROTECTION 


Bresslau (1924) found that Protozoa give off a substance which 
he calls “‘tektin,” a mucin-like body, which is given off in greater 
abundance when the animals are stimulated by heat, pressure, meth- 
ylene blue, iodine, etc. The tektin, when given off, takes up water 
rapidly and exhibits strong surface activity, adsorbing foreign par- 
ticles readily. Bresslau tells of putting 2 cc. of liquid from a culture 
of Infusoria (either Colpidium or Paramecium) containing many in- 
dividuals in one dish and a similar amount from the same culture, 
but with few individuals, into another. Into each he introduced 1 
cc. of t per cent solution of methylene blue. Both produced the 
tektin and adsorbed this poison: the culture with the many animals 
so much more completely that the possibility of surviving the toxic 
action of the poisonous material was greatly increased. 

Carpenter (1927) approached this problem from a wholly different 
angle. Without referring to the work of Drzewina and Bohn, she 


PROTECTION FROM TOXIC REAGENTS 213 


attacked the problem of the toxicity of metallic salts on fishes on 
account of her interest in stream pollution. Using lead nitrate, she 
found that the fatality curve is described by the equation K = 1/tlog 
t/conc., where K is a constant dependent upon the toxic substance 
employed and ¢ is survival time for any given concentration (conc.). 

When a number of minnows (Leuciscus) were killed successively 
by placing them in the same 500 cc. solution of N/1o lead nitrate, the 
survival time was found to be prolonged with each successive fish 
until the eighth had a survival time approximately double the stand- 
ard figure for that concentration and size of fish. The actual sur- 
vival times reported are, in order, 73, 89, 92, 93, 94, 120, and 130 
minutes. Carpenter makes the deduction that the solution was 
successively weakened by the abstraction from it of a certain pro- 
portion of the lethal substance by each fish tested and that the 
lethal efficacy was thus progressively reduced. 

The lethal efficacy of the solution varied in inverse proportion to 
the actual size of the fish exposed and directly according to the 
amount of lead (as Pb) required to kill the fishes. If the original solu- 
tion be relatively weak, a single fish will remove an important frac- 
tion from a small amount of solution, and will therefore show a 
longer survival time than a similar fish isolated into a large volume 
of solution of similar strength. This is in keeping with Huxley’s ob- 
servations (1922) on the action of mercuric chloride on the gill tissue 
of Mytilus. Carpenter concluded that the lead salt causes death, 
due to the formation of a film over the gills and skin of the fish, by 
an interaction of mucus and the metallic ion, which causes death by 
suffocation. If insufficient lead ions are present, the film is shed; and 
the solution being thus freed from its toxic element, recovery en- 
sues. 

An attempt was made to establish the amount of lead which en- 
ters into combination with the mucus, and the amount, if any, 
which enters the blood-stream. Two large minnows were placed 
together in 1-5 liters of distilled water, to which was added sufficient 
lead nitrate to supply 6.21 mg. of lead. Immediately after death, 
the bodies were transferred to fairly dilute acetic acid for 4 minutes 
until the gills were quite clear. After washing the bodies in distilled 


214 ANIMAL AGGREGATIONS 


water and combining both liquids, these were found to carry 4.62 
mg. of lead. The original solution was found, by similar analysis, 
to have 1.8 mg. of lead after the death of the fishes. Adequate 
analysis of the fishes’ bodies showed no lead had penetrated. Car- 
penter points out that the overplus of o.21 mg. found in the analyses 
as compared with the original solution makes the experiment incon- 
clusive in some respects, but that it does show clearly that the fishes 
remove toxic substances by adsorption, just as do our results with 
colloidal silver. 

In later work (1930) Carpenter finds that the survival time of a 
number of North American fishes is directly influenced by the value 
of the ratio of the volume of solution to mass of individual fish so 
long as the solution is of constant molar concentration. When the 
concentration and volume are constant, the survival varies inversely 
according to the mass of the fishes. For equal masses of animals, the 
survival varies directly with the ratio of the volume of the solution 
to the molar concentration. The results upon survival of exposing 
groups of fishes to lethal concentrations and volumes of lead salts as 
compared with isolated fishes, are not yet clear, but the later mem- 
bers of a processionary series killed in the same solution survive 
longer than do the earlier members on account of the using up of the 
‘harmful agent. 

Preliminary experiments in our own laboratory in which the sur- 
vival of groups of 10 goldfish was compared with that of the same 
number of individuals isolated into the same volume and concentra- 
tion of lead nitrate have revealed no significant differences when 
solutions of from N/10 to N/200 were used. The technique used 
in these experiments differed from that of Carpenter; hence we are 
not ready to criticize her results. But our experiments do show that 
the group effects clearly exhibited with colloidal silver are not so 
readily demonstrated with lead nitrate. 

Pawlow (1925) came at the same general problem from still an- 
other angle. Led on by Ostwald’s basic work on adsorption of toxic 
salts, Pawlow worked out theoretical formulas to express the relation 
between toxicity and adsorption on animals. These show that if 
the active toxic substance has a direct effect upon the organism, 


PROTECTION FROM TOXIC REAGENTS 215 


then the survival depends on the volume of the medium for any giv- 
en concentration of the toxic agent and also upon the density of the 
population. The same relationship holds if the active lethal agent 
works through capillary attraction or by division among phase 
boundaries. These relationships are recognized as forming a bio- 
logical aspect of the law of mass action. 

In experiments on the brine shrimp, Artemia salina, exposed to 
reduced or to increased salt solution, Pawlow found that the theory 
given above was verified. The mean life-duration varied from 32.1 to 
6.1 in different salt concentrations when the volume was quad- 
rupled, while halving the numbers of animals present decreased the 
survival between 6.7 and 15.2 per cent. Similar relationships were 
found for the amphipod Gammarus pulex when exposed to certain 
concentrations of NaCl. 

Stimulated to similar studies by their interest in the implications 
of the work of Gurwitsch on mitogenetic rays, Frank and Kurepina 
(1930) have demonstrated mass protection for sea-urchin eggs from 
the action of KCN, both as regards the rate of development and the 
percentage surviving. They report that 10 * N solution exerted the 
following effects: 


INitirt Bete O Re SP Sercaan. cicke aid foes os oc tics sis one tineach 13 drops 68 in 3 drops 
INoicledvare so) ose fas Sede eo yd Sak 85% 20% 
BEDIASEOMMIEKES Hs. yarn ost ee CMe eee 15% 40% 
A SaTASEOMICLES sty. fers hike apes SPS ol a ols. o% 40% 


Another type of protection has already been shown to be operat- 
ing, in at least certain instances, with aquatic forms (chap. vii) and 
is of sufficient importance to be repeated here in some slight detail. 
The discovery of this factor is a result of the work of Fowler (1931), 
who has examined the resistance of Daphnia pulex to a great many 
chemicals. Usually he found the groups more resistant than were 
isolated individuals run in the same volume of the same solutions. 
When he pushed the analysis of this phenomenon, particularly with 
CaCl., he found that the group was consuming less oxygen per in- 
dividual for a given exposure time than were the accompanying 
single individuals. Further, when the experiments were run in such 
a way that the CO, was absorbed as soon as produced, the survival 


216 ANIMAL AGGREGATIONS 


of the animals in the groups became the same as that of the isolated 
animals. When he re-examined his original data in the light of 
these findings, it was discovered that the cases in his early work in 
which the singles lived for a longer time than the group were those 
in which the dilution of the chemical was such that acclimatization, 
rather than direct resistance, was an important factor. In such 
cases, as Child has repeatedly pointed out (e.g. 1915), the animals 
with the higher rate of general metabolism have the greater power of 
acclimatization, and hence of survival. On the other hand, whenever 
the individuals tend to produce conditions which depress their rate 
of metabolism and are exposed to solution strengths lethal within a 
relatively short time before acclimatization becomes a factor, such 
depression has survival value. 

Fowler’s work also demonstrated that when Daphnia are exposed 
in dilute solutions of sodium or potassium hydroxide, the grouped 
individuals lived significantly longer than did similar isolated ani- 
mals, and that under these conditions the hydroxide in the solution 
surrounding the group was reduced in strength as compared with 
that surrounding relatively isolated animals. Eight experiments 
with NaOH showed that 2 animals in 20 cc. solution in 3 hours 
reduced the hydroxide from a mean concentration of 0.00094 N to 
0.000764 N. Under the same conditions 20 Daphnia reduced the 
same amount of the same solution to 0.000606 N, a difference of al- 
most 30 per cent, with a statistical probability of 0.002. Similarly 
significant results were obtained with KOH solutions. Here the car- 
bon dioxide given off by the animals reacts with the hydroxides to 
form carbonates and water, thus weakening the toxic concentration 
of the medium. The groups produce carbon dioxide more rapidly 
than do animals isolated into the same volume of medium, and hence 
decrease the toxicity more rapidly and survive longer. 

The problem of the number of animals present in relation to their 
survival when exposed to toxic agents presents a somewhat different 
aspect when applied to insects placed in containers into which toxic 
gases have been introduced in concentration just sufficient to kill 
only a part of the isolated individuals with the exposures employed. 
Here the production of autoprotective materials presents a distinctly 


PROTECTION FROM TOXIC REAGENTS 217 


different problem from that found in aquatic animals. Discussion 
of experimentation in this connection will be reserved until chapter 
xiv, which deals with aggregations and insect survival. 


GROUP PROTECTION FROM HIGH TEMPERATURES 


Not all adverse conditions in nature are the result of the presence 
of toxic materials in aérial or watery solutions where the types of 
protection we have just been considering might operate. One other 
adverse condition is that furnished by the physical factor of high 
temperature. Concerning the ratio between the mass of exposed 
animals in relation to volume of medium with respect to this physi- 
cal factor, Robertson (1921) records observations upon the Aus- 
tralian infusorian Enchelys farcimen. 

A temperature of 30° C. prevents subcultures of this protozoan 
from multiplying, and the isolated individual almost inevitably dies. 
But shade temperatures of 30° C. are known in pools of South 
Australia in which wild Enchelys live. Wild infusorians brought 
into the laboratory are similarly killed by such temperatures if 
isolated; but if the culture slides are populated by 20-30 individuals, 
they can successfully resist exposure to temperatures of 33-34 C. 
for as long as 7 days in succession without apparent injury or abnor- 
mality. Also, a similar number of individuals put into fresh hay 
infusion at this temperature survive and multiply, while isolated 
individuals perish. 

Typhoid bacilli can be rendered abnormally susceptible to hydro- 
gen peroxide by a degree of heat which is just sublethal; and the 
growth inhibition so caused can be neutralized, as in bacteria natu- 
rally susceptible to peroxide, by the accumulation of certain prod- 
ucts of bacterial growth (Burnet, 1925a). These peroxide-neutraliz- 
ing materials are more rapidly produced the larger the colony of 
bacteria and the more vigorous their growth. This subject will re- 
ceive further attention in chapter xvi. 

Drzewina and Bohn (1926, 1928) report similar results with sper- 
matozoa. Although these will be considered at greater length else- 
where, it should be stated that both of these observers interpret 
the greater protection which they find to be given by the presence 


218 ANIMAL AGGREGATIONS 


of greater numbers, to the conditioning of the water by some sort 
of autoprotective exudate. Robertson brings his observations into 
line with his general theory of the extrusion of an autocatalytic 
substance, while the others regard mass protection from high tem- 
perature as an excellent example of the effect of an autoprotective 
secretion. 


~ 


PROTECTION FROM ULTRA-VIOLET RADIATION 


Another type of experimentation which should yield critical tests 
of the efficacy of group protection and of its mechanism should be 
found in treatment with radiant energy applied as ultra-violet 
radiation. Hinrichs (1927) has reported that Arbacia sperm are Jess 
affected in concentrated than in dilute suspensions (obviously a 
mass protection); and Petersen has obtained similar results in this 
laboratory in radiating Paramecia (unpublished). Accordingly, tests 
(Allee, 1928) were made concerning various aspects of survival 
value of groups as compared with isolated planarians, when exposed 
to the full spectrum of a quartz mercury-vapor arc, with temperature 
controlled during the exposure. The results collected demonstrated 
two facts: 

In the first place, the presence of products of cytolysis produced 
by exposure to ultra-violet radiation are more harmful than bene- 
ficial to those worms that have been exposed to the action of ultra- 
violet radiation or to those that have not been exposed. Similarly, 
water containing products of metabolism or exudates given off by 
the animal, either in its usual laboratory culture or when exposed 
to ultra-violet radiation, shortens rather than lengthens the survival 
of other worms isolated into it. 

In the second place, the exposure of a number of worms in a 
limited amount of water with limited exposed area gives much 
greater protection for the individuals than if they had been exposed 
singly or in pairs. Some of the implications of these results deserve 
consideration and will be discussed in the order just given. 

When the survival of worms in various sorts of worm-conditioned 
water is compared with that of similarly treated planarians in 
aérated well-water, the worms in the conditioned water are found to 


PROTECTION FROM TOXIC REAGENTS 219 


die more quickly. The difference in the survival periods is usually 
small, and the periods themselves are highly variable for different 
lots of worms. Eleven such group comparisons are possible with the 
data at hand; and in these, nine cases definitely favor the well-water, 
one favors the conditioned water, and the other is recorded as a tie 
but with more careful observation it would probably have been 
placed definitely in the majority column. The main inquiry was 
concerned with the possibility of there being a protective value in 
the conditioned water, and there is no doubt of the negative an- 
swer given by the experiments. Rather, the converse is indicated 
though the evidence is not complete concerning the possibility 
that with different water and with smaller proportions of the con- 
ditioning matter, such conditioned water may prove advantageous 
to the worms placed in it. 

With regard to the definite protection furnished when many 
worms were exposed to ultra-violet radiation in a limited amount of 
water and with a limited surface area, the experimental evidence 
indicates that this protection is due to some sort of interference with 
the penetration of injurious rays or to some other biophysical effect 
of numbers, rather than to the presence of some exudate or exudates 
released as a result of the radiation. As has been stated above, water 
containing such exudates produced harmful rather than beneficial 
results. 

In the experiments where the worms were more densely crowded 
(60 to 1 cc. of water), the protection was obviously connected with 
the “shading” which Hinrichs mentions in her studies on the radia- 
tion of sperm suspensions. In the less dense groups (15 to 1 cc.), 
“shading” in the usual sense is less obvious; but exposure at this 
density also resulted in definite protection to the group-exposed 
worms, which suggests that some other factor or factors may have 
been operating. Protection furnished by the presence of so few 
worms may be an illustration of the phenomenon called by Drzewina 
and Bohn ‘“‘catalysis by contact.” Similar shading would undoubt- 
edly have resulted from the exposure of isolated worms in. water 
conditioned by the pressure of products of cytolysis. The experi- 
ment was not tried, since the only information to be gained would 


220 ANIMAL AGGREGATIONS 


have been the relative value of the protection and the toxicity of 
such conditioned water. 

It is worth noting that all the members of the group exposed to- 
gether were benefited by the fact that they were together. In such 
circumstances it might have happened that certain animals at 
the surface would take the brunt of the harmful rays. Their pres- 
ence might serve to protect the other members, and the group might 
thus be of value to the species if not to all the individuals composing 
it. In one experiment, where the subsequent history was taken for 
all the exposed animals, the interval before the first effects of ex- 
posure was observed in the 30 animals exposed as a group ranged 
from 27 minutes to 32.8 hours. Their final cytolysis ranged from 
8.25 to 106 hours. The same conditions were observed for the 26 
worms exposed in pairs in from 1 minute to 23.5 hours, and from 
3.4 to 84 hours, respectively. Thirteen (50 per cent) of those ex- 
posed as pairs were visibly affected before the end of the first hour 
after exposure, while 6 (20 per cent) of the grouped animals were 
similarly affected in the same time. Six of those radiated in pairs 
were dead before the first of the group died. 

I am not prepared to generalize widely from these experiments 
with the relatively large and highly pigmented Planaria doroto- 
cephala that mass protection from ultra-violet radiation is always 
due solely to some sort of physical interference rather than to the 
possibly protective action of some exudate. With small organisms 
such as sperm of sea-urchins or with Paramecia, which are more 
translucent, the mass protection may be due to the latter factor, as 
Hinrichs suggests. Her observations show, however, that even with 
such minute organisms, the fact of physical interference is significant. 

Single planarians, and to a greater extent massed planarians, 
would be more nearly analogous to sperm or protozoan aggregates 
than to a suspension in which distribution is fairly equal in three 
dimensions and each organism is surrounded by a medium of approx- 
imately uniform consistency. In the latter case, it is conceivable 
that a closer equilibrium between cells and medium must be main- 
tained than with larger forms. This subject is one for experimenta- 
tion rather than a priori discussion. 


PROTECTION FROM TOXIC REAGENTS 221 


In general the experiments on the group resistance of Planaria 
to ultra-violet radiation forward our understanding of mass rela- 
tions of individuals in two ways: First, the results emphasize the 
fact that the phenomena of possible protection of individuals by 
chemical exudates, which has been demonstrated for many animals 
exposed to different situations, is not universal. Second, they show 
that even when the massed animals are known to produce chemical 
exudates which are harmful, the massing may still have survival 
value through ‘the changed physical conditions which it produces. 

The work reviewed in the present chapter makes clear that group 
protection from toxic agents may operate in diverse ways. There 
may be group protection solely as a result of the distribution of 
toxic material among so many individuals that no one receives a 
lethal dose, or the toxic substance may be adsorbed on the slime 
which is often produced in copious quantities under the stimulation 
of the abnormal situation. The survival value of the group may be 
due to the depressed physiological condition obtaining among its 
numbers, which favors increased survival when the animals are 
exposed to strongly toxic solutions which kill actively metabolizing 
animals more rapidly than those whose rate of metabolism is lower. 
The group may act in a purely physical manner by altering the 
electrical conditions, or against light and ultra-violet radiations by 
a simple shading phenomenon. The evidence to date does not ex- 
clude the possibility of protection from active toxic conditions by 
the group conditioning the medium through some protective secre- 
tion, although this kind of explanation should be adopted only on 
positive proof. While it is clear that group protection is a fact, it is 
also certain, as might have been expected, that there are many ways 
by which groups accomplish this protection, and that more than one 
may be acting in the same case. 


CHAPTER XIII 
RESISTANCE TO HYPOTONIC SEA-WATER 


We have just seen that when exposed to a variety of toxic agents, 
with the mass of animals in optimal relation with the volume of the 
medium and other conditions being favorable, groups of animals 
will survive longer than will equal numbers of single individuals 
when each is isolated into the same volume of the same medium to 
which the group is exposed. Our experience with colloidal silver 
and with several other reagents indicates that such protection is 
largely, and at times probably completely, furnished by the fixa- 
tion of the toxic substance either directly by the mass, or by slime 
and other products given off, so that it is either removed or dis- 
tributed among so many individuals that each receives a sublethal 
dose. If the animals are closely aggregated, only those on the out- 
side receive the full impact of the toxic agent, so that those within 
are protected by another type of mechanical action of the mass. 
Such an attack on the problem of the protection of the individual 
by the mass, while demonstrating the fact of the protection, does 
not furnish critical evidence concerning the production of an auto- 
protective secretion other than slime, which in these cases presum- 
ably owes its protective action to its adsorptive power. 

A better opportunity to obtain critical evidence is furnished when 
the toxic properties of the solution are due to the absence of easily 
measured materials rather than to the presence of some toxic sub- 
stance added to the water. Such conditions are fulfilled when marine 
animals are placed in hypotonic sea-water. In concluding a note on 
the effect of exposing the marine turbellarian Convoluta roscoffensis 
to hypotonic sea-water, Bohn and Drzewina state (1920): 

“Toutes conditions égales d’ailleurs, les individus isolés sont in- 
finiment plus sensibles que les individus groupés, comme si le fait 
d’étre groupés constituait pour eux une protection. Le contraste 
est souvent saisissant. Soient deux verres de montre, dont l’un 


222 


RESISTANCE TO HYPOTONIC SEA-WATER 223 


contient, dans l’eau deluée a 75 pour 100, quelques individus, et 
Vautre plusieurs centaines de Convoluta; les premiers sont cytolysés 
en quelques heures, les derniers aprés plusiers jours.” 

Elsewhere they state clearly their belief that the observed pro- 
tection is due to the secretion of an autoprotective substance by the 
mass in greater quantity in proportion to the available volume of 
water than is possible for the isolated animals. ; 

Lapicque (1921), in discussion, made the obvious objection that 
the introduction of differmg numbers of marine animals into the 
same quantity of hypotonic sea-water would have a differential ef- 
fect upon the salt content and that the difference in survival might 


Fic. 16.—Procerodes wheatlandi, dorsal view. The posterior end forms a muscular 
sucker by means of which the animal attaches to the under side of rocks near the low 
tide line. These animals are usually found in considerable numbers on a given stone, 
if at all. 


‘be due to this direct action of the greater mass of animals. He 
rightly thought there should be a quantitative determination of the 
salt content at the end of the experiment. 

Drzewina and Bohn (19210, 1928) replied by calling attention to 
the small size of the Convoluta worms, which are about 3 mm. long. 
They calculate that the amount of salt such worms would carry 
would not affect sensibly the salt concentration of the solution, but 
have not refuted this criticism by experiment. 

At Woods Hole I have had an opportunity to test this situation 
with two main objectives: First, the question of fact involved: Is 
there a greater protection furnished by the presence of large num- 
bers of animals in hypotonic sea-water when compared with fewer 
similar animals in the same volume of water? And, second: What 
is the mechanism of the protective action of the group, if it be found? 

For these studies a turbellarian, Procerodes wheatlandi (Girard), 
Figure 16, was selected as representing a form taxonomically some- 


224 ANIMAL AGGREGATIONS 


what related to Convoluta. These animals are normally subjected 
to hypotonic sea-water when a heavy rain occurs at low tide. 

These small worms reach a length of about 5 mm. and are about 
1mm. in width. In nature they are found in abundance on the lower 
sides of stones in small tide pools, near or below low tide line. They 
are not abundant in deeper water. Usually they were taken from 
the protected sides of stones that were firmly located on a sandy 
substratum; evidently they cannot stand the full sweep of the waves. 
They were usually present on a given stone in considerable numbers, 
if at all. As the water went stale in the laboratory, if there were 
numbers of worms present they would collect on the surface film in 
shaded areas in dense aggregations. Here, as in the field, they did 
not occupy all the apparently optimal space. 

Appropriately safeguarded experiments showed that these worms 
will survive exposure to equal amounts of tap-water the better (a) 
if they are present in numbers; (0) if isolated into tap-water in 
which other living Procerodes have previously been exposed; and 
especially (c) if exposed in tap-water in which other Procerodes have 
died and disintegrated in whole or in part, even when such a condi- 
tioned medium is boiled or filtered. As is to be expected, these 
worms live longer if the salinity of the tap-water is increased by as 
much as 0.05 per cent above a minimum of that value. With well- 
washed worms it is possible to demonstrate protection when the 
tap-water contains exudates from living or dead Procerodes in which 
the salt concentration, as measured by the amount of chlorides pres- 
ent, isnot the determining factor. The method used consists in titrat- 
ing with N/1oo silver nitrate, using a 1 per cent solution of potas- 
sium chromate as an indicator. 

The first experiments (Allee, 1928) demonstrated that the pro- 
tection was due neither to sea-water contamination nor to the leach- 
ing-out of electrolytes in the proportions in which they exist in sea- 
water. They did not exclude the possibility that the protection may 
have been given by the leaching-out of electrolytes in some other 
proportion from that found in sea-water. Such an explanation of 
the observed protection is the most simple and obvious one to be 
advanced. The possibility of its operation was tested at the first 
opportunity. 


RESISTANCE TO HYPOTONIC SEA-WATER 225 


For this purpose experiments were repeated, using the technique 
suggested but with the additional precaution of checking the electri- 
cal resistance of the different solutions to which the worms were 
exposed, at the beginning and at intervals during the progress of 
the survival tests (Allee, 1920). 

This method of determining the amount of electrolytes present 
measures all electrolytes, instead of depending on the well-known 
relation between the amount of chlorine and the total salt concen- 
tration in sea-water. The experiments to be reported fall into two 
main groups: those in which the water had an initial resistance of 
about 1,900 to 2,500 ohms, and those in which the initial resistance 
was 5000 to 6,500 ohms. The experiments reported in 1928, as nearly 
as can be told from chlorine titrations and survival values, belong 
to the former level. Preliminary tests showed that with hypotonic 
sea-water at the greater dilutions, and under the conditions of these 
experiments, an initial difference of about 1,000 ohms is needed to 
affect significantly the survival time of these worms. 

All the comparisons made concerning the effect of homotypically 
conditioned medium upon the survival of Procerodes are summarized 
in Table XXIV, which gives the results of nine separate sets of ex- 
periments, each of which consisted of from three to six independent 
sets of tests. In the results down to those of Experiment 71 the 
initial resistance of the conditioned water was regulated by the 
addition of distilled or of tap-water to the conditioned medium. In 
the later experiments the resistivity was controlled by dialysis. 

Direct comparisons indicate that dialyzed Procerodes culture me- 
dium is less effective than is similar dialyzed culture medium which 
also contains the water extract of dead Procerodes worms. These 
results are in keeping with those obtained in 1928 with the methods 
then in use, and in all probability represent the true state of affairs." 

In all, 640 worms were used, which were divided equally between 
Procerodes-conditioned water and fresh water to which enough dilute 

™ Castle (1928) in his work on the life-history of Planaria velata similarly found that 
in placing 50 small fragments of the flatworm in ro cc. of distilled water some will sur- 
vive. If the same mass of the same number of small fragments is placed in 200 cc. of 
water, usually all will die. With small volumes, more than 75 per cent will survive. He 


attributes this protective action of the mass primarily to the rapid conditioning of the 
medium by the products from disintegrating pieces. 


226 ANIMAL AGGREGATIONS 


sea-water had been added to bring it to an equal initial resistivity. 
- These worms showed a mean survival of 18.38 hours longer in the 
worm-conditioned water than in the controls. Each group of ex- 
periments considered singly gave positive results, although some 
individual pairs did not. Despite the variability in procedure used 
in the different experiments, the combined results show a statistical 
significance of 0.03 when considered as nine paired experiments. 

These results are graphically summarized in Figure 17, which 
shows two sets of histograms. That marked A gives in black the 


TABLE XXIV 


SHOWING THE EFFECT OF Procerodes-CONDITIONED WATER UPON SURVIVAL 
oF Procerodes ISOLATED INTO EXTREMELY HypoTonic SEA-WATER 


RESISTANCE IN OHMS SURVIVAL IN Hours 

EXPERIMENT Conditioned Water | Dilute Sea-Water EES Gon Dilute we 
ditioned | _Sea- Pare 
Start Late Start Late Water Water es 
ATI 558 Go CIE ole TE SOO) ||| Tp SIEto | Me ACO || ma AvAG 56 | 88.16 | 28.44 | 59.72 
A SAIS cneysnallelste) ors Dele) || i Agys\ || Ap ietey || 285 4KsKe) 60 SOnO7 1037203) 22204: 
MOMs 5 sion ec coc Ds aisto) |i We yeley || Bo Bkfon || wo sAdo 60 58.1 44.05 | 14.05 
AQw 5iltae chia aieere 2 AON LOZ0) P2400) | eT ooS 56 oy fley || seri 316) 9.20 
KOA ince AAS) |} i He) || Ho (oler) || ayes 60 10.05 owe 2.53 
Tweets Rene ae 5,09) || Boner | Syuge || atCew 58 Aypeie) || wis 0) || BH.2r 
TIL Dreenk eters te weyers SOM 3555 On 45250) Sn 400 58 Ri sans) || 1.28 || QAO) 
U ERDICRE Oe 6,200 | 4,700 | 6,200 | 4,500 118 12.06 eas 4.31 
Tio Loe teats Ghee 6,200 | 4,700 | 5,100 | 4,000 II4 II.30 7.88 3.42 


percentage of worms surviving at the indicated hourly intervals 
when placed in tap-water with an initial resistivity of from 5,000 
to 6,550 ohms. Just above, in the shaded blocks, is given the added 
percentage of survival resulting from the presence of sufficient sea 
salt to decrease the initial resistance to from 1,890 to 2,400 ohms. 
The upper clear blocks give the added survival resulting from the 
presence of water extract of Procerodes worms with the same initial 
resistance as the hypotonic sea-water. 

Graph B shows similarly the survival in tap-water with an initial 
resistivity of from 5,000 to 6,550 ohms, in hypotonic sea-water made 
by adding dilute sea-water to pond or distilled water, bringing it to 
a resistivity of from 5,150 to 6,550 ohms, and finally, at the top, 


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RESISTANCE TO HYPOTONIC SEA-WATER 


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228 ANIMAL AGGREGATIONS 


the survival in dialyzed Procerodes-conditioned water having the 
same initial resistivity. 


HETEROTYPICALLY CONDITIONED WATER 


Having demonstrated in two successive years that Procerodes 
may be protected, at least to some extent, from the harmful effect 
of extremely hypotonic sea-water by being isolated in water condi- 
tioned by exudates or watery extracts of other Procerodes, and that 
whatever the protective device may be, it is not a direct result of an 
increase in total electrolytes present, it becomes of importance to 
inquire carefully concerning the specificity of this protection. 
Evidence will be presented here concerning the species specificity 
only, since to date no work has been done to attempt the analysis of 
possible functional specificity. In the course of this study the effects 
of the following kinds of media were tested against very dilute sea- 
water with the same resistivity: Paramecium culture medium, water 
extract of Planaria maculata (a fresh-water turbellarian) and of P. 
maculata culture media, and water extracts of a marine amphipod. 
The effect of each of these will be discussed in the order given. 

Paramecium culture medium.—The solution used was the clear 
brownish surface liquid from a hay infusion which contained a 
vigorously growing and almost pure culture of Paramecium. The 
general situation can be presented most clearly by describing one 
experiment in some detail and by adding summaries of all. In 
Experiment 74, hay-infusion liquid, such as has just been described, 
containing many Paramecia was boiled and dialyzed against run- 
ning tap-water until it showed a resistance of 6,450 ohms at 20° C. 
Tap-water was brought to the same resistivity by adding 0.25 per 
cent sea-water. Another lot of tap-water was brought to 5,450 ohms 
resistance. 

One hundred and fifty Procerodes were isolated, 50 into each of the 
modified tap-waters and 50 into the dialyzed Paramecium culture 
medium. At the end of 6 hours the water in the more dilute tap series 
had changed from 6,450 to 3,350 ohms and that in the culture me- 
dium had fallen from the same initial level to 3,550 ohms. Both 
were changed to 6,550 ohms in their respective media. After an- 


RESISTANCE TO HYPOTONIC SEA-WATER 229 


other 6 hours, this had fallen to 4,950 ohms for the modified tap- 
water and to 5,750 ohms for the culture media. Again the liquids 
were changed to appropriate solutions, each at 7,400 ohms at 10° C. 

After 24 hours the water from worms just beginning disintegra- 
tion in the dilute sea-water showed 4,850 ohms resistance, while that 
from worms in the same condition in the culture medium showed 


TABLE XXV 


SHOWING THE SURVIVAL TIME, IN Hours, oF Procerodes ISOLATED INTO 1 Mt. 
Eacu oF Paramecium Hay INFUSION AND OF DILUTE SEA-WATER 
OF THE SAME OR GREATER RESISTIVITY 


SURVIVAL TIME IN Hours 


RESISTANCE 
MEDIUM DVM IN i) RANGESIN 
Mean Maximum Minimum Oums 

Controlbli. 4c I- 9 18.05 360.0 Be 
(Cultureseeerer ce II-I9 21.85 38.5 6.0 
Controlblitene ee 21-29 G75 34.0 2.5 
Controleleeeea te 31-40 12.70 34.0 Boks, 
Cultures ss aec sas. 41-50 27.95 58.5 Tr 
Controlpligee ase: 51-60 22.50 58.0 II.o 
Control lee 61-70 17.00 40.0 Si. 
Culltureseeeeee eee 71-80 28.30 40.0 8.5 
Controlligeseeeece 81-90 19.20 64.0 OBS 
Controlleaeee ee XI-I0 16.85 34.0 7.0 
@ulturesee ae. X11-20 25.93 49.3 TO 
(Comuroll Wo ocsao0c- X21—31 18.50 41.0 8.5 
Controlbleee nese: X3I-40 I1I.40 17.0 4.0 
(Gulltureseeer. ee X41-50 25.20 58.5 8.5 
Controle =.--- X 51-60 20.50 41.0 8.1 

or { Control I... 49 15.20 40.0 Bok 7400-3350 

Bee Cultures sa.- 49 25.85 58.5 6.0 7400-3550 

Sei Reaneort. 49 19.30 64.0 25 6150-3550 


4,950 ohms. Again the surviving worms were changed to water 
having a resistance of 7,200 ohms for each solution. Ten hours 
later both showed a resistance of 4,475 ohms. 

The approximate survival time was obtained for 49 worms ex- 
posed singly to 1 ml. each of the two solutions whose history has 
just been given. It will be noted that in no case was the resistivity 
less in the Paramecium culture medium than in the accompanying 
control solution. The survival times are summarized in Table 
XXV, in which Control I is used to mean the modified tap-water 


230 ANIMAL AGGREGATIONS 


with the same resistivity as the culture medium, while Control II 
designates the less-dilute salt solution. 

The summary shows that the worms in the culture medium lived, 
on the average, 25.85 hours, while those in the very dilute sea-water 
with the same initial resistivity lived only 15.2 hours. The differ- 
ence of 10.65 hours, when examined by Student’s method, shows a 
statistical probability of 0.0714, when considered as 5 pairs of tests 
as listed above, but when considered as 49 individual pairs, the 
probability becomes 0.00002, which is clearly significant. 

As convincing as is this experience that Paramecium culture me- 
dium with equal or less total electrolytes than accompanying dilute 
sea-water has a survival value for marine Procerodes isolated into it, 
the same series of experiments furnished still further evidence that 
such is indeed the case. In the series labeled “Control II” the initial 
resistance was 1,000 ohms less, and therefore a less-dilute solution 
of sea-water than Control I. This water was renewed each time the 
others were changed, and was kept at least 1,000 ohms more con- 
centrated at these times. It was never found to be less concen- 
trated than was the culture medium, and only once to have the same 
concentration. The worms in this less-dilute sea-water lived longer 
than did those in Control I by an average time of 4.1 hours, a dif- 
ference which is not statistically significant. 

The worms in the culture medium showed a mean survival time of 
6.55 hours greater than did those in the more concentrated Control 
II. When individual pairs are considered, this has a probability of 
0.02, and hence is statistically significant. Further experiments 
with Paramecium culture medium and with water extracts and 
culture medium of Planaria maculata, and water extracts of marine 
amphipods found in close association with Procerodes in nature, give 
essentially the same results and show that heterotypically condi- 
tioned fresh water has protective value for Procerodes isolated into 
it as compared with the survival when isolated into hypotonic sea- 
water with the same electrical resistivity. 

While this general relation holds, there was not necessarily the 
same degree of protection from each type of solution. Whether this 
is due to the conditions under which the experiments were run, or 


RESISTANCE TO HYPOTONIC SEA-WATER 231 


whether it is an inherent property of the different heterotypically 
conditioned solutions, is not yet apparent. At any rate, the findings 
to date are summarized in Figure 18, which gives the experience 
from comparable experiments run simultaneously. The vertical axis 
shows the percentage of worms surviving at any given time; the 
horizontal axis gives the time elapsing since the beginning of the 
experiment. One finds that there is a marked drop in the percentage 
of survivals within the first 48 hours and that thereafter the curves 
tend to flatten, reaching extinction in from 16 to 20 days. 

The mean survival for the whole group is not plotted on this 
chart, but practically coincides with the graph for Procerodes-condi- 
tioned water, except that it continues just above the base line until 
after the 15-day mark. The graph for hypotonic sea-water having 
the same initial resistivity runs below the lowest graph on the chart 
at all points, except that at the 2- and 3-day periods it is very slightly 
above this lowest graph. 

These results show clearly the lack of species specificity in the 
protection of Procerodes against the lethal effect of hypotonic sea- 
water; and remind one, in this respect at least, of the results ob- 
tained by Allee and Schuett (1927) that protection from such toxic 
substances as colloidal silver also lacks species specificity. The pro- 
tection against colloidal silver appears in a large part to lack func- 
tion specificity. Whether or not the present protective mechanism 
also has other and general functions is not clear at the present time, 
although on general grounds one would be inclined to think that 
such would be the case. 


POSSIBLE FACTORS CONTRIBUTING TOWARD THE OBSERVED 
SURVIVAL VALUE OF CONDITIONED SOLUTIONS 


The facts recorded are plain. The lethal effect of the fresh water 
is clearly less for solutions that have been conditioned by the pres- 
ence of living organisms, when compared with hypotonic sea-water 
having equal initial resistivity. The exact source of this condition- 
ing is not yet clear. In some of the Procerodes-conditioned media 
the survival value is due to exudates from the living worms; in 
others, where water extracts were prepared, the survival value may 


ANIMAL AGGREGATIONS 


232 


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RESISTANCE TO HYPOTONIC SEA-WATER 233 


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have been in part due to the action of bacteria, although this is 
practically ruled out in those experiments in which the solution was 
given a fair pasteurizing treatment in the formation of the extract, 
which was later boiled vigorously. The bacterial growth in such 
solutions must have been slight, at least in the early history of the 
isolations. 

With the Paramecium culture medium the réle played by the 
different elements entering into the conditioning of this medium 
could be told only by experimentation. The Paramecia were grow- 
ing in a hay infusion which contained the water extracts of the hay 
as well as the products of bacteria and of Protozoa other than the 
Paramecia. All these must be considered as possible conditioning 
factors, since the work above has clearly shown that the protection 
is furnished by heterotypic as well as by homotypic conditioning. 

It has been possible to begin experimental analysis of the factors 
leading to increased survival in these biologically conditioned solu- 
tions. To date these indicate that the results are not primarily due 
to the depressing action of the conditioned medium, in so far as 
these are mimicked by dilute alcohol. They are not due to differ- 
ences in pH. Gelatine suspensions do not confer similar benefits. 
The colloids present do not mask the amount of electrolytes in 
solution. 

Preliminary tests have been run to determine whether the protec- 
tive substance would be adsorbed on animal charcoal. These gave 
results of sufficient interest to deserve being presented in outline. 
Animal charcoal was added to the dialyzed culture medium. The 
whole was stirred, boiled, filtered twice with suction and once with- 
out, and, after redialysis to 6,050 ohms, a standard survival experi- 
ment was set up with hypotonic sea-water controls at 6,050 ohms 
and 5,050 ohms. The resistance decreased in all solutions, as usual, 
during the progress of the experiment. The solutions were changed 
twice in the first 20 hours. As usual after standing, the culture water 
had less electrolytes than did either of the accompanying solutions. 

Under these conditions the 50 Procerodes isolated into the treated 
culture medium lived a mean time of 11.03 hours, while those in 
hypotonic sea-water with the same initial resistivity lived 9.21 hours. 


234 ANIMAL AGGREGATIONS 


The difference, 2.72 hours, has a statistical probability of 0.0564, 
which is too great for much significance to be attached to the ob- 
served difference. These values are to be compared with the results 
shown in Table XXV, which were obtained immediately preceding 
the present experiment. That test showed a difference in survival 
of 10.65 hours, with a probability of 0.00002. 

When comparison is made between the survival in the charcoal- 
treated medium and the less-dilute sea-water which had an initial 
resistivity of 5,050 ohms, the difference in survival time is 2.53 hours, 
with a statistical probability of 0.072. These values are to be com- 
pared with a survival difference of 6.55 hours and a probability of 
0.02 in the preceding experiment with the same type of culture 
water similarly dialyzed but not treated with charcoal. 

These results indicate that the charcoal did remove a part of the 
actively protective substance, and suggests the need of further work 
at this point. Perhaps more extended adsorption would have a still 
greater effect. Another possibility must be mentioned, although 
here, too, the evidence available at the present time does not warrant 
the drawing of definite conclusions. I refer to the possibility of the 
differential leaching of materials in raw and in biologically condi- 
tioned water. Early in the analysis of the results here summarized, 
indications appeared that worms isolated into water conditioned by 
living organisms tended to lose electrolytes less rapidly than did 
their controls isolated into hypotonic sea-water with the same initial 
resistance. Accordingly, the resistance records were examined for 
comparable experiments. The different exposures were for varying 
lengths of time, and hence show a marked variation in the change 
in resistance from that given at the beginning of the experiment. 
The initial resistance of the observed solutions stood at different 
levels in the various experiments, but always at the same level in 
both the experiment and the control for any given experiment. The 
mean difference in the whole series of experiments shows that there 
was 4.8 per cent less change in the resistivity of the biologically 
conditioned media than in their accompanying hypotonic sea-water 
controls. Despite the many known elements of variation, this has 
‘a statistical probability of 0.00068. 


RESISTANCE TO HYPOTONIC SEA-WATER 235 


One must be cautious in interpreting this lessened change in elec- 
trical resistivity in the animal-conditioned media as being the 
fundamental cause of the longer survival of the worms isolated into 
such media, because in general we have seen that much greater initial 
differences in resistivity have relatively little effect upon survival, 

and also because gelatine suspensions showed a smaller resistivity 
change than did the controls, and yet had no protective action. 

