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HOMOIOTHERMISM

HOMOIOTHERMISM %

The Origin of W arm-Blooded Vertebrates * "^

BY

A. S. PEARSE AND F. G. HALL

Duke Universiiy

NEW YORK

JOHN WILEY & SONS, Inc

London: CHAPMAN k HALL, Limited

1928

Copyright, 1928,
By Arthur S. Pearse
AND Frank G. Hall

Printed in U. S. A.

PRESS or

BRAUNWORTH A CO., INC.
BOOK MANUFACTURIRS
BROOKLYN, NKWYORK

TO
GEORGE HOWARD PARKER

PREFACE

Many of the problems of biology result from the fact
that changeful organisms live in changeful environments.
Life always involves continual adjustments to environ-
ment. Animals which invade new types of habitats must
be able to change their inherited adjustment patterns or
they must possess inherent qualities which make them
more or less immune to the effects of environmental
changes.

The animals which dominate the earth today origi-
nated in the stable, saline, food-laden ocean. Primitive
animals for the most part remained in the sea, but as
the ages went by several types migrated into fresh
water and a few of the more progressive types even took
up life on land. Migrants into fresh water and land
habitats left the stability of the old marine haunts of
life, where they lived in a medium which resembled the
fluids such as are found in all protoplasms, where there
was no dearth of oxygen and very little change in tem-
perature. In fresh water they received little compensa-
tion, for they were obliged to live where there was often
great variation in salinity, oxygen, and temperature. But
those that attained to the land, although they were
obliged to endure extreme variations in temperature, had
to secure all their body salts from their food, and were
in continual danger of death from desiccation, were also
able to live in a rarer medium which permitted rapid
locomotion and by supplying an abundance of oxygen
made rapid metabolism possible.

vii

viii FREFACE

This book attempts to discuss the physiological and
ecological aspects of one type of adjustment which has
made successful life on land possible and has reached
its climax in the attainment of thermal and chemical
stability within the bodies of animals.

CONTENTS

CHAPTER PAGE

I. Introduction 1

II. The Temperature of the Earth in Time and Space.. . 4

III. Ancestry of Homoiothermic Animals 10

IV. Responses of Poikilotherms to Variations in Tempera-

ture 17

V. Responses of Homoiotherms to Variations in Tempera-
ture 30

VI. Factors Influencing Rate of Metabolism 37

VII. Factors Influencing Body Temperature 50

VIII. The Mechanisms of Temperature Regulation in

Homoiotherms 66

IX. Growth and Longevity 69

X. Periodic Fluctuations in Body Temperature 79

XI. Behavior and Body Temperature 84

XII. Fitness of Temperature Constancy 89

XIII. Man as a Homoiothermic Animal 96

Bibliography 101

Index 116

HOMOIOTHERMISM

CHAPTER I
INTRODUCTION

Warm-blooded, or homoiothermie, animals trnqnes-
tionably had their origin through evolutionary changes
from cold-blooded, or poikilothermic, animals. By ac-
quiring the ability to maintain a comparatively constant
and high temperature, the former have become more or
less independent of temperature changes in the environ-
ment. A homoiothermie animal lives at approximately
the same rate at all seasons and at all latitudes, but a
poikilothermic animal must slow down its life processes
whenever its surroundings become cooler.

Of course warmth is not the cause of metabolism, but
is one of the conditions for it. Homoiotherms have
gained to some degree the ability to ignore the environ-
ment. They have done this by means of certain mechan-
isms for heat production, insulation on the outsides of
their bodies, the transfer of liquids from certain parts
of their bodies to others, and other adaptations. The
cells within their bodies live at rather constant tempera-
tures which are close to the optimum for metabolic activi-
ties. Some poikilotherms which inhabit cold oceans pass
their whole lives below 0 deg. C. These animals must
live on a lower metabolic and psychic level than such
active cold water animals as penguins or whales. On

2 HOMOIOTHERMISM

the other hand there are in parts of the tropics and in
warm springs certain animals which inhabit an environ-
ment that never falls below about 27 deg. C. Lusk (1917)
suggests that homoiotherms probably had their origin
along the shores of tropical oceans, where they lived in
an environment which continually had a rather high
temperature to which they gradually became adjusted.

Metabolism requires a continual supply of energy and
under usual conditions homoiotherms need more energy
from day to day than poikilo therms. However, the change
from the poikilothermic to the homoiothermic condition
has not been related directly to a higher energy require-
ment, but to the use of new mechanisms for tempera-
ture regulation and better insulation. The protoplasm
of the two types of animals is similar in its energy re-
quirements, but poildiotherms are able to live at slower
rates at lower temperatures, and homoiotherms, with the
exception of certain hibernators which become quite in-
active, are not. The homoiotherms have gained a great
advantage in that they are able to live to practically their
full capacity at all times, but they have also been obliged
to assume certain limitations in order to attain this end.
Their method of living requires the continual expenditure
of a considerable amount of energy and the control of
their metabolic processes has to some degree passed
under the domination of various controlling mechanisms.
If there is lack of correlation between these controlling
mechanisms, the result may be death. The mechanisms
for heat production must operate under rather accurate
control and must be correlated with mechanisms for heat
dissipation. If the former run ahead of or behind the
latter, fever or subnormal temperatures result.

Homoiothermic animals have become adjusted more
and more to uniformity in the conditions for metabolism.

INTRODUCTION 3

They are now specialized for a peculiar mode of life, a
life characterized by many mechanisms for maintaining
miiformity in temperature, metabolic rate, salt and or-
ganic content of the blood, and reaction of body fluids.
This specialization has been associated with the migra-
tion of vertebrates from marine and fresh water habitats
to land. Animals which live in the ocean are surrounded
by a medium which is quite constant in its composition
and resembles the blood of vertebrates in its salt content.
This medium in any particular locality does not vary
rapidly or extremely in temperature or in its content of
dissolved gases. Freshwater animals live in habitats
which are often limited in extent and vary widely in
temperature and in content of dissolved substances.
Land animals inhabit a medium which is very constant
in composition and is rich in oxygen, but varies extremely
in temperature. They are in continual danger of desic-
cation and therefore require a continual supply of water.

The homoiothermic condition is an adaptation for
living a life under constant conditions in a highly vari-
able environment. It has enabled birds and mammals to
become dominant in the land habitats on the earth. Some
homoiotherms, like the whales and sea birds, have been
able to return to the home of their remote ancestors and
have again attained a considerable degree of success in
the ocean.

This book is concerned with the physiological and eco-
logical aspects of the evolution of poikilothermic into
homoiothermic animals. In it we attempt to discuss evi-
dence for the origin of homoiotherms, temperature regu-
lation, the factors influencing rate of metabolism and
growth, and other matters which relate to the adjust-
ments associated with evolution from erratic, more or
less intermittent, living to uniform, constant living.

CHAPTER II
THE TEMPERATURE OF THE EARTH IN TIME AND SPACE

The formation of the earth's surface was probably a
prelude to the origin of primitive life. The temperature
of the earth and its atmosphere has furnished the back-
ground which makes the interpretation of heat phenom-
ena in animals intelligible. It has also, we believe, fur-
nished the stage setting in the story of how some
organisms have become homoiothermal.

The Primitive Earth. — The history of the earth is
read in rocks and more recent events are more clearly
recorded than those which were more remote. The late
history is revealed with great fidelity but the earlier his-
tory is indistinct and, if traced back toward its begin-
nings, the indistinctness merges into obscurity. Thus
the picture of the primitive earth is more a matter of
inference than knowledge.

There have been many attempts to explain the origin
of our planet. Astronomers and geologists now gener-
ally agree that the solar system was evolved in some
way from a nebula of one form or another. The Planet-
esimal Hypothesis of Chamberlain (1916) perhaps has
the most wide acceptance of any theory of earth origin
at present. According to this hypothesis, the original
nebula consisted of small bodies, molecules or aggregates,
moving in orbits about a common center and forming a
disk-like system. The bodies are assumed to have been
controlled by revolution about the center of the nebula

TEMPERATURE OF THE EARTH IN TIME AND SPACE 5

and not by impact on one another, as the older hypothesis
of Sir George Darwin required. Evolution appears to
have consisted in the gathering of these small bodies, or
planetesimals, into planets and satellites. The meetings
and unions of planetesimals implied a relatively slow
evolution of nebula into a solar system but the end result
was inevitable. The planetesimal hypothesis therefore
signifies a relatively slow growth of the earth.

What the temperature of the juvenile earth was is
still a matter of speculation. The hypothesis of Cham-
berlain requires a much lower temperature than that of
Laplace. Instead of consisting of a primitive molten
globe the earth probably originated in a nebulous knot
of solid matter on which fell scattered nebulous matter,
and the latter probably did not have a high temperature.
Liquefaction of the rocks only occurred locally as the
result of heat generated by increased pressure and by
radioactivity. This temperature problem thus becomes
fundamental because, according to the opinion of Cham-
berlain, the earth may have had a temperature suitable
to life long before it reached its present size — perhaps
when it had attained to the magnitude of Mars.

The Primitive Atmosphere. — The primitive atmos-
phere was, by early geologists, held to be vast, hot, and
heavy, containing all the water of the globe, all the car-
bon dioxide now in carbonated rocks, all the oxygen which
has been added to the rocks by oxidation, as well as that
portion of all these constituents which is now found in
the atmosphere and in organic tissues. Such a condition
was assumed because heat promotes the expulsion of
gases from liquids or solids. The heat of the primitive
earth was believed to have been sufficient to drive out
all gases. As the earth cooled these gases were re-
absorbed.

6 HOMOIOTHERMISM

^An atmosphere containing much carbon dioxide and
water-vapor should give the earth a warm and equable
climate. Such climates apparently did prevail at certain
times during the earth's early history, and also at cer-
tain times later.

Temperature of the Modern Earth. — Organisms now
live at a time when the earth has marked climatic dif-
ferences, varying between the frigid polar climates and
the tropical humid or dry conditions. The earth has been
colder than it now is, and at times the climates of the
past have been warm and fairly constant the world over.
However, there probably have been, usually if not at all
times, zonal belts and fluctuations in temperature. This
is indicated by the types of ancient animals or plants,
from which may be deduced and plotted the temperature
curves of the past. Thus it is indicated that warm cli-
mates persisted during long geological ages and that the
polar regions were then usually inhabited by plants and
animals which were adjusted to winterless environments.
Apparently temperature fluctuations were often greatest
during the opening and closing of the periods and eras.

Very long, warm times were separated by shorter
periods of cool to cold climates. Geologists have deter-
mined that there have been about seven periods of de-
cided diferences in temperature as indicated by the fossil
remains in sedimentary rocks and by the characters of
the rocks themselves:

1. Early Proterozoic 5. Triassic to Lias

2. Late Proterozoic 6. Cretaceous to Eocene

3. Silurian 7. Pleistocene

4. Permian

At the beginning and ending of the Proterozoic,
Permian, and Pleistocene there were cold periods accom-

TEMPERATURE OF THE EARTH IN TIME AND SPACE 7

panied by extensive glaciation. Cooler climates have
occurred when the land areas were largest, or most
emergent, usually during the closing stages of periods or
eras, and cold climates nearly always existed during or
immediately following times of active mountain for-
mation.

The two principal atmospheric factors in the regula-
tion of climate are carbon dioxide and water-vapor. The
latter is the more important of the two and is also the
more variable. If both of these atmospheric constituents
are present in great abundance, especially aqueous vapor,
a thickened atmospheric blanket will be formed which
will retard the heat loss of the earth and will also absorb
a greater amount of the sun's radiation. When there is
but little carbon dioxide gas and water-vapor, more heat
is lost and less absorbed. Climates then become cooler
and more variable. The amounts of carbon dioxide and
water-vapor in atmosphere have varied much during the
earth's history.

Sources of Heat. — There are two sources of the heat
by means of which the earth surface may be warmed:
(1) radiant energy and (2) the internal heat of the
earth ^s mass. If either of these factors were to be re-
moved, life could no longer exist on the surface of the
earth.

Temperature of the Earth's Interior. — There are
many reasons for believing that the interior of the earth
is very hot. Volcanic phenomena, geysers, and hot
springs show that the earth is very hot in certain locali-
ties, even near the surface. In going down into deep
mines the temperature rises a:bout one degree Fahren-
heit for each 100 feet. While there is little probability
that this rise in temperature which is usual near the
surface would continue at greater depths, yet it indicates

8 HOMOIOTHERMISM

that at a depth of a few hundred miles, temperatures may
perhaps reach several thousand degrees. A slow dissemi-
nation of the internal heat toward the surface of the
earth continually takes place.

Radiant Energy. — The chief source of radiant energy
is the sun. The amount of energy received from the
stars is not sufficient to produce a significant effect upon
the earth, but that received from the sun is very great.
It is measured by the heat produced when a given sur-
face exposed at right angles to a beam entirely absorbs
the radiant energy. The mean value of such determina-
tions is called *^the solar constant of radiation'' and
has been fixed by Abbot as 1.932 calories per square
centimeter per minute. If this value were constant the
earth would receive in a year something like one million
million million million calories. This would be enough
heat to melt a layer of ice 110 feet thick over the entire
surface of the earth, provided there was no reflection.

The amount of radiant energy coming to the earth at
any given point depends upon several factors, among
which are longitude, distance between earth and sun, sun-
spots and other variations in the sun itself. The factor
which influences the heat converted from solar radiation
at any one point on the earth depends upon the intensity
of the solar radiation striking a given area, and the
absorbing capacity of the substance. A ** perfect black
body" is a highly efficient absorber of such radiations.
Obviously, surfaces that reflect radiations well have little
power to absorb. Snow is an excellent reflector and has
much to do with low temperatures during winter months.
Substances which are good transmitters of radiations
likewise absorb little of the radiant energy coming to
them. During volcanic eruptions the transmissibility of
the atmosphere undergoes marked changes, with the con-

TEMPERATURE OF THE EARTH IN TIME AND SPACE 9

sequent diminution of temperature on the earth. Such
changes occurred after both eruptions of Krakatau
(1884-1886; 1903-1904).

The Capture of Solar Radiation. — The internal heat
of the earth's mass and the heat converted from radiant
energy are sufficient to create a favorable physical en-
vironment for life on the earth's surface. Life, however,
could not have evolved to the complexity of organization
manifested today in animals, and especially could not
have climbed the phylogenetic tree to the branches occu-
pied by the homoiothermic animals, save by the capture
of sunlight.

In warming the earth's surface the energy of sunlight
is converted directly from electronic energy to molecular
energy. Living matter has found a way by which the
energy of sunlight may be stored or conserved and trans-
lated into molecular energy at some future time when it
will better serve the organism. Thus there seems to be
a system of conservation of energy in living matter —
almost a closed cycle of energy changes. The energy of
sunlight is captured on one side of this cycle to compen-
sate for the loss of energy on the other. This captured
energy enters the prison gates of living substance
through chlorophyll. It is transformed and mobilized to
places where it awaits release. Thus living substance is
not only a transformer of energy but an accumulator as
well. Once energy has accumulated, plants and animals
become reciprocating transformers of it, so that the
energy wastes of one become the energy sources of the
other. This matter will receive more detailed treatment
in later chapters.

CHAPTEK III
ANCESTRY OF HOMOIOTHERMIO ANIMALS

The general structure of the genealogical tree of ver-
tebrates is now fairly well established. Since Cambrian
times fishes gave rise to amphibians and amphibians to
reptiles. From early reptiles came the birds and
mammals.

Although some fishes, notably the tunnies (Boulenger,
1910), after activity may show a body temperature some-
what above that of the water in which they live, there
are none which maintain a high and constant tempera-
ture comparable to that of birds and mammals and there
is no evidence that any fishes have been able to do so in
the past. Amphibians and reptiles, both aquatic and
terrestrial, also may have body temperatures above the
medium in which they live, but are not homoiothermic
and apparently have never been so.

In speaking of the dinosaurs, Lull (1924) says: ''That
their activity, which implies increased metabolism, raised
the bodily temperature during the time of such activity,
it is, I think, safe to assume — an analogy is seen in the
tuna among the bony fishes today — ^but that they pos-
sessed a mechanism for the maintenance of a constant
bodily temperature irrespective of external conditions,
as with birds and mammals, is sustained by no evidence
thus far oifered. In fact, their entire lack of heat-retain-
ing clothing, such as the feathers of the bird or the hair
of the mammal, negatives such a possibility." The same

10

ANCESTRY OF HOMOIOTHERMIC ANIMALS 11

appears to be true of the pterosaurs, or extinct flying
reptiles.

Birds

Among several groups of ancient and modern reptiles
the habit of standing more or less upright and walking,
running, or leaping with the hind legs has been developed
in various localities and at various periods. These bi-
pedal reptiles are, and were, often quite agile. They
commonly use their tails to help support or balance their
bodies, develop processes on their pelvic bones for the
attachment of muscles, and possess other characteristics
adapted to their method of locomotion. Their fore legs
are often small and delicate and serve more for balancing
than support.

Heilmann (1926) has given a very careful account of
the evolution of birds from reptilian ancestors. He be-
lieves that birds sprang from the Pseudoseuchia, a group
in which the pubic bones already showed a tendency to
extend backward. These reptiles were also related,
though not directly, to the dinosaurs, pterosaurs, and
crocodiles. The particular group from which birds arose
were generally characterized by sharp-clawed first, sec-
ond, and third fingers, and by a tendency to reduction
of the fourth and fifth fingers; adaptations which indi-
cate arboreal, climbing habits.

The evolution of birds apparently took the following
course. Certain pseudoseuchians raised the forepart of
the body off the ground and as they became bipedal the
hind foot centered along the median line. From terres-
trial runners or leapers these animals gradually changed
to arboreal climbers. They leaped from branch to
branch; long, parachute scales developed along the pos-
terior borders of the fore limbs: these later became

12 HOMOIOTHERMISM

frayed into feathers, which later spread over the whole
body. In early stages of this evolution, the climbing
fingers became elongated, and thus favored wing devel-
opment. With the growth of wings a keel for muscular
attachments projected more and more from the breast
bone. The hind toe of birds undoubtedly originated in
connection with adaptation to arboreal life. Heilman
concludes :

The accelerated metabolic process, finally, produced an increased
caloricity, protected by feathering, until the warm-blooded state was
attained. The air-sacs of the lungs have expanded, spreading through
the whole body and filling the bones with air. The increase of all
these activities, moreover, has also resulted in a considerable enlarge-
ment and a somewhat refining evolution of the brain.

In this way the reptile, through millions of years and innumerable
generations, has been changed into a bird.