There is a possibility that if the worms lose contained electrolytes 
less rapidly into the surrounding medium in the presence of materials 
from other organisms, organic materials necessary for the continued 
well-being of the worms, which are not measured by present meth- 
ods, may also escape less rapidly, as Robertson assumes to happen in 
the case of allelocatalysis in Protozoa; and that the observed protec- 
tion may be due to the retention within the organism of a certain 
substance or substances necessary for continued existence, rather 
than to the presence of a definite protective material in the treated 
solutions. 

We are here dealing with some chemical difference in the medium 
unanalyzed as yet, rather than with some sort of physical protection 
of which different types have been described (Allee, 1920, 1926, 
1927; Drzewina and Bohn, 1928). The material behaves in some 
aspects like that which Banta and Brown (1929) find affecting the 
percentage of males in crowded cladoceran cultures, which they 
attribute to the accumulation of excretory products; in others, like 
that which Petersen reports as conditioning Paramecium cultures so 
as to allow higher division rates, especially since the latter has been 
shown not to be species-specific. We are probably dealing here with 
the sort of protection which Drzewina and Bohn originally postu- 
lated in their communication of 1920, quoted at the beginning of 
this chapter. 


CHAPTER XIV 


RELATION BETWEEN DENSITY OF POPULATION 
AND INSECT SURVIVAL 


We have already seen that the insect Drosophila reproduces less 
rapidly the greater the population density (chap. vii), and that, 
on the other hand, the confused flour beetle (Tribolium) during 
initial stages in cultures of flour reproduces more rapidly if the pop- 
ulation for given environments be larger than the minimum (chap. 
x). In later chapters we shall find that crowding exerts certain 
physiological effects upon aggregated insects which are expressed in 
morphological changes. These facts concerning mass relations be- 
tween non-social insects are the more interesting in view of the high 
state of social organization to be found at times in this group. 
Unless we can find a strong substratum of generalized co-operative 
survival values among the subsocial insects, we shall have difficulty 
in demonstrating a connection between the phenomena associated 
with relatively slightly integrated animal aggregations and those 
connected with the more closely knit social communities. 

For the present we desire to consider the relations between num- 
bers of non-social insects and their survival in the face of unfavorable 
conditions. Three lines of evidence exist upon this point: (1) the 
data from laboratory and field experiments concerning the relation 
between numbers of insects present and their survival in the pres- 
ence of toxic reagents; (2) studies on the relation between population 
density and longevity in Drosophila; and (3) the more usual type of 
natural-history observations showing that the survival value of 
crowding in the presence of predators holds for insects as well as for 
the larger animals, for whom it has been more frequently reported. 


MASS PROTECTION FOR GRASSHOPPERS 


Two workers have independently reported mass-survival values: 
for insects exposed to poisonous substances. Deere, for the majority 
236 


DENSITY OF POPULATION AND INSECT SURVIVAL 237 


of his experiments in this laboratory, used different species of grass- 
hoppers, working upon the meadow grasshopper, Xiphidium fascia- 
tus, more than on any other single species. Experiments were run 
upon the effects of ether, carbon tetrachloride, ethylene chloride, 
ethyl alcohol, and hydrocyanic acid. Two types of experiments 
were carried out. In one set the insect containers had a direct 
volumetric relation to the number of individuals per bottle, so that 
if 5 insects were used, the container had five times the volume of 
that containing but 1 insect; and if to were used, the container had 
ten times the volume of that employed with only 1 animal. In 
such experiments the volume percentage of the toxic gas was kept 
constant. 

In the other type of experiment, all tests were run in the same 
size bottle regardless of the numbers exposed. In these, too, the 
volume percentage of the toxic agents was kept constant. The 
amount of gas used was calculated to produce narcosis in most cases, 
without death necessarily following. 

In all experiments the group survived longer; but, as might be 
expected, there is relatively greater group resistance when the vol- 
umes are identical than when they are proportional to the number of 
insects. Similarly, single individuals have been found to have a 
somewhat longer survival when placed in a smaller bottle rather 
than a large one, when each has the same volume percentage of toxic 
gas. The results of one of the constant volume experiments are given 
in Table XXVI. 

The effect of exposure of these animals may be summarized as 
follows: In the group of 30, 82 per cent survived for 24 hours; 
in the group of 20, 90 per cent survived; in the group of 10, go per 
cent survived; while of the ro singles only 60 per cent survived 
for the 24 hours of the experiment. In control lots under similar 
conditions, but without the ethylene chloride gas, 97 per cent of 
the group of 30 survived, and all of each of the other lots lived for 
the period of the test. Apparently the group of 30 suffered some- 
what from the ill effects of crowding, but not enough fully to mask 
the survival value of the group when exposed to this amount of 
narcotic gas. 


238 ANIMAL AGGREGATIONS 


The Xiphidium have cannibalistic tendencies, which caused sev- 
eral experiments to be discarded. When Melanoplus were used, they 
showed greater initial activity but otherwise gave similar results. 

Ethyl alcohol differed from the other vapors used in that the 
effect on the animals was much more gradual, extended over a 
longer period of time, and stimulated them to greater activity. 


TABLE XXVI 


SHOWING THE RELATIVE SURVIVAL OF GROUPS AND ISOLATED INDIVIDUALS OF THE 
GRASSHOPPER Xiphidium, EXPOSED TO EQUAL VOLUMES OF 27.5 VOLUME PER CENT 
OF ETHYLENE GAS, AUGUST 17, 1928; BAROMETER 746, TEMPERATURE 24° C. 


All were placed in 240 cc. bottles supplied with 66 cc. of the gas. 


CONDITION AFTER 12 Hours 


MINUTES UNTIL Minvtes* 
NUMBER OF ANIMALS First Errecr | untit Quiet 

poe | pester asec ey 

BOs ove ap rep Mecaccetta Be 25 (few) 50 (3) 24 3 2 

DO ad re pede Cle 35 (several) 45 (few) 17] I 2 

TOPPA ote eS. 25 (few) 45 (few) GS flake oe I 
Tee er heen ns comers 25 37 be Pee tl Baio ner ds 

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TAREE. Gitta OS Pre ah a 27 eee wil oe enact Tao, \| aeecte cereams ene ee oem 
TAR AO FRAG Pr, ttm ney: 29 40 Ty, ie PBs BAUS ee ee eee 

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Dae fe cee laten ee tee eee ore 7 SOR | |aecgens nee 1a a | Miwon 
TR pa Retest en Ns ores each T: Si og Sage Wik eee tence te cars| lene cep | EP Slt, Sverre: 

Tey a eee 24 AN eel oye. Acces eee IA SN od I 
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* Removed at this time to ordinary air. 


The period of recovery was also longer with this reagent. The dif- 
ference in recovery between the group and the single individuals 
was also greater with alcohol than in the case of the other vapors 
used. 

The outstanding effect noted in the case of insects exposed to 
HCN was the lack of stimulation when the insects were first ex- 
posed. Xiphidium in particular remained motionless from the time 
the gas reached them. Muscular activity could be induced, if at all, 
only by most vigorous shakings. This would loosen their attach- 
ment from the bottle or from whatever object to which they might 
be clinging. Those that were fully narcotized would fall to the 


DENSITY OF POPULATION AND INSECT SURVIVAL 239 


bottom without movement; others might show slight movements 
of the feet, particularly if these came into contact with a solid ob- 
ject, even when there was no other visible movement. The recovery 
period from HCN was also shorter than from exposure to the other 
gases used; but otherwise the results were similar, particularly in 
that the group showed greater survival value than isolated indi- 
viduals. 

Such results from different numbers of insects exposed to the 
same volume of the same concentration of toxic gases are according 
to expectation from the experience previously recorded with aquatic 
organisms exposed to various toxic conditions. The apparent ex- 
planation is that, when the toxic gas is distributed among a number 
_of organisms, no one individual is as likely to receive a dose sufficient 
to cause death as if it were exposed singly to the same quantity of 
the same gas. The explanation of the findings when groups and 
singles are exposed to volumes in proportion to the numbers present, 
indicating that there is group protection in proportion to the num- 
bers present even in the face of similar conditions for each indi- 
vidual, is not clear, and further work is needed at this point. 

It seems probable that under such conditions of crowding, even 
with equal space and an equal amount of the toxic gas present per 
individual, the group may cause a greater concentration of carbon 
dioxide in its neighborhood, or by some other effect may very well 
cause these animals to have a lowered rate of metabolism. Under 
these conditions, and with the concentration of poisonous substance 
at the right strength, one would expect the members of the depressed 
group to survive longer than would the more active single individ- 
uals. This suggestion is in line with the susceptibility work of Child 
(1915) and the resistance of Daphnia to various chemical reagents, 
as developed by Fowler and discussed in a previous chapter. 

Experimental results similar to those of Deere have been obtained 
by Bliss (personal communication) under natural conditions. In his 
field studies Bliss found that the camphor scale insect (Pseudoanidia) 
has a greater natural death-rate the greater the density of the popu- 
lation on a given twig. When exposed to adverse conditions, such as 
are represented by spraying with an oil spray, the opposite condi- 


240 ANIMAL AGGREGATIONS 


tions prevail, and there is decidedly greater survival the more dense 
the population. In making these observations, due care was taken to 
consider only those insects located on trees in comparable situations 
and on comparable parts of the trees. Bliss concludes that death 
from these sprays is not due to suffocation, as has been supposed, but 
is due to the taking-up of some poisonous material by the insects; 
and that when they are present in greater numbers, the toxic mate- 
rial is divided between more insects, no one of which is so likely to 
receive a lethal dose as if the population were less dense. 


POPULATION DENSITY AND LONGEVITY IN DROSOPHILA 


The relation between the density of population of laboratory cul- 
tures of wild-type Drosophila and their life-duration were less ex-. 
pected. Pearl and Parker showed in 1922 that statistical analyses 
of data accumulated in other studies indicated greatest longevity 
from bottles originally stocked with from 35 to 45 of the wild 
stock per bottle. In 1923 the same workers reported the results of 
an experiment made to test out this question of an optimal popula- 
tion. The data on the length of life of 12,382 individuals showed 
that the optimal density of population, when longevity is taken as 
the criterion, is not found in the minimal populations but lies in the 
region of initial densities of from 35 to 55 per 1-ounce vial, and the 
increase in length of life from the lowest density is at a much more 
rapid rate than is the decline of duration of life after the optimal 
density is passed. 

A more complete analysis of the problem is given by Pearl, Miner, 
and Parker (1927). In this experimental work the flies were kept 
in t-ounce vials stoppered with cotton plugs and held at 25°C. 
The bottles were examined daily, the dead flies were removed and 
their age recorded, and the living flies were at the same time trans- 
ferred to fresh bottles of newly prepared food. In the first experi- 
ments banana agar was used as food, but similar results were ob- 
tained with a synthetic medium. 

The extent of the experimental data may be visualized from the 
following statement of the numbers used in one experiment. One 
hundred and fifty vials were started with an initial population of 


DENSITY OF POPULATION AND INSECT SURVIVAL 241 


t pair each; similarly 80 vials contained originally 2 such pairs; 
50 vials contained 3; and 4o vials contained 4 pairs each. Thirty 
vials were started with 5, and 30 more with 6 pairs each, and 20 
vials contained an initial population of 15 flies, or 7.5 pairs. Ten 
vials were started with each of the following populations: 20, 25, 
35, 45, 55, 95, 75, 85, 95, 105, 125, 150, and 200. In another experi- 
ment the initial densities per t-ounce vial were: 5, 25, 50, 75, 100, 
200, 300, 400, and 500. 

The results from the first experiment are given graphically in 
Fig. 19, which shows the mean duration of life of wild-type Drosophi- 


a3 
ea 


10 20 30 #0 50 60 70 80 90 100 110 120 130 10 150 {60 170 180 190 200 2/0 
Initial Density 


Fic. 19.—Showing the mean duration of life in Drosophila with relation to the initial 
density in 1-ounce bottles. (From Pearl, Miner, and Parker.) 


la at different densities of population. The second experiment yield- 
ed similar results and demonstrated that with the higher densities 
there is relatively slight effect upon longevity of marked increases in 
original density. 

Pearl, Miner, and Parker sum up their experience with this sort 
of experiment as follows (1927): 

“The rate of mortality of Drosophila is profoundly influenced by 
the density of the population, that is, by the number of flies to- 
gether occupying a limited universe in which volume of air, volume 
of food, and area of food surface are constant. 

“There is an optimal density of population for Drosophila under 
the conditions of these experiments. This optimal density falls 


242 ANIMAL AGGREGATIONS 


somewhere in the region of 35—55 flies per one ounce bottle contain- 
ing 8 cc. of food substrate. At densities of population above and 
below the optimum, the specific death rates are higher at all ages 
than they are at the optimum.” 

Such results as these make one fairly bristle with questions, of 
which only a few can as yet be answered. Pearl and his associates 
have reported on investigations concerning the effect of changes in 
density during the progress of the experiments. Since an initial 
population of about 35 flies per vial was found to lie near the opti- 
mum under the conditions used, a number of vials were set up 
with this density and apparently were followed according to the . 
practice of the preceding experiments until the sixteenth day, at 
which time the survivors were etherized and the tips of the wings 
of half of them were clipped, after which they were returned to 
their proper bottles. Appropriate tests showed that the wing-clip- 
ping as practiced did not significantly affect life-duration. 

Vials with an initial population of 200 were treated in like manner 
up to the sixteenth day, when they were etherized and counted. 
In part of such bottles the population was then brought back to its 
initial density of 200 by adding marked flies which up to that time 
had lived under the optimal conditions furnished by an initial popu- 
lation pressure of 35 flies per ounce vial. Others were brought to 
the original population density of 200 per ounce bottle by adding 
flies surviving from stocks started from that density. The results 
of such treatment are shown graphically in Figure 20, which gives 
survival curves starting with too flies at 16 days of age, and the 
effect of previous history of crowding upon the duration of life 
after that age. 

In commenting upon these results, the experimenters state that 
here, as before, flies subject to an initial density of 35 per ounce 
bottle survive, on the average, about double the time those live 
that are subjected to an initial density of 200 for the same size vial 
and with the same food supply. They continue: 

“When flies which have lived the first 15 days of their lives under 
the conditions implied by an initial density of 35 are on the 16th 
day of their age submitted to a density of 200, and live out the 


DENSITY OF POPULATION AND INSECT SURVIVAL 243 


remainder of their lives under the conditions implied thereby, their 
average duration of life is reduced in these experiments from the 34 
or 35 days which it would have been had they stayed in the bottles 
with an initial density 35, to 22.83+0.19. This result shows that 


ees ae ‘, Density 35 Throughout 
200 % Wings Not Clipped 
deere 
, 

Spleen 1 
© 40 5 Roe a a a 
{ a ea 
= > 
: a Sees Sie p Density 35 Throughout 


Wings Clipped 


aby 40 shog 9/ 40 00% 


te) 
EH ei 


19 #5 31 37 43 49 55 61 67 73 79 
Age in Days 


Fic. 20.—Showing survival curves starting with 1,000 individuals of Drosophila 
at 16 days of age which had been subjected to different densities of population up to 
that time. (From Pearl, Miner, and Parker, 1927.) 


crowding produces a heavy increase in mortality even though it 
occurs as late as 16 days of age. 

“Flies which have lived the first 15 days of their lives under the 
conditions implied by an initial density of 200, and then at the age 
of 16 days are again subjected to a density of 200, have a significant- 


244 ANIMAL AGGREGATIONS 


ly shorter duration of life than do their companions in the same 
bottles who spent the first 15 days of their lives in bottles of initial 
density 35. The difference is 22.83-+0.19 minus 19.71+0.17, which 
is 3.12+0.25 days. This difference is more than 12 times the prob- 
able error. It may be taken as probable to the point of practical 
certainty that excessive crowding in early life deleteriously affects 
the survivors of 16 days of age so that they are significantly less 
resistant to the effects of heavy crowding again at that time than 
are flies which lived at optimal densities in early life.” 

These Drosophila data indicate that the harmful effect of supra- 
optimal densities of population is most marked near the beginning 
of adult life. This original sensitivity decreases with age, so that 
older flies are less affected either by the original or the average den- 
sity of population. Even so, excessive crowding has been shown to 
increase the death-rate at later ages of these flies and to produce this 
effect almost immediately upon the increase in density. “Further- 
more,’ as Pearl and his collaborators state, “it appears that the 
amount of shortening of life produced by crowding at any age is 
influenced by the previous history of the flies relative to density of 
the population. This suggests that there is a deleterious effect of 
supra-optimal crowding in early life even upon those flies that do 
not immediately die as a result of it, and that this effect endures for 
at least the first 15 days of life.’ Unfortunately for us, Pearl’s 
experiments have not been concentrated at the part of the survival 
curve which would show whether suboptimal crowding will have the 
same effects that supra-optimal densities have been demonstrated 
to have. 

These experiments on Drosophila, like those of Chapman on the 
confused flour beetle, are distinct from the great majority of crowd- 
ing experiments in that they are carried on in a non-liquid environ- 
ment, whereby the temptation to postulate the production of some 
X-substance responsible for the phenomena observed is much re- 
duced. Pearl suggests that the biological explanation for his ob- 
servations lies in an entirely different direction, but the matter has 
not been discussed publicly, even in general terms, by members of 
his laboratory. We know that in the case of effect of crowding on 


DENSITY OF POPULATION AND INSECT SURVIVAL 245 


egg production in fowls, Pearl and his associates regarded the under- 
lying stimulus to lie in the psychological field, and it is possible to 
regard the effects recorded here as expressions of some sort of stimu- 
lus physiology, such as Goetsch has recorded in his experiments 
upon the effect of crowding in retarding growth of rapidly swimming 
animals such as young tadpoles. 

There are, however, other possibilities which must be considered. 
In vials stoppered with cotton there may be an increased CO, ten- 
sion developed which may in part account for the observed effects. 
Then, too, the reduction in length of life at suboptimal densities 
may be an expression of the inability of the small populations pres- 
ent to gain control of “wild” organisms other than the food yeasts 
present in the cultures, while the supra-optimal density effects may 
be related at least in part to food shortage and to excess of excretion 
products. Obviously, the resolution of this situation into causal 
factors is not easy; but it is equally obvious that we have here one 
of the most suggestive of the phenomena yet presented, indicating 
the wide application and fundamental importance of the physiologi- 
cal effects of animal aggregations upon the aggregants. 


MASS PROTECTION IN NATURE 


A different aspect of the effects of numbers on insect survival is 
illustrated by the observations of Haviland (1926) upon the protec- 
tive value of feeding aggregations of certain chrysomelid beetle 
larvae (Coelomera cayennensis). Haviland says: . 

“The larvae are thickly hairy and the last segment of the body is 
expanded into a strongly chitinized, shovel-shaped flange. The lar- 
vae feed in a compact mass on the upper surface of Cecropia leaves. 
Their heads are all directed inwards while the caudal expansions 
thresh to and fro on the outside of the circle. The chief enemy of 
these larvae is a carnivorous Pentatomid bug (Phyllochirus) which 
loiters on the outskirts of the throng awaiting the opportunity to 
impale a larva with his proboscis and drag it from its fellows. As 
long as the circle of shovels is unbroken, the bug stands little chance, 
for his stylets cannot penetrate their polished armor and he cannot 
reach the soft bodies beyond. But as the larvae feed they move out- 


246 ANIMAL AGGREGATIONS 


wards from the original center, the circle becomes wider, and sooner 
or later the enemy slips in between the defenses and secures a 
victim. According to my observations, however, the circle is not 
broken naturally until the larvae are full grown, in which case the 
loss of one or two individuals does not signify, for before the bug 
has digested his meal, the rest of the brood enter the pupal stage 
and so escape.”’ 

These observations bring us back again to the familiar biological 
level of the struggle for existence between predators and their prey. 
They are given here because it frequently appears that such masses 
of feeding larvae are more at the mercy of their enemies than if 
they fed alone, unless they have developed means of detecting the 
approach of enemies and transmitting the information through the 
mass or unless they have developed effective means of active group 
defense like that of the soldier caste of termites and ants. 


CHAPTER XV 
COMMUNAL ACTIVITY OF BACTERIA 


Many of the relationships which we have found to hold for animals 
can also be studied with bacteria. The large amount of work done 
upon these micro-organisms and the excellent technique developed 
both in culturing and in making isolations cause the results obtained 
to be the more significant for our studies. In one way bacteria as 
presented here and spermatozoa as discussed in the following chap- 
ter may serve as test cases for us. The situation may be stated thus: 
We have been presenting certain evidence concerning the réle of 
numbers of individuals present in relation to the physiology of each 
individual. The studies so far have dealt with a wide range of ani- 
mals. If we turn to some totally different organisms, such as the 
bacteria, shall we find similar relationships prevailing there? If we 
do, and if we also find the same conditions holding for the sperma- 
tozoa, we shall have the greater reason for regarding the phenomena 
we are studying as being of universal biological significance.! 

Buchanan (1918) has summarized the life-cycle of a bacterial 
culture into seven periods. These are illustrated in Figure 21, where 
each phase extends to the left of its respective numeral. This analy- 
sis of conditions in a bacterial culture is essentially like that known 
to obtain for many protozoan cultures (cf. Fig. 9) except that the 
analysis with bacteria has been more carefully made. The different 
phases will be discussed briefly here. 

t. The initial stationary phase covers the time during which the 
number of introduced organisms shows no increase. This phase has 
been studied but little. If the bacteria are actively growing at the 
time of transfer, growth may continue on the new medium, indicat- 


* Two summarizing books have appeared dealing with the material presented in the 
early part of this chapter, by Buchanan and Fulmer (1928) and by Henrici (1928). 
These assist a layman in this field to present a more seasoned summary than would 
otherwise be possible. 


247 


248 ANIMAL AGGREGATIONS 


ing that the phase of the parent culture from which the inoculum 
was taken affects the duration of this initial stationary phase. This 
is the equivalent of the “lag phase” of Robertson (1924). 

2. The lag phase is also called the positive growth acceleration 
phase, and is the period during which the rate of increase shows 
acceleration. This period, together with the preceding one, makes 
up the lag phase as so considered by many investigators working 
with bacteria and with Protozoa. Buchanan and Fulmer use it as 
meaning “the period elapsing between the beginning of multipli- 


Ss 


Oo 
un 
) 


15 20 25 30 35 
Units oF TIME 


LOGARITHMS OF NuMBERS OF BACTERIA 
Co _ La) a + uo oO ~ ao © 


Fic. 21.—Showing the life-history of a bacterial culture. Figures on the vertical 
axis give logarithms of numbers of bacteria; those on the horizontal axis give units of 
time; those on the curve itself indicate the close of the respective growth phases, 
(Redrawn, with slight modifications, from Buchanan and Fulmer 1928 by permission of 
Williams & Wilkins.) 
cation and the beginning of the rate of maximum increase per or- 
ganism.”” | 

The phenomena associated with the lag phase interest us particu- 
larly because of the inherent implication that an increase in numbers 
or the continued occupancy of a new medium affect growth condi- 
tions. A number of theories have been advanced in explanation, of 
which the following may have significance in some cases (Buchanan 
and Fulmer, 1928). 

a) The essential secretion theory is an application to the present 
phenomenon of a variant of Semper’s suggestion that some X- 
substance must be necessary for growth. Here it takes the form 
that the transplanted organisms must give off some essential mate- 


COMMUNAL ACTIVITY OF BACTERIA 249 


rial into the new medium before maximal growth can occur. The 
fact that with bacteria, as we have already seen with Protozoa 
(chap. X), transfers from cultures during periods of maximal growth 
do not exhibit a lag period shows that such secretion is not always 
essential. Robertson’s work (1924) for Protozoa and that of Valley 
and Rettger (1926) for bacteria indicate that it may at times have 
significance. 

b) The adaptation theory is based on the idea that acclimatization 
to a new medium requires time. The evidence mentioned above 
shows that this is not always true; further, transfers from old cul- 
tures may show lag even though they are transferred to the same 
medium. Fulmer (1921) presents evidence that adaptation may be 
operative in the case of the transfer of yeast to a medium containing 
ammonium fluoride. 

c) The non-viability theory which supposes that some of the or- 
ganisms die off early may still hold for Protozoa, although it does not 
hold universally for bacteria, since culturing on agar plates reveals 
the lag phenomenon and non-viable bacteria would never be counted 
in this method. 

d) The agglutination theory suggests that bacteria agglutinate on 
the plates, and the resulting count is of clusters of bacteria rather 
than of individuals. This may be true under certain conditions, but 
lag can be shown when agglutination has been avoided. 

e) The theory of recovery from injury holds that exposure to the 
accumulated products of metabolism in the parent culture affects 
the different cells so that time is required to recover when placed in 
a new environment. Buchanan and Fulmer regard this as perhaps 
an “approximation of the truth, though probably not entirely ade- 
quate.” If we cannot admit that the production of such toxic sub- 
stances is universal, we may still think of the injury as being due to 
starvation in the deteriorated parent culture. Somewhat the same 
idea is expressed in the inertia theory, which holds that the trans- 
ferred organism continues to grow for a time in the new medium as 
it would have grown in that from which it was taken. Buchanan’s 
so-called germination theory of lag appears to be another statement 
of about the same concept. 


250 ANIMAL AGGREGATIONS 


f) Penfold (1914) and later Robertson (1924) regarded favorably 
the theory of elaboration of essential chemical substances. This theory 
assumes that some substance, c, may be required for growth which 
is not produced from a already in the solution, but from 6 which 
must be produced by the organism from a. Further, c must be 
present within the organism in optimal quantities before optimum 
growth occurs; and in a medium lacking the proper amount of c 
this substance diffuses out of the bodies of the non-growing cells and 
must be resynthesized before growth can take place. 

The causes of the lag phenomenon may reside both in the medium 
and in the organism. They may differ in different conditions, and 
all the foregoing theories may operate in special cases. 

In the light of the situation concerning Robertson’s phenomenon 
among protozoa reviewed in chapter x, we are interested in the ef- 
fects of number in the inoculum upon the duration of the lag period. 
Robertson (1922) defined “lag” in terms of the initial stationary 
phase and found no effect of numbers in the inoculum upon this 
phase preceding the first division. His “allelocatalytic effect,” in 
which the rate of reproduction was much increased by the presence 
of a second organism in a limited amount of culture medium, be- 
longs in the lag phase as used here, and the entire discussion of chap- 
ter x is pertinent at this point. 

3. The logarithmic growth phase (see Fig. 21 again) is the period 
at which the rate of increase remains constant and at its highest 
value. During this phase the logarithms of numbers of bacteria 
plotted against time give a straight ascending line. 

4. Negative growth acceleration phase covers the time when the 
rate of increase is falling, although the bacteria continue to increase 
in numbers. This phase sets in soon in bacterial cultures but comes 
more slowly in those of Protozoa. The causes contributing to its 
onset and development are those which later lead to the decline in 
numbers. These have already been discussed at some length in 
chapter vi. Briefly, those considered most important are the in- 
crease in concentration of harmful products of metabolism and the 
decrease in available food supply, which lead to cells entering the 
so-called “resting stage’ or cause death. 


COMMUNAL ACTIVITY OF BACTERIA 251 


5. The stationary phase is, as the name indicates, a time of practi- 
cally no change in the numbers of the bacterial population, and, as 
is not indicated by the name, a period when the numbers are at a 
maximum. During this phase the multiplication and the death of 
cells are practically in equilibrium. | 

6. The phase of accelerated death covers the period when decrease 
in numbers begins slowly, and continues with increasing rapidity and 
leads into (7), the so-called logarithmic death phase, which covers 
the mid-senescent period of the cultures at a time when the rate of 
death sometimes remains constant. Henrici (1928) suggests the 
addition of (8), a final phase of negative acceleration in death-rate, 


TABLE XXVII 


SHOWING PENFOLD’S DATA CONCERNING GENERATION TIME 
IN MINUTES DURING THE First 2 HOuRS OF 
GROWTH OF BACTERIAL CULTURES 


EXPERIMENT 
DILUTION OF PARENT 
CULTURE 

A B 
IERAKS.O)s GALE Ca cnet O rics ac 44 45 
TA OO Mer tertcte eens pred oaye soeeeioe 48 47 
TEMA OOO! poste estate igachsie:setuers one 52 48 
TIO; OOO ari siraiers seeps studs ee QI 56 


since in many cases the death-rate decreases after a time and some 
cells remain alive after relatively long periods. 

The form of the growth curve of a bacterial culture is influenced 
by such factors, among others, as temperature, composition of the 
medium, and the numbers, age, and previous history of the seeded 
bacteria. Of these, the effect of the size of the inoculum interests us 
particularly. Evidence clearly indicates that, within limits, the 
smaller the inoculum the longer the lag phase (Rahn, 1906; Penfold, 
1914; Montank, vide Henrici, 1928). Penfold’s data on the point is 
given in Table XXVII. Montank used yeast in his experiments. 
His results are summarized in Table XXVIII. 

Inspection of these tables shows clearly that with the smaller 
seedings the combined initial stationary and lag phases are pro- 
longed. With the yeast the prolongation is in proportion to the size 


252 ANIMAL AGGREGATIONS 


of the inoculum. In these yeast cultures of Montank’s, when the 
logarithmic growth stage is reached, growth is more rapid the smaller 
the seeding, except with the smallest amount used, where the rate of 
increase is distinctly less than with the next higher amount. This 
is in line with Robertson’s findings for the effect of the size of the 
seeding upon rate of reproduction in certain protozoans in which 
TABLE XXVIII 
SHOWING MONTANK’s DATA ON THE INFLUENCE OF SIZE OF SEEDING UPON 
THE GROWTH CURVE IN YEAST 
(Figures Are Given in Terms of Millions of Cells per 
Cubic Centimeter of Medium) 


Fiask No. 
Hours oF INCUBATION 
I 2 3 4 5 

Ore ckeh epee otate chee 20.9 20 0.2 0.02 0.002 
Dies creme Renee hese eae 23.8 2D One 0.02 0.002 
7A RE, CORR RRC CRNA BO Bait One 0.025 0.003 
ORR ree iaetees ereie 58.9 8.3 0.3 0.02 0.003 
Siem eee ioe tetas 105.5 19.6 TO 0.02 0.004 
Ti Olts e tre oye cesta roan 129.0 21.0 Bois) 0.024 0.004 
ee er en ae ON 161.6 54-5 ETE ts) 0.04 0.005 
BY 0) ectiea choise toe cme TA Qmes5 44.8 0.26 0.005 
Nove oe eth SEASON ty Ope? 118.0 89.3 3.0 0.006 
DAN sciait ts reharmcstiere: 18L.0 124.9 T2877 18.3 0.007 
BOR tac icbess hese keeee 183.6 144.1 138.3 114.6 0.289 
Stns DE hls pees 186.0 156.7 146.7 135.0 1.089 
HDs eas sites faeries t ohee, Rieke 185.1 165.6 162.2 160.0 93.264 
QOS Kye ehetetevendtonsce = ake 195.0 75 TOM TOS Re 169.400 
TAA easter eee aoe 197.9 Tose 180.3 17/5) (03 171.408 


the rate of reproduction at early stages of the subcultures was more 
than doubled by seeding with two rather than with one organism. 
It may be remembered that Peskett (1924, 1925) using isolation 
cultures failed to find increased growth-rate with increased number 
of cells present at any number level tested. 

The experience with bacterial cultures parallels experience with 
protozoan and other animal cultures in another respect, in that the 
final yield is practically independent of the seeding as long as the 
seedings are smaller than the normal maximum yield of the medium. 


MASS PROTECTION FOR BACTERIA 


The evidence for mass protection for bacteria to be presented 
here is based almost entirely upon the studies of Churchman and 


COMMUNAL ACTIVITY OF BACTERIA 253 


Kahn (1921) and of Burnet (1925). It is presented in some detail 
both because the results broaden the base of our knowledge of the 
physiological effects of numbers, extending this to the bacteria, and 
also because these results have been attained independently of simi- 
lar work on different material. Verification studies are much more 
impressive when carried on in a different laboratory from that 
bringing out the original report; and when many different investiga- 
tors working in widely separated laboratories and on radically dif- 
fering materials reach similar conclusions independently and about 
the same time, the fundamental nature of the phenomenon under 
discussion becomes the more striking. 

The bacterial studies in question are concerned with the behavior 
of certain types of bacteria in the presence of gentian violet. Bacil- 
lus coli grows equally well on both sides of a divided agar plate, 
one-half of which contains gentian violet, providing both sides are 
stroked with a heavy suspension of the bacteria. As the strokings 
are made with increasingly dilute suspensions, the colonies become 
less numerous on the gentian-violet side and finally disappear com- 
pletely. In part the results may be due to differential susceptibility 
to the dye, shown by different B. coli individuals, for if a plain agar 
plate is inoculated from broth cultures of this bacterl'um which 
have been exposed to gentian violet, the stained organisms appear 
to grow as well as the controls, but if the experiment is repeated 
with increasingly dilute suspensions of B. coli, it will be seen that 
many of the bacteria have not survived the treatment with the 
stain. 

Such results are what might be expected from the well-known 
relations between growth and size of the inoculum. In an ordinary 
bacterial culture, many organisms are known to be dead. Others, 
though living, are more readily affected by the somewhat unfavor- 
able conditions found in the new medium to which they are trans- 
planted, which makes it much more probable that heavy growth 
will occur if the inoculum consists of thousands or millions of organ- 
isms than if it contains merely hundreds. The fact just observed con- 
cerning the better growth of a large than of a small inoculum exposed 
to gentian violet becomes merely the starting-point of our interest. 

The technique developed by Barber, which allowed the transfer 


254 ANIMAL AGGREGATIONS 


of single cells to new media, gives a proper method for attacking 
this problem. This technique was employed by Churchman and 
Kahn. They took their transplants from a strain of B. coli which 
had been isolated from a single colony growing on gentian-violet 
agar and kept growing by frequent transplants for several weeks on 
media containing that dye. 

Care was taken to avoid as much as possible the lag phenomenon. 
When strokes of a heavy suspension were made on gentian-violet 
agar, growth occurred with almost no inhibition; but transplants 
of single cells almost never grew, although only motile organisms 
were picked for transfer. Single transplants to controls lacking the 
dye gave as high as 85 per cent growth. “In the two cases in the 
whole series of cell transplants in which growth occurred, marked 
delay took place, a delay which was never observed for the controls. 
Moreover transplants of small groups of organisms (five to fifteen) 
did not grow, though transplants of thirty individuals grew regu- 
larly.” 

Churchman in another series of experiments, using Bacillus sub- 
tilis, which is definitely susceptible to gentian violet, tested the ef 
fects of repeated inoculations on the ability of this organism to grow 
in the presence of this dye. The susceptibility of B. subtilis is shown 
by the fact that its growth is prevented by dye dilutions of 1: I ,000,- 
ooo; inhibited by 1:2,000,000; and only becomes vigorous at dilu- 
tions of 1:3,000,000. “If, however, the gentian violet half of the 
plate is repeatedly and heavily inoculated on successive days at the 
same place, a fair growth—in some cases a rather vigorous growth— 
may finally be obtained.”’ Such an experiment is shown in F igure 
22. The figure shows the results of two initial strokes of a thick sus- 
pension of B. subtilis on this divided plate, the lower half of which 
is plain agar while the upper half is gentian-violet agar, at a strength 
of 1:100,000, which is ten times stronger than a dilution known to 
prevent growth. 

After the first stroke, nothing further was done to the right-hand 
side, and the usual effect of the dye is clearly shown by the lack of 
growth on the gentian-violet part of the plate. On the left-hand 
stroke organisms were repeatedly introduced on the gentian-violet 


COMMUNAL ACTIVITY OF BACTERIA 255 


agar, and inspection of the figure shows that a fair growth resulted. 
This growth was not due to an acclimatization to the dye, because 
all experiments made looking directly toward such acclimatization 
have been unsuccessful. Further, attempts to reinoculate gentian- 
violet agar from this growth failed, at least insofar as the first smears 
were concerned. 

In discussing these re- 
sults, Churchman sug- 
gests four possibilities: 
The original transplants, 
though not surviving, 
may live long enough to ~ 
effect some change in the 
dye such that subse- 
quent transplants can 
survive. Or, the result 
may be due to some pro- 
tective barrier laid down 
by the dead bodies of 
the bacteria. Or, nutri- 
tional or growth-promot- 


Fic. 22.—Showing the effect of reinoculations on 
; a divided agar plate. The upper part contains gen- 
Ing substance may be tian violet; the lower part is plain agar. Two initial 
provided by these same _ strokes of Bacillus subtilis were made. On the right 
dead bodies. Or, finally, side, nothing further was done. On the left, reinocu- 


lations were made on the gentian-violet agar. (From 
the results may be due Ghanian) 


to communal activity of 

the living bacteria, a possibility which will be examined later. We 
have already discussed possible causes of survival and growth in 
such cases in preceding chapters. 

In experimenting to find which of these possibilities actually 
obtained, Churchman grew B. subtilis on medium containing only 
distilled water and agar, in order that the cells might contain a 
minimum of nutritive material. These bacteria were washed six 
times with distilled water and were killed. In order to test whether 
food material was present under these conditions, these dead bacte- 
ria were smeared over a Petri dish bottom which was then dried, 


256 ANIMAL AGGREGATIONS 


moistened, and inoculated with B. subtilis. There was no growth 
when this was incubated. 

A control tube of gentian-violet agar was inoculated with this 
organism. No growth occurred. Another similar tube was covered 
with a thin layer of the killed and washed organisms just described. 
After this layer had dried, an inoculation of living B. subtilis was 
made on top of it and growth occurred. Evidently the shield of 
dead, washed organisms, without sufficient nutritive value to sup- 
port growth in themselves, served as some sort of a barrier between 
the poisonous dye and the living cells. Similar results were ob- 
tained if dead bodies of Micrococcus aureus or of Bacillus coli were 
used in place of dead B. subtilis, so that the protection is not species- 
specific. These results are suggested graphically in Figure 23. 

These experiments with agar media indicate that there is a real 
difference between the behavior of a single cell and that of a group 
of cells in the presence of gentian violet. This was tested further 
by experiments upon the survival and growth of B. coli in a gentian- 
violet broth at a dilution of 1:100,000. The strain used came from 
one of the two colonies that appeared from single-cell transplants 
onto gentian-violet agar. Not only did these cells come from a 
strain known from several weeks culturing to be gentian-violet 
tolerant, but they all came from a single organism of this strain 
which had successfully produced a colony when isolated onto 
gentian-violet agar. 

When single organisms of this strain were cultured into plain 
broth, 80 per cent growth was obtained. When 30 or more cells were 
cultured together in gentian-violet broth, almost roo per cent 
growth was obtained. The first 140 single-cell inoculations into 
gentian-violet broth yielded no growth. In a final series of 8 inocu- 
lations, delayed growth was obtained in one tube in the second 24 
hours. The chances are at least 147 in 148 that growth will not 
occur under these conditions. These results are to be compared 
with those showing that there are almost too chances in 100 of 
inocula of 30 such cells growing under similar conditions. 

It may be that 30 cells succeed in growing because these 30 cells 
are able to produce some anti-dye substance in an amount sufficient 
to destroy the harmful effect of the dye when a single cell is unable 


COMMUNAL ACTIVITY OF BACTERIA 257 


to do so. Obviously, from the conditions of the experiment we are 
not dealing here with the usual chances that in a large inoculum 
there are more vigorous cells than in a small one. The probabilities 
just mentioned argue strongly against such a conclusion. 
Churchman and Kahn performed a number of experiments to 


Gentian 


violet 
agar 


Fic. 23.—Showing results of implantation of Bacillus subtilis on gentian-violet agar 
without (left) and with (right) a thin layer of killed washed bacteria (6) below the 
implants. C is redrawn from published halftone of the mass a below it. (From Church- 
man.) 


258 ANIMAL AGGREGATIONS 


determine whether the observed facts were to be explained by the 
relation of the number of organisms to the amount of gentian violet 
to which they were transferred. This is the same sort of relationship 
which Allee and Schuett found effective with various organisms ex- 
posed to colloidal silver, and which Carpenter found in the case of 
fishes exposed to lead nitrate. In the language of the original report: 

“Large inoculations of this gentian-negative strain grow in the 
presence of gentian violet without any apparent restraint; so, too, 
do inoculations of 30 cells; whereas single cells, under identical 
conditions, do not grow at all. This might be due to the fact that 


Fic. 24.—Showing effect of size of transplant on amount of growth of a gentian- 
tolerant strain of Bacillus coli on gentian-violet agar. Numbers in the test tubes give 
the number of cells transplanted. (Redrawn from photograph published by Churchman 
and Kahn.) 


groups of cells, even small groups of 30 individuals, were able to 
make some change in the dye, gentian violet being assumed to offer 
a slightly unfavorable medium even for this gentian-negative strain, 
in spite of the absence of any apparent inhibition to the growth of 
groups of cells. Single cells might be unable to effect this change in 
dye in sufficient amount to allow growth to take place.” 