Modern birds generally maintain a constant body
temperature somewhat above that of mammals. This
varies very little at different seasons of the year (Simp-
son, 1911) and is quite similar in primitive and special-
ized birds (Simpson, 1912). However, Stoner (1926)
has recently reported that the temperature of the adult
bank swallow, Eiparia riparia (Linnaeus) may vary as
much as 7 deg. C. and that of the young, 10 deg. C.
Kendleigh and Baldwin (1928) made similar observa-
tions on house wrens. Atkins (1909a) observes that the
osmotic pressure of a developing hen's egg rises from
5.5 to 7.3 atmospheres. The latter figure is about the
same as that for bird's blood. **The view is put for-
ward that birds are descended from organisms with an
osmotic pressure of five atmospheres or less." The os-
motic pressure of the internal fluids in the bodies of
modern reptiles is about five atmospheres.

ANCESTRY OF HOMOIOTHERMIC ANIMALS 13

Mammals

There is a general agreement among vertebrate
palaeontologists that mammals originated from the order
of Triassic reptiles known as the Cynodontia. During
Permian and Triassic times the cynodonts had developed
temporal bony arches, a secondary palate, paired occipi-
tal condyles, heterodont dentition in which incisors,
canines, premolars, and molars were differentiated, and
a functionally important dentary. These reptiles de-
veloped a new type of jaw articulation, and the quadrate
bone, which plays such an important role in lizards, be-
came degenerate.

Hairs are believed to have been derived from reptil-
ian scales and there is evidence which supports this view.
The genital organs of primitive modern mammals, mono-
tremes and marsupials, show transitional stages between
those of reptiles and more specialized placentate mam-
mals. The marsupium probably enabled early mammals
to carry their eggs away from the nest. The eggs were
thus better protected. Later the young mammal prob-
ably took the place of the egg in the pouch. Lactation
probably came in with the acquirement of the homoio-
thermous condition. The oily fluid which perhaps at first
served to lubricate the pouch and help keep the young
warm later came to serve as a regular means for nourish-
ment. The young at first licked milk from hairs ; sucking
succeeded licking and the young became in a sense ex-
ternal parasites on the mother. The placenta developed
and the young were then able to obtain nourishment
w^ithin the body of the mother and could thus grow to
larger size before birth (Gregory, 1910).

In Cretaceous times the dinosaurs and other reptiles
lost their dominant position among the animals of the

14 HOMOIOTHERMISM

earth. Perhaps the mammals ate their eggs, perhaps
they were unable to compete with the mammals on ac-
count of their lack of ability to endure cold, perhaps they
passed away partly on account of their inferior loco-
motor mechanisms and brains, and because of wasteful
methods of reproduction. At any rate, they were exter-
minated or greatly reduced in numbers and reptiles to-
day hold a rather unimportant place. Eepresentatives
of very primitive Cretaceous mammals have recently
been discovered in Mongolia (Gregory, 1927). These
early insectivore-carnivore ancestors of modern mam-
mals were already far from the beginnings of their
careers; already differentiated into marsupials and
placentals.

Among primitive, living mammals, the monotremes
are probably not distinct from other Theria but are ^ * the
much specialized descendants of very early mammals''
(Watson, 1916). They probably represent primitive an-
cestral conditions in their egg-laying habits, the arrange-
ment of their genitalia in relation to a cloaca, and their
somewhat variable body temperature. Among many
types of placentals there are rather elaborate mechan-
isms for maintaining constant temperature, and these
are not so well developed in more primitive mammals
(Martin, 1903). Marsupials depend largely on evapora-
tion through the lung membranes for the lowering of
body temperature, and are the lowest animals to show
heat polypnea. A rabbit also uses polypnea as a means
of temperature regulation and depends largely on vaso-
motor regulation for the dissipation of heat ; carnivores
have somewhat more varied and effective mechanisms.
Among homoiothermic animals there are various means
and degrees of efficiency in the regulation of temperature.

ANCESTRY OF HOMOIOTHERMIC ANIMALS 15

Environment during Evolution of Homoiotherms

Vertebrates had their origin in the water and some
of them have slowly spread into land habitats. The
earliest vertebrates already had a long heritage from
the past. Even their blood had a somewhat distant rela-
tion to sea water, which probably served their remote
ancestors as a body flnid. Types which attained to life
on land were obliged to develop resistance to loss of
water and to endnre great changes in temperature. One
of the means of gaining relief from the continual and
marked changes of land climates was the attainment of
the homoiothermic condition. In other words, the ability
to maintain a constant temperature which permits unin-
terrupted metabolic activity at approximately optimum
rate is an adaptation to life on land, where climate is
variable.

The earliest birds and mammals were carnivores and
insectivores. Homoiotherms and seed plants evolved to-
gether and Berry (1920) believes that the evolution of
the former was to be to a considerable degree dependent
on that of the latter. The development of seeds and
fruits furnished nourishment which was in concentrated
form and which was available at all seasons of the
changeful year. The land animals which took advantage
of the newly developed seeds and fruits as food were
thus relieved to a considerable degree of the necessity
for continually seeking and capturing living prey. This
surety of food supply made easier the maintenance of
metabolism at a continuously high rate. ^*It may be sug-
gested that the changing food supply which is due to
the evolution of the flowering plants, and which is sug-
gested as one of the important factors in the evolution
of the higher mammals, was also one of the factors that

16 HOMOIOTHERMISM

spelled the doom of the overgrown and specialized Rep-
tilia of the Mesozoic/' A giant dinosaur required tons
of plant food, while a small mammal subsisting on seeds
and fruits could remain active at all seasons. The de-
velopment of seeds and fruits greatly increased the food
resources of early mammals. Berry stresses the fact
that differentiation among the flowering plants was soon
followed by the great differentiation of mammals, be-
ginning in the Eocene Period.

CHAPTEK IV

RESPONSES OF POIKILOTHERMS TO VARIATIONS IN
TEMPERATURE

It has long been recognized that, in so far as body
temperature is concerned, animals may be divided into
two large groups. These have been referred to as the
cold-blooded and the warm-blooded animals. The first
group includes animals, poikilotherms, in which body
temperature changes with that of the environment; the
second group includes those animals, homoiotherms,
which maintain at all times a practically constant tem-
perature which is inherently independent of the environ-
ment. In this chapter the responses of poikilotherms to
heat and cold will be discussed.

Body Temperature of the Poikilotherms. — The lower
vertebrates and all of the invertebrates belong in the
group of variable temperature animals. Only the birds
and mammals possess a means of regulating their body
temperature which makes them independent of the tem-
perature of the environment. How close the poikilo-
therms approximate the temperature of their environ-
ment cannot be stated with certainty. It mil be seen
from Table I that not all investigators agree. Many
of the conflicting results reported as to the tem-
perature of poikilotherms are likely due to the methods
employed. By the use of precision instruments many
of the variations may be eliminated and a better under-
standing of temperature relations now is being acquired.

17

18

HOMOIOTHERMISM

TABLE I

BoDT Temperaturb of Certain Cold Blooded Animals

(After Rogers, 1916)

Form

Range

Conditions
and Method

Temperature of

Animal above that

of Environment

Observer

Medusa . . . .
Earthworm.

Leeches.

56 deg. F.
56-54 deg. F.

Worms

Ceylon jungle leech

Echinoderms.
Snails

Several in glass
by thermometer

Several in glass
by thermometer

Snail

Oyster. . .
Crustacea .

Insects. . .

Bees.

Bees.
Bees.

Fishes:
Bonita.
Trout. .

Eel

Fishes (?) .

Fishes (?) .

Fishes (?) .

Fishes (?) .

Carp

Carp

Carp

Mackerel . . .
Herring ....
Amphibia . .

Frog.
Frog.

54 deg. F.

76.25 deg. F.

82 deg.
72-80 deg. F.

62-83 deg. F.

19.2 deg. C.

Several in glass
by thermometer

Individuals ther-
moelectric
Hive

57-69 deg. F.
Below 57 deg.

80.5-78 deg. F.
86-40 deg. F.

Thermoelectric

Thermoelectric

Heart muscle

51 deg. F.

66.5 deg. F

80.5 deg. F.

4.6-6.04 deg. C.

58-86 deg. F.

Stomach
Stomach

Rectal tempera-
ture
In air

In water

0.27 deg. F.
2.0-2.5 deg. F.

1.0-1.5 deg. F.

4.4-5.8 deg. F.

Same as water or

air

0.40 deg. F.

1.25 deg. F.

0.25 deg. F.
Same as water
1 deg. below to

same as water

8 deg. below to

10.5 deg. F.

above

O.lSdeg. C.

0.25 deg. C.
0.55-10.5 deg. C.
Same as air

Form clusters and
raise tempera-
ture

1 . 5-5 deg. F.
28 deg. below to
2 deg. F. above
Same as air
0.55 deg. F.
1.38 deg. F.
6.2-9.3 deg. F.
1.2-6.2 deg. F.
3 deg. F.
0.86 deg. F.
1.9-3.5 deg. F.

8.5 deg. F.

0.0-1.2 deg. C.

3.0 deg. below to

8.5 deg. F. above

Temperature less

than air
Same as water

Valentin
Hunter

Hunter

Davy
Davy

Valentin
Hunter

Davy
Davy
Davy

Davy
Davy

Dutrochet

Dutrochet

Phillips &
Demuth

Phillips &
Demuth

Davy
Davy

Davy

Martin

Kraft

Broussouet

Broussouet

Boniva

Depreta

Hunter

Davy

Simpson

Davy

Berthold

Berthold

The factors which influence the temperature of ani-
mals may be regarded as either extrinsic or intrinsic.

RESPONSES OF POIKILOTHERMS TO VARIATIONS 19

The intrinsic factors are those that lie within the organ-
ism and act to produce a temperature different from that
of the environment. The extrinsic factors are those
imposed on the organism from without. These will be
discussed first. Heat may be gained or lost by (1) con-
duction and convection and by (2) radiation.