Acting on the assumption that this reasoning is correct and on 
the knowledge that the single-cell transplants had been made into 
5 cc. of 1:100,000 gentian-violet broth in which 1 cell would not 
grow while 30 cells transplanted together would do so, transplants 
were made of 30 cells into 150 cc. of this broth. Growth occurred 
with a fair degree of constancy. Even when 30 cells were transplant- 
ed to a liter of such broth, growth took place in 60 per cent of the 


COMMUNAL ACTIVITY OF BACTERIA 250 


flasks. If the purely quantitative relations stated above held here, 
it would be necessary to transfer 6,000 cells into this volume of 
gentian-violet broth in order to obtain growth. Here 30 organisms 
perform not merely 30 times the work of one but at least 200 times 
that amount. This discrepancy between the work actually accom- 
plished by the 30 individuals and that which they might be expected 
to accomplish on a quantitative basis is what is meant by the ex- 
pression ‘‘the communal activity of bacteria.”” Such an expression 
implies interreactions not yet proven for bacteria. Here, as in many 
other places in the present summary, we come upon apparently 
well-demonstrated facts for which the physiological explanation is 
as yet lacking. 

On single bacteria, the amount of gentian violet does have some 
effect. Thus B. coli isolated into more than 0.8 cc. of the broth used 
in these experiments did not grow. If isolated into o.1 cc., 60 per 
cent of the isolations yielded growth. The corresponding ratio for 
30 cells would be 3.0, yet the facts are that 60 per cent of successful 
inoculations occurred when 30 individuals were introduced into a 
liter of such broth, which is more than 333 times the expectation 
based on the performance of a single cell. Volume alone was not 
responsible for these results, as some of the early work of Drzewina 
and Bohn might indicate, for when single-cell inoculations of B. 
coli were made into a liter of plain broth, growth occurred in 75 
per cent of the cases. 

It seems clear, as Churchman and Kahn conclude, that 30 cells 
are able to accomplish much more than 30 times as much as a single 
cell, and that this excess of ability of the 30 is an expression of 
communal activity of bacteria, whatever that may be." 

« The results just recorded are mainly concerned with homotypic relations. Castel- 
lani (1926) has described somewhat similar relationships as holding for heterotypic 
colonies of yeasts and bacteria. Bakers’ yeast, for example, is a mixture of one or more 
yeasts and one or more bacteria and produces fermentation over a wider range of carbo- 
hydrates than will the individual organisms used separately in pure cultures. The mix- 
ture Bacillus typhosus plus B. morgani produces gaseous fermentation in maltose, manni- 
tol, and sorbite; B. typhosus alone produces acidity only, never gas; and B. morgani 
alone produces neither gas nor acidity from these substances. The opposite result may 
also be found. For example, Monila tropicalis ferments saccharose, producing gas; but 


when mixed with Bacillus typhosus, it loses this property. Buchanan and Fulmer 
(1930a) summarize many similar relations between bacteria and other organisms. 


260 ANIMAL AGGREGATIONS 


Burnet (1925, 1925a) has extended and verified certain aspects of 
the work just reported. He found that “a batch of nutrient agar 
plates which had been allowed to remain in the light for some time, 
while still capable of growing staphylococci when heavily inoculated, 
did not allow the growth of isolated organisms. If, however, a 
broth culture, diluted in saline so as to give discrete colonies, were 
spread on such a plate, and an adjacent area were inoculated heavily 
with staphylococci, growth of isolated colonies occurred within a 
range of approximately one centimeter from the region of heavy 
growth.” 

The inhibiting agent in the plates exposed to light was found to 
be hydrogen peroxide, whose effects are destroyed by substances 
produced by an active and heavy growth of staphylococci, so that 
in regions reached by the diffusion of such growth-products, colonies 
can develop from isolated organisms. The growth-facilitating sub- 
stances are in part thermolabile and appear to have some of the 
properties of enzymes; in part they are thermostable, non-enzyme 
materials which Burnet calls “thermostable XY.” This substance will 
neutralize growth-inhibiting powers of hydrogen peroxide and will 
prevent its accumulation when plates are exposed to the action of 
light. ‘There is a definite quantitative relation between the amount 
of thermostable X and the amount of peroxide that can be neutral- 
ized, and it seems probable that the interaction is a simple reduction 
of the peroxide, both substances being destroyed. 

An inhibition of growth due to the presence of potassium cyanide 
can also be neutralized by these substances. Burnet regards the 
mechanism of bacterial mass resistance to KCN to be essentially 
the same as that which gives protection from peroxide introduced 
into cultures directly or by the action of light on culture plates. He 
thinks that the KCN is not destroyed by the thermostable-X sub- 
stance as is the peroxide, but that it is inactivated more indirectly, 
thus: *KCN acts to inhibit the normal mechanism for removing 
peroxide, which accumulates until it is present in toxic amounts. 
The diffusible substances produced by bacterial growth remove the 
peroxide as shown above, and by so doing render the KCN innocu- 
ous. Burnet gives evidence which indicates that the effect of certain 


COMMUNAL ACTIVITY OF BACTERIA 261 


dyes, of which acid fuchsin is an example, is either to increase the 
production of peroxide or to hinder the removal of normally pro- 
duced amounts. 

“A bacterial colony may be merely a fortuitous result of con- 
tinued multiplication, but it seems too to furnish a means whereby 
numerous potentially nocuous influences are rendered less effective. 
An isolated organism may produce diffusible substances as readily 
as one within a colony, but it cannot retain them within its immedi- 
ate neighborhood, and a high concentration of such substances can 
only occur when numerous adjacent organisms collaborate in their 
production. The most important peroxide destroying bodies have 
been shown to be diffusible and will therefore reach a high concen- 
tration only when a colony is formed. By their presence traces of 
peroxide in excess of the amount which can be dealt with by the 
oxidative mechanism of individual organisms are immediately de- 
stroyed and in this way the viability of cells damaged by small 
changes in the environment is retained. It may be considered as a 
primitive maintenance of constant internal environment by ensur- 
ing that the immediate toxic body in many cases of bacterial injury 
(hydrogen peroxide) shall not accumulate. In this respect the bac- 
terial colony may almost be regarded as a metazoan individual.” 

This analogy between the bacterial colony and the metazoan in- 
dividual does not mean that such colonies are on the evolutionary 
high road which leads to multicellular individuals, but it does give 
some insight into certain survival values which favored the develop- 
ment of metazoans from cells which had these and other potentiali- 
ties. Similarly, the facts brought forward in the present compre- 
hensive survey of the survival values of aggregations of organisms 
indicate that groups of animals having such values have the possi- 
bility of becoming what is usually known as “‘social animals.” To 
be sure, they must possess other attributes. An aggregation implies 
that the grouped individuals have tolerance for the presence of other 
organisms in the same limited area, and that they have a reaction 
system which causes them to aggregate or to remain aggregated if 
passively collected. In addition certain other qualities are needed, 
particularly the ability to establish close group integration. The 


262 ANIMAL AGGREGATIONS 


facts indicate that not all animals whose groups show survival values 
are to become more closely social, but that animals, whatever their 
endowments, could not have developed the social habit had the 
incipient social stages lacked the type of survival values which we 
have repeatedly demonstrated for different sorts of animals, and 
now also for bacteria. These values are a function, other conditions 
being equal, of the mass of animals in relation to the volume of 
their effective environment. 


CHAPTER XVI 
MASS PHYSIOLOGY OF SPERMATOZOA 


The relation between numbers of spermatozoa present and their 
functional ability and longevity has long attracted attention, and 
a large literature has been built up about various aspects of the 
subject. The general facts seem well established, but there is much 
discussion concerning causal relations underlying the observed 
facts. 

Spallanzani in 1785 made quantitative studies of the amount of 
seminal fluid necessary to fertilize amphibian eggs. He reports that, 
while the fluid is active in extreme dilutions, the percentage of 
fertilized eggs diminishes with such dilutions. Spallanzani did not 
understand the true nature of the spermatozoon, but his observa- 
tions are essentially correct. Prevost and Dumas in 1824 found 
evidence that the spermatozoa are the essential elements of the semi- 
nal fluid and attempted to repeat Spallanzani’s results. While they 
obtained indications supporting his conclusions as given above, their 
results were too variable to allow a precise statement of quantitative 
relations. 

Gemmil (1900), using sea-urchin sperm, found that the length of 
the functional life of spermatozoa is directly related to the numbers 
present: the greater the concentration the longer the retention of 
ability to fertilize eggs. He records that the duration of vitality of 
sperm of Echinus varies from 3 to 72 hours according to the degree 
of dilution, those in the more dilute suspensions dying first. The 
decrease in vitality he thought to be due to exhaustion by increased 
movement and to the dilution of nutritive strength of spermatic 
fluid. He found similar relations concerning the spermatozoa of 
limpets (Gasteropoda) and of nemertine worms, except that the 
length of vitality varied with different sorts of sperm. As will soon 
appear, the work of recent investigators indicates that one cause of 
the lessened longevity in dilute sperm suspensions is to be found in 

263 


264 ANIMAL AGGREGATIONS 


the relatively greater activity shown under these conditions, as 
Gemmil thought; however, it is doubtful whether longevity is related 
to the dilution of the nutritive fluid, which Gemmil supposed to be 
the main factor involved. 

It is unnecessary to follow here the details of our advance in 
knowledge concerning the mass relations of spermatozoa. The im- 
portance the subject came to occupy may be realized by the fact that 
in 1915 Glaser, as Schiicking had done earlier (1903), questioned 
whether a single spermatozo6n was capable of initiating develop- 
ment of a single egg, even though only one sperm nucleus is con- 
cerned in the biparental inheritance. To explain these relation- 
ships Glaser postulated a mass effect of spermatozoa, as well as an 
individual effect. Lillie (1919), reviewing his own earlier work on 
this. question, states that he can demonstrate that mass effect is not 
necessary if the sperm suspensions have recently been made, and 
that ‘‘the appearance observed by Schiicking and Glaser is found only 
with suspensions not perfectly fresh.” 

Lillie reports that fertilizing power of perfectly fresh sea-urchin 
sperm may extend to 1/90,000,000 of 1 per cent, although at a dilu- 
tion of 1/10,000 per cent one can rarely find a spermatozoon in the 
jelly of the fertilized eggs. He states further, “If one determines by 
comparison the rate of loss of fertilizing power of sperm suspensions 
of different concentrations it is found that sperm suspensions of 
1/240,000 per cent decline to zero in their fertilizing power in about 
six minutes, those of 1/30,o00 per cent in about fifteen minutes, 
those of 1/300 per cent not until after more than two hours, while 
one per cent sperm may retain their fertilizing power for two or 
more days.’ 

The rate of loss of fertilizing power of fresh Arbacia sperm in 


«Tn nature the sperm of the sea urchin, Arbacia, used in many of these studies, are 
shed from the genital pores into the sea-water, where they fertilize the eggs which have 
been similarly shed by the females. In making sperm suspensions for such work as re- 
corded here, the sea urchin is cut around the peristome and placed aboral side down. 
The sperm is then collected in a clean, dry watch glass. Such sperm is designated “dry” 
sperm and may be diluted as desired by adding the appropriate amount of sea-water. 
Thus a 1 per cent suspension is made by adding 1 drop of sperm to gg drops of sea- 
water. 


MASS PHYSIOLOGY OF SPERMATOZOA 265 


relation to dilution is shown in Figure 25, taken from Lillie and 
Just, in which the ordinates give percentage of eggs fertilized and 
the abscissas give a geometrical series of dilutions of 1 per cent 
sperm in powers of 2. 

Lillie holds that fertilizing power of the sperm vanishes before 
motility, and that the spermatozoa tend to lose their fertilizing 
ability in proportion to dilution, apparently because of the greater 
speed of diffusion from the spermatozoa of some substance or sub- 
stances necessary for fertilization. Obviously, this diffusion gradi- 


itp pellit ahs SRG EE EC LE EE 4 15 16 17 18 19 20 21 22 23 24 25 26 27 
90 ! is T 1 

ae 

Sale lala wigisi ae] 
Ane ig i NG Rai 
gues 4 ee sa 
ze ise a 
SCECEEEEEEEE ECCS 
MEE eeoe Mess aie 

10+ miss 

ol M1 a ae | 


Fic. 25.—Logarithmic curves showing the fertilizing power of dilutions of Arbacia 
sperm. The vertical axis shows percentages of eggs fertilized; the horizontal axis 
gives dilutions of sperm in powers of 2. Graph a, perfectly fresh sperm; graph 3, 
sperm suspensions 20 minutes old. (After Lillie and Just, 1924.) 


ent would be steeper the less concentrated the sperm suspension; 
hence the length of functional life would be shorter. 

Hinrichs (1926a), after producing further supporting evidence 
from the greater resistance shown by more concentrated sperm sus- 
pensions to ultra-violet radiation, sums up the situation as follows: 

“Time and dilution are both known to be factors affecting the 
fertilizing power of Arbacia sperm (F. R. Lillie, 1915). Drzewina 
and Bohn (19230) also showed dilution to be a factor in the suscep- 
tibility of sea urchin sperm to the combined action of neutral red 
and light. Motility and fertilizing power were lost more quickly in 
dilute than in concentrated sperm suspensions. Usually loss of 
motility is associated with loss of fertilizing power, but the two do 
not exactly parallel each other. Fertilizing power is not a function 


266 ANIMAL AGGREGATIONS 


of motility alone, and declines more rapidly than does motility (F. 
R. Lillie, 1915; Lillie and Just, 1924). Sperm may be injured in 
such a way that its fertilizing power, as measured by the proportion 
of eggs fertilized and the normality of cleavage and development, is 
materially lessened while motility is not visibly impaired (Hinrichs, 
1926; Lillie and Baskervill, 1922).” 

That such is the case is evidenced by one experiment reported by 
Lillie (1915) and cited by Lillie and Just (1924) in which eggs were 
added to a 1/2’? sperm suspension that was near the point of com- 
plete loss of fertilizing ability. ‘“The sperm were still active and 
entered the jelly of the eggs to such an extent that in ten eggs 
selected at random an average of nine spermatozoa was counted in 
contact with each egg in optical section; the eggs, however, remained 
unfertilized.” 

Had fertilization occurred, and with slightly less stale sperm, as 
there is a possibility that it might, we might perhaps be dealing 
with mass fertilization, as suggested by Schiicking and Glaser. On 
a priori grounds, if the ability of the sperm to initiate development 
depends upon a material which leaches out before the loss of motil- 
ity, then what one spermatozoo6n is unable to supply might be sup- 
plied by many. Barthelmy (1923, 1926), in discussing polyspermy, 
states that the egg reacts in proportion to the stimulus given by the 
sperm, so that when this is weak, as in the case of aged sperm, the 
egg reacts slowly and more sperm enter, producing polyspermy. Al- 
though there is evidence that the defect in polyspermy is in the egg 
rather than in the sperm, there is still a possibility that the phenom- 
enon of physiological polyspermy may also be an instance of the 
same sort of mass fertilization behavior. Such polyspermy is charac- 
teristic of forms with large eggs, such as sharks, some Amphibia, rep- 
tiles, and birds. Insect eggs of several sorts are said to possess 
more than one micropyle as a structural adaptation for polyspermy 
(Henking, 1891). Lillie and Just state that among animals having 
small eggs, physiological polyspermy occurs only in the Bryozoa, 
where the sperm are united in bundles, and where the research 
worker describing the phenomenon (Bonnevie, 1907) thinks that 
polyspermy has definite value for the organism. Lillie and Just do 
not accept her interpretation on this point. 


MASS PHYSIOLOGY OF SPERMATOZOA 267 


Among reviewers of the fertilization problem there is general 
agreement as to the importance of the analysis made by Cohn in 
1918. After demonstrating that fertilizing power of sea-urchin sperm 
suspensions falls off more rapidly in dilute than in concentrated sus- 
pensions, Cohn undertook the measurement of the carbon-dioxide 
production in sperm suspensions of different concentrations by fol- 
lowing the hydrogen-ion potential, with the results shown in Figure 
26. The ordinates give the hydrogen-ion potential of the suspension 


PATA 


PAT 2 | 
° 


PAT.3 


EA 


| 


PAT.S T 
PAT.6 


= 
T 
fe} 
if 
HYDROGEN /0N CONCENTRATION | 
PAT.7 OF 
SPERM SUSPENSIONS 
a 


OF 
ENT CONCENTRATION 
PATB DIFFER. G 


@-1. 0.5% O- 4 0.05% 


° a = a4 — 2, 0.2% 4-5. 0.02% L| 
PAT.9 | ‘| | = — 3. 0.1% DB — 6. 0.01% 

way Le ® — 7. 0.005% 
7) as a ae | J [SS a= ale =I 


2 4 6 8 10 12 14 16 18 20 22 24 26 
TIME 4N HOURS 


Fic. 26.—Diagram from Cohn (1918) showing the hydrogen-ion concentration of 
sperm suspensions of different concentrations after different time intervals. 


in terms of pH; the abscissas, the time in hours. The concentration 
of each suspension is given in the accompanying legend. The in- 
crease in H-ion concentration is due to the carbon dioxide produced; 
and in turn, the rate of production of CO, is a function of the H-ion 
concentration of the suspension; and, as would be expected from 
these conditions, the rate of carbon-dioxide production of sperm 
suspensions decreases with lapse of time. The length of functional 
life as measured by the percentage of eggs fertilized by identical 
concentrations of sperm under identical experimental conditions is 
summarized in Figure 27. 


268 ANIMAL AGGREGATIONS 


An inspection of this figure will show that the spermatozoa lived 
longest in the most concentrated sperm suspensions. Further, Cohn 
undertook to study the total carbon-dioxide production of sperm 
suspensions at different dilutions, with the results shown in Table 
XXIX. These results show that in the less concentrated suspen- 


=e eee ae 
) 

: a ee 
y 80 = = 
nu 
N 
N 70 ala 
K 
S 
kK 60 
i) 
8 ° 
50 | 
S 
8 40 
IN 
= 6 
ee eet Ce | 
X | 
R 

20 hal ia af 

10 N = 

Q a o Bs 
0 4 2 | 0 | 
2 4 6 8 10 12 14 16 18 20 Ze 


T/ME /[N HOURS 


Fic. 27.—Showing the length of life of the different concentrations of sperm sus- 
pensions shown in the preceding figure as measured by the percentage of eggs fertilized 
(ordinates) after given lapses of time (abscissas). (From Cohn 1918.) The legend for 
concentrations is the same as in the preceding figure. 


sions which more nearly approximate normal conditions, spermato- 
zoa that live for longer periods of time produce no more carbon 
dioxide than do those living for only 4 hours. Roughly speaking, 
the total production of carbon dioxide approaches a constant; hence 
the total available energy of spermatozoa must be a constant, and 
the rate at which this energy is expended is a function of the activity 
and an inverse function of the length of life of the spermatozoa. 
These results argue in favor of Gemmil’s assumption that length of 
life is associated with the amount of activity, and against his sug- 


MASS PHYSIOLOGY OF SPERMATOZOA 269 


gestion that the effect of dilution is related to a dilution of possible 
foodstuffs. 

From such observations Cohn concludes that the basic factor con- 
cerned in the greater functional longevity of the sperm composing 
the more concentrated sperm suspensions is to be found in the de- 
pressing action of the self-produced carbon dioxide upon the activity 
of the relatively massed sperm. Similarly cyanides, dilute acids, 
depleted oxygen, or anything that will paralyze the sperm without 
killing them will prolong their ability to fertilize eggs. When dense 
sperm suspensions, either fresh or after a period of some time, are 


TABLE XXIX 


SHOWING THE TOTAL CARBON-DIOXIDE PRODUCTION 
OF DIFFERENT SPERM SUSPENSIONS 


Calculation of 
Relative CO, Approximate 
Sperm Concentration in per Cent | Production per |Life of Sperm in 
Unit Sperm Hours 
Concentration 


OUAS ie ese ey akaie tas ccnoie testis te neces 13 17+ 
Oye ys crower ereieiois ol ohersteyasreue 18 17+ 
(A OE: CiCR DROIT CINCO ELT eee 26 17+ 
ORO An aOR So OCT cere au) to+ 
(CVO) tars DANCERS Oe One 32 6+ 
(Oy MOR MeN As o Perce at One eon eaG 45 6+ 
OOOO Era cteyeversystosuereneta oiyeates 60 4+ 


diluted with large quantities of normal sea-water, the H-ion con- 
centration is markedly lowered, the activity of the sperm is much 
increased, and the fertilizing power of the sperm suspension is like- 
wise increased. But sperm so treated will lose their ability to fer- 
tilize ripe eggs in sea-water long before spermatozoa that have been 
relatively inactive in more acid sperm suspensions. Here we have 
our fourth explanation of the unquestioned mass effect upon func- 
tional longevity of spermatozoa, and the end is not yet. 

Cohn recognized that there is an H-ion aspect of these relations 
not necessarily identified with the CO, factor. Smith and Clowes 
(1924) present direct evidence that such is the case, based on experi- 
ments of CO,-free sea-water. They also show that here, too, there 
is a definite extension of ability to fertilize eggs in higher H-ion 


270 ANIMAL AGGREGATIONS 


concentrations, associated with increased density of the sperm sus- 
pension used, as shown in Table XXX. 

The smallest quantity of sperm was so thin that it was unable to 
fertilize all eggs even in sea-water, and the largest was so thick that 
it was opalescent; but the block to development did not shift beyond 
6.9 to 7.1. Increasing the quantity of sperm increases the number 
of eggs fertilized in acid solution, but the shift is not so great as 
expected if the failure to fertilize were attributable to impairment 
of sperm. The slight shift favors the belief that the block is due to 
an alteration of the properties of the egg. Unless the sperm are in- 
jured by toxic action of egg secretions, all eggs which are fertilized 


TABLE XXX 
SHOWING THE EFFECT OF DIFFERENT H-ION CONCENTRATIONS IN CO2-FREE 
SEA-WATER UPON FERTILIZING POWER AT DIFFERENT SPERM DILUTIONS 
(Data from Smith and Clowes) 


PH 
AMOUNT OF SPERM IN 25 
cc. OF PH SOLUTION 


6.6 6.7 6.8 6.90 7.0 Fiat! 72 3 7.4 8.15 


fo) fo) fo) 50 | 100 | 95 | 95 | 100 | 100 | 100 TCH 1/20 

fo) ° ° o | 35 | 80] 95 | 100 | 100 | 100 “1/200 

fo) ° fo) ° fo) 20 | 85 87 | 95 | 90 Fi DCS) 
° ° ° “ 1/20,000 


Oh 1th Ao! |. 453 2518535," ge 


in these solutions develop normally, indicating that the fertilization 
reaction, when once initiated in the neighborhood of the block, is 
completed without impairment. 

Before continuing with this search for further suggested explana- 
tions of this phenomenon, it is of interest to stop for a moment over 
the controversy concerning the mechanism of aggregation of sper- 
matozoa in sperm suspensions. Lillie (1919) points out that with 
active spermatozoa in a suspension we should expect to find the 
individual spermatozoa colliding with each other and with the walls 
of their container, but that on the whole their distribution is ap- 
proximately uniform throughout, much as is supposed to be the 
case with gas molecules. This arrangement is found in perfectly 
fresh suspensions, but usually lasts only a short time before various 
kinds of aggregations occur. 


MASS PHYSIOLOGY OF SPERMATOZOA 275 


Figure 28, from Lillie, shows a photograph of aggregations of 
sperm of WVereis in sea-water. His account of the formation of these 
aggregations is as follows: 

“Tf a drop of dry sperm from a mature Nereis is mixed in about 
6 cc. of sea water in a Syracuse watch crystal it makes a uniformly 
milky suspension; in a few seconds clouds begin to appear, and in 


Fic. 28.—A sperm suspension of Nereis taken go seconds after mixing the sus- 
pension. (Drawing by Toda from photograph published by Lillie, 1919. 


15 to 45 seconds these usually draw together into white solid masses 
uniformly spaced through the fluid. The intervening fluid becomes 
quite clear and the masses quickly settle on the bottom. The rate 


of formation of these masses and their number and size depend on 
the condition of the animal furnishing the sperm, temperature, 


‘freshness’ of the sperm, reaction of the medium, etc. Sperm sus- 
pensions of most animals do not, however, exhibit such marked 
aggregations.” 

Lillie believes, as a result of well-planned experiments, that these 


272 ANIMAL AGGREGATIONS 


aggregations form as a definite positive reaction of the individual 
spermatozoa to regions of self-produced higher concentrations of 
carbon dioxide; that is, they are tropistic reactions. Cohn and 
others believe that the aggregation is the result of a trap action 
whereby the spermatozoa, on invading a region of higher CO, con- 
centration, have their activity reduced and tend to remain trapped. 
We are not, at the present time, interested in the relative merits of 
these two points of view. It is of interest, however, to note that, 
according to both, the aggregations form without any evidence of 
what we have called “‘secondary”’ or “social” reactions, but merely 
as a result of the primary or individual reactions to the environment, 
just as we have already seen that aggregations of animals frequently 
form. It is also of interest that here with the spermatozoa, as in the 
case of animal aggregations, far-reaching physiological effects may 
result from these groupings. 

In the reaction of spermatozoa another aspect gives their behavior 
more direct significance. Lillie and Just summarize evidence which 
shows that among the other effects of egg secretions upon homotypic 
sperm there is, at least in the sea urchin, a marked tendency to 
aggregation in which “the spermatozoa are attracted to the drop of 
egg secretion and gather in or around it, depending on its concen- 
tration.” Such reactions are not easily demonstrated, if they can be 
demonstrated at all, by Pfeffer’s capillary tube method, but can be 
directly shown by introducing a drop of egg-water in a sperm sus- 
pension on a slide covered by a raised cover slip. Under these con- 
ditions, Lillie and Just report: “If a drop of egg water of Arbacia, 
with agglutinating substance removed, be injected into a sperm 
suspension of the same species, a ring of active spermatozoa forms 
around the drop, separated by a clear zone almost devoid of sperma- 
tozoa from the main suspension. If the clear zone be examined care- 
fully, spermatozoa may be seen swimming directly across it from 
the general suspension to the drop of egg water for some minutes. 
The clear zone thus gives the range of some directive influence pro- 
ceeding from the drop.” This same phenomenon is exhibited clearly 
by Nereis sperm when a drop of 1 per cent CO, sea-water is similarly 
introduced, and is illustrated in Figure 29 and explained in the 


MASS PHYSIOLOGY OF SPERMATOZOA 2 


accompanying legend. The significance of the reaction to emana- 
tions from a homotypic egg in relation to fertilization is obvious, but 
does not constitute a part of our present problem. 

Lillie (1921) studied the effect of copper chloride upon fertiliza- 
tion in connection with his analysis of the fertilization reactions. 
Briefly, he found that fertilization was markedly inhibited at as low 
a concentration as 1 part of copper chloride in 2,500,000 parts of 
sea-water, using a normal sperm suspension. Most of his experi- 


Fic. 29.—Reaction of a sperm suspension of Nereis to a drop of 1 per cent CO, 
sea-water (natural size). The preparation (a) is mounted on a slide beneath a raised 
cover slip. a, Showing the form of the reaction after 15 seconds; b, after 75 seconds; 
c, after 105 seconds; and d, after 195 seconds. In d the general suspension has aggre- 
gated. The drop to the right in @ is a control drop of sea-water. Note that in a the 
spermatozoa also withdraw from the margin of the preparation, thus increasing the 
CO. tension. (Figure and legend from Lillie, 1919.) 


ments were done at a copper concentration of 1/500,000 copper 
chloride in sea-water, a concentration at which no eggs fertilize at 
normal sperm concentrations. If much higher concentrations of 
sperm are used, small percentages of fertilization are obtained, the 
number varying somewhat in different similar experiments. Lillie 
comments: ‘‘There is thus a certain virtue in mass action of the 
sperm in the presence of this inhibitor of fertilization; this is some- 
what difficult to understand, because only one spermatozoon pene- 
trates normally.’”’ He suggests the possibility that an excess of 
sperm protects the eggs to a certain extent by combining with the 
copper and thus reducing the amount acting directly on the eggs, a 
suggestion he and Just repeat (1924). 


274 ANIMAL AGGREGATIONS 


Drzewina and Bohn, in their studies upon the effects of dilution 
of sperm upon their physiology, record (1923) another interesting 
phenomenon. As we have seen before, dilution of sperm in sea-water 
hastens their loss of activity, and the greater the interval after dilu- 
tion the greater is the loss of activity. Likewise, the greater the 
dilution the more rapid is the loss of activity. Drzewina and Bohn 
found that sperm immediately after dilution (1/5,000~1/ 100,000) 
are very active, but are reduced in vitality so that, while fertiliza- 
tion is effected, abnormal development and cytolysis result. If an 
interval is allowed between dilution and fertilization, normal de- 
velopment is obtained. It is believed that the sperm, when diluted, 
undergo a disturbance of equilibrium, exhibiting a “differential sen- 
sibility.” There is little, if any, change in fertilizing power, but a 
considerable change in cytolyzing power. These phenomena are 
exhibited only within certain limits of dilution and vary with the 
season and locality. 

Treatment of sperm of sea urchins, Echinus microtuberculatus or 
Strongylocentrotus lividus, with KCl or distilled water shortens the 
duration of fertilizing power in proportion to duration of treatment 
and concentration of the sperm, and also in proportion to the light in- 
tensity; the weaker the dilution the more rapid the loss. One or 2 
drops of normal KCl in distilled water, mixed with 1 cc. of sea-water 
is a rapidly lethal dose for Infusoria, Planaria, etc. Sperm diluted to 
1/100 survive for 24 hours, but dilutions of 1/1,000 and 1/5,000 
succumb much more rapidly. Certain of these relations are illus- 
trated in Table XXXI 

Further, Drzewina and Bohn observed that in KCl, in fresh water, 
and in the controls, within certain limits of duration of treatment 
and of dosage of injurious substance, the nature of the liquid has 
little effect, but that the concentration of the sperm is the decisive 
factor in duration of fertilizing power. A given concentration in 
any of the foregoing media will, after a certain time, give about the 
same percentage of fertilization. This condition is compared to a 
colloidal suspension in that the nature of the solvent does not make 
as much difference as the number of particles per unit volume. 

They pointed out that the spermatozoa behave as if a substance 


MASS PHYSIOLOGY OF SPERMATOZOA 2S 


were emitted which, when the sperm are numerous enough or when 

the volume of liquid is small enough, is sufficient to protect the 

group. This action is regarded as being similar to that obtaining 

when Procerodes are isolated into fresh water in which other organ- 

isms have lived and died. This is Drzewina and Bohn’s hypothesis 

of the production of an autoprotective substance applied here to 
TABLE XXXI 


EFFECT OF DILUTION OF SPERM SUSPENSION IN THREE DIFFERENT MEDIA 
(Data from Drzewina and Bohn) 


4 Hours 8 Hours 

1/1,000 sperm in sea-water No effect 25% fertilization 
1/1,000 sperm in sea-water plus 2 drops of No effect 50% fertilization 

KC@lipentces: 
1/1,000 in 2/3 sea-water plus 1/3 distilled No effect 10% fertilization 

water 

20 Minutes 1 Hour 4 Hours 

1/5,000 sperm in sea-water No effect {Almost 100% <1% 
1/5,000 sperm in KCl No effect 90% o% 
1/5,000 sperm in 2/3 sea-water plus 1/3| No effect 70% o% 

distilled water 


* It is noted here that in weak doses and certain concentrations of sperm KC] increases survival of sperm. 


spermatozoa, and adds the fifth explanation of the greater longevity 
of relatively heavy sperm suspensions. 

The same workers (1926a) reinvestigated the effect of carbon 
dioxide upon the longevity of spermatozoa. They exposed sperm 
suspensions of different dilutions, from 100 to 5,000 times diluted, to 
sea-water kept saturated with CO,, with results which may be sum- 
marized as follows: 

Wiluted(x/ TOO... Misses c No paralysis; sperm immediately gather around 
the eggs, but no fertilization membrane is formed. 
Segmentation is irregular, and larvae cytolyze 
easily. 

Morevdiluter saec.n.04 etees Paralyzed 4 hour, but then recover and fertilize 
with normal membranes and development. 


276 ANIMAL AGGREGATIONS 


Apparently the sperm most severely affected give the better re- 
sults in fertilization. The explanation of this paradoxical situation 
is that CO, determines a crisis. In the former case, the fertilization 
occurred during the crisis with abnormal results; in the latter case, 
fertilization took place after the crisis was over, and the results 
were normal. 

By a longer exposure of 20 to 60 minutes the contrast between 
concentrated and dilute suspensions is reversed. The concentrated 
recover gradually and in proportion give normal development, while 
the dilute sperm suspensions lose fertilizing power more and more. 
The same results are obtained with one-half the dose of CO,. It is 
scarcely possible even to suggest more rapid exhaustion of toxic 
substance as the cause for the protective effect of numbers in this 
case, since the CO, is kept constant. Obviously Cohn’s explanation 
that the more concentrated sperm suspensions survive the longer 
because of the greater CO, tension which, by inhibiting activity, 
promotes functional longevity, does not represent the whole story 
of the protective action of the mass of spermatozoa, however true 
it may be under many conditions. 


BIOELECTRICAL EXPLANATION OF MASS PROTECTION 


Again Drzewina and Bohn (1926) call attention to their experi- 
ence that the more concentrated suspensions of sperm resist a raised 
temperature better than the more dilute (see Young, 1929, on mam- 
malian sperm). Also, with low temperature (1° C.), if one compares 
the effect on 1/100 or on 1/1,000 sperm, the cold activates the first 
and inhibits the second, as if there were a change in the sign of the 
reaction with the change in mass. Drzewina and Bohn report simi- 
lar effects from darkening of sea-urchin sperm. They are particu- 
larly interested in this protection furnished by the more concen- 
trated masses of sperm, not only because of the similarities to their 
observations on mass relations with various animals previously 
cited, but the more so because they believe that the spermatozoa are 
the bearers of electrical charges, which are not only associated with 
the fertilization phenomena but are also concerned with the problem 
of mass physiology of spermatozoa with which we are dealing here. 


MASS PHYSIOLOGY OF SPERMATOZOA 277, 


Here we have a suggested sixth explanation of the greater longevity 
of the more concentrated sperm suspensions. Some of the further 
evidence at hand concerning the electrical nature of this relation- 
ship, taken from the summary by Drzewina and Bohn (1928) of 
their work, will be given. 

Sperm of sea urchins lose fertilizing power rapidly if a medium 
concentration be placed in a silver dish. The greater the concentra- 
tion the longer they retain this power. Diluted 100 times they 
fertilize eggs up to an hour’s exposure; diluted 10,000 times, up to 
20 minutes; while at a dilution of 100,000, fertilizing power is lost 
immediately. The effect is in part a function of the mass of the 
silver acting: a thin silver foil is completely inactivated after three 
consecutive exposures. It is reactivated if placed over a block of 
silver but not if placed over glass. The effect on sperm is paralleled 
by the action on the flatworm Convoluta. 

Further, hydroquinone is active when very dilute, and there is 
good reason for belief that its activity is, at least in part, electrical 
in nature. If so, the effect upon suspensions of sperm should be 
illuminating. It is found that if sperm are diluted 100 times and 
exposed to hydroquinone diluted 1,000,000 times, and then given a 
chance within 30 seconds to fertilize normal eggs, normal fertiliza- 
tion membranes are formed. This is not the case when sperm sus- 
pensions diluted 1,000 times are given the same treatment; and if 
the hydroquinone, rather than the sperm suspension, is made more 
dilute, it also loses its effect. 

Convoluta placed in a dish lined with stearin survive only half 
an hour, although if placed in a similar dish lined with paraffin 
they live normally and even longer than in a glass dish. The par- 
affin appears to exert a protective effect for the Convoluta, while the 
stearin has a destructive action. When exposed to silver in a glass 
dish, all Convoluta die, while those exposed similarly in a paraffined 
dish are still alive. A paraffined dish favors survival of sea-urchin 
eggs but is harmful to sperm. Stearin, on the other hand, is harm- 
ful to eggs. 

Both paraffin and stearin are more chemically inert than glass 
and are less soluble in water; yet it is sufficient merely to line the 


278 ANIMAL AGGREGATIONS 


containers with these substances in order to change the survival 
time. Drzewina and Bohn suggest that in such cases we are dealing 
with an electrical phenomenon which they designate as “catalysis 
by contact.” They suggest that animal groups or masses of sperma- 
tozoa, by the mere fact of their presence in numbers in a limited 
space, influence each other, causing the individuals to become more 
or less sensitive, just as they are activated or disactivated above 
paraffin or stearin; somewhat as inorganic chemicals are more or less 
sensitive according as they are placed over paraffin or stearin. 

From an entirely different approach Gray (1915) had suggested 
that at least a part of the phenomena connected with the physiology 
of spermatozoa is electrical in nature. We have stated above that 
the spermatozoa of sea urchins become quiet in acid of proper 
concentrations and are again activated by making this sufficiently 
alkaline. He thinks that the movements of the spermatozoa are 
dependent upon the electrical properties of the cell and of the sur- 
rounding medium, and cites two lines of supporting evidence. 

If sperm are suspended in a neutral isotonic cane-sugar solution, 
their activity ceases, but is recovered if a trace of alkali is added. 
If an electric current of appropriate strength is passed through the 
neutral solution just mentioned, the sperm tend to collect rapidly 
about the positive pole. Those remaining around the negative pole 
become very active. This activity is probably associated with the 
fact that the region becomes definitely alkaline. If the neutral solu- 
tion is made slightly acid, neither the migration to the positive pole 
nor the activation about the negative one takes place. In such a 
solution the action of the electric current causes the spermatozoa 
to collect in a well-scattered netlike aggregation. The activity 
shown can readily be interpreted on the assumption that the active 
spermatozoa carry a negative surface charge which is lost when free 
hydrogen ions are present. 

Further (Gray, 1920), if a drop or two of a dilute solution of cerous 
chloride is added to a weak suspension of Arbacia sperm, the sperm 
become highly active and aggregate rapidly. The aggregated sperm 
remain active for some time and then become motionless. These re- 
sults are attributed to the action of the trivalent cations present. 


MASS PHYSIOLOGY OF SPERMATOZOA 279 


Their effect can be neutralized by treatment with sodium citrate. 
Gray suggests that these trivalent ions have this effect upon the 
spermatozoa because of the electric charges carried by the former. 

Gray returned to a consideration of the whole problem of sperm- 
suspension dilution in relation to functional longevity in 1928. He 
repeated the well-known observations that as long as seminal fluid 
of a ripe sea urchin remains undiluted, little or no movement occurs 
on the part of the sperm. If a small drop of this fluid comes into 
contact with sea-water, the cells at the surface at once become in- 
tensely active, and eventually the whole lot of spermatozoa exhibits 
lively movement. Gray suggests four possible causes: (1) There 
may be an inhibiting substance in testicular fluid. (2) Sea-water 
may contain some element absent from testicular fluid necessary 
for movement. (3) The viscous resistance of testicular fluid may 
be too high to allow movement. (4) Each spermatozo6én may exert 
some form of inhibition on the movement of its neighbors. 

If undiluted sperm be centrifuged, they can be separated from the 
surrounding testicular plasma. If a drop of undiluted sperm be 
added to this plasma so that it becomes diluted thereby, activity 
results as in sea-water. Appropriate checks show that the results 
are not due to changes in CO, or O, tension due to centrifuging. - 
Gray thinks, therefore, that the activating effect of dilution in sea- 
water is due to mechanical dilution whereby each spermatozo6n is 
given more space, allowing free movement. The spermatozoa do 
not move when closely packed; but what is more striking, they make 
no effort to do so; this can be demonstrated by comparing O, con- 
sumption of undiluted and diluted sperm. As movement becomes 
retarded in the diluted sperm due to increased CO, tension, artificial- 
ly produced, or to the passing of time, the rate of O, consumption 
anproaches that of the originally undiluted sperm. In this connec- 
tion it would be interesting to know if the same effect can be ob- 
tained by diluting the sperm and then packing it together again in 
sea-water with a low CO, content. 

Except in very dilute suspensions the respiratory level at the 
beginning of active life is not simply proportional to the number of 
spermatozoa present, but also depends on the degree of dilution of 


280 ANIMAL AGGREGATIONS 


the original testicular fluid. The greater the dilution the greater is 
the initial activity of individual sperm; but as dilution increases, the 
effect on activity is less marked. In very dilute suspensions the ac- 
tivity of a spermatozo6n is more or less independent of dilution, but 
in stronger suspensions the activity is much affected by further 
dilution. That the reduced activity in strong suspensions is not due 
to lack of O, is shown by the fact that in one of Gray’s standard 
6 cc. of a suspension containing 25 mg. nitrogen’ equivalents of 
sperm, there is actually less O, consumption than is shown by the 
same volume of a suspension containing 5 mg. nitrogen equivalent; 
and also by the fact that strong aération failed to increase activity. 

Gray’s interpretation is that inactivity of sperm in concen- 
trated suspensions is due to lack of free space. Over a considerable 
range of dilutions the specific activity of a spermatozoon is a linear 
function of the cube root of the volume of sea-water. The total 
initial activity is proportional to the number of sperm and to the 
average free space for each cell. The initial degree of activity is 
thus lower in the less diluted suspensions; but, interestingly enough, 
the total energy expended during a period of at least 2.5 hours is 
also distinctly less. The relation between density of spermatozoa 
and rate of oxygen consumption may be accurately expressed by the 
same formula used by Pearl to show the effect of population density 
of the rate of reproduction in Drosophila (Pearl, 1925). 

Gray states, without supporting reference or evidence, that mu- 
tual inhibition of activity of unicellular organisms is known among 
the Protozoa, and particularly with Paramecia. The decreased ac- 
tivity with massed spermatozoa may be the result of inhibitions due 
to continual collisions, or it may be comparable to the effect of 
thigmotactic reaction to foreign bodies in Paramecia. The behavior 
of spermatozoa almost suggests a voluntary phenomenon in which 
contractile effort is proportional to free space in which the organism 
can move. Until more is known of the mutual effect of one cell 
upon its neighbors, the phenomenon which Gray calls “‘allelostasis,” 
that is, the mutual depressing effect of one cell upon another, must 


« Total nitrogen present is used as a measure of sperm concentration. Oxygen con- 
sumption is used as a measure of activity of sperm in suspension. 


MASS PHYSIOLOGY OF SPERMATOZOA 281 


remain obscure. This last explanation of the effect of dilution upon 
spermatozoan physiology is fundamentally the same as Gemmil’s 
first assumption. 

It is a well-known fact that the fertilizing power of sperm falls 
more rapidly when sperm are kept in dilute suspension than when 
they are concentrated. The usual explanation is that in stronger 
suspensions activity is inhibited by CO, given off by the sperm, and 
so the cells conserve their energy (Cohn, 1918). Gray’s data show 
that this explanation is only partially correct at best and may be 
quite erroneous. In his experiments the CO, product was continu- 
ously removed, and the respiration per unit of sperm suspension in 
the concentrated mass at no time equals that in the diluted sus- 
pension. 

If the conditions of a dilute sperm suspension are such that the 
CO, generated is allowed to accumulate, the rate at which energy is 
expended will decrease, which will tend to increase the length of 
life during which the cells are motile when the CO, is removed by 
subsequent dilution. Such conditions appear to have existed in some 
of Cohn’s experiments where the suspensions were very dilute and 
the full initial activity was mechanically possible. In such suspen- 
sions it should be possible to show that the rate of respiration in 
relation to the percentage of sperm was the same in all cases. 

Gray’s conclusions are: 

t. Relative inactivity of undiluted Echinus sperm is not due to 
physical and chemical constitution of their natural medium in the 
testis, since the cells are intensely active in this medium when the 
majority of the spermatozoa are removed by centrifuging. 

2. Total activity of any suspension, as measured by its O, de- 
mand, is proportional to the number of sperm present and to the 
average space in which each cell is free to move. Inactivity in the 
testis appears to be due to mechanical overcrowding, each cell ap- 
pearing to exercise a restraining, or allelostatic, effect on the activity 
of its neighbors. 

3. Total energy expended during life of sperm, as well as the 
level of activity exhibited immediately after activation, depends on 
the degree of dilution of the suspension examined. 


282 ANIMAL AGGREGATIONS 


4. The relatively long life of concentrated suspensions is not due 
to the narcotic effect of accumulated CO, but is the result of an 
incomplete state of activation on the part of each spermatozo6n. 


MASS PHYSIOLOGY OF MAMMALIAN SPERM 


The experiments reported so far have dealt with spermatozoan 
studies in those sea-dwelling animals where the sperm are shed 
freely into the sea-water and where fertilization occurs in this me- 
dium. There is a small amount of data at hand concerning the 
physiology of mammalian sperm where fertilization takes place 
within the body of the female after introduction by artificial or 
natural means. It has already been indicated that in regard to 
resistance to high temperatures the relation between volume of 
sperm and degree of resistance is similar in the two types. The 
work of Lloyd-Jones and Hayes (1918) and of Walton (1927) allow 
these comparisons to be carried further. The latter performed ex- 
periments upon the physiology of the spermatozo6n, using insemina- 
tion as a test for fertility. One of the variables found to enter is that 
of the mass of sperm present per given volume of medium. 

Walton, in his work, obtained spermatozoa from a buck rabbit 
killed by a blow behind the ears. The abdomen was opened im- 
mediately, the testes removed, and the cauda epididymi were opened 
in about 5 cc. of 0.15 NaCl solution. This gave a dense suspension 
of spermatozoa. After examination under the microscope to check 
for motility, these were diluted in ascending powers of to. 

Doe rabbits were allowed to copulate with vasectomized males to 
insure ovulation and were then artificially inseminated immediately. 
Directly thereafter the densities of the sperm solutions used were 
determined with the aid of a haemocytometer for those between 
ro° and 107, and the others were estimated from the dilution in 
comparison with those counted. The results from 130 experiments 
performed are given in Table XXXII. 

Walton recalculated the data obtained by Lloyd-Jones and Hayes 
from studies on the effects of insemination after excessive sexual 
activity of male rabbits. The results are shown in Table X XXIII. 

Even in these internal impregnations a large number of spermato- 


MASS PHYSIOLOGY OF SPERMATOZOA 283 


zoa is apparently necessary to insure fertility, and there is a signifi- 
cant decrease in percentage of fertile matings as the artificial or 
natural sperm suspensions become less dense. In the artificial in- 
seminations just summarized a mark of decline came when the 
number of spermatozoa was less than ro° per cc., and complete steril- 
ity occurred when this value fell below about 1o* per cc. In the 
series with overworked males the point of complete sterility was not 


TABLE XXXII 
NUMBER OF SPERM PER 3 Cc. PLACED IN VAGINA OF DOE 


103-104 Io4-I05 1os—108 10-107 1o7—108 1o8—109 
Total inseminated........ 14 19 27 28 30 12 
Hentilewy seer cerkoe on, oar fo) I 6 14 19 7 
IRErcenta genase seine fo) 5-3 2D sP 50 63.3 58.4 

TABLE XXXIII 
Copulation I 5 10 15 20 

Wolumescceeeen en Onsd 0. 20 ©.22 0.13 O.1 
Density per cc... ..} 104.5 X10°] 39.5 X10°} 14.7 X10} 4.2 X10°| 3.5 Xr108 


Total number of 
sperm ejaculated.| 35.6X10°| 8.4 X108| 3.2 X10°| 0.6 X10°} 0.3 X10° 
Percentage of fertil- 
AY Bouc.ane lomo 72.00 61.1 41.37 41.37 B5 E55 


reached. It is of interest that the size of the litters is reduced after 
the first, fifth, and fifteenth service. 

In the latter series there are complicating factors other than 
density of the sperm population. Thus, there is not only a decrease 
in the density but also in the percentage of functionally mature 
sperm, a decrease in progressive motility, and a decrease in the 
duration of the motility of the sperm. That the latter effects are 
not due to mere dilution is shown by the fact that similar sperm 
diluted with 10 times its volume of isotonic solution showed greater 
duration of longevity than did sperm in the natural medium at 
the density existing when ejaculated. The probable cause of this 
phenomenon is that in the natural semen the by-products of me- 
tabolism and of developing bacteria more quickly reach a harmful 


284 ANIMAL AGGREGATIONS 


or fatal concentration for the sperm than in the case of artificial 
dilutions. 

In Walton’s cases, where all dilutions were under controlled arti- 
ficial conditions, three separate factors, each influenced by quanti- 
tative considerations, may influence the result. (1) The probability 
of any one spermatozoo6n reaching the fertilizable ovum within the 
time that the ovum is fertilizable is small; and the probability of 
such success is greater, within limits, the greater the number of 
sperm introduced. (2) Spermatozoa are variable and are not all 
equally capable of fertilizing, so that, again, the probability of fer- 
tilization is greater, within limits, the greater the number of sperm 
present. Finally (3), the toxicity of the medium may well act dif- 
ferentially on sperm suspensions under natural conditions, just as 
it has been shown to act in laboratory experimentation. This as- 
sumes that there is a general toxicity to sperm in the female genital 
tract which would act more vigorously upon the more dilute sperm 
suspensions. 

In summary, we have found uniform agreement concerning the 
greater functional longevity of the more dense sperm suspensions, but 
a not unexpected lack of uniformity in the explanations advanced 
for this phenomenon. Recapitulating the latter, we find the more 
rapid exhaustion in dilute suspensions attributed to— 

t. More rapid movement, exhausting sperm energy (Gemmil). 

2. Dilution of nutritive spermatic fluid (Gemmil). 

3. Greater diffusion of substance from spermatozoa necessary for 
fertilization (F. R. Lillie). 

4. Effect of self-produced CO, (Cohn). . 

5. Decreased production of an autoprotective secretion (Drzewina 
and Bohn). 

6. Unfavorable electric relations (Drzewina and Bohn). 

7. Allelostasis (Gray). 

Of these, evidence has been produced that throws grave doubt 
upon the second. Cohn’s observation that the total amount of CO, 
per unit is practically the same, regardless of dilution, is strong 
negative evidence; and the supporting evidence produced by Gem- 
mil as a result of treating sea-urchin sperm with nutritive solution 


MASS PHYSIOLOGY OF SPERMATOZOA 285 


has been shown by Cohn to be what one would expect from the pH 
changes known to accompany this treatment. The third explana- 
tion seems to be well established for a number of animals and is 
supported by various observers. If this be sound, the movement 
hypothesis—the first and in a modified form the seventh on our 
list—is not closely pertinent in the problem of functional longevity, 
although still of value in the matter of total longevity. There seems 
to be no doubt but that self-produced CO, slows down metabolic 
processes of crowded spermatozoa, and so delays their final loss of 
fertilizing power; but following the experience of Drzewina and 
Bohn and of Gray, with controlled CO, tensions, it is no longer pos- 
sible to believe that this is the whole story. The autoprotective 
secretion hypothesis is not strongly supported by direct evidence. 
A final decision concerning the applicability of electrical phenomena, 
as advanced in our sixth hypothesis, must await the further develop- 
ment of biophysics in this field. 

Finally, we may pause for a moment to consider the justification 
for bringing the mass relations of spermatozoa into a discussion of 
the physiology of numbers as a foundation for the formulation of a 
consistent view of general sociology. If the phenomena with which 
we have been dealing have general rather than special significance, 
we shall expect them to appear wherever there are collections of 
living material, whether these are gametes or zygotes or the products 
of zygotes. The advanced state of investigation into the physiology 
of the spermatozoa allows us to make this an important test case of 
the general application of the relationships already found to hold 
rather generally throughout the animal kingdom. The fact that 
they also operate here strengthens by that much our belief in their 
general applicability. A similar inquiry into the same set of rela- 
tions in the culturing of bacteria and in tissue culture has shown 
that in those fields also there are definite protective values in opti- 
mum mass relations, and that, as in many animal relationships, 
the optimum usually does not coincide with the minimum popula- 
tion either of spermatozoa, of bacteria, or of cells in tissue culture. 


GENERAL EFFECTS OF AGGREGATIONS 


CHAPTER XV It 


INFLUENCE OF CROWDING UPON SEX 
DETERMINATION 


We have just seen that when gametes are shed free into the sur- 
rounding sea-water, their period of life is distinctly limited. If two 
gametes of opposite sex are unable to meet during this fertilizable 
period, death results for the spermatozoa, probably from starva- 
tion, and for the egg, perhaps from suffocation (Child, 1915). This 
means that animals of different sexes must be relatively close to- 
gether in order that there may be a successful union of the shed 
gametes. Grave and Downing (1928) give us a chance to estimate 
some of the space requirements for the successful operation of this 
system in sea-water. They report that the most vigorous sperm of 
the sea urchin, Arbacia, and of the mollusk, Cumingia, can travel 
30 cm. by their own effort in still water. Hydroides sperm are less 
active. Spermatozoa of the first two animals in suspensions of 
1/2,000 to 1/10,000 per cent survive from 3 to 12 hours. Sperma- 
tozoa under natural conditions in sea-water may survive and may 
fertilize eggs for this period, but many die after 3 hours and the 
majority succumb after 7 hours. Where currents are present, these 
survival periods would allow of much greater distribution than 30 
cm.; but the fact remains that even in the most favorable sea-water, 
animals must be relatively closely aggregated for fertilization to be 
successful. In fresh water the life of the shed gametes is quite short. 
After ro minutes (Reighard, 1893) the eggs of the wall-eyed pike 
lose the power to be fertilized. The same observer has stated in a 
lecture at Woods Hole that the sperm of certain fishes loses fertiliz- 
ing power within a half-minute after shedding. With animals re- 
quiring internal impregnation, the necessity for close co-operation 
of at least two individuals is obvious. These considerations must 
be fundamental for the long-recognized breeding aggregations of 
animals, particularly of animals shedding their gametes into the sur- 
rounding water during the breeding season. 

289 


290 ANIMAL AGGREGATIONS 


The aggregation phenomenon may be of still greater importance 
in sex biology than the foregoing illustrations would indicate. The 
following considerations strongly suggest that it may have been an 
essential element in the evolution of sex itself. Presumably, this 
evolution started with a time when all gametes of any one species 
were similar. Under these conditions a first step toward union of 
two isogametes could be supplied by the greater well-being fostered 
by the presence of more.than one gamete within a limited area, such 
as we have seen holds, under certain conditions, with the Protozoa 
and bacteria. From the survival value thus present before actual 
union took place, we can find a logical beginning for the action of a 
selection, which would in time, and with present known values, re- 
sult in the establishment of the sexual phenomena as they appear 
today. 

These fields, important as they are, have not yet been explored 
sufficiently to allow more than this suggestion of their fundamental 
significance. In the matter of sex determination, however, there is 
a mass of evidence concerning the importance of the close associa- 
tion of animals which merits presentation. 


THE EFFECT OF CROWDING ON SEX IN THE 
MONSTRILLID COPEPODS 

The copepod crustaceans of the aberrant family Monstrillidae are 
marine and free-living as adults, but in their larval stages certain 
of them have been shown to be parasitic in the blood-vessels of 
marine annelid worms. Malaquin (1901) has described in detail the 
life-history of Haemocera danae, which passes its parasitic stage in a 
serpulid worm. The adult males and females are highly dimorphic. 
Malaquin reports that when a single parasite is found in the host 
worm it may be either a male or a female, but that when two or more 
are present they develop into males, with the exception of very rare 
cases; In two cases only, out of some thousands examined, were two 
females found in the same host. The crustaceans gain entrance to 
their host in the nauplius stage, and it is extremely unlikely that the 
observed sex distribution is the result of a differential penetration of 
predetermined sexes. Since, when a single parasite is present, either 


CROWDING AND SEX DETERMINATION 291 


sex may develop, the chances that the first parasite to enter may 
become male or female are 50 to 50. It is also highly improbable 
that a second or third nauplius enters only those worms which have 
been previously parasitized by a copepod predetermined for male- 
ness. The possibility of differential mortality, however, is not so 
easily dismissed. . 

Malaquin interprets the observed results as being due to crowd- 
ing, and suggests that the effective agent is the decrease either in 
amount of food or in available space. With the higher numbers of 
parasites per host he finds a marked decrease in size of the parasites 
and, accompanying this, partial or nearly complete suppression of the 
testes, which he regards as an example of nutritive castration. The 
statistical facts concerning the tendency toward maleness of crowded 
monstrillid parasites appear plain, but the physiological explanation 
is not so clear; and until necessity compels us to change, we must 
keep in mind the possibility that the results are due to differential 
mortality. In this regard, at least, the effects of crowding on sex 
ratios will be more clear in cases to be reported immediately. 


SEX DETERMINATION IN BONELLIA 


Bonellia is an aberrant annelid worm with strong sexual dimor- 
phism. In the female, the proboscis is long and extensible and is 
bifurcated at the anterior end. The enlarged body contains a well- 
developed alimentary canal, a pair of nephridia, and a single anterior 
nephridium which is enlarged and serves as a uterus. The male is a 
small turbellarian-like worm, about 1 mm. in length, ciliated, lack- 
ing a proboscis, and with a reduced alimentary canal which lacks 
both mouth and anus. These and other details are well illustrated 
in Parker and Haswell’s textbook of zodlogy. Although the adult 
males differ so much from the females, their structures can be shown 
to be homologous. A series of studies, chiefly by Baltzer, has pre- 
sented us with an analysis of the factors leading to this dimorphism, 
which he conveniently summarizes (1928). 

According to this account, the male living within the uterus of 
the female fertilizes the eggs there. These are laid, and in about 4 
days develop into free-swimming larvae about 1 mm. long. These 


292 ANIMAL AGGREGATIONS 


larvae are at first sexually indifferent. About to-12 days after fer- 
tilization the first signs of transformation into females may appear, 
providing the larvae remain out of contact with the proboscides of 
older females, although the larvae can remain indifferent from 2 to 
4, or even 5 weeks, and then develop into females. 

When they develop into females, the larvae first lose their ciliary 
bands, the posterior part of the body enlarges, due to the develop- 
ment of the coelom, and the peristaltic contractions, characteristic of 
mature females, also begin. The alimentary tract differentiates, setae 
and anal vesicles are formed, and the animal sinks to the bottom, 
where it lives from now on. It normally depends on yolk for its 
food for about another week, before it begins active feeding on mud 
and detritus. 

Male development is almost always initiated by the effect of a 
special metagametic factor or factor complex present in the proboscis 
of the female. If the indifferent larvae settle upon the proboscis 
lappets of a female, male development starts. It is not necessary 
that the female should be adult, for immature females have the same 
effect. This sessile period lasts some 3 days. During this period 
the typical male pigmentation develops, together with the shorten- 
ing of the forward end, which is also a characteristic male feature. 
In the later part of the sessile period spermatogenesis is initiated. 

After 3 or 4 days on the proboscis lappets the developing male 
creeps into the foregut of the female, where development as a male 
is completed; and after 16 or 18 days from the beginning of male 
differentiation the adult male makes its way into the uterus of the 
female by way of its external opening. During the time spent on 
the proboscis the developing male takes no food, living on its stored 
yolk. The food relations of the male during its life in the gut and 
later in the uterus, though presumably of a parasitic nature, are not 
exactly known. 

There is good evidence, both from vital staining and from experi- 
mental removal of the sessile males at different periods after attach- 
ment to the proboscis lappets of the female, that the attached larvae 
receive material from the female which induces male development. 
If the larvae are removed after a very short time on the proboscis 


CROWDING AND SEX DETERMINATION 293 


and then are carried along as free-swimming larvae, there is less 
tendency toward male development than is shown by other larvae 
left longer on the proboscis. However, the sex determination in the 
attached larvae is very rapid, and in a few hours can progress far 
enough to produce intersexes. In one experiment 16 larvae left on 
the proboscis for between 35 to 8 hours after attachment showed no 
ccmplete females, 2 slightly female intersexes, 2 male hermaphro- 
dites, and 12 complete males. 

Experiments with stains show that male characteristics begin to 
appear at the anterior end and gradually spread through the body 
(Baltzer, 1928a). Larvae which have been left for several hours on 
proboscides dyed blue and then transferred for 2 or 3 days to pro- 
boscides colored with a red stain, show the blue limited to the ante- 
rior part and the red distributed over the whole body. So with the 
male-determining substance, a smaller quantity gives a narrower 
action radius while a longer attachment with more absorption acts 
over the whole body. 

When larvae are experimentally reared in vessels lacking females, 
and when those starting to change in the female direction are re- 
moved, it is found that the great majority of the animals become 
females but that those transforming near the end show intersexual- 
ity, including even spermatogenesis. Under similar conditions 
Herbst (1928) found male development to be brought about by 
the use of a weak solution of HCI; the best results were obtained with 
an N/400 solution. In these experiments not all larvae transformed 
into males. This observation of Herbst may account for the occa- 
sional transformation in the male direction in Baltzer’s experiments 
just recorded. 

When larvae are reared in the presence of older females, or in the 
presence of female proboscides, the majority of the larvae settle 
upon the proboscides and develop into males. Similarly, extracts 
of proboscides or of intestinal issue of females placed in sea-water 
with the indifferent larvae bring about male differentiation. Ex- 
tracts from muscle tissue made in the same manner did not have 
this effect. Extract of intestinal tissue gave better results than 
did that from proboscides, but in both instances there was slower 


2094 ANIMAL AGGREGATIONS 


initiation and the final results were less complete than when attach- 
ment to living proboscides was allowed. 

The situation, so far as Bonellia viridis is concerned, has been 
summarized by Baltzer as essentially follows: 

Sex determination is partly predetermined, partly epigenetic. 
Both sex tendencies are probably predetermined in the fertilized 
egg in varying degree, with the male tendency predominant. The 
fertilized egg, as well as the indifferent larvae which is not yet sex- 
ually differentiated in its organization, 1s most probably hermaphro- 
ditic. Only in their further development do they become wholly 
male or female. In this development the evolution of the organiza- 
tion in males or in females proceeds in an unlike manner, cor- 
responding to the strong sexual dimorphism of the species. 

With few exceptions the development of males is possible only 
when the indifferent free-swimming larva finds an opportunity to 
live parasitically on the proboscis of an adult female. During this 
period there is an absorption of sex-determining substance from the 
host by the larva. If this opportunity for parasitism exists, all the 
larvae usually become males. If it is lacking and the larvae are 
forced to live freely, females drise almost exclusively, though there 
are occasional hermaphrodites. Females appear only after a longer 
indifferent period during which development is approximately at a 
standstill; during this time the female tendency gradually gains in 
intensity and finally assumes ascendancy over the male tendency. 
The females that arise late are protandric hermaphrodites and 
contain sperm in the coelom, but in further development become 
typical females. 

If opportunity for parasitic development is given to swarming 
indifferent larvae, and if the parasitism is prematurely interrupted 
by separating the larvae from the proboscis of the host, then, after 
further culture in the free state, hermaphrodites arise, together with 
a few males and females. The duration of the parasitic period de- 
termines whether hermaphrodites actually bisexual are obtained, 
or gynandromorphs in which only the secondary sexual characters 
are combined. In Bonellia these secondary sexual characters include 
most of the organizational characters. 


CROWDING AND SEX DETERMINATION 295 


The method of effecting male determination in the case of the 
Bonellia larvae appears not unlike that which effects the transfor- 
mation of a zygotically determined female calf embryo which is in 
blood-stream connection with a twin of the opposite sex. The well- 
known work of F. R. Lillie (1917) has shown that under these con- 
ditions a transformation in the male direction takes place, due to 
the effect of a hormone produced by the male upon the organs of 
the developing female. 


SEX IN CREPIDULA 


In the gasteropod mollusk, Crepidula plana, Gould (1917) has 
shown that the sexual life of adults of this marine snail may be divid- 
ed into (a) the male phase, (6) the transitional phase, and (c) the 
female phase. In other words, these animals are protandric her- 
maphrodites with the opposed sexual phases completely separated. 
The development of the male condition does not a!ways take place 
at the same stage with respect either to age or to size of the individ- 
ual, and at times may be omitted entirely. 

Male development occurs, if at all, during the early life of the 
adult. The growth of the animal during this period is highly vari- 
able, depending, among other factors, on the amount of movement, 
the extent of available space, and the season of the year. Clearly 
distinguishable primordial male and female germ cells are both to 
be found in the gonads of these animals from the postlarval stage 
up to the time of complete female development, and can be dis- 
tinguished from each other under the microscope. During the period 
of transformation from the male to the female phases, the testis 
degenerates; and finally all the primordial male germ cells disappear 
as the gonad becomes reduced in size. With the assumption of the 
female phase the gonad again enlarges. 

The same duct serves for the passage of the sperm in the earlier 
phase and the eggs in the later, but it undergoes marked changes in 
the transition. Secondary male characteristics, such as the penis, 
the sperm groove, and the seminal vesicles, are present only when 
the testis is developed; they appear when the testis appears and dis- 
appear when it degenerates. 


296 ANIMAL AGGREGATIONS 


The variability of the male phase is summarized by Gould (19172) 
as follows: ‘“‘A number of specimens of the same size and apparently 
of the same age, taken at the same time of year, may show widely 
different sexual states. One may be a fully developed male; one 
may exhibit evidence of having been a male, though the male 
characters are being lost; and one may furnish no suggestion that 
any male characters have ever developed.” 

An analysis of these conditions shows that the development of 
the male phase is dependent on the nearby presence of a larger 
individual of the same species, which is usually, though not neces- 
sarily, a female. The greater the difference in size between the 
smaller and the larger animal, the more certain and complete is 
the development of the smaller in the male direction. A greater 
stimulus is necessary to complete male development than to initiate 
it. 

In nature these snails develop from free-swimming veliger larvae 
which settle within the gasteropod shells occupied by hermit crabs. 
If the snail which has formed the shell has recently died, the Crepidu- 
la population will be small; but in the shells long occupied by the 
crabs, a large number of Crepidula may collect. Those located near 
the outer margin will show little sign of movement. Their own shells 
will have grown to fit the irregularities of the substratum. Usually 
these are large females. Crowded about them, occupying vacant 
patches, and back in the deeper recesses of the shell, the Crepidula 
are smaller; and, since their shells are not intimately fitted to their 
surroundings, it may be assumed that they move about. 

If a larval Crepidula plana settles into a shell where no larger 
members of its species are present, the male phase is normally 
omitted. In cases where Gould found neuters and returned them 
to hermit crab shells free from large individuals; he recovered at the 
end of 34 days 24 specimens, which, when sectioned, showed 3 with 
adult testis, 2 with spermatids, 1 with spermatocytes, 11 with sper- 
matogonia only (that is, no further development toward sperma- 
tozoa), 5 sexually inactive, and 2 with odgonia. Of 2 specimens 
found to be neuters, isolated on hermit crab shells and sectioned 
after 67 days, 1 had early odcytes and the other had ova with yolk. 


CROWDING AND SEX DETERMINATION 297 


Of 4 others tested after 75 days, 3 were definitely females, while the 
other, the smallest of the lot, which had been closely associated 
with a larger specimen, was a fully developed male. 

In contrast with the preceding experience, when known neuters 
were transferred to a colony containing larger individuals, there was 
a definite development in the male direction regardless of the sex 
of the large individuals. In one experiment Crepidula examined 
and found to be in a neutral stage were transferred to the vicinity of 
large females. At the end of 34 days the specimens were sectioned 
for microscopic examination. Of 27 individuals so treated, 18 had 
adult testis, 6 were in the spermatid stage, 2 had spermatocytes, 1 
showed spermatogonia only, while none of the entire lot remained 
in the sexually indifferent stage and none had developed in the 
female direction. 

When small males were placed near larger males in an attempt to 
find the effect of such proximity upon the continuation of the male 
phase, it was found that fewer small males underwent degeneration 
of the sexual organs than when they were completely separated from 
larger animals. More showed degeneration than if they were in the 
presence of large females. Even a large animal with a degenerate 
testis will give a stimulus toward male development to a smaller 
one near-by. An immature female, formerly a male, will also have 
the same effect. After degeneration of the testis and the accompany- 
ing secondary organs, these may be regenerated if the individual 
comes under the influence of a larger animal; but the largest animal 
in an experimentally arranged colony never shows regeneration of 
the testis following degeneration. There is no evidence that a small- 
er animal can affect a larger one or that a group of smaller ones can 
have this effect, although the last point does not seem to have been 
specifically investigated by Gould. 

A mature or nearly mature female does not undergo degeneration 
of sexual organs however placed; but if a partially developed female 
is placed near a large, mature female, the female sex organs degen- 
erate and the male organs develop. 

The degeneration of the testis in the absence of a large female 
is not due to the lack of opportunity for copulation, for degeneration 


298 ANIMAL AGGREGATIONS 


does not take place when a small male is near a larger one. Though 
males are normally more motile than females, movement is not nec- 
essary, as is shown by those males the conformation of whose shells 
proves that they have not recently moved about. Neuters will de- 
velop into males without showing movement. Male development 
here is not a matter of food, for neuters will develop into males re- 
gardless of the richness or scantiness of the food supply, provided 
only they are near large animals. Starving or feeding mature males 
does not affect their state of sexual development if they are near 
large females. 

Preliminary experiments designed to test for the presence of a 
secretion from the larger animal which would affect the development 
of the sexual organs of the smaller have been run without positive 
results. There is no specific ovarian secretion concerned, since the 
presence of a large male has somewhat the same effect as that of a 
female. Males in one finger bowl were covered with water from 
another finger bowl which contained a number of females. The 
water was changed daily and was replaced with water in which the 
females had stood. Degeneration of the male organs occurred. Simi- 
lar results were obtained when water from a dish containing 20-25 
large females was led into a finger bowl containing 20-25 males. 
The experiment ran for a month, and during this time the males 
were kept separated as carefully as possible. Again the degenera- 
tion of the sexual organs was shown by the condition of the penes. 
A similar experiment in which water from the mature females flowed 
over a number of small neuters did not reveal any tendency for penis 
development. Neither was there any initiation of male development 
from adding extract of crushed adult females twice daily to finger 
bowls containing neuters. 

The stimulus to sex development or recession does not depend 
on the presence of the hermit crab with which these Crepidula are 
normally associated in nature, since the whole gamut of sex can be 
run under the artificial conditions of the laboratory, with glassware 
in place of shells, and with hermit crabs entirely absent. 

Gould recognizes that his experiments concerning the causal rela- 
tions involved in sex determination in Crepidula are inconclusive. 


CROWDING AND SEX DETERMINATION 299 


They do not make clear whether the transformations affected by 
the presence of the large individual can take place in the absence 
of physical contact. They are not conclusive concerning the possi- 
ble presence of some chemical product or the effect of such a product 
transmitted through the sea-water; and the biophysical possibilities 
remain to date unexplored. All we know is that in some manner the 
presence or absence of larger individuals affects the state of develop- 
ment of the primary and secondary sex organs of associated smaller 
individuals, which is exciting enough information to deserve greater 
experimental attention than has been so far given to it. 


SEX IN NEMATODE PARASITES 


While working on grasshoppers to determine the fatal dose of the 
eggs of one of their parasites, the nematode hairworm, Mermis 
subnigrescens, Cobb, Steiner, and Christie (1927) discovered that 
the sex of the resulting adult parasite was male or female according 
to the size of the dose. The parasite is an important factor in the 
control of grasshoppers, and the experiments were primarily con- 
cerned with this aspect. 

The parasite M. swbnigrescens is found quite commonly in a num- 
ber of varieties of common grasshoppers: for example, the red- 
legged grasshopper Melanoplus femurrubrum and its relatives. The 
grasshoppers become parasitized by swallowing eggs of the parasite 
that have been deposited on their food plants; these eggs contain 
well-developed hairworms. 

Cobb, Steiner, and Christie (1927) state that their researches have 
shown that “when immature females of the hairworm are kept in 
solitary confinement before they can have copulated with males 
they produce viable eggs.’”’ In thousands of observations made by 
these workers the average number of hairworms per infested grass- 
hopper in nature has been from 1 to 3, and these are always females. 

Usually overinfestation of a grasshopper by hairworms resulted 
in the premature death of the grasshopper as well as of the para- 
sites, though on rare occasions highly infested grasshoppers have 
been found in nature containing more than a hundred hairworms. 
In all such extreme cases the parasites were small and young. These 


300 ANIMAL AGGREGATIONS 


observations led to experiments designed to determine the dose of 
hairworm eggs that would be fatal to the host; for the present pur- 
pose it may be said that for young grasshoppers in the second instar 
it averages well under 50 eggs. These later experiments led to the 
discovery that when a slightly sublethal dose of eggs is given, the 
resulting parasites are all males. In one case where 20 hairworm 
eggs were fed to a grasshopper first freed from this parasite, 19 para- 
sites were recovered, all males. Many tests and observations were 
made, and all led to the same conclusion: Feeding a very few eggs 
resulted in female parasites; feeding a large number, but not quite 
enough to kill, resulted in male parasites. 

Similar experiments in treating the larvae of a midge (Chirono- 
mus) with the hairworm Pseudomermis zykoffi gave similar results. 
High parasitism resulted in the parasites being all males. Similar 
observations were made on the hairworm A gamermis paradecaudata 
infesting the tea bug Helopeltis theivora. No artificial infestation was 
made in this case; the observations were confined to the parasite as 
it existed in nature. Here, again, when there were a large number 
of parasites all of them were males. 

Observations on the mirmithid Allomermis sp. infesting the 
common dooryard ant Lasius niger americanus showed that when 
parasitism is high the parasites are all or nearly all males, and that 
when it is low the parasites are all females. 

The phenomenon is the same in hairworms of four different gene- 
ra, found in host insects of four different orders. Between the ex- 
tremes of parasitism in all these cases there is a gradient with mix- 
tures of males and females, the proportion of the males increasing 
with the severity of the infestation. 

In the case of the hairworm Mermis subnigrescens, Cobb and his 
associates think (Christie, 1929) that by appropriate experiments 
they have excluded other possible explanations of these results; 
that they were not brought about by using the eggs from an individ- 
ual hairworm, nor from a definite part of the uterus having a uniform 
sex-potentiality, nor by death or other selective elimination. The 
most convincing data at hand on these points are those collected by 
Christie (1929). Table XXXIV shows the results from one set of 


CROWDING AND SEX DETERMINATION 301 


experiments in which he fed a known number of M. subnigrescens 
eggs to the common New England grasshopper, Melanus femur- 
rubrum. 

In four cases of relatively heavy infestation a total of 100 eggs 
were fed, from which 86 parasites were reared, all of which were 


TABLE XXXIV 


SHOWING THE RESULTS OF FEEDING GRASSHOPPERS WITH 
KNown NuMBERS OF MERMITHIDAE EGGS UPON THE 
SEX OF THE DEVELOPED NEMATODES 


(Data from Christie) 


Number of 
Number of Eggs Fed Parasites Males Females 
Obtained 
3 Onna erin Suunto ae 25 25 ° 
BON evshetagaya ten (ons foe aravs it 24 24 ° 
BOM Ger TM nyse tcl nih aie 19 19 fo) 
DOr Eaters: Mes Oe oe 18 18 fe) 
re a ne eee 2 ° 2 
GPa cree nie ete eyevecnegne 2 fo) 2 
Tas Ger Pa Rene See 5 ° 5 
Ce eRe een ee 4 ° 4 
Ginn ciel arep Me Se lews ese ee 8 3 ° 3 
I Pca SERECR OE eto A ° 3 
em meee ER Ean Scart B ° 3 
it: HERO ERE 5 I 4 
SO aetn eie oie cle a ae 4 I 3 
Ger stae ope aiere sieges I fo) I 
ec as eae ae 5 I 4 
5. Pre Meio 4 I 3 
A ern yaeas eat eee eel 4 fo) 4 
Le GaN PRA RS SE ee 4 fo) 4 
SEL: OA, IAW Ss B ° B 
BE ete es sooner. ane 5 I 4 
araering eee aes Reed aa 4 ° 4 
[te ae a te eae 4 I 3 
ite Valances ei ot tence 4 fe) 4 
hea eed nera a tet eae one 2 fo) 2 
Se isin hy chswmerom init 2 ° 2 


* Camnula pellucida used as host insect. 


males. In the relatively light infestations, 102 eggs were fed, usual- 
ly 5 to each grasshopper; of these, 73 parasites were reared, of 
which 6, or 8 per cent, were males. 

Caullery and Comas (1928), stimulated by the early reports of 
the work just reviewed, examined the distribution of sexes in the 
nematode worm, Paramermis contorta, which is a parasite upon a 


302 ANIMAL AGGREGATIONS 


chironomid larva. A part of their results are summarized in Table 
XXXV. They interpret their findings and the findings of Cobb, 
Steiner, and Christie (1927) as meaning that in these parasitic 
nematodes sex depends largely on the number of parasites simultan- 
eously present, and on the resulting nutritive values of the host. 
When a single parasite is present, some genotypic males appear to 
have been inverted into phenotypic females. When two are present, 
the normal genotypic relation is not sensibly altered. With higher 


TABLE XXXV 
SHOWING THE NUMBER OF NEMATODE PARASITES PER CHIRONOMID 
LARVA IN RELATION TO SEX DISTRIBUTION 
(Data from Caullery and Comas) 


No account is given here of the intersexes also reported. 


Number of Parasites Number of Cases Ratio of Females 


per Larva Observed Females Males to Males 
LOG COTE ne oe 272 255 17 15.0 
DUR crsievaie aiteancoteeute 107/83 180 166 1.14 
Bitoni tare eee 43 47 82 0.57 
MAU c cusses then ererers eee 16 23 41 0.56 
Graded yn Sector 6 5 25 0.2 
Oars arses tun ores fe 3 3 15 0.2 
1 Bekok ALTER RE ECR 2 2B II 0.27 
Optics athe ster I I 8 0.125 
NOs vaeasyaeeers usec nk 3 4 20 0.154 
Tele eC eer I 2 9 0.22 
Ly fe ach nm Sree Presi Biot I 2 15 0.133 


numbers in the nematode population of individual bloodworm hosts, 
genotypic females apparently were transformed into phenotypic 
males. The production of females is not quantitative at any level 
in Caullery and Comas’ work, but it may be summarized as showing 
that the more parasites present in a given host, the fewer females 
in the population. 

There seems no escape from the conclusion that we are dealing 
here with cases where the numbers of individuals of the same species 
in the immediate environment is a sex-determining factor for all 
those present. Here, even less than in the preceding similar case of 
Crepidula, has analytical work been done. We do not even know, 
for example, whether nematodes of different species present in the 


CROWDING AND SEX DETERMINATION 303 


same host will have the same effect as if all were of the same species; 
nor do we know whether the effect of malnutrition or other adverse 
environmental influences upon the host will affect the sex ratio of 
the parasites. Christie has noted that with heavy infestations there 
is a reduction in size of individuals present, whether males or fe- 
males, but that there is no tendency toward a differential suppression 
of the gonads. He finds that there is a tendency for the parasites in 
a heavily infested host to complete their parasitic development more 
rapidly than those in a lightly infested animal. While this applies 
to both sexes, it is more apparent with the females. 


SEX IN CLADOCERA 


The sex situation in the cladoceran Crustacea differs from the 
instances previously discussed in that with these animals the usual 
method of reproduction is parthenogenetic: the females produce 
eggs that develop without fertilization to form other females. In 
nature, after a period of purely parthenogenetic reproduction of this 
sort, there may occur an outbreak of bisexuality in which parthe- 
nogenetic eggs develop into males,’ and at the same time, or, more 
usually, slightly later, eggs are produced which require fertilization 
before development. The eggs so produced are resistant to many 
adverse conditions, such as drying or freezing. With favorable con- 
ditions some of them, after a dormant period, produce partheno- 
genetic females, and the usual type of reproduction begins again. 
In the other cases cited, the effect of crowding upon sex dealt with 
the effect on the expected sex ratio; here we are interested in its 
effect on the transfer from the production of parthenogenetically 
produced, self-sufficient females to the production of parthenogene- 
tically produced males. 

It is interesting to note that Banta and Wood (1928) report genetic evidence that 
males of the cladoceran, Daphnia longispina, are diploid, and that E. Allen (1928) finds 
in cytological studies of Moina macrocopa: “After the egg is laid, the first division occurs 
in the parthenogenetic egg without reduction in the number of chromosomes. In the 
sexual egg, the first maturation division results in the haploid number, which is eleven. 
The diploid number is twenty-two in both types of egg. In the eggs of crowded mothers, 
which should produce a high percentage of males, no evidence has yet been obtained 


indicating that the male number is haploid. Several such crowded mothers have been 
studied.” 


304 ANIMAL AGGREGATIONS 


Many workers have united in believing that the change from 
parthenogenetic to sexual reproduction is due to internal factors, 
perhaps to an innate sexual cycle. The problem has attracted much 
attention because of the frequent coincidence between the appear- 
ance of sexual forms, the production of the resistant sexual egg, and 
the beginning of periods of special environmental stress. The whole 
relationship appears to be highly adaptive in that the resistant egg 
allows the species to survive periods of drought or of low tempera- 
ture which would otherwise be fatal to it. The phenomenon is the 
more impressive when we find that if a given pond contains more 
than one cladoceran species, all show development of sexual repro- 
duction at approximately the same time. 

In the laboratory, Banta has shown that one cladoceran, Moina 
macrocopa, can be reared for at least 780 asexual generations with 
complete vigor, and that another, Daphnia pulex, can be grown 
similarly for at least 767 generations without the appearance of 
sexual forms, providing cultural conditions remain favorable. Thus 
it appears that the hypothesis of an innate sexual cycle will not hold 
as a step toward a universal solution of the problem. 

Grosvenor and Smith (1913) suggested, as a result of their experi- 
ence with another species of Moina, that the production of males is 
initiated by the accumulation of waste products due to the crowded 
condition of the mothers. Langhans (1909) had shown that accumu- 
lations of excretory products reduce growth and reproduction and 
hence presumably favor the production of sexual forms. Papani- 
colau (1910) confirmed an earlier report of Langhans that repeated 
transfers of the Daphnia to fresh water postpones the appearance of 
sexual forms indefinitely; but as a result of further work (1910a) he 
concluded that crowding serves only to decrease numbers. Mc- 
Clendon (1910) also observed a hastening of sexual reproduction 
which he interpreted as being due to excretions. Smith (1915) 
believed the effect of crowding is due to the excretions which ac- 
cumulate, rather than to deficiency of food. Hartmann (1919) con- 
sidered the accumulation of excretions a probable cause of the out- 
break of gamic reproduction. 