Conduction. — Conduction means the loss of heat from
a body at a higher temperature to one at a lower tempera-
ture by passage from particle to particle. For example,
if one end of a copper rod is placed into a dish of hot
water, heat will pass into the rod and along it until the
other end becomes hot. This is molecular transference
of energy. The amount of heat gained or lost by a
poikilothermal animal through conduction will depend
upon several factors; the most important of these are
the amount of surface exposed, temperature gradients,
and atmospheric humidity. The amount of heat gained
or lost will vary directly with the area of the surface
exposed. Since the area of the surface of a body in-
creases to the second power when the mass is increased
to the third power, a larger animal will gain or lose heat
more slowly than a smaller one. The gain and loss of
heat also depend upon the moistness of the surface.
Most of the poikilotherms live in the water and are
little influenced by this factor, but amphibians and rep-
tiles which live in the air of course show marked differ-
ences in this respect. Amphibia nearly always have
moist skins whereas Reptilia have dry scaly skins. The
latter therefore lose heat more slowly. The nature of
the surface is likewise a factor in the gain and loss of
heat. A layer of fat at or near the surface will consid-
erably decrease the rate at which heat arrives at the sur-
face from the interior. The low thermal conductivity of
fat is thus an important factor in the retardation of heat

20 HOMOIOTHERMISM

loss. The subcutaneous tissues often cause a tempera-
ture lag or thermal gradient, which in a large animal will
be several degrees. This factor is of most importance
in the homoiotherm but nevertheless must not be con-
sidered as a negligible factor in the poikilotherms.

The heat lost by a body in a unit of time is propor-
tional to the difference between its temperature and that
of the surrounding medium. A warm body loses heat
and becomes colder, while the environment becomes
warmer. This transference of heat goes on until the
organism and the environment are at the same tempera-
ture. The time required for this equilibrium to become
established depends mainly upon the gradient between
the integument of the animal and the immediately sur-
rounding medium. The circulation of the blood tends
to make the gradient greater and consequently a living
poikilotherm will come more quickly to the temperature
of its surroundings than will a dead one. This we have
shown to be true of a turtle immersed in a water bath of
constant temperature. The temperature was taken by
a thermocouple placed in the coelomic cavity very near
the center of the body. The surrounding water was kept
in rapid circulation and the turtle was held stationary.
The turtle weighed 278 grams and at the beginning of
the experiment was transferred from a water bath which
was 10 deg. C. higher than the experimental bath. The
absolute temperatures were read with a standard mer-
curial thermometer calibrated to one-hundredth of a
degree centigrade. The temperature differences were
determined with copper-constantan thermo-couples.
After one determination was made of the rate of heat
transference from the living turtle to the water bath,
the experiment was repeated with the same turtle killed
by ether anesthesia. The experiment was then reversed

RESPONSES OF POIKILOTHERMS TO VARIATIONS 21

with a turtle weighing 278 grams with heat absorbed first
by a living turtle and then with the same turtle dead.

Figure 1 illustrates how blood circulation apparently
influences the temperature gradient between the surface
of an animal and the surrounding medium. It is also
interesting to note that, in consequence of the influence

\io

i 6

A\ \B

10

20

SO

TIME IN MINUTES

40

50

Fie. 1. — Rate of response to an altered environmental temperature in
a living and dead turtle. A — living turtle; B — same turtle dead.

of temperature on the rate of heart beat and velocity of
blood flow, the temperature of the turtle more quickly
approaches the temperatures of the water bath at higher
temperatures than at lower. If the medium is kept
in motion the gradient will be higher than if it is stag-
nant. A terrestrial animal will gain or lose heat
more rapidly if it is exposed to convectional cur-

22 HOMOIOTHERMISM

rents or winds than if the air is quiet. In the absence
of air currents a layer of air will form an envelope
around the animal, decreasing the temperature gradient
and preventing rapid conduction of heat. Humidity is
obviously a factor only in the terrestrial animals. Water
is a better conductor of heat than air. The more humid
the atmosphere, the more rapid will be the gain and loss
of heat if other factors do not change. The humidity of
the atmosphere also influences the rate of evaporation
of water from the surface of an animal's body. The
maximum rate of evaporation will take place in an at-
mosphere containing no water vapor and the minimum
in an atmosphere saturated with water vapor, assuming
that temperature and other factors remain constant.
Evaporation has a marked cooling effect on animals.

Radiation. — The transmission of heat by radiation is
fundamentally different from that of conduction. Radia-
tion implies vibratory movements of particles or elec-
trons depending on their kinetic energy. If the tempera-
ture of the particles is increased, the velocity of the
vibrations will increase. Conduction requires molecular
continuity. In other words, one body must be in contact
with another in order to have transmission of heat energy
by conduction. One body may affect the thermal state
of another body by radiation without being in contact
with it, and also without affecting appreciably the inter-
vening medium. Radiation is not dependent upon a con-
tinuous medium, such as air or water. In fact, it takes
place quite readily in a vacuum.

The heat gained or lost by an animal through radia-
tion depends on the area of its surface, color, and tem-
perature. It has been proved experimentally that the
absorptive and emissive powers of a body are equal.
Therefore if the temperature of an animal is higher than

RESPONSES OF POIKILOTHERMS TO VARIATIONS 23

that of its environment it will radiate energy more than
it absorbs from it, but if it be at a lower temperature,
it will absorb more than it will radiate. The total emis-
sive and absorptive power of a body is directly propor-
tional to the area of the surface. A perfectly black body
will absorb all radiant energy that falls on it ; and, since
a body cannot absorb more than is incident on it, the
black body is considered as unity. There are no animals
which have a skin that is perfectly black and conse-
quently any animal can absorb only a part of the radiant
energy incident. Likewise there are no perfectly white
animals and therefore all must absorb some radiant
energy. However, there are a few animals, such as the
marine Coelenterata, which are such good transmitters
of visible light that the radiant energy absorbed by them
is negligible. Buxton (1923) gives an admirable account
of the color of desert animals and emphasizes the im-
portance of color as an adaptive feature to physical
rather than organic environmental conditions. Desert
reptiles with a scaly integument reflect most of the light
that falls upon them and thereby resist to a large extent
the radiant energy of the sunlight.

Pigmentation plays an important part in the gain and
loss of heat in animals. For example a turtle, with its
highly pigmented shell, may, when basking in the sun,
have a body temperature higher than that of the. atmos-
phere about it. The junior writer has performed aa ex-
periment which demonstrates this. A turtle was placed
on a table in the laboratory beneath a source of radiant
energy (an infra-red generator) and its resulting body
temperatures are shown in Fig. 2. It will be observed
that, while the atmosphere was not warmed appreciably,
the temperature of the turtle rose constantly to a point
higher than that at which a turtle could live. It appears.

24

HOMOIOTHERMISM

therefore, that poikilotherms may have a higher body
temperature than that of their immediate environment
through the absorption of radiant energy. In a very
warm environment highly pigmented animals commonly
find long exposures to sunlight deleterious, and seek the
shade or are nocturnal. In a colder environment pig-
mentation may be of considerable value because it would

40
35
30
25

.^'"^^^^

^

y

X

/

/

/

10

20

50

60

30 40

TIME IN MINUTES

Fifl. 2. — ^Rise in body temperature of a turtle when placed three feet
away from a radiant source of energy. The circumjacent tempera-
ture remained unchanged.

aid in the absorption of heat. The temperature of a
body is a determining factor in the amount of radiation
from it. The total heat-loss by radiation is proportional
to the difference between the fourth powers of the abso-
lute temperatures of the body and its environment, if
the area and nature of the surface remain constant. It
follows then that a poikilotherm would lose little heat

RESPONSES OF POIKILOTHERMS TO VARIATIONS 25

by radiation if its own temperature differed little from
the environment.

Poikilotherms respond to thermal alterations in the
environment mainly in four ways: (1) torpidity, (2)
migration, (3) acclimatization, and (4) tolerance.

Torpidity. — Metabolic processes vary with tempera-
ture, being slowed as an animal becomes colder and
quickened when it is w^armed. The limits at which an
animal can carry on life processes lie between the freez-
ing point of protoplasm and the coagulation of certain
proteins by heat. As animals approach the limits of
their toleration, certain of them may suspend their ac-
tivities and go into a state of torpidity. Protozoa encyst ;
many insects become torpid during cold and dry weather ;
snails may seal up their shells ; many fishes hibernate or
aestivate during extreme climatic conditions; certain
amphibians and reptiles hibernate during winter, but
apparently do not hibernate if kept warm the year
around.

Migration. — Many insects are known to migrate dur-
ing changing climatic conditions. Fishes, such as herring
and sardines, in summer live near the surface and store
fat, but in winter descend to greater depths and lose
fat. Marine fishes are sensitive to differences of
0.2 deg. C. Ward (1921) states that the migration of
salmon is primarily controlled by temperature, the fishes
choosing the cooler branches of the rivers as they move
upstream. Amphibians such as Necturus move into the
deeper and colder water during the summer months, and
many frogs regularly alternate between aquatic and
terrestrial habitats as the seasons change.

Acclimatization.— Dallinger (1880) showed that by
raising the temperature of a culture of flagellates, he
could raise their limits of tolerance from 23 deg. to

26 HOMOIOTHERMISM

70 deg. C. Miss Behre (1918) has found that Planaria
will quite readily change its limits of tolerance to high
or to low temperatures. Acclimatization of vertebrates
has been investigated by Davenport and Castle (1895).
They tested toad tadpoles and found that the limit of
toleration could be increased several degrees. Loeb and
Wasteneys (1912), using Fundulus, demonstrated an in-
creased resistance to heat and cold after continued ex-
posures to extreme temperatures. Davenport (1897) sug-
gested that increased tolerance to extreme temperature
may result from lowering of the water content of proto-
plasm. Miss Behre believes that an adjustment of meta-
bolic rate is an important factor in acclimatization.
Hathaway (1928) has shown that continued exposures to
high oir low temperatures progressively raised or lowered
the limits of tolerance of several species of fishes. He
states%lso that fishes which inhabit shallow water (bass,
bluegill, and sunfish) undergo a change of tolerance by
acclimatization much more readily than the perch, which
is typically an inhabitant of deep, cool water.

Tolerance. — Many animals are very susceptible to
temperature changes and succumb if the thermal varia-
tion of their environment exceeds a few degrees; other
animals resist or endure extreme temperature changes.
Those in which the optimum range is narrow are said
to be stenothermal. Those which endure the great varia-
tion in external temperature are eurythermal. Land
animals as a rule are eurythermal to a greater degree
than those in the water. In the latter group are such
animals as the corals, which live largely in tropical seas.
The carp and the goldfish are examples of freshwater
forms which usually feed only between about 6 deg. and
30 deg. C. Eurythermal animals are illustrated by forms
like the brine shrimp {Arternia salina). This crustacean

RESPONSES OF POIKILOTHERMS TO VARIATIONS 27

thrives in salt marshes and in pools which are opposed
to the full radiating and evaporating power of the sun
for the purpose of making salt. Centipedes are reported
to range from hot plains and to the snow-covered peaks
of mountains on the island of Cyprus. The common
European eel is found from Ireland to the Nile. The
American eel is found from the Caribbean to the Great
Lakes. The water snake {Tropidonotus natrix) is found
from Sweden to Algeria. Many other species of animals
range through a variety of temperature conditions.

The extreme minimum temperatures which poikilo-
thermal animals can tolerate are near the freezing point
of water. Below this, metabolic activity ceases. Some
marine animals in arctic seas live in water which is a
degree or two below zero and are protected from freezing
by the salts in their body fluids. Extremely low tem-
peratures during a considerable part of the year do not
prevent the successful existence of land animals if there
is a warm season to allow for growth and reproduction.

The problem of toleration of high temperatures is
unlike that of low temperatures for the reason that the
effect of the former is principally chemical and usually
involves a permanent change in the living protoplasm,
while that of the latter is physical and produces a sus-
pension of activities which are resumed with the return
of favorable circumstances. The toleration of high tem-
peratures is usually accomplished by means of altera-
tions in the physico-chemical state of the protoplasm.
Insects, for example, lose water in hot surroundings.
Brues (1927) reports many insects living at 30 to 40
deg. C. in hot springs. He found ''blood worms" devel-
oping at 50 deg. C. Snails, amphipods, and isopods were
also observed living in similar conditions. Vertebrates
were found to be rather uncommon, rarely survived in

28 HOMOIOTHERMISM \

water over 25 to 30 deg. C, and apparently never lived
at temperatures of more than 40 deg. C. Tadpoles, how-
ever, were found living in water very near this limit.

It is a very interesting ecological point that nearly
all groups of animals that are represented in freshwater
contain a few species known to inhabit hot waters. The
forms that adjust their tolerance to high temperatures
are extremely varied.

Aquatic animals may have great difficulty living in
warm water because of the small amount of dissolved
oxygen. Many of the species which inhabit hot springs
obtain their oxygen from the air. It is perhaps signifi-
cant that the animals living in such situations where the
essential elements, salts, oxygen, ^^H, etc., may be very
unlike the ordinary freshwater conditions, show a greater
resemblance to freshwater forms than do the marine
forms which have always lived in the sea, where they
are never obliged to resist great fluctuations in the tem-
perature of their environment. Apparently animals
which now live in waters which have high temperatures
have been derived rather recently from typical fresh-
water animals and not from their more marine ancestors.

Body Temperature of Poikilotherms. — While it ap-
pears from the observations of many of the older ob-
servers that the body temperature of poikilotherms may
differ as much as several degrees from that of the en-
vironment, later workers have found that such tempera-
tures may resemble each other very closely, even to
hundredths of a degree. Eogers (1927) found that the
earthworm, salamander, clam, and goldfish in rapidly cir-
culating water, had body temperatures of almost exactly
the same temperature as the surrounding water. In the
experiment summarized in Fig. 1 the body temperature
of a turtle soon came to the same temperature as the

RESPONSES OF POIKILOTHERMS TO VARIATIONS 29

surrounding water bath. The same has been found to be
true when salamanders and frogs are used. Thus it
appears that with precise temperature measurements,
little or no heat regulation obtains in the poikilotherm.
Hence the temperature of the poikilo-organism is appa-
rently determined through the gain and loss of heat
controlled by aforementioned extrinsic factors.

CHAPTER V

EESPONSES OF HOMOIOTHERMS TO VARIATIONS IN
TEMPERATURE

For every animal there is apparently an optimum
range of body temperature within which life-processes
are adjusted to proceed smoothly. This range is com-
paratively wide in the poikilotherms, but rather narrow
in the homoiotherms. Since homoiotherms maintain an
almost constant and optimum internal temperature, they
are little affected by external temperature changes un-
less the alteration is great enough to overtax or break
down their heat-regulating mechanisms. Thus it appears
that all animals have an optimum range of external as
well as of internal temperature. Poikilotherms have a
relatively wider range of internal optimum temperature
compared with the homoiotherm, but a relatively nar-
rower range of external optimum temperature (Fig. 3).

The Body Temperature of Homoiotherms.— The aver-
age temperature of mammals is about 39 deg. C. That
of man is about 37 deg. C. That of the duckbill (Orni-
thorhynchus) and spiny ant-eater {Echidna) is about
25 deg. C. and is much more variable than in other mam-
mals. Birds have an average temperature about 5 deg. C.
higher than the mammals.

The variation in the body temperature of the more
primitive mammals is more approximately and imper-
fectly regulated in comparison with the higher mammals.
There are few references to transitional stages between

30

RESPONSES OF HOMOIOTHERMS TO VARIATIONS 31

the poikilo therms and the homoiotherms. Martin (1903)
has discovered that certain of the primitive mammals of
Australia have an imperfect means of heat regulation.
He found the duckbill to be almost cold-blooded, and the
Australian anteater showed a fluctuation of about 10
deg. C. in body temperature with a change of 30 deg. C.
in the surroundings. The kangaroo and other marsupials
exhibit a somewhat better control. The placental ani-

BODY TEMPERATURE DEG. C.

0 10 20 30 40

-40 -30 -20 -10 0 10 20 30 40 50 60

ENVIRONMENTAL ^TEMPERATURE

Fig. 3. — Difference between body temperature and environmental tem-
perature in poikilotherms ( ) and homoiotherms ( ).

mals have the most highly perfected regulatory mechan-
ism. The sloth {Bradypus cuculliger cuculliger Wagler),
one of very primitive placental forms, shows a rather
imperfect mechanism of heat regulation. Kredel (1928)
has shown that variation in the air temperature sur-
rounding this animal will produce a relatively marked
change in the body temperature.

Homoiothermal animals respond to environmental
temperature variations by: (1) physiological regulation
of gain or loss of body heat, (2) morphological adapta-

32 HOMOIOTHERMISM

tions, (3) parental care of young, (4) migration, (5)
hibernation, (6) storing of food, (7) occupation of homes
which furnish more or less insulation, (8) the wearing
of clothes.

Regulation of Body Temperature. — The regulation of
body temperature will be discussed in Chapter VIII and
hibernation in Chapter X. Many mammals protect their
young from adverse temperature conditions. New-born
rats and mice are unable for some days to resist changes
in atmospheric temperature. The human infant must be
protected for several weeks after birth and is easily
chilled by exposure to cold. The young of the marsupials
are carried in a pouch and are thus kept warm within
the mother's body. Among many species of birds the
young are kept warm by the heat from the mother for
some time following hatching. Apparently the mechan-
ism of temperature control is not completely functional
at the time of birth, even in the higher mammals.

Migrations. — Many animals, notably the birds, mi-
grate during certain seasons of the year and thus escape
temperatures which might be deleterious. Migration,
however, is apparently not always brought about directly
by temperature variation and the advantage that may
be derived by migration as a means of protection against
extreme temperature changes is probably secondary to
those associated with the securing of food and repro-
duction.

Morphological Adjustments. — Animals are provided
with certain morphological adaptations for restraining
the loss of heat through the skin. The more important
are: (1) the subcutaneous adipose tissue and (2) the
natural hairy or feathery coverings of the body. These
vary greatly in different environments and with the de-
gree of temperature fluctuations. The importance of the

RESPONSES OF HOMOIOTHERMS TO VARIATIONS 33

subcutaneous fat is seen in the case of the homoiothermic
marine animals in the Arctic. Such animals live habit-
ually among the ice floes in a medium which conducts
heat at least twenty times better than air and yet they
are able to maintain a body temperature of between 35
and 40 deg. C. The outer layer of skin is subjected to
conditions that would lead to an enormous loss of heat
were it not for the layer of fat insulating the deeper
tissues from the skin.