Banta and Brown, reporting in a series of papers beginning in 


CROWDING AND SEX DETERMINATION 305 


1923 and not yet finished, have found that crowding of the mothers 
in 2 species of Daphnia, 3 of Simocephalus, 3 of Moina, and 1 of 
Ceriodaphnia causes a large number of males to appear. Agar has 
recently informed me personally that he has confirmed these results 
“to the hilt,” although his published work (1914) showed that males 
will appear without crowding and with frequent changes of medium. 
Shelley (1929), working with another species of Daphnia (D. magna) 
in this laboratory, has obtained results similar to those reported by 
Banta and Brown. There can be no doubt of the fact. 

The later work of Banta and Brown gives an opportunity to show 
the extent of the changes produced in the sex of the offspring. In 
all they have performed some 2,900 experiments dealing with the 
effect of crowding on male production in cladocerans. One lot of 
rog uncrowded mothers which produced 1,954 young, averaged 
only o.3 per cent males; 5 males were produced by one mother, and 
a single male by another. On the other hand, 33 moderately crowd- 
ed culture bottles which contained from 7 to 14 females, usually 
ro per culture bottle, gave an average male production of 41.6 per 
cent out of 3,638 young whose sex was determined. Six of these 
bottles failed to produce males; 2 produced too per cent males. 
In heavily crowded bottles, with 15 to 24 mothers to the bottle, 
from 2,240 young whose sex was determined, 62 per cent were males. 
One bottle failed to produce males, and 4 produced too per cent 
males. They regard these as typical results. It will be noted that 
the effects of crowding are not uniform and that quantitative male 
production does not occur in all the bottles of a densely crowded 
series. 

We find that Banta and Brown conclude from their experiments, 
as Grosvenor and Smith did earlier, that the chief factor in stimu- 
lating male production in Moina is the accumulation of excretory 
products. Further, they present evidence which shows that this 
effect of crowding can be produced by the presence of other genera 
of Cladocera or by other aquatic animals, such as Planaria, Asellus, 
Physa, insect larvae, small fish, or frog tadpoles; monotypic excre- 
tory products are unnecessary. Here, in the heterotypically pro- 
duced adverse conditions, there is a lack of species specificity of 


306 ANIMAL AGGREGATIONS 


effects, just as we have already seen a lack of species specificity in 
group protection from toxic solutions or hypotonic sea-water. These 
excretory products effective in male production are apparently not 
volatile, but they are unstable compounds readily made non-effec- 
tive by a variety of treatments. Standing for a short time, or treat- 
ment with NaOH or H.SO,, or a greater dilution of the medium, is 
effective in causing a lack of potency for male production. 

The factors causing production of sexual eggs are apparently 
different from those causing males to appear. Sexual eggs are 
produced only rarely in the crowded bottles under the conditions 
used to produce males. In Moina macrocopa they are produced by 
rearing females in clear pond water, or in very dilute food, or in old 
culture media. Banta and Brown believe their experience shows 
that scarcity of food controls the production of the sexual egg, as 
the accumulation of excretory products as a result of crowding 
controls the appearance of males. 

Further studies have indicated that the time at which the sex 
of the forthcoming young of this cladoceran is determined lies about 
4 hours before the parthenogenetic eggs are laid, when the tempera- 
ture stands at 20° C. (Banta and Brown, 1929)). Observation that 
there was some relationship between the time of the release of a 
female’s first brood and the sex of the young led to definitive exper- 
iments concerning the relationship between the rate of the mother’s 
development and the sex of her offspring. The evidence from these 
experiments is summarized in part in Figure 30, which indicates 
that the production of males is closely associated with a reduction 
in the rate of development of the females producing them. Appar- 
ently this is due to the accumulation of excretory products in the 
crowded bottles, since the reduction in time of production of the 
first clutch of young is proportional to the degree of crowding, and, 
further, the percentage of males produced is proportional to both. 

Tests were made (Banta and Brown, 19209d) to find whether 
experimentally changing the rate of development would affect the 
sex ratios of the produced young. Treatment with small dosages of 
ethyl alcohol and with filtrates from dried adrenal cortex, thyroid, 
thymus, and muscle tissue serves to increase the rate of development 


CROWDING AND SEX DETERMINATION 307 


of M. macrocopa females as measured by the time before the pro- 
duction of the first (parthenogenetic) brood. When tested under 
crowded conditions, the retarding effect normally accompanying 


80 
72 


64 


o 


oung in per cent 
3 


he 


Male 
8 


Flours of retardation 
9 10 Wl 
12 16 24 


No. mothers per bottle 


Fic. 30.—Diagram, from Banta and Brown 1929), showing the relation (A) between 
the amount of retardation of the production of young and the percentage of males pro- 
duced in Moina macrocopa, and (B) between the number of mothers per bottle and the 
percentage of males produced. 


308 ANIMAL: AGGREGATIONS 


crowding was largely counteracted by the foregoing treatment, and 
the percentage of males expected from crowded cultures was ma- 
terially reduced. Considering the wide range of tissue extracts used, 
the authors appear to be correct in regarding the observed results 
as due to some generalized factor, such as a change in the bacterial 
flora, rather than to the action of a specific tissue product. 

The converse experiment, in which the general metabolic rate was 
experimentally depressed by the use of chloretone or of potassium 
cyanide in proper concentrations, caused the Moina tested to reach 
the reproductive age more slowly. The females so retarded produced 
a much higher percentage of males among their first-broods than did 
their untreated sisters, which were subjected to similar conditions 
except for the treatment with depressing agents. 

Stuart and Banta (1929, 1931) report controlled bacterial studies 
which indicate that sister Moina females cultured in equal numbers 
have the sex of their offspring determined in part at least by the 
numbers of bacteria present. ““These females in the highly bacterized 
bottle produce only female young. Between these two extremes there 
is a very uniform gradient in male percentage.” These authors in- 
terpret their results as indicating that quantity of food appears to be 
the determining factor, which causes speculation as to whether the 
previous analysis of the situation by Banta and Brown is final or 
whether their findings may not be bound up with as yet unexplored 
differences in bacterial flora accompanying crowding. The latter is 
indicated by the conclusion of Stuart and Banta (1931) that: “Sex 
of the young produced by experimentally crowded Moina macrocopa 
mothers can be controlled by the amount of available food (bacteria) 
present in the medium. The bacteria present in the medium do not 
appear to influence the control of sex by the adsorption of excretory 
substances.’ Here, as so frequently in this survey, we can be certain 
of the facts of the effects produced by crowding, but, even with a 
large amount of analytical work, we are not yet sure of the causal 
factors involved. 

There is no question but that sex is one of the most nearly funda- 
mental properties of animals. On this account its determination has 
aroused great interest, and an enormous literature on the subject has 


CROWDING AND SEX DETERMINATION 309 


resulted. Much evidence has accumulated of recent years that sex 
determination at or before fertilization is not the hard and fast fact 
it was regarded to be in the first blush of enthusiasm over the dis- 
covery of the sex chromosomes. The work of Lillie, Goldschmidt, 
Crew, and Domm has shown a much greater fluidity than was at 
first anticipated in the light of the chromosome theory. The results 
of metabolic effects produced by crowding, reported here for mon- 
strillid copepods and Cladocera among the Crustacea; for Bonellia, 
the aberrant annelid worm; Crepidula, the gasteropod mollusk, and 
certain of the nematode worms, are not out of harmony with other 
instances of epigametic sex determination produced by other means. 
From our present knowledge it appears that sex determination, at 
least for the majority of animals, is normally associated with the 
chromosome mechanism, but that chromosome determinations may 
be overruled by other factors, among them the effects produced by 
crowding. In certain cases these effects appear to be produced by 
the transfer of material from one individual to another, as in 
Bonellia; in others, as Cladocera, by the effects of crowding upon 
animals that are not necessarily in physical contact with each other. 
From our point of view the important thing is that they occur at 
all and can be controlled, in certain animals of widely distributed 
taxonomic position, by the degree or kind of aggregation obtaining. 

When we examine the possible survival value of the effect of 
crowding upon sex, we find a mixed situation. In the monstrillid 
copepods crowding has negative survival value, in that a normal sex 
ratio among uncrowded animals gives way to an overproduction of 
males among crowded individuals. With the parasitic nematodes 
the high proportion of males accompanying heavy infestations would 
appear to be as definitely harmful for the race as their absence in 
cases of low infestation. Here is evidence of an optimum popula- 
tion, where males and females are in fairly equal ratio, well above 
the minimum population, as we have seen in other relationships. 
It may be suggested that there is a more remote survival value 
in the overproduction of males, in that the neighborhood is less 
likely to be overstocked, to the destruction of all. 

With Bonellia and Crepidula the tendency for an isolated indi- 


310 ANIMAL AGGREGATIONS 


vidual to become a female and for those appearing later in the im- 
mediate vicinity to become males has survival value. Baltzer con- 
sidered this point in 1914. He reports that Bonellia is not an abun- 
dant animal, and occurs in numbers in only a few places in the Bay of 
Naples. If one supposes that the males remained parasitic but, un- 
like their present condition, were unalterably predetermined for 
maleness, then a male larva would be useful only if it found an 
adult female. All males which did not by chance come into the 
neighborhood of an adult female must die without issue. This would 
include a large number of larvae, not only because the species is 
relatively rare, but because the larvae are positively phototactic at 
the beginning of their free period and frequently swarm away from 
their egg mass, even though they are able at this early age to 
attach to the proboscis of a nearby female. (It appears that these 
larvae may not attach to the proboscis of their own mother.) By 
means of their special method of sex determination this potential 
loss of large numbers of males is avoided, since if they reach a suit- 
able environment lacking females they develop directly into females 
and are able then to effect the transformation in the male direction 
of the next larvae to arrive in that vicinity. The survival values of 
the Crepidula sex situation are similar. 

Essentially the same situation is found in Cladocera. With these 
animals the tendency to change, as the culture becomes crowded, 
from parthenogenetic eggs rapidly produced but not resistant, to 
the sexual eggs which can withstand adverse conditions, has sur- 
vival value. In nature such crowding usually precedes the drying-up 
or freezing of small bodies of water in which the animals have been 
living; and either usually follows a long reproduction period which 
has given time for the increase of the cladoceran population to ef- 
fect a definite change in the environment. Under these conditions 
the production of resistant eggs has definite value. 

It appears, therefore, that in 4 out of these 5 cases, in which 
crowding is known to affect the sex ratio, the result has survival 
value. 


CHAPTER XVIII 
MORPHOLOGICAL EFFECTS OF CROWDING 


In addition to its effect upon primary sexual characters through 
sex determination, which has been discussed in the preceding chap- 
ter, crowding may produce morphological changes in secondary 
sexual characters especially noticeable in animals such as Bonellia, 
which have a strong sexual dimorphism. Crowding also exerts de- 
cided influences upon structures, entirely apart from its effect upon 
sex. Casual observation shows that a tree grown in the open coun- 
try has a different growth form from that shown by the same sort 
of tree grown in a forest. Similarly, when sessile marine animals, 
such as barnacles, ascidians, corals, sea anemones and sea mussels, 
grow in closely packed masses upon a rock or wharf piling, their 
growth form differs from that shown if they grow separately. With 
these sessile animals the changes associated with crowding appear 
to be due largely to the limitations imposed by the physical contacts 
established under conditions of close aggregation. 

The effect of crowding upon physical form is not limited to sessile 
organisms. There is the well-known case of the free-swimming pelagic 
tunicate, Salpa, with its alternation of the asexual solitary, casklike 
form with the quite differently shaped aggregated form of the mem- 
bers of a Salpa chain. These latter are sexual individuals that, ap- 
parently because of crowding, have lost their regular barrel-like ap- 
pearance and are rather rounded, ovoid, or fusiform. 

With non-sessile animals which lack the physical connections of 
a Salpa chain, even though the crowding be less dense, there are 
cases of marked morphological changes other than mere decrease 
in size. Whitefield (1882) observed that Lymnaea, a snail, when 
grown in a small volume of water for three successive generations, 
had not only become much reduced in size but had suffered other 
physical changes as well: the male organs did not develop, and the 
liver was also much smaller in proportion to the other organs, as 


311 


Bie ANIMAL AGGREGATIONS 


compared with that of related snails grown under uncrowded con- 
ditions. Whitefield says that the shell proportions were so changed 
that experienced conchologists recognized the dwarfed forms as 
sufficiently distinct to be placed in a separate species. 

Differential morphological effects associated with partial starva- 
tion are well known. Child (1915) has shown that starving adult 
planarian worms will cause them to return to a juvenile condition. 
The culture water in which Planaria have been crowded definitely 
depresses normal head formation in regenerating planarians (Child, 
to1r). In analyzing this effect, Child reports (1911a): “The pres- 
ence of metabolic products of Planaria in the water undoubtedly 
decreases the rate of metabolism, and the effect on regulatory 
morphogenesis is similar to that of starvation or low temperature, 
though it may be greater and in extreme cases it approaches that 
obtained with anesthetics.” 

Vernon (1903) has reported differential inhibition in echinoderm 
larvae due to crowding. He attributes this to food deficiency. 
Warren (1900) reports modifications of the spine of Daphnia brought 
about by crowding. Other such cases could be cited, but it will be 
more profitable to examine in some detail the evidence that crowd- 
ing affects (a) the appearance of winged forms in aphids, (0) colora- 
tion and changes in bodily proportions of certain grasshoppers, and 
(c) the suppression of certain bristles and eye facets in some races of 
Drosophila. 

WING PRODUCTION IN APHIDS 


Aphids show two changes in the course of their normal seasonal 
cycle, which may or may not be associated. Their annual history, 
briefly, runs as follows: In.the autumn sexual forms appear, and 
an overwintering fertilized egg is formed which hatches out the 
following spring into a wingless female capable of producing young 
parthenogenetically. These, too, are usually wingless females capa- 
ble of parthenogenetic reproduction. After a time winged forms 
appear which migrate to new host plants where they give rise to 
wingless females with parthenogenetic powers. The production of 
winged forms is thus a process distinct from the production of the 
sexual forms. In the case of the latter in most, but not all, aphids 


MORPHOLOGICAL EFFECTS OF CROWDING hig 


the sexual female is wingless but the male is winged. The problem 
with which we are especially concerned here is the relation of crowd- 
ing to the production of the winged (alate) parthenogenetic forms 
from apterous mothers. 

Grassi (1907; vide Shull, 1929) ing reported that crowding of 
aphids of the genus Phylloxera on their food plant was followed by 
wing production. Davidson (1914), in speaking of his own work with 
the aphid Schizoneura and other species, attributes the appearance of 
winged forms to some change in the constitution of the cell-sap 
products of the overpopulated plant. He summarizes his work by 
saying: “Throughout my experiments it was observed that when 
the plant had finished its active growth, or became heavily infested 
with aphids, the changes resulting either in the quality or the quan- 
tity of the cell sap (or both) seemed to induce the production of 
winged forms.” 

Wadley (1923), using Rophalosiphum, reports in detail experi- 
ments which show that when aphids were allowed to multiply for 
two or three generations and to overcrowd the plants in their experi- 
mental cage, a high percentage of alate aphids were invariably pro- 
duced. Even when apterous aphids were removed from these crowd- 
ed plants and placed on new and flourishing host plants, their prog- 
eny, reared with abundance of food, gave from 30 to 86 per cent 
alate individuals, while the control gave from 7 to 20 per cent. 
Wadley attributes these results to the effect of limited nutrition, 
since he was able to obtain similar data from starvation without 
crowding. In both instances, alate forms tend to give rise to apter- 
ous forms, as they do normally under optimum conditions. 

Ackerman (1926) confirmed the results obtained by Wadley, using 
the same species of aphid. In one experiment with 776 aphids reared 
under crowded conditions, 34 per cent were winged; while with no 
overcrowding, all of 280 aphids were wingless. Ackerman also found 
that partial starvation tended to produce a high percentage of winged 
forms in the offspring. 

Reinhard (1927), using another species of A phis, has undertaken 
the most extensive set of experiments to date dealing with the effect 
of crowding upon wing production in aphids. He first tested the 


314 ANIMAL AGGREGATIONS 


effect of parentage upon wing production to see if there is an innate 
cycle which causes winged forms to appear at certain intervals. 
During a period of 12 months of continuous work, 59 complete 
generations were reared without the appearance of winged forms, 
except in 3 generations when the aphids became crowded. These 
results are supported by other data and indicate that with this 
species of aphid there is a normal tendency to be wingless and that 
the production of wings depends upon environmental influences 
rather than on an innate cycle, as many have thought (Shull, 1929). 
Reinhard’s experience with starvation supported that of previous 
workers. He found that starvation of apterous parents increased 
the number of alate individuals, while starvation of alate parents 
did not affect the normal tendency of such animals to produce 
wingless forms. 

Reinhard also found that with the species of aphid which he ob- 
served, the appearance of its winged phase was not determined by 
the temperatures to which the animals were exposed, nor by humid- 
ity. Other workers (vide Shull, 1929) have found that temperature, 
and light relations as well, do affect the appearance of winged aphids 
of other species. Having cleared the way by these preliminary ex- 
periments, Reinhard turned to more intensive experimentation upon 
the effect of crowding upon wing production. 

In his long experiments, he found no winged forms except when 
the plants were allowed to become crowded. Subcultures from this 
experiment were made at different generations. The aphids were 
isolated on uninfested plants, and they and their progeny were 
undisturbed until a crowded condition resulted. In each case these 
subcultures yielded alate aphids, even though the uncrowded main 
experiment did not. The results of these experiments are summa- 
rized in Table XXXVI. 

In another series of experiments wingless aphids of unknown 
parentage were placed on uninfested plants and allowed to develop 
crowded conditions. In 14 tests which are reported by Reinhard in 
detail the history of 438 aphids is given. Of these, 221, or slightly 
more than 50 per cent were alate; 5 winged aphids, or about 2 per 
cent, appeared among the 243 individuals reared under conditions 


MORPHOLOGICAL EFFECTS OF CROWDING 315 


similar, except that there was practically no crowding. Normally, 
as we have stated, the alate aphids give rise to wingless forms. 
Reinhard found that crowding these alate individuals caused an 
increase in the percentage of winged offspring in the next generation. 
In the light of these experiments, Reinhard concluded that “‘crowd- 
ing is a potent, if not the dominant, factor in controlling wing de- 
velopment in A phis gossypii.” While this conclusion appears entire- 


TABLE XXXVI 
DATA ON THE EFFECT OF CROWDING ON WING DEVELOPMENT IN APHIDS. 
THE CHECK PROGENY WERE SISTERS TO THOSE CROWDED, AND 
OF THE SAME GENERATIONS 


(From Reinhard) 


; CHECK PROGENY 
+ First WINGED = z 
DATE BOG YOuNG ForMS AFTER Not Crowbep 
GENERATION PARENTAGE Tareas Tee OBSERVED PROGENY 
SOEATED SO" | DEVELOPING WELL eee 
LATED WINcs CROWDED ee Alate 
1925 1926 
Oe Sivers cots Sh, | Apterous | Dec. 9 9 | Jan.9 Many 25 ° 
TO Metal nsosci bs fst | Apterous | Dec. 20 17 | Jan. 15 Many 19 ° 
1920 
ToS erase, 2 ie cia | Apterous | Jan. 9 Ir | Jan. 20 Few 32 4 
EO eects eeercee os Apterous | Feb. to 20 | Feb. 16 Many | 75 6 
EOE yer teas oe Apterous | Feb. 16 65 | Feb. 24 Predomi- 16 ° 
nate 
BON svar otaca) See es3 Apterous | April 17 1g | April 23 | Few II ° 
Ai eiavstatetetavees ‘she Apterous | July 25 13 | Aug.o Few 16 ° 
AS Wr arent rar < Apterous | July 28 to | Aug. 18 | Many 17 fe) 
Gitta pee ; Apterous | Sept. 15 9 | Sept. 23 | Many soit ° 
BO lsropanyavsicet yore o> = | Apterous | Oct QO) |) Oct: 19 Few 14 ° 
i i 


ly reasonable, it must be remembered that the factors involved in 
bringing about this result are not fully revealed. Observers agree 
that starvation is also an effective agent in producing winged aphids, 
and anyone who has undertaken to rear aphids under crowded con- 
ditions will appreciate the difficulty of separating these two factors 
with certainty. One of Reinhard’s observations indicates that 
crowding is the more effective agent of the two. Starvation did 
not cause winged forms to increase the number of winged progeny 
in the next generation, while crowding did do so. Other possible 
factors to be tested include the physical effect of the presence of 


316 ANIMAL AGGREGATIONS 


other aphids and the possibility of aphid secretions being transferred 
from one to another through the plant. Merely stating these alter- 
native hypotheses helps to emphasize the probability that changed 
or decreased nutrition is the dominating factor in the situation. 

This recital of references to observations by a number of workers 
in widely separated regions, and usually upon distinct species, indi- 
cates that the effect of crowding upon wing production must be a 
widespread phenomenon among aphids. The racial importance of 
this phenomenon is apparent when one remembers the added 
migratory power thus conferred upon members of a crowded colony 
living upon a host plant which may soon become exhausted from 
the feeding activities of its aphid population. 


CROWDING AND THE PHASE THEORY OF LOCUSTS 


In the following discussion it will be convenient to follow Uvarov 
(1928) in applying the name “locust” to gregarious members of 
the short-horned grasshoppers, family Acrididae, which migrate in 
swarms, while “‘grasshoppers,” in a restricted sense, will be used in 
speaking of the non-gregarious, non-migrating members of the same 
group. The gregarious collections of adult locusts will be spoken 
of as “swarms,” and the similar collections of immature hoppers 
(nymphs) will be called “bands.” 

In order to have clearly in mind the morphological relations of 
locusts and grasshoppers, it will be necessary to pass hurriedly in 
review some of the available knowledge concerning the behavior of 
these animals. The egg pods of the gregarious locusts are deposited 
close together so that when the young hatch and emerge from the 
ground they are immediately in close contact with each other. After 
their intermediate molt these recently emerged hoppers soon form 
primary bands, due largely to the reaction of the young animals to 
light and heat. On warm sunshiny days these gather in exposed 
sunny places; on cool wintry days the bands collect in sheltered 
spots (La Baume, 1918). In addition to these place aggregations 
there is said to be a distinct gregarious tendency which has not yet 
been analyzed to see whether it is a behavior unit or whether it may 
be split into more elementary reaction complexes. 

The alternation of behavior, night and day, appears to be related 


MORPHOLOGICAL EFFECTS OF CROWDING 207 


to that on cool days and on warm sunny days. The young hoppers 
spend the latter in groups basking in the sun, while at night or on 
cool days they crawl under stones or other shelter, or climb some 
plant. In either case they may be found in dense collections. Anal- 
yses to date do not show the relative importance of heat and of light 
in these reactions; but both, and particularly the former, are known 
to be of decided importance. Movement stops at night when the 
body temperature falls below the threshold for torpor, and begins in 
the morning when it rises above that level. A further rise in tempera- 
ture will send the hoppers into greater activity, which results in such 
a scattering that the surface covered by a band in full daytime activ- 
ity is three times that occupied by the same numbers at night. 
Comparison with the observations of Boyer and Buchsbaum, which 
have been summarized in chapter iv, in connection with slumber 
aggregations of insects, suggests that the temperature threshold of 
activity will be found to be lower on sunny than on dull days. On 
cool days the hoppers have been observed to keep their nighttime 
aggregation throughout the day. Uvarov (1928), from whose book 
much of the present account is summarized, cites observations which 
indicate that the temperature threshold for activity is higher in 
older nymphs. 

With still further increase in temperature the bands start on their 
irregular and apparently aimless wandering. Some become restless 
and make small irregular jumps. These seem to initiate jumping 
on the part of others which at first is aimless, but which at length 
settles into a definite direction. Uvarov considers favorably the 
suggestion by Loeb (1918), based upon the work of Lyon (1904) 
and others, that there is a tendency of an animal to move so as to 
stop the movement of images of surrounding objects on the retina. 
One hopper jumping thus starts others seeing it to jump in such a 
way that there will be no movement across the retina. In a band 
this is taken up continually and passes along as a sort of automatic 
restimulation. Further, Grassé (1923) has shown that even non- 
gregarious grasshoppers give greater activity when several indi- 
viduals are experimented upon together than when one is taken 
singly. 

If some such explanation holds, we have the common direction of 


318 ANIMAL AGGREGATIONS 


the movement of the band determined by chance or by environmen- 
tal factors. La Baume (1918) thinks that the bands of the Moroccan 
locust move downhill because of a positive geotropism. Most of 
the obvious environmental factors, such as direction of the sun’s 
rays, direction of wind, location of lush vegetation, do not seem to 
be definitely related to the direction of movement. 

The appearance of one of these bands on the march is shown in 
the accompanying figure from Uvarov, from which it may be seen 
that the line of movement tends to be broad and shallow and with 
an irregular front (Fig. 31). When two such wandering bands meet, 
they usually fuse and go off in the direction formerly taken by the 
larger band. Such migrating bands recognize no obstacles other 


Fic. 31.—A band of locust hoppers on the march. (From Uvarov 1928, by permis- 
sion of the Imperial Bureau of Entomology.) 


than smooth vertical walls, and these merely cause a deflection. 
Inequalities of the surface are filled by the bodies of the first comers, 
and those following pass over the smoothed surface. Rivers are 
crossed by swimming with the same hopping motions that carry 
the insects along on land. The bands stop at noon if the heat be- 
comes sufficient to produce heat torpor, and at night when cold 
torpor sets in. 

We have already seen that the beginning of the wandering, as well 
as other movements of these hoppers, is largely determined by the 
temperature. We shall see later that, according to the phase theory 
of Uvarov, living together in dense bands tends to cause an altera- 
tion in the coloration, so that black pigment develops. Buxton 
(1924) reported that the body temperatures of a black form of 
Calliptamus coelesyriensis, a grasshopper of Palestine, was from 4° 
to 5° C. warmer than buff individuals of the same species under the 
same conditions. If Uvarov’s theory is correct, it would appear 


MORPHOLOGICAL EFFECTS OF CROWDING 319 


that the collection in bands produces a coloration which increases 
the internal heat by increased absorption of the sun’s rays, and that 
this in turn increases activity of the animals and is responsible, at 
least in part, for a greater tendency to wander. Apparently, the 
behavior and the coloration may be found to be inextricably inter- 
mixed with various other physiological processes and their morpho- 
logical expressions, which have as yet escaped analysis. 

The transition from a wingless band of hoppers to a winged swarm 
is gradual. The first winged forms to appear continue to move as 
hoppers with the wingless nymphs. Later, when more are molting 
to the adult form, the behavior of the band is modified; they rest 
much and are easily disturbed. After some days, isolated winged 
forms take off, fly in a circle, and again settle. When they pass 
over other winged individuals, these too may take to the air, per- 
haps as a response to air vibrations, as suggested by Vayssiére 
(1921), since blinded locusts will respond, although those with eyes 
intact fail to do so if they are inclosed in glass (De Lepiney, 1928). 

Faure (1923) gives almost the same account for the brown locust, 
Locustana pardalina, of South Africa. He reports that the bands of 
hoppers may be composed of three or four distinct nymphal stages, 
although they are more usually of the same stage. The members of 
a band do not all molt to form flying insects on the same day, but 
the winged males and females remain with the main band, probably 
until there are enough winged individuals to make a separate swarm. 
These precocious flyers camp with the main band at night even 
though they have ranged widely through the day. 

Very large flying swarms travel for hundreds of miles. Small 
ones tend to remain near where they became winged. The distance 
covered is greater during the first few weeks of adult life. Later, as 
the females become heavy with eggs, the swarm tends to break up 
into sections. At night these flying swarms collect in clusters which 
are not so dense as those of the hopper bands. In South Africa 
brown locust swarms may fly on moonlit nights, particularly if 
harassed by birds. It is noteworthy that compact swarms leave 
large deposits of eggs behind. 

Ordinarily, these locusts which Faure describes feed upon sweet 


320 ANIMAL AGGREGATIONS 


grasses; but if food is scarce, they will eat almost anything. They 
readily become cannibalistic, eating injured members of the swarm. 
Mating does not begin until a day or two after the insects become 
winged; it continues at intervals thereafter until death. Nymphal 
aggregations are evidently not due to sex attraction; nor is the 
aggregation of the newly emerged winged adults, either with each 
other or with the nymphs which have not yet molted. Mating takes 
place immediately following egg-laying during the daytime; hence 
overnight aggregations, even of the mature adults, are not primarily 
due to mating reactions. 

Uvarov cites cases which demonstrate that the migration of the 
adult swarms is not related to flood supply, since they will leave 
dense stands of vegetation upon which they normally feed and mi- 
grate out into arid regions. Neither is it due to a search for suitable 
nesting sites, for they will leave the regular nesting grounds and 
deposit their eggs wherever the physiological urge becomes suffi- 
ciently strong, regardless of the fact that the place may be entirely 
unsuitable for the development of the eggs. Further, he does not 
believe that the migration is a negative reaction to high parasi- 
tization, since the heavily parasitized individuals do not migrate, 
and since the others carry along with them their destructive red- 
mite and fly-larva parasites. Swarms have been known to stop 
and deposit their eggs on a barren hillside, where their eggs will 
develop poorly, if at all, and within sight of dense growths of one 
of their principal food plants growing in the type of habitat where 
their eggs would develop well. Uvarov believes that the emigra- 
tion flight is both induced and regulated mainly by internal physio- 
logical factors. 

The non-gregarious grasshoppers are solitary, not in the sense 
that there is but one or, at most, a few in a considerable area, but 
in the sense that for some unknown reason these Acrididae lack the 
tendencies which lead to mass movements. Their individual re- 
actions to different environmental stimuli seem approximately like 
those of the individuals from the gregarious locusts, with the excep- 
tion that their reactions are not so closely dependent upon those 


MORPHOLOGICAL EFFECTS OF CROWDING 321 


given by nearby grasshoppers. In behavior there is no hard and 
fast line that can be drawn between the two types, and in matters 
of form and color it appears that they also intergrade to a consider- 
able degree. 

This lengthy introduction to Uvarov’s theory of phases of locusts 
is needed in order to have a proper background to understand the 
relations considered in that theory. To this, another known rela- 
tionship should be added. After a period when non-gregarious 
grasshoppers have been no more than the usual agricultural pest, 
taking a relatively light toll of the available plants, a locust outbreak 
may occur either suddenly or after building up for a year or so; 
and this may be so serious as to present a great agricultural problem 
for an entire district. Such an outbreak may disappear as suddenly 
as it appeared, leaving behind only the normal population of grass- 
hoppers. These outbreaks do not appear to have any definite pe- 
riodicity. They are probably conditioned by a favorable combina- 
tion of climatic and biotic factors. Weese (1924), working with 
spiders and their parasites, found that the climatic conditions which 
favored the development and survival of the. hosts differed from 
those which were most favorable for the development and low mor- 
tality of their parasites. Uvarov is probably right in saying that 
“neither climatic factors in themselves nor the activity of natural 
enemies can be regarded as sufficient for a satisfactory explanation 
of the rapid increase in numbers of locusts in their breeding grounds 
at the beginning of an outbreak”’; but he probably underestimates 
the possibilities of the two factors working together to supplement 
and reinforce each other. 

In order to account for locust outbreaks and their sudden subsi- 
dence, Uvarov has put forth his theory of locust phases (1921,1928). 
The essence of this theory is: “Various species of gregarious locusts 
cannot be considered absolutely stable in all their characters, either 
morphological and (sic) biological; on the contrary, there are good 
reasons for regarding each species as exceedingly plastic and liable 
to fluctuations in all essential characters. These fluctuations have, 
of course, certain limitations, but in some cases the bounds are so 


322 ANIMAL AGGREGATIONS 


wide that the extreme forms have been recognized as distinct 
species.” These diverse forms of apparently related stock Uvarov 


calls ‘‘phases.” 


Fic. 32.—Showing morphological differences between phases of different species of 
acridid grasshoppers or locusts. 1, 2, Pronotum of danica phase of Locusta migratoria L. 
3, 4, Ditto of migratoria phase of the same. 5, 6, 7, Pronotum and wing of pardalina 
phase of L. pardalina Walk. 8, 9, 10, Ditto of solitaria phase of the same. 11, Pronotum 
of gregaria phase of Schistocerca gregaria Forsk. 12, 13, Ditto of flaviventris phase of the 
same. (After Uvarov 1928, by permission of the Imperial Bureau of Entomology.) 


Uvarov was led to this theory of phases by a consideration of the 
interrelations between the two supposedly good species, Locusta 
migratoria and L. danica. In his early work, on account of distinct 


MORPHOLOGICAL EFFECTS OF CROWDING 323 


differences in morphology, color, and ecological relations, he con- 
cluded that these are distinct species, as many others have regarded 
them. In hunting for delimiting structural characters, two were 


Se ee ee 
LSS (eae ees 
ee S| 
eee 
JSR Ss NSS eae 
75021 She Coo 
a000 0008 Gee 


MUMBER OF SPECIMENS 


(SN C= Su U3 NN co CE oy (C—O Lr STE” Cs LN = UNC CD = 
ie) 3 = ep 2 & bk tS é S oy ey Rey ey) 


PRONOTAL PROPORTION 


Fic. 33.—Graph of the range in variation in the pronotal proportion in 358 speci- 
mens of Locusta migratoria L. (After Uvarov 1928, by permission of the Imperial Bu- 
reau of Entomology.) 


found—the proportions and shape of the pronotum and the relation 
between the length of the elytra and the hind femora. The pro- 
notum differences of these two forms are shown in Figure 32. The 
differences may be summarized by saying that in L. migratoria (3, 4) 
the pronotum is relatively shorter and broader than in L. danica 
(1, 2) and has a low median keel. There is a more definite constric- 
tion in the middle of the pronotum, and the keel is straighter in pro- 


324 ANIMAL AGGREGATIONS 


file. Locusta danica has the pronotum relatively longer and more 
compressed laterally; the midconstriction is less pronounced; the me- 
dian keel is higher and is convex in outline. The elytra of the former 
are relatively longer and the hind femora relatively shorter than in 


NVUMBER OF SPECIMENS 


0.46-047 
0.48-049 
0.50-0.51 
OFS: 2)=0:535 
0.54 -0.55 
0.56 =0.57 
0.58-0.59 
0.60-0.61 
0.62-0.63 


0.40 - 0.41 
0.42-043 
0.44 -0.45 


FEMORAL PROPORTION 


Fic. 34.—Graph showing the range of variation of the femoral proportion in 358 
specimens of Locusta migratoria L. (After Uvarov 1928, by permission of the Imperial 
Bureau of Entomology.) 


L. danica. These differences are summarized in numerical values 
in Figures 33 and 34. A study of Figure 33 shows that, so far as the 
pronotum characters are concerned, there is no evidence to indicate 
that here are two species, or even two forms of one species, but that 
with the femoral proportions there is a distinct difference in the 
erasshoppers shown by the two-humped curve. The hump to the 
right represents the L. danica type. Proportions taken for many 


MORPHOLOGICAL EFFECTS OF CROWDING 325 


individuals of both types show that there is no interval between 
figures for L. migratoria and for L. danica. 

In L. migratoria both sexes are about the same size; the males 
are about 4 per cent smaller than the females. On the other hand, 
L. danica males are about 20 per cent smaller than females of that 
species. In both, the proportions between different parts of the 
body remain constant, independent of the absolute measurements. 

The coloration of adults of the two forms is variable both in 
color and in pattern, so that they cannot be separated accurately 
on color characters. In general, L. migratoria is less variable in 
color than is L. danica; the color markings tend to be less sharp, 
and the general coloration tends to be more uniform. The hind 
tibiae are usually yellowish, though occasionally they are red. 
Locusta danica is more variable in adult coloration: bright-green 
forms are common; dark-brown and black forms are frequent; and 
the pattern is usually distinct, even if variable. The hind tibiae are 
frequently red, but this cannot be taken as absolutely diagnostic. 
During the breeding season males of L. migratoria become a bright 
yellowish, while males of L. danica show no color change when adult. 

The situation regarding coloration of the hoppers is different. 
Locusta danica nymphs may be uniformly green, fawn, gray, brown, 
or even black. ‘Quite the opposite is the case with migratoria, in 
which each larval stage exhibits its constant color characters. Their 
coloration presents a combination of black and orange-red (or yel- 
low), the earliest stages being almost entirely black, while orange 
or yellow appears later and extends with each molt without, how- 
ever, entirely replacing the black.” While there is variation even 
in this form, Uvarov says: “The main point is that this type of 
coloration never occurs in hoppers of danica in spite of the wide 
range of coloration in the latter.”” Somewhat similar conditions are 
exhibited in Figure 35 for the solitary and swarming phases of the 
South African locust, Locustana pardalina, and for the desert locusts, 
which have previously been known respectively as Schistocerca flavt- 
ventris and S. gregaria. 

In L. pardalina, of South Africa, there are approximately the 
same differences between the swarming and solitary phases as have 


Fic. 35.—A black-and-white copy of a color plate by Faure (1923) showing solitary 
and swarm phases of the brown locust (Locustana pardalina). Black in this plate repre- 
sents black or bluish black in the animals; heavy stippling represents dark brown; light 
stippling represents light or golden brown except on the head, prothrox, and meta- 
thoracic femora in Nos. 7 and 9 which are green. 

The numbered drawings are: 1, 2, 3, 4,5, swarm phase, first, second, third, fourth, and 
fifth stages of nymphs (or hoppers) respectively; 6, 7, 8, solitary phase, fourth, fourth, 
and fifth stages of nymphs (or hoppers) respectively; 9, adult female, solitary phase; 
10, adult male, solitary phase; 11, adult female, solitary phase; 12 adult female, swarm 
phase, representing coloration of the young flier. 


MORPHOLOGICAL EFFECTS OF CROWDING 227 


just been described for Locusta migratoria and L. dancia. In addi- 
tion Uvarov, using specimens sent him by Faure, discovered dif- 
ferences in wing venation. With both Locustana and the Locusta 
we have been discussing, these characters are subject to great varia- 
tion, and intermediate forms occur. Adults of the desert locust 
Schistocerca flaviventris have a distinctly higher crest of the pro- 
notum than those of its supposed gregarious phase, usually known 
as S. gregaria. The coloration of the hoppers of the two phases for 
all three series is suggested in Figure 35 and may be compared with 
the description for the Locusta forms given above. The solitary 
nymphs are like Locusta danica, while the gregarious nymphs might 
be mistaken for L. migratoria, since they are of the same color and 
pattern. 

The evidence that these phases of particular species may be 
transformed from one to the other does not as yet appear entirely 
conclusive. It does strongly suggest that such transformations may 
take place, and for that reason this relatively large amount of space 
has been devoted to the consideration of these locust phases. 
Uvarov observed in 1912 in the northern Caucasus that a swarm 
composed entirely of L. migratoria deposited eggs in nature in posi- 
tions that were marked by those engaged in locust control work. 
The following year the hoppers were mainly of the parental type, 
but there was a considerable admixture of L. danica nymphs which 
showed the typical behavior of non-gregarious hoppers. There were 
many nymphs intermediate between the two. Similar field observa- 
tions were made in the two following years in a different locality. 

Plotnikov (1924; and, vide Uvarov, 1915, 1927) has carried on 
rearing experiments of the L. migratoria and L. danica nymphs. The 
account of the most convincing of his experiments that have come 
to my attention is summarized here. On May 26, he took 80 larvae 
of the third nymphal stage, and with typical migratoid coloration, 
from an ordinary cage of 0.02 cu. m. volume and transferred them 
to an open-air ground cage which covered 4 sq. m. of surface and 
was 60 cm. high. An equal number of similar larvae were left in the 
original cage for controls. After a time the larvae in the ground cage 
began to turn green, which Plotnikov regards as an intermediate 


328 ANIMAL AGGREGATIONS 


color between migratoid and danicoid phases. They attained the 
fifth nymphal stage by June 11, when they had become quite green 
with the exception of 6 larvae, which were dark gray, almost black. 
In the original cage all the control larvae retained their typical mi- 
gratoid coloration. The same experiment was repeated with dani- 
coid nymphs, and only green forms appeared in the ground cage. 

Plotnikoy performed 15 experiments with larvae of the second 
brood of ZL. danica reared under conditions of crowding. When the 
larvae were kept in small cages or in glass jars, with 30—50 nymphs 
to 450-675 cc. space, a typical migratoid coloration was invariably 
obtained, but no dark specimens were to be found. When the 
nymphs were kept singly in the glass jars, they began turning green 
as early as the second stage. They were quite green by the fourth 
or fifth instar. When groups of 4 were put into too cc. of space in 
glass jars the fifth-instar nymphs were a mixture of migratoid and 
green, with some transitional forms. 

In these experiments, in which the developing nymphs retain 
migratoid coloration only when they are crowded, Plotnikov reports 
that the nymphs having the migratoid color also lack the keeled 
pronotum typical of the L. danica nymphs, so that their structure 
as well as their color is affected by crowding. 

In another series of experiments with non-migratoid nymphs, 86 
second and third instar nymphs were placed in a 2,000 cc. cage for 
12 days, at the end of which time they were in the fourth and fifth 
instars. Forty-five of these had definite migratoid coloration, while 
9g more showed tendencies in that direction. Again, when 44 non- 
migratoid nymphs were put into a 1,000 cc. cage, a few typical mi- 
gratoid animals were obtained. One regrets that there is no record 
of controls for these experiments reared under conditions such that 
crowding would be impossible. 

Plotnikov considered the question of factors causing the migratoid 
color to appear in crowded cages. He eliminated cannibalism as a 
causal factor by feeding the isolated migratoid nymphs with killed 
hoppers. Under these conditions the isolated animals lost their typi- 
cal migratoid coloration. He considers, too, that he has eliminated 
the humidity factor, since broods in highly humid jars gave the same 


MORPHOLOGICAL EFFECTS OF CROWDING 320 


results as those in an open outdoor cage with lower humidity. The 
same results were obtained in the hot midsummer as in the cooler 
autumn. This eliminated to some degree the factors of season, il- 
lumination, and temperature. Further, crowding experiments car- 
ried on in darkness produced migratoid coloration just as did rearing 
nymphs in open-air cages. Uvarov, in commenting on these experi- 
ments, thinks that the results are due to influences, as yet unknown, 
connected with the density of the larvae but not concerned with 
the density of the eggs or of the adults. One wonders whether the 
question of nematode infestation and its effects has been carefully 
checked in this work. Plotnikov (1927; vide Uvarov, 1928) has 
done work that confirms his earlier results that rearing under 
crowded or isolated conditions causes the changes above recorded; 
but he now regards these as aberrations, not as true transformations. 
The intermediate forms found in nature he interprets as hybrids 
between the two distinct races. These so-called “hybrids” do not 
differ in appearance from intermediate forms produced by cultural 
methods. In this connection we should record that Faure reports 
having seen a small, solitary-phase male of L. pardalina copulating 
with a large, swarm-phase female. 

Faure (1923), working on the South African locust Locustana 
pardalina, observed transitions from the swarming phase to the 
solitary phase. Eggs collected from a field, where they had “‘in all 
probability” been deposited by a swarm, hatched out the usual 
black of the swarming nymphs of the first instar. If a number of 
these, some 60 or more, were kept together, swarm-phase colors 
held during the succeeding instars. If small numbers, not more 
than ro or 12, were kept together in a cage, the color changed to 
that common for the solitary-phase nymphs after the first molt, and 
remained so during subsequent molts. The opposite change, from 
solitary to swarm phase, came on more gradually with crowding. 
Although without definite evidence, Faure believes, as a result of 
field and laboratory observations, that about 300 in a group will 
produce swarm-phase characteristics. Faure reconstructs the ap- 
pearance of these two phases in nature as follows: After an epidemic 
of swarm-phase locusts, great numbers are killed off by man or other 


330 ANIMAL AGGREGATIONS 


enemies, or by changed environmental conditions or overcrowding, 
and the species persists in the solitary phase. When conditions are 
again favorable, as at the end of a severe drought, these scattered 
grasshoppers multiply rapidly. As numbers permit, they gather 
into loose swarms, which deposit eggs in compact lots. From these 
compact deposits the swarming type of nymphs arises. 

Another link in the chain of evidence connecting the two phases 
is furnished by Johnson (1926), working with the desert locust 
Schistocerca gregaria and S. flaviventris. Uvarov (1923), as a result 
of his studies on the phases of Locusta migratoria, suggested, from his 
inspection of museum specimens of these two supposedly good spe- 
cies of desert locust, that Schistocera gregaria was the swarm-phase 
and S. flaviventris the solitary phase of one and the same species. 
Field observations recorded by Johnson suggested that this was 
indeed the case, and these observations were followed by breeding 
experiments which gave the same results as have similar tests with 
other species, since the experimenter was able to control the appear- 
ance of either phase by regulating the density of the crowding. A 
part of this test was made in nature and on a large scale. Bands of 
hoppers thinned out by poison turned into the solitary phase so far 
as coloration and behavior were concerned. There is also prelimi- 
nary evidence (Dampf, 1925, 1926) that the South American locust 
S. paranensis can be turned into the solitary phase usually known as 
S. americana by controlling the density of the population. 

The phase theory of Uvarov remains to be tested even in a pre- 
liminary manner upon two other species, which, with the four dis- 
cussed, make up the larger swarming locusts of the world. These 
are the African red locust Nomadacris septemfasciata, of which two 
types are known, differing in pronotal characters; and Patangia 
succincta of India, which apparently rarely swarms. Of the smaller 
swarming locusts, there is a suggestion that the Moroccan locust, 
Dociostaurus maroccanus, has a solitary phase which has been de- 
scribed as a pigmy race. Finally, there is the case of the Rocky 
Mountain locust, Melanoplus spretus, now apparently extinct in its 
typical form. In 1878, the United States Entomological Commis- 
sion, composed of Riley, Packard, and Thomas, in their first annual 


MORPHOLOGICAL EFFECTS OF CROWDING 331 


report, record that they considered and rejected the possibility that 
the offspring from the breeding swarms of M. spretus change in the 
direction of a morphologically related species, M. atlanis, which 
differs from M. spretus by being less gregarious in its habits. The 
nymphs of the latter have coloration suggesting that of other typical 
swarming-hoppers, and the pronotum of the adult is proportionately 
shorter than that of M. ailanis. Somes (1914) questioned on mor- 
phological grounds the validity of the separation of the two species; 
and Parker (1925) and Hebard (1925) have suggested that the two 
are phases of the same species. This suggestion offers an interesting 
possibility of putting the whole phase theory to the test of critical 
experimentation without the necessity of making an AEA, time- 
consuming journey. 

In final criticism of the phase theory, it does not seem to be clearly 
proved that the transformations from one phase to another do 
actually take place on a large scale and to a convincing degree. 
But as one reads through the descriptions of the different workers 
from different parts of the world and finds that independent students 
have thought that they have obtained these transformations, and 
when one learns that the same suggestion had been considered in 
1878, long before Uvarov first stated his phase theory, it becomes 
impossible to dismiss the evidence entirely, although for the time 
being it must rest with a verdict of unproved but a promising open- 
ing for further work. 


DROSOPHILA CULTURE EXPERIMENTS 


The work of the Drosophila students probably presents the great- 
est mass of carefully controlled work upon the culture of a single 
animal species yet performed. Unfortunately, certain phases of the 
environment within the culture bottles cannot be controlled, but on 
the whole these workers have succeeded in creating standard con- 
ditions for their breeding experiments. For this reason one may ac- 
cept their results without the mental reservations just indicated in 
the case of work on the transformation of locusts from one phase to 
another under laboratory conditions. 

Bridges (1921) called attention to the necessity of having opti- 


332 ANIMAL AGGREGATIONS 


mum environmental conditions in order to eliminate distortion in 
ratios in experimental work with Drosophila, and lists the effect of 
overcrowding as one of three main sources of disturbance. In the 
early Drosophila work, he says, this was the largest source of diff- 
culty. 

Eigenbrodt (1925) presents evidence that overcrowding the larvae 
of a homozygous race of bar-eyed Drosophila decreases the weight 
attained by the adult flies and decreases the size of the whole animal 
and of such individual parts as the thoracic length and length of 
wing. The number of eye facets, of hairs on or around the eye, and 
the number of teeth in the sex combs (found in the male only), are 
also decreased. Flies reared under overcrowded conditions do not 
exhibit the correlation of size with the temperature at which they 
are reared that is characteristic of normal flies. The rate of devel- 
opment is retarded by the overcrowding; variability tends to be 
increased, and sexual dimorphisms tend to disappear. In order to 
obtain standard results, Eigenbrodt concludes that Drosophila 
should never be reared under overcrowded conditions. He finds 
that the flies are normal in the foregoing relations if from 8 to 20 
hatch out in an 8-dram vial which contains 9g grams of standard 
food. Under these conditions a higher population than 20 flies rep- 
resents overcrowding. 

Plunkett (1926), in his attack on the problem of how genes pro- 
duce their effects, undertook to study experimentally the effects of 
various combinations of genetic and environmental factors upon the 
development of Drosophila bristles. These have the advantage for 
this sort of work of being discrete units that can be counted with a 
minimum of observational error. Their number is known to be 
affected by different genes, and also by such environmental factors 
as temperature and nutrition. The bristles have been plotted and 
named for the normal wild-type Drosophila and for various other 
stocks. Plunkett, in his experiments, used a mutant stock known 
as Dichaete, selected for low bristle number for many generations, 
and took care to insure uniformity in the different culture bottles, 
’ except for the factors under observation. » 

Under these conditions it was found that the number of flies 


MORPHOLOGICAL EFFECTS OF CROWDING 333 


developing in a bottle, other conditions being equal, has a pro- 
nounced effect on the mean bristle number. This effect is summa- 
rized in Table XX XVII for populations from eggs laid during 4 days 
for the first 7 groups, and during 8 days for the last 5 groups. The 
temperature was held at 25° C. throughout. 

In commenting on these results, Plunkett says: “It is evident 
from the table that, under these conditions, there is no correlation 
between bristle number and density of population up to about forty 

TABLE XXXVII 


SHOWING DATA FOR THE EFFECTS OF DENSITY OF POPULA- 
TION UPON BRISTLE NUMBER IN DIcHAETE Drosophila 


(After Plunkett) 


Flies Mean Number of 

per 5 Posterior Dorsocentral 
Bottle Bristles per Half-Fly 
1 UR ea it i aay eae a ae 0.234+0.030 
Dyin Cian et ates aie Ad Aig ARI ONES Dicicen ue 0.284+0.017 
SY ale feat ota: oe REC TIO 0.28440.012 
ASUS Teneo eta g ae es EY, once Hee fe nN s 0.183 +0.009 
SO Meee Ta ioe ec PE the cee ny Wee SAS etree ©, 13020-0151 
OG 7 rere Rae unm eer Apo 0.120+0.011 
7 ONO Meera Weck Tea ee PRR © crt es 0.056+0.009 
Olas aeRO Re cis ice eee 0.055+0.008 
LOSES en cele ee es ee 0.051+0.007 
Lo hs 1S Aik An pete. Re ae A ag ee 0.027+0.006 
22 ORO Pea Lpet tsess eke ae sctint tera Sei 0.022+0.005 
B15 Ten Obra riser tect es ete oe ancl sie isttoexonna sys 0.017+0.003 


flies per bottle; but above this the bristle number falls off rapidly 
with increasing population, reaching almost zero (for these bristles) 
when the population is much in excess of 100 flies per bottle, as in 
ordinary ‘stock’ bottles. This ‘crowding effect’ makes it unsafe to 
draw quantitative conclusions as to the effects of other genetic 
factors or other environmental factors (e.g., temperature) on bristle 
numbers in flies raised under these conditions; 1.e., more than forty 
offspring per bottle from eggs laid over a period of several days.” 
In later experiments, when parents were kept in the bottles for not 
over 24 hours, t were no obvious effects of crowding up to about 
80 or 100 strep hie per bottle. 

Plunkett continues: ‘Experiments designed to analyze the factors 


334 ANIMAL AGGREGATIONS 


responsible for these ‘crowding’ and ‘age of culture’ effects, indicate 
that they are due largely, perhaps entirely, to competition for food. 
This factor seems to affect especially the younger larvae when in 
competition with older ones.” 

When virgin F, females from a cross of a wild type by vestigial 
winged Drosophila were backcrossed to vestigial males, an equal 
number of eggs destined to produce hybrid and homozygous vestig- 
ial flies was laid. Under conditions of overcrowding, Harnly (1929) 
found that the proportion of vestigial to the wild-type hybrid flies 
was reduced, thus demonstrating a pronounced selective effect of 
overcrowding. Similar results were obtained whether crowding was 
produced by using a reduced surface area of food or by increasing 
the length of the egg-laying period with the food area (and depth) 
remaining constant. Clausen (1924) reported a similarly low survi- 
val value for vestigial flies reared under crowded conditions. 

Evidence from these students of genetics concerning the effective- 
ness of an environmental factor is the more trustworthy since they 
do not have the reputation of being easily convinced of the effective- 
ness of environmental factors upon morphological characters. 


CONCLUSION 


CHAPTER XEX 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 


We have seen something of the kinds and the extent of the aggre- 
gation phenomena in nature and in the laboratory, among members 
of the animal kingdom and among specialized cells such as bacteria 
and spermatozoa, as well as cells under the artificial conditions that 
obtain in tissue cultures. We have reviewed the action of the direct 
and indirect environmental forces which control the formation of 
such aggregations. We have found that methods and different de- 
grees of integration exist, which may serve to organize a group Close- 
ly, although it lies well below the division-of-labor level of social life. 

The well-known harmful effects of crowding have been summa- 
rized, as has the newer evidence that, despite the menace of over- 
crowding, many aggregations have survival values for their members 
and may even produce morphological changes as well as the more 
easily induced physiological effects. A further discussion of the ex- 
tent and implication of the co-operation involved in these loosely 
integrated groupings is reserved for the final chapter. It is sufficient 
to state here that aggregations formed without sexual stimuli and 
at the lowest level of group integration may have survival value for 
their members and, under certain conditions, are essential for the 
survival of the race. 

Although the matter has not been discussed, it has been the open 
as well as the implied suggestion throughout that these aggregations 
are important in social evolution. This question must now be faced 
directly. One avenue of approach to such a consideration might lie 
through an extensive review of the evidence concerning the origin 
of the social habit, but we shall limit this aspect of the discussion by 
citing summaries of generally accepted points of view and testing 
these against some of the known facts. 

We have already suggested that sex may have evolved from the 
mutual stimulation which has been demonstrated to occur under 

337 


338 ANIMAL AGGREGATIONS 


certain conditions when two similar asexual cells are crowded to- 
gether within a limited volume of medium. If this suggestion is 
sound, it would indicate that mass physiology of animals is much 
more primitive and fundamental than can be considered to be the 
case under the assumption that the gregarious or social habit in 
animals is at bottom an outgrowth only of the association of young 
individuals with one or both parents, and therefore usually a result 
of sexual reproduction. It is assumed that in special cases or at 
critical periods in social evolution the period of the association be- 
comes lengthened and the family comes to react as a unit under 
many conditions. Students of social life in insects, especially as it 
exists among wasps, bees, and ants, usually adopt this explanation 
in some form for the origin of the social habit. Wheeler, in his 
summaries of studies on ants (1913a, 1918), and more recently in his 
review of social life in insects (1923, 1928), regards the insect colony 
as the result of an extension of the affiliation of mother and off- 
spring. Wheeler’s particular contribution is his theory that mutual 
feeding, which he calls “‘trophallaxis,” is the bond that unites parent 
and offspring in the social insects; the mother feeds on secretions 
from the larvae and so is bound to them through self-interests, 
while they receive food from the mother to their own advantage. 
Wheeler shows that the social habit, meaning thereby a more or 
less prolonged association of young and adults, has arisen de novo 
at least thirty different times among insects alone, in nearly that 
many natural taxonomic families or subfamilies, belonging to five 
different orders. The gradual development of the mother-offspring 
family from the solitary insects is shown almost diagrammatically 
by the growth of the social habit among the solitary wasps (Wheeler, 
1923). 

Herbert Spencer’s suggestion that colony life arose from the con- 
sociation of adult individuals for nonsexua!, co-operative purposes 
was an early recognition of that type of social unit and, at the time 
it was advanced (1893-94), was not well grounded on proved fact. 
Spencer suggests that in some cases permanent swarms arise from 
such consociation and that natural selection establishes such of 
these groupings as are advantageous. In terms of human society, 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 339 


this view would stress the importance of the gang, rather than that 
of the family, as a preliminary step in the evolution of the social 
habit. It is important to note that the gang cuts across family lines 
in its formation, just as do the sleeping collections of male robins. 
which occur in our parks and orchards during the breeding season. 
Unfortunately for Spencer’s principle, which may be correct in many 
instances, he limited his theory by a concrete example in ants, where 
we now know that social development is probably due to an exten- 
sion of mother-offspring relations; but the easy dismissal of his illus- 
tration does not necessarily wreck the underlying principle. 

Wheeler (1913a) expressed the usual attitude toward these con- 
sociations in relation to social origins when, after describing some 
cases of aggregations in ants, he dismissed them as entirely fortuitous 
instances, which would occur wherever ants might be abundant and 
places of refuge scanty; or as the manifestation of highly developed 
social proclivities, and not of such proclivities in the process of de- 
velopment. More recently (1930) he said: “Societies really repre- 
sent very different emergent levels from the associations and have 
arisen in a different way, though, of course, ancient aggregative 
or associative proclivities may have been retained by many species 
and may serve to reinforce their specifically social behavior.”’ Highly 
developed social life demands well-developed sense organs, central 
nervous system and muscular apparatus, and in addition to these, 
according to Wheeler, there must be a development of the family. 
‘All the societies of insects,’ he says, ‘‘are merely single families 
I OLISTIE se as The family origin of the flocks and herds of birds 
and mammals and hordes and tribes of primitive man is also ap- 
parent; though in these societies the family is open and not closed 
as in insects, and there is a retention in the flocks, herds and hordes 
of primitive aggregative or associative tendencies which seem to 
hark back to the ancestral fish and tadpole stages.” 

This attitude may be entirely correct so far as the highly inte- 
grated societies are concerned; but even in these closely knit organ- 
izations there is evidence, as has just been recognized, that the 
numerous aggregations, an account of whose formation, integra- 
tion, and physiological effects have made up the body of the present 


340 ANIMAL AGGREGATIONS 


discussion, have social significance. In more loosely organized social 
life we would expect the cohesiveness shown by animals at the 
aggregation level to make up a greater part of the total unifying 
forces which operate to produce a social unit. Further, we must 
recognize that at least a part of the forces which operate to bind 
members of families together are similar to those which operate in 
the case of other kinds of social groupings. 

It is worth re-emphasizing, as Child (1924) and Alverdes (1927) 
have recognized, that both the congregation and the family bases of 
societies are in fact but two different types of aggregations, so that 
at all events aggregations of some sort are essential for the develop- 
ment of the social habit. In other words, this phase of the problem 
of social origins is not the question whether the social habits as seen 
among birds, ants, men, and others arose from aggregations such as 
we have been discussing in earlier sections of this book or from some 
sort of family; but rather it becomes a question as to the kind of 
aggregation which gave immediate rise to them, since the family 
type is only one of a number of kinds of aggregations, as we have 
seen from Deegener’s outline. 

The main value of the detailed classification of social organiza- 
tion given by Deegener lies in his recognition and elaboration of the 
essential unity of sex-conditioned and asexually conditioned social 
tendencies. Throughout his two main categories of loose accidental 
unions, or associations, and essential groupings, or societies, Dee- 
gener has recognized important subdivisions, differentiated accord- 
ing to whether the groups are formed on a sexual basis or prolonga- 
tion of some sort of family relations, or whether by the gathering 
of individuals from different parents into more or less well-integrated 
groups. The former type, while common enough in homotypic 
groupings, does not compose all such groups; while the latter is 
most characteristic, though not necessarily exclusively so, of social 
bands or flocks which contain more than one species. 

In order to appreciate the importance of social organizations 
which are not fundamentally united on a sexual basis, one should 
review Deegener’s classification to find the extent to which he recog- 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 341 


nizes these asexually conditioned groupings. Among the homotypic 
categories alone are to be found such aggregations as the following: 
all collections in a favorable locality of limited extent; hibernating 
groups; animals collected about food; individuals joined in migrat- 
ing bands; aggregations brought together by tropistic reactions 
which lead the responding organisms into a limited space; collec- 
tions due to unfavorable conditions, whether passive, as in drift 
lines of beetles formed by wind action, or active, due to the moving 
together of stimulated individuals. From heterotypic categories one 
finds such social mutualism as exists between flocks of cowbirds 
and herds of cattle; one animal living upon the shell or covering of 
another without being parasitic; two different species occupying the 
same runways, even though these runways are made by only one 
of the species involved; the well-known relationship: between ants 
and aphids, where the former feed on excretions of the latter and 
in turn afford them some protection; the same sort of relationship 
where narcotic material is supplied rather than food; the relation- 
ship between so-called ‘“‘robber guests” and their hosts; or that of 
harmless guests which feed upon fragments dropped by a larger or 
more active species; or of the animals which make their small nests 
in the large nest of a larger individual; or of the small individuals 
which remain in the neighborhood of a larger one without being 
attacked, and thus avoid attacks from others; or the cases of ani- 
mals cemented into the built-up covering of another, as caddis-fly 
larvae use small mussels; or those which live within the body of 
other animals without becoming parasitic; and, finally, the different 
types of parasitism. 

All the foregoing list must be dismissed, wholly or in part, as fall- 
ing outside the range of social phenomena, if the latter is to be 
regarded as limited solely to those relations which depend upon 
some sort of a sexual or family basis. While there are many who 
would be willing to dismiss a part of these as falling without the 
field of the social life of animals, I know of no student of social life 
who would dismiss them all. From such considerations as these we 
are drawn to the conclusion that, important as sex and the family 


342 ANIMAL AGGREGATIONS 


are as integrating social factors, they do not form the sole outlet 
for the expression of the fundamental social appetites, nor are they 
the only foundation upon which social structures have arisen. 

Observations on bird behavior (Allee, 1923a; Sherman, 1924) 
furnish interesting information concerning the problem of the ex- 
tent to which social groups originate through individual, and to 
what extent through family, behavior. The question at hand is, 
Do these annually or semiannually recurring bird flocks form by 
the coming together of individuals or by the collections of some 
sort of family groups? The answer is that both methods occur. 
There is much evidence that ducks and geese migrate in flocks in 
which family units can be recognized (McAtee, 1924) and that in 
the tropical rain-forests parrakeet flocks are made up of pairs rather 
than of individuals. With the whistling swan and the Canada goose 
supposed family-groups have been identified in mid-winter (Miner, 
1923). The large flocks of bronze grackles make a conspicuous 
feature of summer bird life in the Mississippi Valley. On July 3, 
M. Nice’ reports seeing one of the birds of a flock beg from another 
as young ones do from their parents, and interprets this as evidence 
that a family joined the flock before the young were entirely in- 
dependent. 

On the other hand, heterotypic flocks are frequent in which a 
species may be represented by a single specimen. An extreme in- 
stance of such a flock is furnished by Beebe (1916), who records a 
flock of 28 birds composed of 23 different species. He comments 
upon the common occurrence of heterotypic flocks in British Guiana. 
Such extremely heterotypic groups could scarcely have been formed 
by family rather than by individual units. 

Sherman (1924), known, like Nice, to be a careful observer of bird 
habits, gives much detailed information to show that not all flocking 
of birds is on a family basis. Proof of this is easily given by calling 
attention to the flocking habits of the cowbirds. The female cow- 
bird deposits her egg in the nest of some other bird, usually smaller, 
and leaves it there to be cared for by the latter. This socially 
parasitized foster-mother frequently hatches and rears the young 


t Personal communication. 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 343 


cowbird. When able to fly, these young cowbirds, reared separately 
and by foster-parents of different species, join in the well-known 
cowbird flocks. Such flocks can form only by the collection of in- 
dividuals. 

Bobolinks and goldfinches begin the formation of their autumn 
flocks by the congregating of old males. Chimney swifts, a pre- 
eminently flocking species, leave their nests by one’s and two’s to 
join the immense late summer flock. Many birds rear more than one 
brood a season; and Nice writes that the only young of the first 
brood that she ever knew to stay with their parents throughout the 
raising of the second brood were one set of bluebirds. 

Family ties apparently sit more lightly with birds than some 
would have us believe. Passerine birds may change mates for the 
second brood. Nice reviews the literature on banded birds and finds 
‘that in 7 pairs of 3 species there was a known shifting of mates in one 
season, as contrasted with 20 pairs of 11 species in which there was 
none.* 

Nice writes further that she does not believe that song spar- 
rows retain the family unit when flocking or for flocking, and she 
cites trapping experience with a robin to show that the female 
outstayed her mate and her three sets of offspring, and so evidently 
could not have joined in a postbreeding-season flock. Much similar 
evidence exists that with such relatively highly social individuals as 
birds, which are capable of forming well-integrated flocks that ex- 
hibit definite physiological division of labor and concerted group 
action, the flocking may occur by the congregation of individual 
birds, just as has been observed for the sleeping aggregations, where 
the male birds may come together to sleep even during the breeding 
season. 

It has just been stated that these bird groups form well-integrated 
social units. Something of the intricacy of the social organization 
possible among them has been revealed by the work of Schjelderup- 

‘In this connection Miss Sherman contributes the following pertinent observation in 
a personal letter: “My chimney swifts led a perfectly upright life for ten seasons, but 
on the eleventh a shocking scandal occurred. An unprincipled female supplanted the 


mate that had helped build the nest and had begun to lay her eggs, which were thrown 
from the nest in which the interloper raised her brood.” 


344 ANIMAL AGGREGATIONS 


Ebbe (1922) in his analysis of the intragroup relationships shown by 
a flock of domestic hens. Such a group forms not a closed but an 
open society; that is, new members are admitted as occasion de- 
mands. The flock is organized by what Schjelderup-Ebbe calls the 
“neck-right,” or the “peck-order.”’ 

The rank of individuals within the group is indicated by their re- 
action when another member pecks or threatens to peck them. A 
given hen will submit to pecking by certain individuals without 
expressing resentment, and will in turn have the right to treat others 
similarly without their showing protest reactions. Hens with this 
power are said to have the “‘peck-right” over those submitting to 
the pecking. The “peck-order”’ decides which birds may peck others 
without being pecked in return. The ranking is determined by com- 
bat or by passive submission. A newcomer can win a position above 
the bottom of the peck-order only by fighting. 

Sometimes the peck-order within the flock is in a simple con- 
tinuous series, thus: A pecks B; B pecks C; C pecks D; and so on 
down to the humblest member of the flock. But it may happen that 
the peck-order is more confused. A may peck B; B may peck C; 
and still C may have the peck-right over A. Frequently, strong 
hens are pecked by weaker ones. This is due to the fact that young 
ones are attacked by older members of the flock, newcomers by 
old-timers, sick by healthy; and the order, once established, tends 
to remain permanent. A revolt or a fight may either change or 
confirm a previously existing peck-order. If the original order was 
accepted passively rather than as the result of a combat, a rebellion 
is more likely to occur. A hen revolting against a previously recog- 
nized superior fights less fiercely than at other times. This indi- 
cates a psychic obstacle to such an attack. Once a hen has accepted 
an inferior position, it is more difficult for her to return to superiority 
than if she had fought for the position at the start. 

Position in the peck-order is associated with certain behavior 
traits. The hen which is entitled to peck all others is usually the 
least malicious, and a threatening note usually suffices for a peck. 
A hen low in the scale is usually cruel to the remaining hens. Katz 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 345 


and Toll (1923) tested the intelligence of hens and found that their 
position in the social scale corresponds roughly with their IQ. 

During breeding time a hen is more easily irritated by other hens, 
and frequently against her superiors. A hen with chicks is very 
courageous; but if the chicks are removed, she may become timid 
and retiring in her behavior. Cocks are said by Schjelderup-Ebbe 
to behave in a manner similar to hens, but more ferociously. A cock 
may interfere with the fighting of two hens, or even of two other 
cocks, if he is superior to both. When among hens, he stands at the 
top of the peck-order. 

This is not the picture of a simple society, though many of the 
complexities of the organization have only been suggested in the 
preceding summary. It may be objected that we have turned from 
the formation of flocks in nature to the examination of the integra- 
tion of artificial flocks of domestic fowls. It does not necessarily 
follow that the details of flock organization are the same in nature, 
but fortunately there is evidence bearing exactly on this point. The 
same observer reports (1923) that in flocks of wild ducks essen- 
tially similar group organizations exist. 

Another complex type of organization of a bird community in 
nature has been worked out in detail, particularly by Allen (1911, 
t913) and by Howard (1920). This is the matter of territory in 
bird life. The evidence shows clearly that the males pre-empt fairly 
definite spaces before the breeding season begins, and maintain their 
position during the breeding season, driving off intruding males be- 
fore and after a female has appeared to accept the territory and the 
male as her mate. Such spaced community organizations are ap- 
parently widespread among birds and again indicate clearly a dis- 
tinct social development. These territorial relations are not limited 
to birds, but are also known for fishes (Reighard, 1920), as well as 
for mammals. 

Among the fishes we have further evidence of the kind we have 
been reviewing for birds. This is well known in the case of the 
black catfish minnows observed by Bowen (1930). While it is true 
that the group of young originates from the eggs guarded and ferti- 
lized by a single male, yet, at least in the absence of the male, the 


346 ANIMAL AGGREGATIONS 


young separate each night and come together one by one to form a 
group as the light increases the next day. Further, the group does 
not keep its original character of being made up of the young of one 
male; but the groups readily mix, either by the fusing of two sepa- 
rate groups or by the junction of individuals with groups with which 
they have not previously had acquaintance. In both cases, the 
group, once formed, is fully as well integrated as though all belonged 
to the original family, and shows a high degree of group protection, 
particularly through the multiplicity of eyes, which may detect 
danger, and the transmission of stimuli, which lead to quick separa- 
tion and flight. The members of the group are safer than if they 
swam alone. This case is the more instructive since the group re- 
forms daily, and since, as Bowen has shown, the coming-together 
is largely conditioned by the possession of a weak social appetite 
combined with certain reciprocal reflexes. 

These illustrations suggest the possibility of the formation of 
groups of decided social integration as the result of the congregation 
of individuals, without a family aggregation of any sort appearing 
in this particular type of organization. 

Social organizations among birds include phenomena of leader- 
ship, of group integration, of division of labor, at least to the estab- 
lishing of sentinels, of joint action in common defense by spreading 
an alarm and by joint attacks, so that we are warranted in ranking 
them as well-developed social groups. Even so, we repeat, they 
may arise from the congregation of individuals as well as by the 
coming-together af families. In so emphasizing the possible social 
development from the aggregations of individuals with which this 
book is mainly concerned, I have no intention of underestimating 
the other well-known method of social development by the extension 
of the family type of aggregation, which may be seen in process of 
development among the solitary wasps and which comes to a high 
state of perfection in the social wasps, bees, ants, and termites. 

Students of human sociology are generally agreed that our com- 
plex modern social organization rests primarily on family groups 
(Thomas, 1909; Lowie, 1920; Malinowski, 1927). Miller in 1928 
reviewed a part of the literature appearing just preceding that date 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 347 


dealing with the sex relations of non-human anthropoids and of 
man. Miller concludes that a precultural gregariousness, not sexual- 
ly conditioned, but including sexual promiscuity, existed among 
early man and has influenced the course of development of human 
institutions. He emphasizes observations on the higher primates 
which indicate sexual promiscuity in horde life rather than family 
organization whether of the polygamous or monogamous type. 
Such observations give point to the strongly intrenched system of 
human taboos and laws which are directed against promiscuous 
horde sex-relations, and bear out the theory that these taboos and 
laws are in themselves a universal, tacit recognition of the fact that 
such promiscuous tendencies are strong enough to be a menace to 
human society as now organized. The firm hold of the tendency 
toward promiscuity among modern men even under these taboos 
and laws is witnessed by the prosperous condition of prostitution, 
which from this point of view is a lusty survivor from primitive 
horde life rather than a recent development conditioned by modern 
economic or other social pressure. The high percentage of occur- 
rence of gonorrhea and syphilis, diseases not excessively easy to 
acquire, also bears evidence that the sexual behavior of man, at 
least as shown under the prevailing Europeo-American civilization, 
consists in part of elements which cannot be distinguished from the 
activities which Miller thinks are characteristic of the ape horde. 
While there are many indications that horde life stands in the 
remote human background, it appears that other anthropoids than 
man have made the transition to some sort of family life. Yerkes 
(1929), after commenting frequently on the incompleteness and 
uncertainty of our knowledge of the social traits of the great apes, 
concludes his detailed presentation of existing evidence as follows: 
‘“‘Gregariousness and degree of dependence of the individual on 
the group tend from lemur to man to diminish, and at the same time 
to give place to a more definite and stable social unit, the family. 
There is a great diversity within the several types. Lemurs and 
their kind may live in bands or as mated pairs. Monkeys almost 
invariably constitute bands, as do also gibbons and siamang, but 
by contrast the anthropoid apes live either as families or in groups 


348 ANIMAL AGGREGATIONS 


constituted by temporarily associated families. The transition from 
pronounced gregariousness to family life seems to occur between 
gibbon and chimpanzee. 

“Leadership, the dominance of one individual, and variation in 
social value and influence in accordance with individual traits, ap- 
parently tend to become increasingly important from lemur to man. 
The transition is from the leader of the herd to the patriarchal head 
of the family. 

“Likewise, sociability and social dependence, in certain at least, 
of their social aspects tend to increase from lemur to ape, but so 
does individualism; and whereas the chimpanzee is extremely soci- 
able and dependent therefore upon its social environment, both 
orang-outan and gorilla are markedly less so than are certain mon- 
keys and lemurs. Obviously knowledge does not permit of safe 
generalization. With respect to mutual aid and like expression of 
sympathetic interest there can be no doubt of marked increase: 
lemur, monkey, ape, man. 

“Permanency of mating, although not definitely established for 
any infrahuman primate, is rendered increasingly probable from 
lemur, through monkey, to ape, by the nature and abundance of 
pertinent evidence. The same may be said of monogamy, for al- 
though it may exist in any of the four groups which we are comparing, 
so also, according to pertinent observations, may polygamy. Wheth- 
er there is definite phylogenetic tendency toward the one or the 
other type of family it is impossible to say. But in any event the 
family as a social unit seems to become more prevalent and also 
more stable as we progress from lemur to man.” 

Men have other aspects of group life in common with these non- 
human anthropoids. Thus the monosexual human gangs and 
clubs seem to have their counterpart in the sleeping group of male 
gibbons which Spaeth (see Yerkes, 1929) found in the Siamese 
jungle. Such human and gibbon behavior appears to be an expres- 
sion of the widespread phenomenon of the formation of monosexual 
groups which have been recorded notably among various other 
mammals (seals, for example), among birds, and among the so-called 
solitary bees and wasps. As in man, the young orang-outans con- 


ANIMAL AGGREGATIONS AND SOCIAL LIPE 349 


gregate in gangs; and Yerkes records various observations of groups 
of chimpanzees engaging in play. 

The high degree of group integration and the division of labor 
which may exist in social groups of monkeys are well illustrated by 
the following extract from a personal communication from J. F. V. 
Phillips, stationed in Tanganyika territory, East Africa. In writing 
about the social habits of baboons, he quotes from Sclater (1900), 
who says that these animals associate in groups of varying numbers 
up to about a hundred individuals; that, when moving from place 
to place, the old males are usually seen on the outskirts and always 
form a rear guard; and that, when resting, a sentinel or two is 
always placed on top of a rock to warn the troop of approaching 
danger. 

Phillips comments: “This is entirely correct; the sentinel is ex- 
ceedingly sharp and detects the least noise, scent, or appearance of 
man or leopard. In East Africa I have seen another species of ba- 
boon behaving in the same manner. The sentinels are often the 
largest, strongest males, that is, with the exception of the real leader 
of the group; they will remain faithfully at their post ‘waughing’ 
(the typical note of danger is ‘waugh,’ ‘waugh,’ very guttural and 
somewhat alarming) despite the proximity of danger. Upon these 
notes of warning reaching the ear of the leader, he will immediately 
assemble the leaders of the group, marshaling the males at the rear 
and along the sides, the females and the young at the forefront, or 
within the cordon of the males; he himself will alternately lead or 
bring up the rear, according to the plan of flight or the degree of 
danger. When things get too hot for the sentinels, they scamper off 
a short distance, mount some high position, and give further warn- 
ing to the leader. In times of slaughter, the young are protected 
by the parents, often with great danger to the latter.” 

The comparison of the behavior of this baboon horde with certain 
aspects of human behavior of which we are justly proud is too 
marked to need emphasis. Like behavior is frequently exhibited 
in some degree by herds of various sorts of mammals. The group 
defense of eggs or young by birds or of the nest by social insects 
shows similar behavior elements. The baboon example is particu- 


350 ANIMAL AGGREGATIONS 


larly valuable in that it dramatizes the possibilities of co-operation 
under group organization, even in non-human communities. 

Restating the general argument of the present chapter, we may 
say that it seems quite possible that sex arose originally from the 
beneficial stimulation received as a result of the aggregation of two, 
or more, simple asexual organisms. Sex, once originated, became 
one of the integrating factors in further social development. In sex- 
conditioned society the offspring of one pair or of one parent may 
have remained in association with their parents for the immediate 
mutual benefit of all concerned, or there may have intervened a 
sexually promiscuous horde life from which the consociation of off- 
spring with their individual parents arose as a further protective 
evolution. When these lengthened associations of parent and off- 
spring continued long enough, a division-of-labor type of society 
could evolve. At first this would occur between parents and mono- 
morphic offspring; later, dimorphic and even polymorphic offspring, 
as in ants and termites, might develop. However it arose, the family 
and the highly integrated division-of-labor society which may origi- 
nate through it is only one type of expression of the fundamental 
tendency of animals to aggregate. There are other social phases of 
animal life which have developed on this same foundation of animal 
aggregations as a result of forces not centering about sex; these 
have produced social units of importance in animal life. 

A part of the difficulties we have encountered in discussing the 
role of different types of animal aggregations in the evolution of 
social groupings may be avoided if we recognize that there are 
many levels of social organization and that these overlap. Among 
the groups which we may fairly call ‘“‘social” there are: (1) those 
that show their social habit merely through the toleration of the 
close proximity of other similar individuals in the same restricted 
space—these may exist without any positive mutual attraction and 
may be called the toleration level; (2) those that form groups 
which react more or less definitely as units—the group integration 
level; (3) those which show physiological division of labor; finally 
(4), those that show morphologically distinct castes, each associated 
with some phase of the division of labor. The animals on the higher 


ANIMAL AGGREGATIONS AND SOCIAL LIFE 351 


planes of social development continue to show the group attributes 
characteristic of the lower levels. Survival values have been demon- 
strated throughout this whole series and extend well below the 
toleration social level to the threshold of primitive life. 

The first indication of structurally modified castes is to be found 
in the dimorphism accompanying sex, and the division of labor 
associated with sex runs not only through all the distinctly social 
levels but also through the majority of all types of animals as well. 
As we have suggested, sex itself, and therefore the divisions of labor 
associated with it, may have arisen as an outgrowth of certain bene- 
fits associated with primitive asexual aggregations. Sex phenomena 
aside, we have just seen that the different types of social organiza- 
tion, including even the physiological division of labor type, may 
have developed from aggregations formed by the coming-together 
of individuals without direct sexual causation as well as from those 
collections which have resulted from sexual appetite or from repro- 
ductive activities. The most permanent societies appear to have 
arisen when sexual and parental integration have operated in addi- 
tion to the more elemental aggregation tendencies. 


CHAPTER XX 


THE PRINCIPLE OF CO-OPERATION 


Espinas (1878) approached his task of organizing the materials 
available concerning animal societies with much the same point of 
view that we have developed from a behavioristic and ecological 
approach to the problem, the study of which has led to the present 
volume. Espinas says in his introduction to Des sociétés animales, 
“No living being is solitary. Animals, especially, sustain multiple 
relations with the organisms of their environment; and, without 
mentioning those that live in permanent intercourse with their kind, 
nearly all are driven by biological necessity to contract, even if 
only for a brief moment, an intimate union with some other member 
of their species. Even among organisms devoid of distinct and 
separate sexes, some traces of social life are manifested, both among 
animals that remain, like plants, attached to a common stock, and 
among the lowly beings which, before separating from the parental 
organism, remain for some time attached to it and incorporated in 
its substance. Communal life, therefore, is not an accidental fact 
in the animal kingdom; it does not arise here and there fortuitously 
and, as it were, capriciously; it is not, as is so often supposed, the 
privilege of certain isolated species in the zodlogical scale, such as 
the beavers, bees, and ants, but, on the contrary—and we believe 
we are in a position to prove this statement abundantly—a normal, 
constant, universal fact. From the lowest to the highest forms in the 
series, all animals are at some time in their lives immersed in some 
society; the social medium is the condition necessary to the conser- 
vation and renewal of life. This is, indeed, a biological law which it 
will be expedient to elucidate. Moreover, from the lowest to the 
highest stages in the series, we detect in the development of social 
habits a progression which, if not uniform, is at least constant, so 
that each social group carries the perfecting of these habits a little 
farther in one or another direction. Finally, social facts are subject 

352 


THE PRINCIPLE OF CO-OPERATION 353 


to laws, and these are the same everywhere that such facts appear, 
so that they constitute a considerable and uniform domain in nature, 
a homogenous whole thoroughly integrated in all its parts.’” 

More recently Kropotkin, Deegener, Alverdes, and Wheeler have 
contributed to the development of this thesis. The latter, in Social 
Life among Insects (1923) expressed his point of view as follows: 
“All living things are genetically related as members of one great 
family, one vast living symplasm, which though fragmented into 
individuals in space, is nevertheless absolutely continuous in time. 
In the great majority of organic forms each generation arises from 
the co-operation of two individuals. Most animals and plants live 
in associations, herds, colonies or societies and even the so-called 
‘solitary’ species are obligatory, more or less co-operative members 
of groups or associations of individuals of different species. Living 
beings not only struggle and compete with one another for food, 
mates and safety, but they also work together to insure to one an- 
other these same indispensable conditions for development and sur- 
vival.” 