Insulation. — ^Homoiotherms living in the air are pro-
tected from heat loss by feathers and hair. These struc-
tures inclose a layer of air close to the body. By restrict-
ing the movement of this insulating air, much less heat
is carried away from the body. Rubner (1903) de-
termined the value of the hair of the guinea pig in saving
heat. He found that normally the average loss of heat
by radiation and conduction was 3.37 Cal. per hour.
After the guinea pig was shorn the hourly loss was 4.19
Cal., Le,, 33.3 per cent more. Hair or feathers have little
value to animals living in water, for when the air spaces
between these structures are filled with water heat is
readily absorbed by the surroundings. Water birds ob-
viate this difficulty by having oily feathers which do not
readily become wet and these obstruct the passage of
water to the air spaces next to the skin. They are often
also well insulated with subcutaneous fat.

Resistance to Extreme Temperatures. — When a mam-
mal loses heat to excess it is no longer able to maintain
its temperature at a constant level. The lower the tem-
perature of the body falls, the greater will be the dis-
turbances thereby produced. The highest nerve centers
are the first to suffer from such cooling. The medulla,
where the important centers are located for the main-
tenance of various vital physiological processes, is not

34 HOMOIOTHERMISM

affected until there is considerable reduction. Tiger-
stedt (1906) observed a man who retained consciousness
with a temperature of only 26.7 deg. C. and discusses
persons who recovered after extreme exposure during
which the temperature of the body fell to 24 deg. C.

An increase of the temperature beyond certain limits
involves general disturbances in the health of an organ-
ism. In general animals stand a decrease better than an
increase in temperature. A rise of only 2 and 3 deg. C.
may cause very severe disorders. A temperature of 41
or 42 deg. C. in the human body is regarded as a very
dangerous symptom if prevailing for more than a short
time.

Constancy of Body Temperature. — While a given in-
dividual is said to have a certain body temperature, it
should not be assumed that the entire body is at the same
temperature; in fact, such is not the case. The differ-
ences between various parts of the body may amount to
several degrees. This has been notably demonstrated by
Bazett (1927) and his co-workers. By means of specially
designed thermocouples he has shown that temperature
gradients exist in the body, the internal temperatures
highest and decreasing toward the exterior. The gradi-
ent is greater when the temperature of the environment
is lower.

Homes. — The usual functions of the homes built by
animals are as refuges from enemies, often with particu-
lar reference to guarding and rearing the young, and as
places for storing food. In many cases homes also serve
for protection against the elements, a shelter against
storms, and an inclosure for protection against the ex-
treme temperature changes of the surroundings. Some
animals build their homes of vegetation; others burrow
beneath the ground beyond the frost line, where they may

RESPONSES OF HOMOIOTHERMS TO VARIATIONS 35

hibernate. Man enjoys more advantages from his homes
than any other animal and of course provides special
means to prevent extreme changes in temperature.

Clothing. — Man alone among animals has become in-
dependent of temperature fluctuations by inventing
clothes. In cool countries he endeavors to protect him-
self by keeping his skin covered. Only about 20 per cent
of his skin is normally exposed to air. He varies the
amount of surface exposed and the thickness of this arti-
ficial covering with the climatic changes. The air in and
within clothes is a much poorer conductor of heat than
the fibers from which fabrics are made and furnishes
excellent insulation. This is especially true of that in the
furs which man finds ready-made on the bodies of other
mammals. The greater the thickness of clothing, the
thicker is the layer of contained air and the greater is
the prevention of heat loss from the body. Moisture ab-
sorbed by the clothes and filling up the air spaces de-
creases the effectiveness of the clothes. Savage races
living in Tierra del Fuego in a cold moist climate usually
do not wear clothes but a layer of oil over their bodies..

Temperature Regulation. — The mechanism of tem-
perature regulation is not fully developed in many
species of mammals until after birth. Some mammals,
however, have the ability to maintain a very constant
temperature from the first. For example this is true of
the guinea pig, which has a well developed nervous sys-
tem at birth. Other species, like the rat and the pigeon
which are born helpless and require parental care for
some time, do not come into possession of the power to
regulate their loss and gain of heat for many days after
birth. A newborn child is not able to control its bodily
temperature well for several weeks. It is probable that
this post-embryonic development of the regulatory mech-

36 HOMOIOTHERMISM

anism is dependent upon the development of the nenro-
muscular apparatus which goes on at the same time.
Kendeigh and Baldwin (1928) found that the resistance
of young house wrens against cold follows the sigmoid
growth curve. Such development of temperature resist-
ance, according to these authors, is due largely (1) to
the mass of body increasing faster proportionately than
the external dissipating surface; (2) to the development
of a feathery covering; (3) to the development of an
internal dissipating surface, which probably is under
nervous and respiratory control; and (4) to the produc-
tion of heat by the metabolism. They believe the third
factor is the most important.

CHAPTER VI
FACTORS INFLUENCING RATE OF METABOLISM

Metabolism is the chief source of the heat which main-
tains the constant temperature of homoiotherms. An
animal may promote the retention of heat within its body
by insulation or the transfer of body fluids and it may,
if febrile, lose heat by surface conduction, radiation, or
evaporation. The chief source of the energy which con-
tinually supplies animal heat is the oxidation of carbo-
hydrates, fats, and proteins. If the surroundings of a
homoiotherm become colder, its body usually responds
by a greater degree of internal chemical change and
energy is thus supplied to keep the body temperature
constant.

The quality of metabolism is quite similar in poikilo-
therms and homoiotherms, but the quantity is strikingly
different. The daily energy requirement of a fish is only
4 per cent of that of a mammal of the same weight. The
life intensity, or rate of living, of a rabbit is 25 times
that of a pike (Rubner, 1924). Because a homoiotherm
lives at a more rapid rate, it must continually have more
fuel for its metabolic processes and this is largely sup-
plied in its food. A small animal requires porportionally
more energy than a large animal of the same type be-
cause its surface permits more rapid dissipation of
energy. On account of the comparatively slow rate of
metabolism in poikilotherms such mass-surface relations

37

38 HOMOIOTHERMISM

are correspondingly less important. A monse requires
mucli more food per gram of body weight than an ele-
phant, but the differences between the food requirements
of a little lizard and a giant crocodile are less significant.

As metabolism has been and is of prime importance
in the origin and continuance of homoiotherms this chap-
ter is devoted to a brief consideration of the factors
which are chiefly concerned with its variations in rate.
In this connection age, temperature, respiration, food,
glands, desiccation, rhythms, and nervous control will be
considered.

Age. — In all animals the rate of metabolism probably
varies more or less at different ages. The fertilization
of a sea urchin egg is soon followed by an increase in
metabolic rate. As development proceeds the rate may
increase or decrease. The development of a yolk-laden
fish egg may proceed slowly until there are mechanisms
for transporting food and then progress at a more rapid
rate. When inert substances are being assimilated or
when profound reorganizations are taking place in the
bodies of animals, the metabolic rate may be slow. A
Drosophila pupa shows a decrease in its metabolism dur-
ing its second day of development, then the rate gradu-
ally increases up to the time of the emergence of the
adult fly (Bodine and Orr, 1925). Various arthropods
and chick embryos show differences in rate of heart beat
and respiratory movements which are characteristic of
particular ages (Crozier and Stier, 1927a; Henderson,
1927).

In youth a fish (Knauthe, 1898) or a mammal
(Lusk, 1917) has a high rate of metabolism and this
gradually grows slower, with some variations at such
times as the annual spawning or the age of puberty. On
the other hand the metabolic rate of amphibian tadpoles

FACTORS INFLUENCING RATE OF METABOLISM 39

which are undergoing metamorphosis progressively in-
creases (Helff, 1923). Such changes in tadpoles are
associated with profound bodily modifications. The
stomach and intestine grow shorter through autotomy,
autolysis, and phagocytosis ; the food changes from vege-
tation to flesh. Of course other influences may be asso-
ciated with age in affecting the rate of metabolism — ^men
commonly have a higher rate than women; various
species of salamanders appear to show characteristic and
consistent differences (Helf, 1928).

Temperature. — ^In general poikilotherms live faster
at higher temperatures and slower at lower tempera-
tures, while homoiotherms live continually at approxi-
mately the same rate, but expend more energy at lower
temperatures in order to furnish bodly heat. The meta-
bolic rate of a fish, amphibian or reptile is rather closely
associated with outside temperatures. The respiratory
exchange, which is a reliable indicator of the rate of
metabolism, in a turtle is approximately related directly
to the temperature of the surrounding medium (Hall,
1924). Van't Hoff's Law for the velocity of chemical
reactions in relation to temperature holds for the rate
of heart beat of salamander embryos (Laurens, 1914).
As the temperature of the water in which a fish lives
rises, the amount of nitrogen excreted steadily increases
(Knauthe, 1898). At temperatures below 6 to 9 deg. C.
adult freshwater fishes cease to eat altogether, but young
fishes continue to eat at lower temperatures than adults
(Haussman, 1897; Knauthe, 1898; Pearse, 1919; Hath-
away, 1927). However, there are species of fishes in
some localities which pass entire life cycles at or near
0 deg. C, and others which never know temperatures
below 45 deg. C. (Murray, 1914, 1914a). Particular
species may have peculiarities or have become adjusted

40 HOMOIOTHERMISM

to life at extreme temperatures, but in general the state-
ment at the beginning of this paragraph is true.