We are not concerned here, as some recent writers have been 
(Wheeler, t911, 1930; Child, 1924), with renewing the Spencerian 
analogy between the living and the social ‘organism’? however much 
we are impressed by the remarkable similarities between the inter- 
relations of the organelles of a cell or the organs of the body, and 
the individuals composing a heteromorphic hydroid colony or a con- 
sociation of free individuals. Rather, it has been our task to present 
material gathered by numerous workers in the field of ecological 
physiology, particularly that collected within the last decade, and 
to focus it upon the present problem. The results obtained show 
that the generalizations of Espinas, of Deegener, and of Wheeler 
rest upon a broader base than that furnished by observational be- 
havior studies concerned primarily with the struggle for existence 
as conceived in the nineteenth century, and with the more obvious 
survival values, or upon the lives of insects admittedly social in 
habit. 

Certain of these problems with which we have been dealing be- 


t Translated by Wheeler, 1928. 


354 ANIMAL AGGREGATIONS 


come more clear if presented in the simplest possible form. In order 
to free our minds of the perplexities brought in by the special re- 
quirements of our present-day species, with their long history of 
past adjustments to environments, or of mutational changes, let us 
try to consider conditions existing when living molecules first 
evolved from their non-living antecedents. 

However, whenever, and wherever life first appeared on this 
planet, considerations which we have given in detail in the preceding 
chapters make it highly probable that, unless the first living mole- 
cules appeared in considerable numbers approximately simultane- 
ously within a limited microhabitat, there would have been little 
chance of survival; a single isolated living particle. must have suc- 
cumbed to the unconditioned unfavorable environment. If this 
occurred, a certain slight modification of the environment would 
result as the particle disintegrated. In doing so, it might free some 
X-substance, such as various workers, from Semper down to Robert- 
son, Drzewina and Bohn, and Burnet, have assumed to be necessary 
for the well-being of living organisms; or the decomposing proto- 
plasmic molecule might fix, by adsorption or otherwise, some of the 
elements of the environment harmful for a living system. In other 
words, the living protoplasm itself or the products of its metabolism 
during life, or freed by death and disintegration, would probably 
condition the immediate environment in such a manner that if an- 
other particle of living matter appeared soon in that niche it would 
have a better chance of survival. 

If, on the other hand, several of these living molecules appeared 
approximately simultaneously in the same restricted microhabitat, 
_ then by the processes of metabolism they would tend to condition 
their environment similarly, and by fixation of toxic substances, 
or by some one of the other communal activities, such as the produc- 
tion of an X-substance, or the modification of the electrical condi- 
tions, this primitive aggregation of living particles would show the 
survival value which we have demonstrated is frequently exhibited 
by present-day animal aggregations of approximately the same in- 
tegration level. 

It may be that numerous transitions from the non-living to the 


THE PRINCIPLE OF CO-OPERATION 355 


living would occur one after the other in the same micro-niche, with 
a successive conditioning and a progressively greater longevity of 
some or all of the particles, until finally conditions would become 
sufficiently favorable for permanent survival. Whatever the details, 
it seems probable that this mechanism was operative from the very 
beginning of life and is a fundamental trait or property of living 
matter. 

In order to discuss this trait more easily, it should be named. A few 
years ago it might have been called “unconscious co-operation”’; but 
since many modern psychologists have discarded the concept of con- 
sciousness, the idea of lack of consciousness is less helpful than form- 
erly. It may be regarded as an automatic mutual interdependence 
among organisms, or, for the sake of simplicity, as the principle of 
co-operation. The only trouble with calling this relationship one of 
co-operation, which it is, lies in the fact that the word carries with 
it an idea of conscious effort (cf. Durkheim, 1922) possible only 
after long ages of organic evolution, and then only in certain favored 
types of animals, while the evidence appears to be clear that the 
sort of co-operation of which we are speaking is a fundamental trait 
of living matter. As in all the other fundamental properties of living 
organisms, there is probably no hard and fast line to be drawn here 
between the living and the non-living. The mutual interdependence 
of the living must have grown out of similar but simpler interde- 
pendence in antecedent non-living matter, and may, in fact, be 
merely a highly specialized biological application of the mass law of 
chemistry. 

If this analysis be sound, as it appears to be, the potentiality of 
social life is inherent in living matter, even though its first manifes- 
tations are merely those of a slight mutual interdependence, or 
of an automatic co-operation which finds its first biological expres- 
sion as a subtle binding link of primitive ecological biocoenoses. Lest 
we be accused of having been carried too far by enthusiasm, it may 
be well to pause for a moment to examine the extent to which this 
automatic co-operation has been demonstrated to exist among ani- 
mals. Are we, in fact, dealing with a phenomenon known to be 
sufficiently widespread to be thought of as having general rather 


356 


tha 
list 


ANIMAL AGGREGATIONS 


n special biological significance? Chart I gives a diagrammatic 
of the portions of the animal kingdom in which this principle of 


automatic co-operation has been demonstrated to date. 
CHART I 
DIpHYLETIC TREE OF RELATIONSHIPS WITHIN THE ANIMAL KINGDOM* 
Birps Mammars INSECTA 
REPTILES 
AMPHIBIA 
ARACHNIDA 
Myriapopa CEPHALOPODA 
VERTEBRATA 
CEPHALOCHORDATA 
CRESTACEA 
UrocnorpatTa 
OnycoPuor: 
HEMICHORDATA 
CHORDATA ARTHROPODA 
MOLLUSCA 
ANNELIDA 
/BRACHIOPODA, | 
ve io 
oat | 
ECHINODERMA 7 ROZOA 
| 
SAGITTA | TROCHELMINTHES 
| Le 
NEMATHELMINTHE ? Lea ee 
| PLATYHELMINTHES 
| ee SS 
| 
] CTENOPHORA 
Wee 
| 
COELENTERATA Bacteria 
PORIFERA Plant 
kingdom 
PROTOZOA PROTOPHYTA 


eae, Primitive plants 
/ 


PRIMITIVE PLANT-ANIMAL 


* Phyla are given in larger, classes in smaller, capitals. 


THE PRINCIPLE OF CO-OPERATION 357 


This chart shows a diphyletic tree giving the relationships within 
the animal kingdom (Allee, 1926a). The distance from the base 
represents the relative degree of specialization. The phyla and 
classes underscored are those in which survival values from aggre- 
gations have been demonstrated, other than those known to occur 
in connection with normal sexual reproduction. Without reason- 
able doubt proper tests would reveal that aggregations of animals 
in all of the divisions still unchecked also possess survival value, 
at least when the animals are exposed to unusual or unfavorable 
conditions, such as those which would be furnished by hypotonic 
sea-water for the marine forms or by distilled water for the Tro- 
chelminthes. Exposure to various toxic agents would undoubtedly 
reveal group survival, providing the group were not too large nor the 
concentration too great. In addition to the group survival values 
known to be so widely distributed among animals, taxonomically 
considered, we have seen that similar survival values have been 
demonstrated for such diverse organisms as bacteria, for the sper- 
matozoa of several kinds of aquatic animals and of mammals, and for 
tissue-culture cells. Evidently mutual interdependence, or automat- 
ic co-operation, is sufficiently widespread among the animal kingdom 
to warrant the conclusion given above that it ranks as one of the 
fundamental qualities of animal protoplasm, and probably of proto- 
plasm in general. 

Even if we are prepared to grant the foregoing conclusion, it does 
not necessarily follow that the principle of automatic co-operation 
is of great importance, though it may be exhibited by all known 
major groups of animals. Before we can satisfy ourselves that we 
are dealing with an important as well as a universal principle, it is 
necessary to find how commonly it is exhibited in nature. Running 
through the preceding pages and building up a summary of the 
various organisms whose aggregations have been discussed in these 
pages, we find, even when the different species of such animals as 
planarians and grasshoppers are lumped together, about 125 such. 
Aggregations of all these have been found in nature, with the ex- 
ception of echinoderm larvae, which have not been reported in the 
density in which they may be found in laboratory containers. With 
about 14 exceptions, these aggregations are exhibited in addition 


358 ANIMAL AGGREGATIONS 


to the congregation of the two sexes during the breeding season. 
Less than one-third of these forms possess sufficient social appetite 
to allow them to be classed as social animals in the usual sense of the 
term. Definite racial survival values have been demonstrated for 
about one-half, again excepting ordinary bisexual breeding relations; 
and of this half, about two-thirds are usually called “non-social.” 
Even this brief survey shows that in addition to being a widespread 
phenomenon, taxonomically considered, survival values frequently 
accrue from animal aggregations in a state of nature, and often much 
below the level of group integration usually called “social.” 

The field naturalist, interested in observing a wide range of ani- 
mal life, is familiar with the widespread occurrence of aggregations. 
Inland waters are notoriously poorer in population than is the sea; 
but in California, during the breeding season, I have seen ponds 
paved with the pebble-like clusters of salamander eggs. In mid- 
Great Salt Lake our boat ploughed through surface-covering masses 
of aggregated Ephydra flies that rose in choking numbers. Aldrich 
(1912) calculated 370,000,000 of these were to be found along every 
mile of Salt Lake beach. In the nearby mountain ponds of Utah 
aggregations of ostracods of the size of a walnut were to be found, 
at times occupying a portion of each cow track with which the bot- 
tom of the ponds were stippled; and similar collections of annelid 
worms occur in Indiana ponds. The collections of Hydra in favor- 
able spots along Lake Michigan remind one of the abundance of 
marine organisms; and in some portions of spring-fed watercress 
swamps the supply of Planaria dorotocephala seems exhaustless. 

Along the seashore, in such favorable locations as part of the 
California coast, the supply of animal life is appalling. One cannot 
step on the rocks exposed at low tide without crushing sea urchins, 
sea anemones, barnacles, or mollusks. Even in the less prolific re- 
gions around Cape Cod every available rock or solid timber washed 
by the tidal currents is the base for a densely packed aggregation, 
composed of many or of few species. Favorable bottom areas are 
similarly packed; and Mytilus and Crepidula fornicata, if proper 
substratum be wanting, form chains of animals, attaching to each 
other in the absence of solid objects. A suitable bit of mud flat may 


THE PRINCIPLE OF CO-OPERATION 359 


be packed with Mya so that the openings of their siphons fairly 
crowd the surface of the mud. At times and in favorable locations 
jelly fishes, or even minute copepods, may discolor the sea for miles; 
the entrance to the White Sea may be covered by red streaks pro- 
duced by the presence of multitudes of starfish eggs (Mesiacev, 
1927). 

Brues (1926) estimated that the Hymenorus beetle population of 
a single panicle of Florida yucca would be about 15,000, and cites 
case after case of well-established insect aggregations. Some of the 
more striking include the hibernating aggregations of ladybird 
beetles (Hippodamia convergens) in northern California, of which 
Carnes (1912) records that two men in a single day can gather from 
1,200,000 to double that number from the hibernating masses 
among the pine needles. A thousand chinch bugs have been found 
in the shelter of a single tuft of grass 3 inches in diameter (Headlee, 
1910). Howard (1898, 190r) records flights of a chrysomelid beetle, _ 
one of which formed a belt 15 feet thick and a hundred yards wide 
over the course of the Gila River. The flight continued for 2 days. 
Cicadas, monarch butterflies, migratory locusts, and many Diptera, 
including Polenia rudis, Muscina, house flies, midges, and other 
insects, are known to collect in great numbers. In this survey I 
have not mentioned the collections of insects about electric lights, 
or the insects in the shore drift of lakes, or the vast collections of 
the more strictly social species, or the type of relationship usually 
called “symbiosis.” 

The recapitulation we have just made summarizes the evidence 
on two points.. Aggregations of animals with little or no group or- 
ganization, which possess survival values for the aggregants, have 
been demonstrated sufficiently widely throughout the animal king- 
dom to indicate that if studied in the other taxonomic units with the 
proper methods they can probably be demonstrated to occur in all 
the larger taxonomic divisions. Certainly they have already been 
demonstrated in groups sufficiently widespread to indicate that the 
absence of such group protection, other things being equal, is to be 
regarded as an exception rather than the rule. Such aggregations 
are ecologically as well as taxonomically widespread, and they are 


360 ANIMAL AGGREGATIONS 


abundant in nature, as well as being widely distributed. It is upon 
such evidence that we may conclude that the mutual interdepend- 
ence, or automatic co-operation, of which we are speaking is a funda- 
mental and important principle in biology. 

There is nothing in this recent work which displaces the earlier 
conclusion that overcrowding is harmful; but this newer evidence 
which we have been interested in presenting does show that under 
proper conditions, and entirely apart from breeding or hibernation, 
beneficial results may follow aggregations, in many organisms of the 
same or of different species, within a limited space. This means 
that in groupings caused by the tropistic reactions of individuals to 
environmental factors there may be a natural co-operation effective 
long before the physiological organization of the group has: reached 
the level of development which occurs in the groupings usually des- 
ignated as being truly social. 

Symbiosis, commensalism, and intra-organismal relations aside, 
such unconscious co-operation was unknown to Espinas or to 
Kropotkin, who were much impressed by the evidences of mutual 
aid among insects and the larger animals. It was unknown to Wheel- 
er when he wrote the 1923 conclusion quoted above, to which he was 
led by the studies of the ecologists and by his own knowledge of the 
behavior of ants and other social insects. The knowledge which we 
have summarized, showing that such general co-operation exists 
among loosely organized, or among apparently unorganized, groups 
of animals living even temporarily in the same region, gives us much 
clearer evidence than has been available to these students of social 
life, that their conclusion that co-operation is one of the major 
biological principles is correct, and that its roots extend far below 
the level of well integrated social activity. 

From this point of view the first step toward the development of 
societies had probably already been taken when life came into 
existence on this planet. These first living particles were probably 
dependent on each other for the final adaptation of their physical 
environment so that they could continue to live; In the course of 
evolution they became more independent of close proximity to each 
other. A further advance was made when such more or less solitary 


THE PRINCIPLE OF CO-OPERATION 301 


animals developed, in addition to the general automatic co-opera- 
tion inherent in living matter, a new toleration for close aggregation 
in a limited area, where they had collected not as a result of a social 
appetite but on account of their individual reactions to the sur- 
rounding environmental conditions. Such collections occur fre- 
quently as the result of forced movements in which the animal re- 
acts, apparently mechanically, to the forces operating upon it, and 
may persist only because of the inertia of toleration. These tropisti- 
cally conditioned groupings show survival values in addition to those 
resulting from the general co-operation of which we have spoken so 
frequently. Such additional survival values may be shown either by 
the effect of the group upon the individuals, rendering them more 
resistant to adverse environmental conditions, or conversely by so 
effecting the environment by the removal of toxic materials, or by 
some other ameliorating device, that it becomes more favorable for 
the continued existence of the animals. \Group survival values can 
slip into the background as animals become well adjusted to the 
environment, to reappear apparently afresh when conditions of exist- 
ence become again less favorable, These new survival values may 
be qualitatively as well as quantitatively different from those shown 
previously. 

( The last advance in this series comes when individuals cease to 
react as separate units and respond only as members of a group— 
when, as in the case of ants or termites and, rarely, with men, they 
are largely group-centered rather than self-centered.) Many of the 
so-called “altruistic” drives in man apparently are the development 
of these innate tendencies toward co-operation, which find their 
early physiological expression in many simpler animals. 

_With the development of the nervous system, closer co-operation 
becomes possible and larger numbers are affected. There is much 
reason for thinking that many of the advances in evolution have 
come about through the selection of co-operating groups rather than 
through the selection of individuals. This implies that the two great 
natural principles of struggle for existence and of co-operation are 
not wholly in opposition, but that each may have reacted upon the 
other in determining the trend of animal evolution. 


362 ANIMAL AGGREGATIONS 


As a result of the working of these two principles, man has de- 
veloped social groups, the scope of whose organization has been 
constantly extended until at the present time we are confronted 
with the problems centering about national versus international 
organization. Now, as in each stage of the social evolution of man, 
the proponents of the narrower organization maintain that the type 
of groupings. they advocate satisfies the natural instinctive and 
traditional drives of man, while the more inclusive grouping is an 
abnormal desire for an idealistic utopia. So might the conservative 
primitive-living molecules, the protozoans, flatworms, isopods, or 
ants have argued, had they the wit, at each stage of their co-opera- 
tive evolution. It may be helpful, and restful as well, to remember 
that the great majority of the evolution of social life has been 
brought about, not by conscious effort on the part of those under- 
going evolution, but by the natural working-out of these two funda- 
mental principles of struggle and co-operation. 

We have been concerned in this book in tracing the earliest be- 
ginnings of these secondary (group) reactions (whether shown in 
overt acts or more subtly revealed), exhibited only under restricted 
conditions in nature which may be mimicked by properly controlled 
laboratory conditions. We have found that the physiology of the 
group considered independently from that of the individuals of which 
it is composed, begins simply and shows stages in development which 
can be arranged in various sorts of ascending series and which culmi- 
nate in the group-centered, division-of-labor type of society that at 
first glance seems impossibly remote from the life of the so-called 
“solitary” animals. 

Brilliant students of the highly social life of insects, like Wheeler, 
have found evidence that the behavior of these societies, taken to- 
gether with observations on ecological associations and the various 
activities that center about reproduction, indicate the existence of 
a fundamental tendency toward co-operation. It has been much 
more easy for a student beginning with the humbler group levels 
to follow, from the social beginnings which he learns to recognize in 
almost unintegrated animal aggregations, the possibilities of the de- 
velopment of great social structures; and to trace their growth slowly 
and as yet imperfectly, but surely. 


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3608 ANIMAL AGGREGATIONS 


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374 ANIMAL AGGREGATIONS 


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376 ANIMAL AGGREGATIONS 


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BIBLIOGRAPHY 377 


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384 ANIMAL AGGREGATIONS 


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386 ANIMAL AGGREGATIONS 


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406 ANIMAL AGGREGATIONS 


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408 ANIMAL AGGREGATIONS 


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INDEX* 


Abbott, C. H., 71 
Achtenberg, H., 117 
Acids, effect on sperm suspensions, 269, 
278 
see a pH, HCl, H.SO, 
Ackerman, L., 313 
Acrididae, see Grasshoppers 
Adsorption, of colloidal silver, 208-12, 214, 
222 
of salts, 213-15 
of material from Procerodes experi- 
ments, 233 


Aération, effect on crowded snail cultures, 
LO, Tr2 


Aestivation, of land isopods, 71 
of quail, 72 


Agamermis, see Nematodes 
Agar, W. E., 305 


Agglutination, of bacteria, 249 
of spermatozoa, 272 


Aggregations, see also Crowding 
beneficial effects of, group defense, 102 
catching food, 102 
stimulate growth, 147, 148, 149, 153 
stimulate rate of regeneration in 
tadpoles, 148 
increase rate of reproduction, 161 
166, 178 
increase survival, 181 
decrease starfish autotomy, 182, 189, 
190 
protect from colloidal silver, 201, 222 
specificity of, 210 
protect, from toxic salts, 213 
high temperature, 217 
from ultra-violet, 218 
hypotonic sea-water, 222 
heterotypic protection, 228 
factors contributing to, 231 
in insects, 236 
in bacteria, 253 
in spermatozoa, 263, 284 
see also Survival value of groups 
breeding, 289, 358 
of isopods, 65 
of mosquitoes and midges, 66 
of frogs, 66 


’ 


of Ambystoma, 67 
of fish, 68 
of snakes, 68 
lunar periodicity in, 69 
classification, 14, 35, 340 
during ‘‘sleep,” 74 
integration, 81, 87 
effects on growth form, 311 
in aphids, 313 
in grasshoppers, 316 
in Drosophila, 331 
formation, 38, 42, 65 
due to, social appetite, 46, 51, 52 
instinct, 47, 48, 49, 52 
moisture control, 71, 181 
harmful effects on growth, ror 
by food exhaustion, 102 
on bacterial growth, 103 
on Lymnaea, 108, 123 
on echinoderm larvae, 110, 117, 153 
on Daphnia, 110 
on tadpoles, 112 
on Hydra, 112 
on Planaria, 113 
on fish, 113, 116, 117 
summary of factors, causing, 118 
on grasshoppers, 319, 329 
on Drosophila, 332 
harmful effects on longevity, 136 
of planarians, 140 
of Daphnia, 141 
in electrolytes, 141 
by pollution, 184 
harmful effects on rate of reproduction, 
I20 
of Paramecium, 120 
of Stylonychia, 122 
of Daphnia, 126 
of hens, 126 
of Drosophila, 131 
of hookworms, 135 
of Cladocera, 306 
origin of sex and, 290 
origin of society and, 339, 340, 350, 363 
types, 12, 36 
aestivation, 71 
hibernation, 70 
Asellus, in nature, 52 
gyrinids, analysis of, 60 
caterpillars, analysis of, 48, 49, 50 


* Author citations in the index do not include reference to bibliography pages. 


413 


414 


Aldrich, J. M., 358 
Alkali, effect on sperm in sugar solution, 
278 
see also pH 
Allee, W. C., 43, 44, 56, 65, 71, 73, 181, 
TO2) LOA 202, 20o 224 225 ea. 
235, 258, 342, 357 
Allelocatalysis, 161 
negative evidence, Jahn’s, 162, 170, 171 
Cutler and Crump’s, 167, 168 
Peskett’s, 167, 168 
Calkin’s, 168 
Greenleaf’s, 169, 170 
Myers’, 170 
Petersen’s 172 
Robertson’s original data, 163 
statistical analysis, 164 
Robertson’s hypothesis, 166 
relation of bacteria, to, 165, 178 
Peterson’s data, 170, 171, 172 
statistical analysis, 175 
Yocum’s data, 171 
exudates and, 235 
Allelostasis in spermatozoa, 280, 281 
Allen, A. A., 79, 80, 88, 345 
Allen, E., 303 
Allen, G. M., 79 
Allomermis, see Nematodes 
Alverdes, F., 7, 8, 9, 26, 27, 33, 34, 349; 
353 
Alytes; breeding behavior, 26 
Ameiurus, aggregation analysis, 62 
stimuli causing aggregation, 90, 95 
social significance, 336 
Ammophila, ‘‘sleep” aggregations, 74 
Amphibia, monogamy 27 
heterotypic aggregations in, 29 
polyspermy, 266 
Amphipods, sex recognition in, 89 
Andrews, E. A., 89 
Annelida, as hosts of monstrillid copepods, 
290 
Antelopes, herds of males, 28 
organized open societies, 34 


Anthropoid society, 27, 35, 347 


Ants, 12, 17, 24, 330, 340, 346, 352, 360, 
361, 362 
associated animals, 14 
mixed families, 27, 30, 32 
relation, to termites, 29, 31 
to aphids, 30, 341 
myrmecocoles, 30, 31 
hibernation, 70 
reaction of Solenopsis, to floods, 72 


ANIMAL AGGREGATIONS 


contact-odor reactions, 89 
reaction to sounds, 93 
soldier caste, 246 
Apes, breeding relations, 27 
social organization, 347 
Aphids, 19 
relations to ants, 30, 341 
harmful effects of crowding, 102 
wing production, 313 
Appetite, social, 11, 46, 51, 52, 60, 64, 80, 
342, 346, 361 
Arbacia, see Sea urchin 
Archusia, 150, 152 
Area, available, see Crowding 
Arenicola larvae, reactions, 38, 40, 42 
Aristotle, vil 
Armadillidium, see Isopods, land 
Artemia, mass protection from salts, 215 
Arthropods, hibernation, near Chicago, 70 
Ascidians, 12, 13, 16 
crowding on growth form, 311 
Asellus, formation of aggregations, 52, 
58, 181 
sex ratio in aggregations in nature, 55 
rheotropism, 56, 59, 65 
breeding behavior, 65 
effect of crowding on growth, 108 
oxygen consumption in natural aggrega- 
tions, 194 
protection from colloidal silver, 202, 
210, 211 
excretions and male production in 
Cladocera, 305 


Ashby, E., 159 


Association, 5, 6, 7, 8, 353 
ecological, 8, 362 
homotypical, 15 
kormogene, I5 
primary, 15 
Deegener’s definition, 15 
secondary, 20 
heterotypical, 22 
Wheeler’s definition, 34 
subdivisions, 35 
food chains, 35 
mimetic, 35 
commernal, 35 
mutualistic, 35 
relation to society, 339, 340 


Augustine, D. L., 33 
Autocatalyst, Robertson’s 
166, 177, 178, 218 


Autodestruction by crowding (Drzewina 
and Bohn), 141, 191, 204, 209 


hypothesis, 


INDEX 


Autolytus, asexual reproduction, 16 


Autoprotection (Drzewina and Bohn), 
Tit, Ali, Aey, Air, ins, Wr, 7) 
223, 235, 274, 285 

Autotomy of starfish arms affected by 
crowding, 182, 189, 190 

Autotoxins, bacterial, 103 

specificity, 106 
. “Auximones,” 159 


Baboons, 349 


Bacillus spp., mass protection against 
gentian violet, 253 
causes of, 255 
heterotypic protection, 256 


Bacteria, 337, 357 
cessation of growth, 103 
effect on allelocatalysis, 165, 178 
communal activity, 167, 247, 250, 261 
mass protection for, 217, 252, 260, 285 
in Procerodes experiments, 233 
culture, life-cycle of, 247 
lag, theories for, 248 
colony compared with an individual, 
261 
in mammalian semen, 283 
effect on male production in Cladocera, 
308 
Baker, 79 
Baker, L. E:, 150 
Balbiani, G., 120 
Baltzer, F., 291, 293, 294, 310 
Banks, N., 75 
Banta, A. M., 67, 68, 89, 235, 303, 304, 
305, 306, 307, 308 
Barber, M. A., 253 
Barbour, T., 26 
Barkow, H. C. L., 70 


Barnacle, 21, 358 
polyandry in Alcippe, 26 
food habits of Alcippe, 31 
growing on whales, 32 
crowding on growth form, 311 


Barthélémy, H., 266 

Baskervill, M., 266 

Bass, black, community relations, 83 
food, 84 

BateswAt EC. 70 

Bats, 12 
breeding behavior, 27 
hibernation, 70 


roosts, 79 
tactile integration, 88 


415 


Beavers, 352 

Beebe, W., 93, 342 

Bees, 12, 346, 348, 352 
solitary, 13, 18 
as heteromorphic societies, 24 
honey, drone behavior, 27 
hibernation, 28, 70 
Mellisodes, 28, 75 
“sleep” aggregations, 75 


Beetles, drifts of, 22, 341 
Necrophorus, 19, 28 
passalid, 24, 27, 92 
coccinellid, hibernation, 28 
gyrinid, aggregations, 28, 60 
dermestid, food habits, 31 
staphalinid, preying on ant larva, 31 
Dermestes, food exhaustion, 102 
population equilibrium, 138, 179 
reproductive rate stimulated by crowd- 
ing, 179, 236, 244 
chrysomelid larvae, group protection in 
nature, 245 
abundance, 359 
Belostoma, aggregation formation, 52 
Bilskiv he) £12, 104) ILLS) DLS, DLO; 132; 
148, 149 
Bilski’s formula for effect of crowding on 
growth, 117 
Biocoenosis, 5, 9, 23, 35, 355 
Bioelectrical, see Biophysical 
Biophysical integration, 96, 299 
effects of containers, 125, 277 
protection, 219, 276, 285 
conditioning of environment, 354 
“Bios,” 119, 157, 159 
Biota, 36 
of lake, 83 
Biotic balance, 83, 85, 179 
Birds, 12 
young, as homomorphic societies, 24 
breeding behavior, 27, 343 
common roosts, crows and robins, e.g., 
_ 28, 79, 339 
mixed groups, 30, 342 
migration societies, 31 
partially closed communities, 33 
songs, significance, 92 
polyspermy, 266 
flocking behavior, 339, 349, 342, 343; 
344, 348, 349 
Bison, American, breeding behavior, 27 
Blair, K. A., 92 
Blanchard, F. N., 67 


Bliss, C. I., 239, 240 


416 


Bluebirds, 343 

Bobolinks, flocks, 343 

Bohn, G., 125, 149, 142, IQI, 201, 202, 
203, 206, 207, 208, 209, 212, 217, 
210, 222, 223, 235, 250, 205, 274, 
275, 276, 277, 278, 284, 285, 354 

Bonellia, polyandry in, 26 

sex determination in, 291, 309, 311 

Bonnevie, K., 266 

Borodin, D. N., 96 

Bottomley, W. B., 158, 159 

Boulenger, G. A., 67 

Bowen, E. S., 62, 63, 90, 95, 345, 346 

Tyo Keg, IDs “Xs 1Do5 Oy W775 BEF 

Bradley, J. C., 76 


Breeding behavior, 358 
of Asellus, 65 
of dipterans, 66 
of frogs, 66 
of Ambystoma, 67 
of toads, 67 
of fish, 68 
of snakes, 68 
of Nereis, 69 
controls bunching in A sellus, 181 


Bresslau, E., 202, 212 

Brewster, W., 79 

Bridges, C. B., 331 

Brown, C. R., 28, 61, 62 

Brown, H., 117 

Brown, L. A., 235, 304, 305, 306, 307, 308 
Brownlee, J., 115, 142, 143 

Brues, C. T., 359 

Bryozoa, polyspermy in, 266 
Bryozoan colonies, 23 

Buchanan, R. E., 247, 248, 249, 259 
Buchsbaum, R., 76, 77, 317 
Bullheads, see Ameiurus 

Burgess, E. W., 5 

Burnet, F. M., 217, 253, 260, 354 
Burrows, M. T., 150, 152, 153, 166 
Buttel-Reepen, H. von, vii 


Butterflies, 21, 3590 
migrating swarms, 34 
“sleep” aggregations, 75 


Buxton, P. A., 94, 96, 318 


Caddis-fly larvae, snails, etc., worked 
into case of, 32, 341 


Calkins, G. N., 168 


ANIMAL AGGREGATIONS 


Calliptamus, see Grasshoppers 
Cancer cells grown in vitro, 166 
Candolle, A. P. de, 103 
Capillary tubes, effect of growing Pro- 
tozoa, in, 122, 136 
relation of kind of glass to effect 
produced, 123 
Carbon-dioxide production, affected by 
crowding, 184 
protects from salts, 215 
from toxic gasses, 239 
possible réle in Drosophila crowding, 


245 
in sperm suspensions, 267, 279, 281, 285 
reaction of sperm to, 272 
saturated sea-water and sperm, 275 


Carbon tetrachloride, mass protection 
against, 237 

Carnes, E. K., 359 

Carpenter, K: E., 212, 213, 214) 258 

Carrel, A., 106, 149, 150, 151, 152 

Castellani, A., 259 

Castes, 351 

Castle, W. A., 225 

Catalysis by contact, 125, 219, 278 


Caterpillars, 18, 20 
migrating swarms, 34 
Szymanski’s analysis of social behavior, 
48, 50, 60 
tropisms, 48, 50 
Deegener’s analysis of social behavior, 


49 
synchronous behavior, 51, 94 
response to sound, 94 
response to substratal vibrations, 96 
menace of crowding, 102 
Catfish, see Ameiurus 
Caullery, M., 301, 302 
Cell aggregates, formation, 45 
Cells, isolated in tissue culture, growth of, 
150 
Ceriodaphnia, see Cladocera 
Cerous chloride, effect on spermatozoa, 
278 
Chalybion, “sleep” aggregations, 75 
Champy, C., 157 
Chapman, R. N., 138, 139, 140, 178, 179, 
180, 244 
Chemotropism, 60 
in sex recognition, 89 
in aggregation integration, 89 
Chesney, A., 105 


INDEX 


Cimilel, (Co Iles AGE AG, (iP, silts ale, Ono}, 
239, 289, 312, 340, 353 
Chimney swifts, flocks, 343 
Chimpanzee, 348 
Chironomidae, swarms of males, 66 
breeding behavior, 66 
host for nematodes, 300 
Chironomus, see Chironomidae 
» Chlorine content of hypotonic sea-water, 
225 
Christie, J. R., 299, 300, 301, 302, 303 
Chromosomes in Cladocera, 303 
Chrysomelid beetle abundance, 359 
Church, F., 113, 116, 119 
Churchman, J. W., 167, 252, 254, 255, 
257, 258, 259 
Cicadas, abundance, 359 
Cladocera, mass protection from colloidal 
silver, 202, 210 
male production in, 235, 303 
chromosomes, 303 
asexual generations, 304, 309 
GlarkeNepas, 525 250 
Classes of animals, 356 
Clausen, R. E., 334 
Climatic factors and animal numbers, 321 
Clowes, G., 269, 270 
Cobb, N. A., 299, 300, 302 


Coccinelid beetle hibernation, 28 
abnormal breeding behavior, 29 
Cockerell, T. D. A., 77 
CohnesEe)e 207)208,)2005 272) 270,2or, 
284, 285 
Colloidal silver, recipe, 201 
mass protection from 201, 222, 258 
Planaria, 202 
Cladocera, 202 
Asellus, 202 
reconditioned solutions, 203, 204, 207 
Paramecium, Colpodium, Stylonychia, 
Stentor, Hydra, Convoluta, leeches 
and tadpoles, 204 
goldfish, 209 
specificity of protection from, 210, 231 


Colonies, primary, 16, 23, 35 
homomorphic, 16, 23 
heteromorphic, 16, 23 
concrescence, 16 
polymorphic, 23 
reciprocal, 23 
irreciprocal, 24 
secondary, 24 


417 


Coloration, crowding in grasshoppers and, 
319, 325, 328 

Col pidium, 161, 166, 167, 168, 212 

Col poda, 161, 204 

Colton, H. S., 111, 119, 147, 148 

Comas, M., 301, 302 

Commensalism, 29, 341, 360 

Common interests of animals, 86 


Communal activity of bacteria, 167, 247, 
259, 261 
Communities animal, 4, 5 
plant, 4, 5 
biotic, 8 
Community, 5, 6 
ecological, 23, 36 
definition, 80 
integration, 80 
organization, 81 
man-dominated, 82 
black-bass-dominated, 83 
of bacteria, 247, 261 
Comte, A., vil 
Conditioned medium, 
Ameiurus to, 62, 90 
reconditioned against colloidal silver, 
207 
heterotypic conditioning, against col- 
loidal silver, 210 
protection against hypotonic sea-water, 
221 
against hypotonic sea-water, 225 
factors concerned in, 231 
Paramecium cultures, 176, 177, 235 
in allelocatalysis, 165, 176 
see also Exudates 


Conditioning of environment by Asellus, 


reaction of 


ZS, 
at origin of life, 354 
Conklin, E. G., 123 
Contact-odor responses, 89 


Convoluta, effect of glass on, 125 
autodestruction in KCl, 140 
mass protection from, colloidal silver, 
204 
hypotonic sea-water, 222, 223, 224 
metallic silver, 277 


Co-operation, 337, 338, 350, 352, 362 
Copepoda, 12, 358 
influence of crowding on sex, 290, 309 
Copper chloride, effect on spermatozoa, 
272 
Coral colonies, associated animals, 32 
Corymor pha, cell aggregations, 45 


418 


Courtis, S. A., 67 


Cowbirds, 13, 341, 342, 343 
breeding behavior, 27 
mutualistic relations with cattle. 29 


Crabb, E. D., 111, 112, 119 


Crabs, 21 
hermit, 21 
relation to Hydractinia, 29 
Pinnixa, lives in mollusk burrows, 31 
Craig, W., 95 
Crampton, G. C., 122, 123, 124 
Crayfishes, sex recognition in, 89 
Crepidula, size relation to hermit crab 
shell, 123 
sex determination in, 295, 302 
abundance, 358 
Crew, F. A. E., 309 
Crow roosts, 28, 80 
Crowd, group defense by, 102 
see Crowding 
Crowding, see also Aggregations 
Crowding— 
affects sex determination, 289 
in monstrillid copepods, 290, 309 
in Bonellia, 291, 309 
in Crepidula, 295, 3090 
in nematodes, 295, 309 
in Cladocera, 303, 3090 
harmful effects on growth, of plar ts, 
102 
of bacteria, 103 
of Protozoa, 107 
of snails. 108, 123 
of echinoderms, 110, 117, 153 
of Daphnia, 110 
of tadpoles, 112 
of Hydra, 112 
of Planaria, 113 
of fish, 113, 116, 117 
summary of factors causing, 118 
increased death-rate, and, 136 
work of Drzewina and Bohn, 140 
and contagion, 144 
in Ophioderma, 205 
in grasshoppers, 319, 329 
in Drosophila, 332 ; 
increases rate of reproduction, 161 
in tissue culture, 166 
in Tribolium, 178 
menace of, 184, 203, 360 
morphological effects, 311 
wing production in aphids, 313 
in grasshoppers, 316 
in Drosophila, 331 
protection, from colloidal silver, 201, 
222 


ANIMAL AGGREGATIONS 


from toxic salts, 213 
from high temperature, 217 
from ultra-violet, 218 
from hypotonic sea-water, 222 
heterotypic, 228 
factors contributing to, 231 
survival values in insects, 236 
protects, from desiccation, 182 
affects metabolic rate, 181, 184, 186 
Ophroderma from autotomy, 182, 
189, 190 
bacteria, 253 
in spermatozoa, 263, 284 
retarding effect on reproduction, 120 
in Paramecium, 120 
in Stylonychia, 122 
in Daphnia, 126 
in hens, 126 
in Drosophila, 131 
in hookworms, 135 
in Cladocera, 306 
stimulates growth, 147 
in tissue cultures, 149 
heterotypic, in tissue cultures, I51, 
157 
in echinoderms, 153 
stimulates rate of regeneration in tad- 
poles, 148 


Crump, L. M., 138, 166, 167, 168, 169, 
173 
Ctenophora, 12 
Culicidae, swarms of males, 66 
breeding behavior, 66 
Cumingia, distance sperm travel, 289 
Cummins, H., 66, 67 
Curran Eee Re 105 ; 
Cutler, D. W., 138, 166, 167, 168, 169, 173 
Cyanide, effect on sperm suspensions, 269 
see also KCN 
Cylisticus, see Isopods, land, 
Cytotropism, 45, 46 


Dampf, A., 330 
Daphnia, crowding, effect on growth, 110, 
312 
effect on rate of reproduction, 126 
loss of fertility, death and, 137 
survival value of crowding, 141, 215, 
239 
chromosomes, 303 
asexual generations, 304 
male production, 305 


Darter, rainbow, breeding behavior, 68 
Davenport, C. B., 110, 118 


INDEX 


Davidson, J., 313 

Davis, W. T., 79 

IDeguaacin, JR, GO, We bh Os Us él, mG, Wy 
UG), We}, BA, Pe AG, AAS), Diy oy Ao, Zio) 
31, 33, 34, 36, 38, 47, 49, 61, Tor, 
349, 353 

Deer, 13 

Deere, E. O., 236, 239 

Demuth, G. S., 70 


Dendrocoelum, mass protection from col- 
loidal silver, 203, 210, 211 


Dermestes, food exhaustion, 102 


Desiccation, land isopods, 
from, by crowding, 182 


Desmones, 150 

De Varigny, H., 109, 110, 112, 119 
Diphyletic tree, 356 

Diptera abundance, 359 

Ditmars, R. L., 68 

Division of labor, 337, 343, 346, 350, 362 
Dociostaurus, see Grasshoppers 
Domn, L. V., 309 

Downing, R. C., 289 

Downs, V. L., 43 

Drew, A. H., 157 


Drosophila, 20 
aggregations conditioned by moisture, 


protection 


73 
crowding, reduces rate of reproduction, 
116, 131, 280 
affects growth form, 331 
affects Mendelian ratios, 332 
factors affecting rate of reproduction, 
134, 236 
optimum population for growth, 148 
density of population and longevity, 
240 
Drzewina, A., 125, 140, 142, I9I, 201, 202, 
BOS 200782075 205,08 200,) 212, 217 
BOs 522282295, 235,250, 205,107, 
275, 276, 277, 278, 284, 285, 354 
Ducks, flocks of, 342, 345 
Duckweed, ‘‘auximones”’ and, 159 
Dumas, J. B., 263 


Durkheim, E., 355 


Earthworms, laboratory aggregations, 74 
Earwigs, heteromorphic societies, 24 
Ebeling, A. H., 106, 151, 157 
Echinoderma, 12, 357 
growth, limited by crowding, 110, 117, 
118, 312 


419 


promoted by crowding, 153 
eggs, development hastened by crowd- 
ing, 156 
mass protection, from KCN, 215 
of sperm, 218, 263 
see also Ophioderma and Sea urchins 
Echinus, see Sea urchin 


Ecological succession, in plants, 103 
in protozoan infusions, 107 
Ecology, 4, 8, 52 
Egg production in hens, reduced by 
crowding, 126 
seasonal values, 129 
Eigenbrodt, H. J., 148, 332 
Eijkman, C., 104, 106 
Electrical resistance of hypotonic sea- 
water, 225 
Electrolytes, crowding on resistance to, 
140 
Elephants, mixed families, 28 
heterotypic groups, 30 
Ellicott, E. L., 68 
Emergence of society, 339 
Emerson, A. E., 30, 95, 96 
Enchelys, allelocatalysis, 161, 160, 
mass protection from high tempera- 
tures, 217 
Enchytraeis, 32, 51 
Ephydra (Diptera), 358 
Espinas, A. V., vil, 6, 33, 34, 47, 352; 3535 
360 
Essenberg, C., 52 
Ether, mass protection against, 237 
Ethyl alcohol, mass protection against, 
237, 238 
Ethylene chloride, mass protection against, 
237 
Euglena, 171 
Evermann, B. W., 72 