At lower temperatures a poikilothermic animal often
has a body temperature which is somewhat above that of
the environment. The tunny may thus show a tempera-
ture more than 3 deg. C. above that of the surrounding
water (Murray, 1914). In turtles, Baldwin (1925) found
a non-critical range between 10 to 27 deg. C, where body
temperatures were at times from 1.5 to 3 deg. C. above
the surroundings, but at lower temperatures, and espe-
cially at about 4.5 deg. C, the body temperature showed
a marked tendency to remain higher than the environ-
ment. He believes there is thus '*a slight tendency to
compensate for critical changes in the environment."
McClendon (1918) and others who have studied the rela-
tions of temperature inside and outside various inverte-
brates and aquatic vertebrates find no such tendency.
This is quite a contrast to conditions in the body of a
mammal. A man, for example, has a high degree of
ability to compensate for environmental changes in tem-
perature. He maintains a constant temperature and
gives off no more heat at 18 deg. C. in a room under a
blanket than when in water having a temperature of 37
deg. C. (Benedict, 1914). In passing from fishes through
the various classes of vertebrates to mammals and birds
there is increasing control of rate metabolism and body
temperature by nervous mechanisms. For example, the
rate of respiration in a turtle is controlled largely by
the temperature of the animal's neck. The temperature
of the blood going to the head affects the respiratory
centers in the brain. The temperature of the center cells
is thus a more potent influence than the general activity
or temperature of the body as a whole (Lumsden, 1924).
In mammals the presence of carbon dioxide in the blood

FACTORS INFLUENCING RATE OF METABOLISM 41

passing to the respiratory centers influences the rate of
breathing movements.

At high environmental temperatures, above about 37
to 40 deg. C, many homoiothermic and poikilothermic
animals do not flourish (Eubner, 1924a). The reasons
for this are not altogether clear, but they are, of course,
quite different from those related to the slow metabolic
rates which limit the activities of most poikilotherms at
low temperatures.

Respiration. — There are two types of respiration: ex-
ternal, which is concerned with the bringing of oxygen
into the body of an animal and with the elimination of
waste gases ; and internal, which supplies oxygen to tis-
sue cells and permits such cells to eliminate their waste
products. The oxygen consumed is perhaps the best in-
dex of the metabolic rate of an animal and is, of course,
the chief agent, through oxidation, for producing animal
heat and maintaining body temperature. Judged by
oxygen consumed and carbon dioxide eliminated, there
are considerable variations in the metabolism of individ-
uals and of species. There are also many influences
which temporarily affect respiratory movements and
the exchange of gases for respiratory processes. The
oxygen-carrying capacity of the blood of various marine
fishes is apparently related to the degree of activity
which each species normally shows. These fishes are
each adapted to a certain metabolic level and the limiting
factor to some degree is the rate at which oxygen can
be supplied to the tissues. If two bullheads are put in
an aquarium together they will show higher rates of
exchange for respiratory gases than if they are left by
themselves. The presence of one individual apparently
influences the rate of respiration in the other. The oxy-
gen consumption of different species of salamanders

42 HOMOIOTHERMISM

differs characteristically and widely (Helff, 1928).
Planarians, insects, and mammals after feeding show
an increase in respiration which takes place before
the food is being digested and appears to be dne to in-
creased activity of entoderm cells (Amberson, Mayerson,
and Scott, 1924). When animals are desiccated carbon
dioxide production increases rapidly when about half the
vital limit is exceeded (Caldwell, 1925). In many inverte-
brates oxygen tension is related more or less directly to
oxygen consumption, but this is not true of a vertebrate
like Fundulus, which reaches a critical point and dies
when the oxygen supply decreases. The metabolism of
a fish cannot fall below a certain minimum limit (Amber-
son, Mayerson, and Scott, 1924). Some vertebrates, how-
ever, like turtles, which have a large storage capacity in
their lungs, can get along for several hours at rather
high temperatures without carrying on any external
respiration (Lumsden, 1924).

Many factors influence respiratory processes directly
or indirectly. Numerous invertebrates are able to reduce
their respiration or cease to breathe altogether for long
periods of time when oxygen is lacking in the environ-
ment (Pearse, 1926). More highly organized animals,
in which more authority has been delegated to nervous
control, cannot do this to such an extent and homoio-
therms of course quickly suffocate if a considerable sup-
ply of oxygen is not continually available. Gesell (1926)
has recently suggested that ** changes in the hydrogen-
ion concentration of the respiratory center rather than
of the blood constitute the prime factor in respiratory
control." He believes that oxygen supply controls the
amount of lactic acid present and that the effects of this
substance and carbon dioxide exert a combined influence
indirectly when present in the blood and directly when

FACTORS INFLUENCING RATE OF METABOLISM 43

present in the respiratory center itself. In primitive
protoplasm respiration is the direct result of and condi-
tional for metabolism, but in more specialized animals
there is much circumlocution in control. Individual
idiosyncrasy, racial pecularity and physiological, en-
vironmental, psychological, and other factors may mod-
ify its rate or method.

Food. — Food supplies the fuel for metabolism. At low
temperatures poikilotherms require little or no food, but
homoiotherms under the same circumstances need an
increased supply. However, an isolated heart of a poi-
kilotherm requires two or three times as much sugar at
low as at high temperatures. In a normal animal the
sugar in the blood varies in accordance with such needs
(Mansfield and Pap, 1920). This indicates that the
tissue cells of poikilotherms, as well as those of homoio-
therms, tend to oxidize more food at lower temperatures,
but in the former the rate of metabolism is so much
slower at lower temperatures that there is an actual
decrease. Another characteristic of metabolic processes
which intensifies this effect is the general tendency of
animals to store fat during warm seasons and utilize this
for metabolism during cooler parts of the year (Fage
and Legendre, 1914; Pearse, 1924).

The amount of food required to carry on a particular
amount of metabolism at a given temperature does not
appear to differ markedly in poikilotherms and homoio-
therms. The protoplasms in the two types of animals
have similar needs. Page (1895) determined that a
trout could subsist on food equal to about one per cent
of its own body weight daily. Centrarchid and silurid
fishes apparently need about six per cent (Pearse, 1924).
The former eat about three times as much at 20 deg. C.
as at 10 deg. C, and if kept at 10 deg. C. for several

44 HOMOIOTHERMISM

weeks gradually eat less (Hathaway, 1927). A young
smelt in May needs daily 1 mg. of food ; in July, 2 mg. ;
and in September, 4 mg. A young herring in July re-
quires 26.6 mg. ; in September, 49.9 mg., and in October,
38.8 mg. (Putter, 1909). Even with lower body tempera-
tures the food requirements of poikilotherms are similar,
in some eases at least, to those of homoiotherms. An
average man requires organic food equal to only one to
two per cent of the weight of his body each day.

Food is prepared for assimilation by digestion and
its energy is released by the activity of catalytic agents.
It has long been known that the digestive enzymes of
homoiothermic animals have their optimum for activity
at about 37 deg. C. and the recent work of Kenyon (1925)
is of particular interest because it shows that the same
is true for poikilothermic vertebrates. When reptiles
gave rise to mammals and birds there were apparently
no radical changes in the methods of digestion, assimila-
tion, or energy release through metabolism.

The character of the food which any animal eats may
have a profound influence on metabolism. It is well
known that various foods have varying values as pro-
ducers of energy. For example, Kubner's standard cal-
oric, or heat producing, values are, in calories per gram
of food; protein, 4.1; fat, 9.3; and carbohydrate, 4.1
(Lusk, 1917). Proteins, of course, have greater value
as tissue builders than carbohydrates or fats but are
more expensive as heat producers because more work
is required to release their energy. Certain foods like
vitamins, and certain protein compounds such as trypto-
phane, do not appear to have their chief value in the
amount of energy or material they furnish, but in pecu-
liar qualities which are as yet not wholly understood.
There are also certain secretions like thyroidin and

FACTORS INFLUENCING RATE OF METABOLISM 45

pituitrin which exert a profound influence on metabolic
activity and growth. There is at present no evidence
that poikilotherms differ essentially in their digestive
and assimilative processes from homoiotherms. They
are susceptible to injury by lack of vitamins, monotonous
diets, and other dietary deficiencies in the same way that
homoiotherms are (Pearse, Lepkovsky, and Hintze,
1926). They are affected in much the same way by thy-
roid extracts and those of other ductless glands (Helff,
1923).

Starvation. — Among vertebrates there are many
species of fishes, amphibians, reptiles and mammals
which regularly pass long periods without food during
aestivation or hibernation, and there are many which
can endure long fasts. Both these types of quiescence
are associated with changes in metabolic rate. An eel
has been kept without food for 657 days. During this .
time the body cells decreased 10 per cent in size, but their
number remained approximately unchanged (D'Ancona,
1928). Two species of centrarchid fishes were kept in
filtered water at about 20 deg. C. for two months without
food (Pearse, 1925). Turtles (CJirysemys) lived for
nearly a year under the same conditions (Pearse, Lep-
kovsky, and Hintze, 1926). Some birds when starved,
lived for a month. Fasting dogs have lived for 96 to 117
days, and men have survived for over seventy days.
Unfortunately, no careful records have been kept of the
temperatures at which fasting poikilotherms have been
kept and hence no direct comparison with homoiotherms
is possible. The former probably can usually endure
longer periods without food because their metabolic rate
is lower at low temperatures.

Fasting animals, of course, lose weight. In lobsters
the organic body constituents fell to one-half their origi-

46 HOMOIOTHERMISM

nal amount after seven months without food. At first
protein, carbohydrate, and fat were oxidized equally, but
after a time the carbohydrate was exhausted and fat,
though present in small quantity, was not used up
(Moore and Herdmann, 1914). Starved centrachid fishes
lost about one-third of their body weight in two months,
but their fat did not fall below .43 per cent of their net
weight at the end of the experiment (Pearse, 1925). A
starved trout lost one-fifth of its body weight in four
weeks at the expense of its protein content. Frogs dur-
ing starvation did not decrease in weight because of the
accumulation of water in their lymph spaces. Most of
their organs decreased in weight, but the integument,
brain, spinal cord, eyeballs, spleen, kidneys, and testes
did not (Ott, 1924). Hibernating woodchucks lost about
3.25 grams per kilo during 110 days (Easmussen, 1917).
In a starving mammal glycogen spares protein if it is
available. The nitrogen output decreases during a pro-
longed fast in a fish or a mammal. This indicates a
decrease in metabolism. There is no doubt that such a
decrease takes place, for not only does nitrogen excre-
tion decrease, but the oxygen intake and carbon dioxide
output is also reduced.