Evolution, animal, 361 
social, 362 


Excretions retard growth of bacteria, 
103, 107 
of snails, r10 
or echinoderm larvae, 110, 117 
of Daphnia, 110 
of Planaria, 113, 312 
of fish, 113, 116 
retard rate of reproduction, 120 
of Paramecium, 120 
of Stylonychia, 122 
of Daphnia, 126, 304 


420 


snails, stimulate growth, 147 

affect male production in Cladocera, 
235, 304 

Planaria, affects growth form, 312 

as food, 341 

see also Exudates 


Exudates, protect Procerodes, 225, 231 
Planaria from distilled water, 225 
Enchelys from high temperatures, 217 
bacteria, 217 

fail to protect Planaria from distilled 
water, 218 
in allelocatalysis, 165, 176 
in Ameiurus aggregations, 62, 90 
see also Allelocatalysis, Autodestruc- 
tion, Autoprotection, Carbon di- 
oxide, Chemotropism, Conditioned 
solutions, Excretions, Metabolic 
wastes, Slime, Tektin 


Fabre, J. H., 74, 75 
Faeces, stimulate growth in snails, 147 
Families, reciprocal, 24 

irreciprocal, 25 
Family as basis of society, 338-51 
Farr’s law, 115, 132, 142 
Haine ween 7031310 32053271329 
Fertility, mortality and, 136 
Fertilization, survival value, 289 
Fibroblasts, culture 7m vitro, 150, 166 
Fielde, A. M., 93 
Fireflies, synchronous flashing, 90 
Fischer, A., 88, 150, 151, 152, 157, 166 
Fischer-Sigwart, H., 68 


Fish, 12, 345 
monogamy among, 27 
groups of female sticklebacks, 28 
nests inhabited by other species, 31 
relation to Portuguese-man-of-war and 
to coral colonies, 32 
aggregations of Ameiurus, 62, 345 
breeding behavior, 68 
of Amia, 25 
of bullheads (A meiurus), 25, 345 
of black bass, 25 
of sticklebacks, 25 
general, 27 
hibernation, 70 
growth limited by crowding, 113, 116, 
Tet) 
size in relation to size of lake, 117 
mass protection, from colloidal silver, 
209 
metallic salts, 213 


ANIMAL AGGREGATIONS 


excretions and male production in 
Cladocera, 305 


Fisher, R. A., 164 
Flatworms, see Planaria 
Flocking, of birds, 13, 36, 339, 340, 342, 
343, 344 
Floerscheims, C., 75 
Food, exhausted by groups, 102, 111, 113, 
119 
of bacteria, 103 
of Protozoa, 107 
relation to population cycles, in Para- 
mecium, 137 
in Tribolium, 138 
snail faeces increase microflora of cul- 
tures, 147 
Food chains, 35 
Forbes, S. A., 83 
Forced movements, aggregations due to, 
38 
Forel, A., vil, 13, 31 
Formation, biotic, 8 
Formulas, Bilski’s, crowding on growth, 
115 
Pearl’s, crowding on reproduction in 
Drosophila, 132 
Chapman’s for biotic potential, 140 
Farr’s law, 142 
Carpenter’s fatality curve in fish, 213 
Fowler, J. R., 141, 142, 215, 216, 239 
Fox, K. M., 69 
Foxes, monogamy, 24 
Frank, G., 156, 215 
Freemartin, 295 
Frisch, K. von, 76 
Frogs, 12, 13, 27 
attempt to mate with toads or fish, 29 
hibernation, 66, 70 
spring migration, 67 
breeding behavior, 67 
of Hylodes, Pipa, and Rhinederma, 26 
community fertilization, 68 
laboratory aggregations, 74 
sex recognition in, 89 
regeneration rate of tadpoles affected 
by crowding, 148 
Fulmer, E. I., 247, 248, 249, 259 
Fulton, B. B., 93, 94 


Galtsoff, P. S., 45, 46 
Galvanotropic reactions, of Paramecium, 


39; 41 
of spermatozoa, 278 


INDEX 


Gametes, distance traveled in sea-water, 
28 
ene in sea-water, 289 
in fresh water, 284 
allelocatalysis, 290 
Gammarus, mass protection from salts, 
215 
Gang as basis of society, 339, 348 
Geese, Canada, flocks of, 342 
Gelatine, protection from hypotonic sea- 
water by, 233 
Gemmill, J. F., 263, 264, 268, 281, 284 
Gentian violet, mass protection for 
bacteria against, 253 
Gibbons, 347, 348 
Giraffes, heterotypic groups, 30 
Glaser, O., 264, 266 
Glass, effect, on protozoans, 124 
on chemical reaction, 125 
Glass rods, and oxygen consumption in 
Ophioderma, 192 
Glossiphonia, relation to young, 25 
mass protection from colloidal silver, 
204, 211 
Gnats, 12 
Goetsch, W., 112, 113,.114, 116, I19, 125, 
245 
Goldfinch, flocks, 343 
Goldfish, mass protection from colloidal 
silver, 209 
from lead nitrate, 214 


Goldman, E. A., 79 
Goldschmidt, R., 309 

Gorilla, 348 

Gould, H. N., 295, 296, 297, 298 
Gounelle, E., 95 

Gowell, 126 

Grackles, 13, 342 
Graham-Smith, G. S., 106 
Grassé, P., 317 


Grasshoppers, migrating swarms, 34, 78, 

316 

synchronism in, 94 

mass protection in, 236 

host for nematode parasites, 299 

morphological changes due to crowding, 
316 

phase theory, 316 

mass menace, 319, 329 

experimental evidence, 327 


Grassi, B., 313 


421 


Grave, B. H., 69, 289 
Gray, J., 278, 279, 280, 281, 284, 285 
Greenleaf, W. E., 167, 169, 170, 177 


Gregarious tendency of grasshoppers, 316 
see Instinct, social 
Gregariousness, in anthropoids, 347 
Grosvenor, C. H., 304, 305 
Growth, limited by crowding, ror 
snails, 108, 123 
echinoderms, 110 
Daphnia, 110 
tadpoles, 112 
Hydra, 112 
Planaria, 113 
fiSh penn ss OTOL 7 
summary of retarding factors, 118 
stimulation by crowding, 147 
in tissue cultures, 149 
in echinoderms, 153 
form, affected by crowding, 311 
in aphids, 313 
in grasshoppers, 316 
in Drosophila, 331 
Growth-inhibiting substance, 110, 117, 
118, 153, 155, 165, 304 
Growth-promoting substance, of Semper, 
108, 118, 248 
in embryonic extracts, 150, 159 
in hen’s egg, 151 
in echinoderms, 153 
in yeast, 157 
in green plants, 158 
in allelocatalysis, 165, 166, 167, 178 
Guest relations, 341 
Gurwitsch, A., 96, 156, 215 
Gynopaedium, 18, 24, 25 
Gyrinid beetles, aggregations of, 28, 60 
reaction to patterns, 61 


Haberlandt, G., 151 

Haemocera, see Copepoda 

Hajés, K., 105 

Halictus, “‘sleep” aggregations, 76 


Harmful effects of aggregations, see Ag- 
gregations, harmful effects 


Harnly, M. H., 134, 135, 334 
Harrison, R. G., 150 
Hartmann, O., 304 

Haswell, W. A., 201 

Hatch, M., 28, 61, 62 
Haviland, M., 245 


422 


Hay infusion protects Procerodes from 
hypotonic sea-water, 228 


Elayes sleet 252 

HCl, effect on male development in 
Bonellia, 293 

HCN, mass protection against, 237, 239 

Headlee, T. J., 359 

Heaton i B= roo. 1 1S5)053 

Hebard, M., 331 

Hegel, vii 

Hegner, R. W., 33 

Helopeltis (tea bug), host for nematodes, 
300 

Hempelmann, F., 70 

Henking, H., 266 

Henrici, A. T., 103, 104, 105, 106, 247, 251 

Hens, crowding affects rate of egg-laying, 
126 


seasonal values, 129 
flock organization, 129, 344 
Herbst, C., 293 
Herds, 12, 13, 27, 30, 34, 35, 330, 341; 348, 
353 
Hermit crabs, 123, 296, 298 
Hess, W. N., go, 92 
Messe Re nr7 
Heterotypic crowding in tissue cultures, 
151) 157 nee 
protection, against colloidal silver, 210, 
231 
against hypotonic sea-water, 228 
in bacteria, 256, 2590 
in yeast, 259 
excretory products and male produc- 
tion in Cladocera, 305 
Heterotypic flocks of birds, 28, 30, 79, 


339) 342 ; ; 
see also Birds, flocking behavior 


Hibernation, 66, 70, 341 
temperature of bee cluster, 70 


En Re Bess 

Hine eS 075 

Hinrichs, M., 218, 219, 220, 265, 266 

Hippodamia (beetle) abundance, 359 

Hoffbauer, C., 115 

Hogg, J., 108, 110, 112, 118 

Hogs, wild, mixed families, 27, 28 

Holmes, S. J., 42, 89 

Holmquist, A. M., 70, 71, 79 

Hookworm, crowding affects rate of re- 
production, 135 


ANIMAL AGGREGATIONS 


Horde life in anthropoids, 347, 349, 350 
Hormone, wound or division of Haber- 
landt, 151 
effect on sex in freemartin, 295 
Houseflies, 359 
Howard, H. E., 345 
Howard, L. O., 359 
Howell, A. B., 79, 80 
Huxleyan|s.203 
Hybridization, phase theory of locusts 
and, 329 
ratios affected by crowding, 331 
Hydra, 17, 24, 358 
effect of crowding on growth, 112 
mass protection from colloidal silver, 
204 
Hydractinia, polymorphism, 23 
relation to hermit crab, 29 
H ion, effect on electrical charge of 
spermatozoa, 278 
see pH 
Hydrogen peroxide, mass protection of 
bacteria against, 260 
H.SO,, effect on male production in 
Cladocera, 306 


Hydroides, distance sperm travel, 289 
Hydroids, cell aggregations, 45 
Hydrozoan colonies, 23, 353 

growing on crabs, 32 
Hyman, L. H., 186 
Hymenorus (beetle) abundance, 359 
Hypotonic sea-water, protection from 


222, 300, 357 py. 
method of measuring hypotonicity, 
224, 225 
heterotypic protection, 228 
factors contributing to, 231 


Thering, H. von, 72 
Indian tree swift, 12 


Individual, metazoan compared with a 
bacterial colony, 261 
with society, 353 


Infusion, protozoan, sequence of forms, 
107 
Infusoria, see Protozoa 
Insects (general), migrating swarms, 34, 
317, 341 
societies, 36, 338, 349, 353, 360, 362 
“sleep” aggregations, 74 ; 
density of population and insect sur- 
vival, 236 


INDEX 


polyspermy, 266 
larvae, excretions and male production 
in Cladocera, 305 
drifts, 3590 
Instinct, 9 
social, 7, 47, 48, 49, 52 
Integration of aggregations, 81, 87, 344, 


346, 
water vibration, 63, 90, 95 
tactile, 88 
odor, 89 
sight, 89 
sound, 92 
substratal vibration, 94, 96 
biophysical, 96 
mitogenetic rays, 97 
Intersexes, in Bonellia, 293, 204 
in Crepidula, 295 
in nematodes, 302 
Isopods, land, 13, 361 
formation of aggregations, 43, 51, 60, 
71, 181 
aestivation, 71 
stage of social development, 87 
relation of bunching, to water con- 
tent, 182 
to oxygen consumption, 181, 184 
relation of bunching, to water con- 
tent, 182 
water, formation of aggregations, 36, 52 
aggregations in nature, 52,71, 194 
laboratory aggregations, 71, 74 


Jackals, marauding packs, 28 

Jahn; DLs 162) 170,171, 172 
Jellyfish, 12, 350 

Jennings, H. S., 39, 40, 41, 42, 61, 62 
Jewell, M. E., 117 

Johnson, C. G., 150 

Johnson, H. B., 330 

Johnson, W. H., 42 

Jorstad, L. H., 153 

Just, E. E., 60, 70, 265, 266, 272, 273 


Kahn, M. C., 253, 254, 257, 258, 259 

Kalmus, H., 123, 124, 136 

Kant, vii 

Katydid, synchronism in chirping, 94 

Katz, D., 344 

Kawajiri, M., 116, 117 

KCl, crowding and resistance to, 139, 209, 
274 

KCN, mass protection from, 215, 260 


423 


affect on male production in Cladocera, 
308 
Kennealy, A. E., 116, 132 
Kennedy, C. H., 42, 89 
Knab, F., 66 
KOH, mass protection from, 216 
Krajnik, B., 185 
Krizenecky, J., 22, 51 
Krogh, A., 186 
Kropotkin, P., 353, 360 
Kuczynski, R. E., 136 
Kuester, E., 106 
Kulagin, N. M., 120 
Kurepina, M., 156, 215 


La Baume, W., 316, 318 


' Ladybird beetle abundance, 350 


see also Coccinellidae 
Lag period, 161, 172, 248 
Lake as a microcosm, 83 
Langhans, V. H., 304 
Lapicque, L., 223 
Lasius, host for nematodes, 300 
Lead nitrate, mass protection from, 213, 

258 

Leadership, 92, 344, 345, 348, 349 
Leeches, 12 

relation to young, 25 


mass protection from colloidal silver, 
204, 211 


Legendre, R., 69, 111, 119 
Leibnitz, vii 
Lemna,growthpromotionby “‘auximones, 
159 
Lemurs, 347, 348 
Leopard, 349 
Lepiney, J. de, 3190 
Leuciscus, protection from lead nitrate, 
213 
Leucocytes, culture 7m vitro, 150 
Liebig, J., 103 
Life, origin, 354 
Light, and temperature, relation to activ- 
ity, 77 : 
mass protection, of bacteria against, 
260 
of spermatozoa against, 276 
and wing production in aphids, 313 
see Temperature and light 


” 


424 


Lillie, F. R., 69, 70, 264, 265, 266, 270, 
271, 272, 273, 284, 295, 309 

Limpet sperm, mass physiology of, 263 

Limulus, animals on shell, 22 

Liobunum (phalangid), synchronous be- 
havior, 88 

Lipoids, growth-inhibiting, 153, 155 

Lizards, breeding behavior, 27 

Lloyd-Jones, O., 282 

Locust swarms, 78, 316, 359 

Locustana, see Grasshoppers 

Locusts, see Grasshoppers 

Loeb, J., 38, 42, 52, 317 

Lowie, R. H., 346 

Lunar periodicities, in breeding behavior, 
69 

utz; Feek., 625,93 

ante aca of crowding on growth, 
se) 

on growth-form, 109, 311 
factors causing growth limitation, 109 
faeces, stimulate growth, 147 


yon Ha be 307, 


McAtee, W. L., 342 
McClendon, J. F., 304 
Maeterlinck, M., 97 
Malaquin, A., 290, 291 
Malinowski, B., 346 


Mammals, 12, 24, 339, 348 
breeding relations, 27 
African, migration societies, 31 
hibernation, 70 
size in relation to size of range, 117 
sperm physiology, 276, 282 
Man, breeding behavior, 27, 347 
social classification, 37 
animals associated with, 82, 349 
social origins, 339, 340, 347, 348 
social relations, see Society, human 
Marmots, mixed families, 28 
hibernation, 70 
Mass action, law of, biological applica- 
tion, 215, 355 
Mass physiology, importance, 338 
summary, 362 
Mass protection, from colloidal silver, 
201, 222 
specificity, 210 
from toxic salts, 213 
from high temperature, 217 
from ultra-violet, 218 


ANIMAL AGGREGATIONS 


from hypotonic sea-water, 225 

heterotypic protection, 228 

factors contributing to, 231 

from gentian violet (for bacteria), 253 

from hydrogen peroxide (for bacteria), 
260 

see Aggregations, beneficial effects 


Mast, S. O., 21, 39, 40 
May flies, 21 

nymphs, laboratory aggregations, 74 
Melanoplus, see Grasshoppers 


Mellisodes, bees, place societies, 28 
“sleep” aggregations, 75 


Mendelian ratios affected by crowding, 332 

Mercuric chloride, reaction with Mytilus 
gills, 213 

Mermis, see Nematodes 

Mesiacev, I., 359 


Metabolic rate, affected by aggregations, 
181, 184, 186, 239 
affects male production in Cladocera, 
308 
in other animals, 309 
affects growth form in Planaria, 312 


Metabolic wastes, effects on growth, of 
bacteria, 103, 107, 249 
of snails, 110 
of echinoderm larvae, 110, 117 
of Daphnia, 110 
of Planaria, 113, 312 
of fish, 113, 116 
effects on rate of reproduction, 120 
of Paramecium, 120 
of Stylonychia, 122 
of Daphnia, 126 
protect bacteria, 217 
condition primitive environment, 354 
see also Excretions 
Metallic colloids, mass protection from, 
201, 208-12, 214, 222 
Metallic salts, mass protection from, 
213-15 
Metazoén individual, compared to a 
bacterial colony, 261; to society, 
353 
Methylene blue, mass protection of 
Protozoa from, 212 
Micrococcus, see Bacillus 
Midges, swarms of males, 66 
breeding behavior, 66 
humidity control of swarms, abundance, 
359 
Migration, of insects, 34, 317, 341 
direction taken, 317 


INDEX 


Miller, G. S., 27, 346, 347 
Miller, N., 67 

Miner, J. R., 240, 241, 243 
Miner, J. T., 342 
Minnich, D. E., 94 


Minnows, as homomorphic societies, 24 
protection from lead nitrate, 213 


Mitogenetic rays, 97, 156 

Mockeridge, F. A., 158 

Moina, chromosomes, 303 
asexual generations, 304 
male production, 305 


Moisture control of aggregations, 71 
Mole cricket, heteromorphic societies, 24 
Molluscs, 12, 358 
Monkeys, breeding behavior, 27 

mixed families, 28 

bands, 347, 348 
Monogamy, 26, 348 
Monstrillidae, influence of crowding on 

SEX, 290, 309 

Montank, 251, 252 
Montesquieu, vii 
Montgomery, T. H., 89 
Morphological effects of crowding, 311 

wing production in aphids, 313 

in grasshoppers, 316 
in Drosophila, 331 

Morrison, T. F., 90 
Mortality, fertility and, 136 
Mosier, C. A., 66 
Mosquito larvae, 20 


Mosquitoes, swarms of males, 66 
breeding behavior, 66 


Moss, pond, protects animals from col- 
loidal silver, 211 


Moth, sex recognition in, 89 
larvae, response to substratal vibra- 
tions, 96 
Mottram, J. C., 106 
Muscina, see Diptera 
Mutual aid, 348 
Mutualism, 29, 341 
Mya, 359 
Myers, E. C., 137, 138, 170, 173, 177 
Myrmecocoles, relation to ants, 30 
Mytilus, 13, 16, 36, 311, 358 
Pinnotheres in mantle cavity of, 32 


Mytilus, gills, reaction with mercuric 
chloride, 213 


425 


NaCl, mass protection from, 215 


NaOH, effect on male production in 
Cladocera, 306 
mass protection from, 216 


Nematodes, sex determination in, 299, 309 

Nemertina, spermatozoa, mass _ physiol- 
ogy, 263 

Nereis, lunar rhythm in breeding be- 
havior, 69 

aggregations of sperm, 271, 273 

Nests, animals inhabiting those of others, 
32 

Newman, H. H., 88 

Nice, M., 342, 343 

Nikolsky, V. V., 79 

Noble, G. K., 67 

Nomadacris, see Grasshoppers 

Notonecta, aggregation formation, 52 

Norrish, 125 


Obelia, 35 
Odor integration, 89 
Oecanthus, synchronous chirping, 93 
Ohaus, F., 92 
Oniscus, see Isopods, land 
Ophioderma, formation of aggregations, 
36, 44, 51, 60, 73, 87, 181 
menace of crowding, 144 
autotomy affected by aggregation, 182, 
190, 184 
oxygen consumption, affected by ag- 
gregation, 186, 198 
affected by glass rods, 192 
mass protection from colloidal silver, 
202, 205, 207, 208, 209 
Orang-outan, 348 
Organism, definition and integration, 80 
Organismal analogy of Spencer, 353 


Origin, of life, 354 
of society, 338 
Ostracoda, 12, 358 
Ostriches, heterotypical groups, 30 
Ostwald, W., 214 
Overcrowding, 184, 203, 360 
see Crowding 
Over-wintering aggregations, 70 
Oxygen consumption, affected by crowd- 
ing, 181, 184, 186, 192, 195, 280 
rate of, in isopods, 186 
in Planaria, 186 
in Musca, 186 


426 


in A pis, 186 
Oxygen tension, effect on sperm sus- 
pensions, 269, 279, 280 
in stream affected by mass of water 
isopods, 194, 200 
Oxytricha, 171 
Oysters, 16, 36 


Packard, A. S., 330 
Palolo worms, 21 
Papanicolau, G., 304 
Parafiin, effect on contained organisms, 
125, 277 
Paramecium, 12 
galvanotropic reactions, 39, 41 
trial and error reactions, 42 
trapped in acid, 61, 62 
growth inhibition, 107 
reproduction retarded by 
120, 136 
population cycle, 137 
allelocatalysis in, 167, 169, 170, 172, 
235 
mass protection, from colloidal silver, 
204 
from methylyene blue, 212 
from ultraviolet, 218, 220 
culture medium, protects Procerodes 
from hypotonic sea-water, 228 
factors producing, 233 
allelostasis in, 280 
Paramermis, 301 
Parasites, of ants and myrmecocoles, 14 
social, 35, 341, 342 
sex ratios affected by crowding, 290, 
294, 209 
Parasitism, Deegener’s definition, 32 
relation to prototaxis, 57 
Park wReg Ens 
Park, T., 180 
Barker Gab. 1,03 
Parkerp leeks 
[PEN eIE, Sh Wig, MBI, WGI, Isl Oko), Auli, We 
Parker, I. J., 201 
Parrakeet flocks, 342 
Passalid beetles, auditory integration, 92 
Patangia, see Grasshoppers 
Patrogynopaedium, 19, 24 
Patropaedium, 25, 26 
Pawlow, P. N., 214, 215 
Rear eRe fro. 20.0n27— 02S r2O Nel 30. 
E3E; 153; 2350246, 241, 242.0243, 
244, 245, 280 


crowding 


ANIMAL AGGREGATIONS 


Pearse, A. S., 89, 117 
Pearson, J. F. W., 78 
Peck-order in bird flocks, 344 
Vetetel ME Mle, wie, wits}, WiWe), HEC, Tyh, TG 
Penfold, W. J., 250, 251 
Pentatomid bug attacks beetle larvae, 
245 
Feo ae lunar, in breeding behavior, 
9 
Peskett, G. L.,-167, 169, 170, 252 
Petersen, Wes 70. laa 7 250072 7 Aan 
170; 177, 178, 180, 218, 235 
Pieffer We s272 
pH, relation to autodestruction, 141, 142 
affected by mass of water isopods, 194, 
200 
relation to mass protection from hy- 
potonic sea-water, 233 
in sperm suspensions, 267, 284 
Phalangidae, synchronous behavior, 88 
Phase theory of locust forms, 316, 322 
experimental evidence, 327 


Phases of bacterial cultures, 247 
Phillips, E. F., 70 
Phillips, J. F. V., 349 
Phototropic reactions leading to aggrega- 
tions, 38, 74, 181 
of Asellus, 199 
Phyla of animals, 356 
Pickering, S., 103 
Pieron, H., 203 
Pigeons, voice in social control, 95 
Pinnotheres, in mantle cavity of Mytilus, 
32 
Phylloxera, see Aphids 
Physa, protection from colloidal silver, 
210 
excretions and male production in 
Cladocera, 305 


Plague, locust, 321 


Planaria, 12, 362 
growth limited by crowding, 113 
autodestruction in KCl, 141 
protection, from colloidal silver, 202, 
203, 204, 205, 210, 211 
from ultra-violet, 218 
from distilled water, 225 
protect Procerodes from hypotonic sea- 
water, 230 
killed by KCl, 274 
excretions affect male production in 
Cladocera, 305 


* 


- Portuguese-man-of-war, 


INDEX 


crowding on growth form, 312 
abundance, 358 
Plant toxins, effect on succession, 102 
Plants, relation to black-bass food chain, 


competitor of black bass, 85 
Platypoecilus, growth limited by crowd- 
ing, 113 
Pleurotricha, 167, 169 
Plotnikov, V. I., 327, 328, 329 
Plunkett, C. R., 332, 333 
Polenia, see Diptera 
Polyandry, 26 
Polygamy, 26, 348 


Polygyny, 26 
Polyspermy, 266 
Popovici-Baznosanu, As, IL, 110, 147, 
148 
Population density, human, effect on 
death-rate, 115, 142 
effect on size, 118 
Drosophila, effect on reproduction, 132 
optimum for growth, 148 
hookworms, optimum for growth, 1 35 
general importance, 135 
equilibrium in flour beetles, 138, 179 
death-rate in scale insects and, 239 
longevity in Drosophila and, 240 
Population equilibrium, in beetles, 138, 
179 
polymorphism, 
23 
Predators and prey, 31, 246 
Prevost, J. L., 263 
Prionyx, sleep aggregations, 75 


Procerodes, formation of aggregations, 36 
mass protection from hypotonic sea- 
water, 223, 275 
heterotypic protection against, 228 
factors contributing, to 231 


Promiscuity, sexual, examples, 27, 347, 
356 

Protein, associated with growth promo- 
tion, 150, 155 

Prototaxis, 46 


Protozoa, 12, 14, 15, 362 
colonial, 15, 16 
sexual societies, 26 
growing on other animals, 32 
growth inhibition of, 107 
rate of reproduction 
crowding, 120 


retarded by 


427 


volume affects rate of reproduction, 122 
allelocatalysis, 161, 250 

lag, 161, 248 

mass protection in, 204, 212, 218, 220 
culture cycle compared with bacteria, 


| 247 
killed by KCl, 274 
allelostasis in, 280 

Pseudomermis, see Nematodes 


“Psychological” influence of numbers, on 
snails, 110 
on hens, 129, 245 


Quail, in dry season, 72 


Rabbits, signaling, 96 
sperm physiology, 282 
Racing record, relation to length of 
course, 116 
Rahn, O., 105, 251 
Rana, breeding behavior, 68 
tadpoles, growth limited by crowding, 


112 
regeneration rate increased by crowd- 
ing, 148 
mass protection from colloidal silver, 
207 
Random movements, aggregations, due 
to, 42 


Rau, N. and P., 75, 76, 80, 102 
Reaumur, R., 70, 
Reeves, C. D., 68 
Reflexes leading to Ameiurus aggrega- 
tions, 64 
Regeneration, tadpoles, 
crowding, 148 
Reighard, J., 32, 68, 289, 345 
Reinhard, H. J., 313, 314, 315 
Reproduction, retarding of by crowding, 
120, 165, 304 
rate increased by crowding, 161 
Reptilia, 12, 21, 27, 68, 70 
polyspermy, 266 
Resistance, electrical, of hypotonic sea- 
water, 225 
Respiratory exchange, affected by crowd- 
ing, 181, 184, 186, 192, 195, 198 
Respiratory rate in sperm suspensions, 
207, 279 
Rettger, L. F., 249 
Rheotropism in Asellus, 56, 50 
Rhythms, lunar, in breeding behavior, 69 
in social appetite, 80 


affected by 


428 


Riley, C. F. C., 52 
Riley, C. V., 330 
Robertson, T. B., 120, 122, 138, 161, 162, 
163, 164, 165, 166, 167, 168, 170, 
172) 1735 170,170) Loo, 200, 209, 
235, 248, 249, 250, 252, 354 
Robertson’s phenomenon, in Protozoa, 
161, 178 
in Tribolium, 178, 180 
Robin roosts, 28, 80, 339 
postbreeding-season flocks, 343 
Roosts of birds, 28 
Root, F. M., 33 
Root excretions, 103 
Rophalosiphum, see Aphids 
Rossman, B., 97 
Roux, W., 45, 46 
Russell, E. J., 103 
Ruthven, A. G., 68 


Salamanders, 21 
spring migration, 67 
eggs, 358 
Salmon, 21 
Salpa chains, 16, 311 
Salts, KCl, crowding and resistance to, 


139, 209 
metallic, mass protection from, 213 
CaCh, KCN, NaCl, crowding and 


resistance to, 215 
Sarcina food for Colpidium, 166 
Sarles, M. P., 135 
Scale insect, mass relations, 240 
Schistocerca, see Grasshoppers 
Schizoneura, see Aphids 
Schjelderup-Ebbe, T., 34, 129, 343, 344; 
345 
Schnigenberg, E., 116, 118, 119 


Schools, of fish, survival valve, 36 
analysis of young Ameiurus, 62 


Schrittky, C., 75 
Schiicking, A., 264, 266 
Schuett, J. F., 202, 231, 258 
Schulz, R., 20, 25, 28, 61 
Schwarz, E. A., 75 

Sclater, W. L., 349 
Scyphozoa, strobila, 16 


Sea anemones, 16, 358 
crowding on growth form, 311 


ANIMAL AGGREGATIONS 


Sea-urchin larvae, growth, inhibited by 
crowding, 117, 153 
promoted by crowding, 153 
eggs, development hastened by crowd- 
ing, 156 
mass protection, from KCN, 215 
of sperm, 218, 210, 263, 264, 267; | 
272, 274, 276, 278, 279, 281 
egg secretions, effect on sperm, 272, 278 
Sea urchins, 358 
distance sperm travel, 289 
Sea-water, hypotonic, protection against, 
222 
heterotypic protection against, 228 
factors contributing to, 231 
Seals, mixed families, 27 
herds of males, 28, 348 
Segmentina, mass protection from col- 
loidal silver, 211 


Selous, F. C., 73 

Seminal fluid, function, 263 

Semper, K., 108, 109, 115, 117, 118, 191, 
248, 354 

Sentinels, 346, 349 

Seton, E. T., 28 

Save, 18h, (Ca, Be 

Severin, H. H. P., 52 


Sex, importance of, 4, 337, 340, 341; 347, 
350 
as basis of societies, 24, 26, 29, 87 
ratio in Asellus aggregations, 55 
relations in “sleep” aggregations, 75, 
78, 80 
factors in sex recognition, 89, 92 
determination influenced by crowding, 
289 
influence of crowding on, 289 
in monstrillid copepods, 290 
in Bonellia, 294, 309 
in Crepidula, 295, 309 
in nematodes, 299, 309 
in Cladocera, 303, 309 
determination of, 289, 309 
origin of, 290, 337, 351 
promiscuity, 347 
Sexual egg production in Cladocera, 303, 
306 


Shackleford, M. W., 4, 8 
Shaw, G., 113, 116, 119 
Shelley, F. C., 305 
Sherman, A., 92, 342, 343 
Shull, A. F., 93, 313, 324 
Siamang, 347 


INDEX 


Silver, colloidal, see Colloidal silver 
metallic, mass relations, of spermatozoa 
LO; 27,7 
of Convoluta, 277 
Silver nitrate, as a measure for hypotonic 
sea-water, 224 


Silvestri, F., 94 

Simocephalus, see Cladocera 

“Sleep” aggregations, 74, 78, 80, 317 

Slime, mass protection from colloidal 
silver and, 210, 211, 212, 221, 222 


Smith, G., 304, 305 
Smith, H., 269, 270 
Smith, V. G., 4, 8 


Snails, 12 
growth limited by crowding, 108 
growth form, ro9 
factors causing growth limitation, 1o9g 
faeces stimulate growth, 147 
protection from colloidal silver, 210, 211 


Snakes, 21 
breeding behavior, 27, 68 
hibernation, 70 


Snyder, T. E., 66 
Social appetite, 11, 46, 51, 52, 60, 62, 64, 
80, 342, 346, 361 


Social behavior, criterion for, 48 
in caterpillars, 48, 49, 51 


Social instinct, 7, 33, 47, 48, 49, 52 


Societies, animal, 3, 4, 8, 23, 352, 353 
ecological, 8 
Deegener’s classification, 15 
with sexual basis, 24, 29 
of Protozoa, 26 
without sexual basis, 27, 29 
Alverdes’ classification, 33 
Espinas’ classification, 34 
Wheeler’s classification, 34, 35 
classification on basis of integration, 34 
open, or closed, 34 
reproductive or protective, 35 
anthropoid, 35 
see Family as basis of society 


Society, definition of, 5, 6, 7 
human, 17, 37, 82, 361, 362 
origin, 339, 340, 346, 350 
Deegener’s definition, 23 

homotypical, 23 
kormogene, 23 
heterotypical, 23 
primary, 24 
secondary, 26 
reciprocal, 29 
irreciprocal, 31 


429 


adoption, 28, 30, 81 
integration, 82, 344 
role of voice in, 95 
origin, 338, 355, 360 
as an emergent, 339 
analogy to organism, 353 


Sociology, general, 4, 33, 37, 285 
comparative, 86 

Solenopsis (ant), behavior when flooded, 

72 

Somes, M. P., 331 

Song sparrows, 343 

Songs, bird, sexual significance, 92 

Sound integration, 92 


Space, free, effect on spermatozoa, 280 
distance spermatozoa swim, 289 
Spaeth, R., 348 


Spallanzani, L., 263 


Specificity, of echinoderm excretions, 110 

of Daphnia excretions, 110, 126 

of protozoan excretions, 122, 173 

of autodestruction, 142 

of mass protection against colloidal 
silver, 210 

of mass protection against hypotonic 
sea-water, 231 


Spencer, H., vii, 338, 339, 353 
Spermatozoa, 337, 357 
mass protection for, 217, 218, 219 
general mass physiology of, 263 
summary, 284 
functional life related to numbers 
present, 263 
mass effect in fertilization, 264 
mammalian, 276, 282 
Gray’s explanation of decreased move- 
ment in masses, 279 
distance sperm travel, 289 
Sphex, “sleep” aggregations, 74 
Spiders, young of Hpeira, 20 
breeding behavior, 27 
polyandry in, 26 
sex recognition in, 89 
climatic control, 321 
Spinoza, vil 
Spirostomum, retarding effect of crowding, 
123 
Sponges, 16 
growth from gemmules in fresh water, 
24 
animals living in canals of, 32 
cell aggregations, 45 
Sprays, mass protection from, 239 
Springer, M. G., 117 


430 


Staphylococci, effect of isolation, 260 

Starfish, see Ophioderma 
growth-promoting substance, 155 
eggs, 358 

Starvation and wing production in 

aphids, 315 

Stearic acid, effect on animals, 125 

Stearin, bio-physical effects, 277 

Steiner, G., 299, 302 

Stentor, 17, 204 

Storks, African, hunting societies, 30 

Strongylocentrotus, see Sea urchin 


Struggle for existence, 86, 246, 353 
relation to co-operation, 361 
Stuart, C. A., 308 
“Student,” 164, 165, 187, 196, 230 
Stylonychia, retarding effects of crowd- 
ing, 122, 169 
mass protection from colloidal silver, 
204 
Succession, ecological, 102 
of plants, 193 
of Protozoa, 107 
Sugar solution, effect on sperm, 278 
Surface, Ha Me 126; 127, 128; 120; 130, 
131 
Survival value of fertilization, 289 
Survival value of groups, 36, 87, 181, 200, 
222, 262, 316, 337, 351, 354, 361 
through co-operation, 30, 346, 349 
of sleep aggregations, 75, 79 
negative values, 1o1, 136, 140, 184 
metabolism and, 181, 182, 186, 230, 
319, 334 
autotomy of bunched starfishes and, 
182, 189 
factors contributing to, ror 
protection from colloidal silver, 201 
specificity of, 210 
from toxic salts, 213 
from high temperatures, 217 
from ultra-violet, 218 
against hypotonic sea-water, 222 
in sex determination, 309 
see also Aggregations and Crowding 


Swarm flocks, 342 


Swarms, of Diptera, 66 
lunar, of Nercis, 69 
of grasshoppers, 316 


Symbiosis, 47, 166, 359, 360 
Sympaedium, 18, 2 
Symphagium, 21, 28 
Symporium, 21, 28 
Syncheimadium, 21, 28 


ANIMAL AGGREGATIONS 


Synchorium, 21, 28 
Synchronous behavior, of caterpillars, 51, 


04 . 

of phalangids, 88 

in tissue culture, 89 

of fireflies, go 

of tree crickets, 93 

of grasshoppers, 94 

of termites, 95 
Syngynium, 17, 28 
Syntropium, 22, 38 
Sysympaedium, 20, 27 
Szymanski, J. S., 48, 49, 50, 51, 56, 60, 88 


Tabanid flies, morning swarms, 66 
“sleep” aggregations, 75 
Tactile integration, 88 
Tadpoles, growth limited by crowding, 
112 
regeneration rate stimulated by crowd- 
ing, 148 
mass protection from colloidal silver, 
204, 207 
excretions and male production in 
Cladocera, 305 


Tanner, F. W., 157, 158 
Tatham, 142 
Taylor, T. H., 66 
Tektin, a means of mass protection in 
Protozoa, 212 
Temperature, and _ light, 
activity, 77, 317 
high, mass protection from, 217, 276 
low, mass protection from, 276 
and wing production in aphids, 314 
effect of body color on, 318 
Terminology, 5, 8 
Termites, 17, 24, 346, 361 
relation, to ants, 29, 31 
to termiticoles, 30 
sound production, 94 
response to substratal vibrations, 95 
soldier caste, 246 
Termiticoles, relation to termites, 30 
Territory in bird, fish, and mammal life, 
345 
Terry-McCoy, E., 201 
Thigmotropic reactions leading to aggre- 
gations, 44, 52, 60, 63, 74 
Thomas, C., 330 
Thomas, W. I., 346 
Thompson, J. A., 83 
Tissue cells, aggregation, 45 
Tissue culture, effect of crowding, 149, 
157, 106, 285, 337 


relation to 


INDEX 


Tissue extracts, effect on male production 
in Cladocera, 306 


Toads, voice as sex call, 67 
tadpoles, growth limited by crowding, 

112 

Tolerance for other animals, 43, 60, 87, 
182, 261, 350, 361 

Toll, A., 345 

Tracheoniscus, see Isopods, land 

Tree-cricket, snowy, synchronism in 
chirping, 93 

Trephones, 150 

“Trial and error’ reactions, 39, 42 

Tribolium, Chapman’s work on, 138, 178, 
236, 244 

Trophallaxis, 338 

SCOvEED 10, 38, 48, 50, 61, 182, 194, 341, 
360 


Ultra-violet, mass protection from, 218 


Ungulates, 12 
African, collection about water, 73 


Uroleptus, 168 


Uvarov, B. P., 78, 79, 316, 317, 318, 320, 
321, 322, 323, 324, 325, 327, 329; 
339; 331 


Valley, G., 249 
Vayssiere, P., 319 
Vernon, H. M., 110, 117, 119, 312 
Vibration, relation, of catfish to, 63, 90 
of termites to, 95 

integration, 90 

substratal, 94, 96 
Visual integration, 89 
Vitamin A, growth-inhibiting, 153 
Vitamin B, growth-promoting, 153 
Vitamins, effect on growth, 119, 157, 159 


Volume, limits growth of snails, 108, 123 
ineffective with clean medium, 112 
limits growth of tadpoles, 112 
of fish, 113 
limits reproductive rate of Paramecium, 
120, 122 

of Stylonichia, 122 

of Crepidula, 123 

of Drosophila, 134 

affects population cycle in Paramecium, 
137 

population equilibrium in beetles, and, 
138 

Volume relations in allelocatalysis, 167, 

168, 169, 172, 173, 178 


431 


Volvox, 23, 24 
Vorticella, 17 
Vultures, method of collection, 89 


Wadley, F. M., 313 

Wallin, I. E., 46, 47 

Walton, A., 282, 284 

Warner, L. H., 11 

Warren, E., 110, 119, 126, 136, 312 
Wasmann, E., 30 

Wasps, social, 346 


solitary, 13, 338, 348 
sleeping groups, 28, 74 


Water loss, land isopods, relation to 
crowding, 182 

Water striders, aggregation formation, 52 

Web-of-life, 9, 83 

Weese, AU OX 321 

Wellman, G. B., 67 

Wheeler, W. M., vii, 7, 10, 11, 13, 14, 30, 
31, 34, 35, 72, 80, 92, 338, 339; 
353, 360, 363 

Whitefield, R. R., 109, 311, 312 

Widmann, O., 79 

Wildiers, E., 158 

Willem, V., 110, 111, 119 

Willer, A., 116, 118, 119 

Willmer, E. N., 150 

Wilson, H. V., 45 

Wing production in Aphids, 313 

Wolfe, H. S., 159 

Wolves, marauding packs, 28 

Wood, T. R., 303 

Woodruff, L: L., 107, 111, 120, 121, 122 

Woodworth, W. M., 69 

Wright, G. P., 151 


Xiphidium, see Grasshoppers 
Yeast, growth promoted by “bios,” 119, 


157 
wild, effect on Drosophila, 134, 148, 145 
allelocatalysis, 167, 168 
lag in, 249, 251, 252 
heterotypic crowding in, 259 
Yerkes, R. M., 67, 347, 348, 349 
iVocom He Be 7.78 
Young, W. C., 276 
Yung, E., 109, 119 


Zebras, heterotypical groups, 30 


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