Glands, Enzymes, and Hormones. — The general
scheme for the maintenance and control of metabolism
appears to be quite similar in homoiotherms and poikilo-
therms. A fish (Knauthe, 1898) or a turtle (Pearse,
Lepkovsky, and Hintze, 1925) requires a certain balance
in its food ration and this is apparently quite similar to
that which is essential for the proper nutrition of a bird
or a mammal. Frogs when fed on white bread develop
beri-beri and recover from the disease when supplied
with a variety of fruits and insects (Hoffman, 1926).
The digestive enzymes of fishes, salamanders, frogs, tur-

FACTORS INFLUENCING RATE OF METABOLISM 47

ties, and snakes are apparently as effective for digestion
as those of homoiotherms and show their greatest degree
of activity at about the same temperature, 37 deg. C.
(Miiller, 1922; Kenyon, 1925). The action of such sub-
stances as thyroidin, epinephrin, and insulin is appar-
ently quite similar throughout the series of chordate
animals. There are here and there exceptions to these
general statements concerning dietary requirements,
enzymes, and hormones, but they do not on the whole
seem to point to any essential diiference between poikilo-
therms and homoiotherms. For example, vitamin defi-
ciency will not cause scurvy in rats, but pigeons and rab-
bits are quite subject to such disease. Magath and Mann
(1923) believe that the metabolism of carbohydrates is
perhaps different in homoiotherms and poikilotherms
because they found that after complete excision of the
liver the giving of glucose would restore a dog or a
goose somewhat, but had no effect on a gar, frog, or
turtle. In any vertebrate the mass of protoplasmic sub-
stance determines the height of metabolism and the rate
and amount of heat production. The rate of metabolism
in poikilotherms is in general correlated with the tem-
perature at which it takes place, but in homoiotherms it
usually takes place at a rather constant and compara-
tively high temperature. Enzymes and hormones may
modify the rate, but they are not the fundamental cause
of it.

Desiccation. — ^Lack of available water may retard the
rate of metabolism. Arthropods and vertebrates com-
monly prepare for hibernation or aestivation by losing
water. A dry rotifer may live in a dormant state for
months and, when wet again, complete its life cycle of a
few weeks. Desiccation probably causes poor elimina-
tion of wastes and inadequate distribution of food and

48 HOMOIOTHERMISM

oxygen to cells. Lack of water also slows down oxidative
processes. When about half of an animal's vital limit
is exceeded, there is a relative increase in carbon dioxide
production (Caldwell, 1925). In land animals which
have well developed mechanisms for temperature regula-
tion, climate influences the type of regulation which is
used and this indirectly influences metabolism.

Nervous Control of Metabolism. — In all vertebrates
the nervous system exerts more or less influence on
metabolic processes, and on body temperatures. The
ability which nervous domination gives to control tem-
perature is, of course, most important for land animals,
which live in a thermally variable environment, but it is
present also in vertebrates which never leave the water.
The diminution of the oxygen supply has little effect on
the rate and strength of gill movements of a Necturus.
They are controlled reflexly from the brain, and condi-
tions in the brain centers are the effective agents (Stew-
art, 1923). There are also **heat centers" in a turtle's
brain. "The respiratory rhythm is more powerfully
affected by variations in the temperature of the cells
composing the respiratory centers than by variations in
the activity of the general metabolic processes or in the
gases inspired." When temperature of the environment
is constant, the amount of carbon dioxide in the blood
appears to be the chief factor in regulation (Lumsden,
1924). In homoiotherms, various factors maintain a
constant body temperature — insulation, surface cooling
through vaso-dilation and evaporation, increased oxida-
tion, etc. The nervous system plays an increasingly im-
portant role in passing from more primitive to more
specialized vertebrates and brain centers take on more
complicated relations to temperature regulation. In a
mammal the result of the removal of cerebral lobes is a

FACTORS INFLUENCING RATE OF METABOLISM 49

fall in blood pressure; if the thalamus is also removed
there is a further loss of ability to regulate body tem-
perature (Rogers, 1920). The nervous control of me-
tabolism and body temperature may be exercised in indi-
rect ways — such as the stimulation of a gland which
gives off a secretion which in turn increases oxidation
or changes blood pressure.

Rhythms. — ^All living things show more or less rhyth-
mical changes in metabolic rate. Such rhythm is one of
the fundamental properties of living substance: the vis-
cosity of protoplasm hinders discharge of energy ; proto-
plasm is unstable and trigger action discharges energy
during katabolism ; endothermic anabolic stages are con-
cerned with reconstruction; katabolism and anabolism
must alternate more or less. Periodicity is in part a
consequence of the colloidal state (Sager, 1923). Ani-
mals are generally adjusted to rhythms. These are
characterized by the alternation of periods of rest and
activity. They are no more characteristic of poikilo-
therms than of homoiotherms, except that metabolism in
the former is more directly under the control of the en-
vironment.

Conclusions. — ^Poikilotherms do not appear to differ
essentially from homoiotherms in the methods employed
to carry on metabolic processes. The former show
greater variations in metabolic rate because they are
more influenced by variations in environment. The lat-
ter are more independent of environment because they
have controlling mechanisms which permit them to live
at a uniform rate if sufficient food is available to supply
energy.

CHAPTER Vn
FACTORS INFLUENCING BODY TEMPERATURE

The body temperature of an animal may be influenced
by a number of factors. Those which are more or less
directly related to metabolism have been considered in
Chapter VI and will therefore be only briefly discussed
again here. Metabolism is, of course, the chief source
of the heat which keeps the bodies of animals more or
less warm; but other influences such as food, insulation,
temperature of environment, adaptation to environ-
mental rhythms, and glandular or nervous control may
also be important.

Metabolism. — ^In a general way the rate of metabolism
of poikilotherms varies directly with the temperature of
the environment (Krogh, 1914; Powers, 1923; Hall, 1925)
and at suboptimum temperatures the opposite is to some
degree true of homoiotherms (Pike, 1923). However,
poikilotherms have a slight tendency to adjust their rates
of metabolism to temperature changes in the environ-
ment. A planarian, if changed from an environment of
20 to one of 10 deg. C, at first shows a reduced rate of
metabolism, but after two or three weeks returns to about
its former rate at 20 deg. C. (Behre, 1918). A turtle
which is cooled to 4 to 5 deg. C. shows a check in the
gradual decrease in metabolic rate (Baldwin, 1925).
Both the planarian and the turtle have a slight tendency
to compensate for environmental changes in temperature,

50

FACTORS INFLUENCING BODY TEMPERATUKE 51

but this is too slow and too limited to permit either to
maintain a constant temperature as a homoiotherm does.
The ability of fishes to adjust themselves to temperature
changes depends on the type of environment in which
they have previously lived (Hathaway, 1927, 1928).
Overheating of an animaPs body may result in acidosis
and death (Barbour, 1921). An animal may exercise to
produce heat if its body becomes too cool. Bees flap their
wings when their hive becomes too cool ; a mammal shiv-
ers when chilly. In mammals the chemical production
of heat becomes important only when the temperature
of the environment is as low as 14 to 15 deg. C. (Barbour,
1921; Britton, 1922).

Glandular secretions are potent factors in regulating
metabolism and body temperature in vertebrates. In
pigeons the thyroid grows larger in autumn, when there
is an increase in heat production (Riddle, 1925; Riddle
and Fisher, 1925). Less thyroid secretion is apparently
needed by tropical animals than by those in cooler
regions (Sundstroem, 1922). Glandular secretions, such
as epinephrin and thyroidin, may affect body tempera-
tures by changing the water balance in an animal's body
(Barbour, 1921). The blood of an animal in an ice bath
becomes concentrated (Barbour and Tolstoi, 1921), and
is more dilute when the temperature is high (Barbour,
1921). In the tropics mammals have relatively fewer
white and more red blood corpuscles (Sunstroem, 1922).
Greater concentration of the blood at lower temperatures
makes animals better able to endure suboptimum tem-
peratures and permits them to remain active. Among
aquatic poikilotherms the character of the water sur-
rounding the body may influence the rate of metabolism,
and heat production. For example, a planarian shows
a considerable increase in respiratory activity in alkaline

52 HOMOIOTHERMISM

water (Anderson, 1928). The amount of heat given off
by an animal varies according to the relation between
mass and surface area. On account of this, small homoio-
therms require more energy to maintain body tempera-
ture than larger. This is not the case among poikilo-
therms because they expend comparatively little energy
to maintain body heat.

Food. — Foods vary in their caloric value and carbo-
hydrates are the best sources of heat. More sugar is
needed by animals at lower temperatures (Mansfield and
Pap, 1920). It is present in greater quantity in the blood
and, of course, furnishes more heat. Associated with
this increased consumption there is an increased use of
oxidase. Fat and protein may be used for heat produc-
tion in an animal's body but both are less productive
of calories than carbohydrates, and protein is a rather
poor source of heat because of the difficulty with which
it is broken up and oxidized.

A developing hen's egg uses foods in the following
order: carbohydrates, proteins, fats. Needham (1926)
discovered this fact and believes that it has phylogenitic
significance as an indication of the order in which foods
have been utilized by vertebrates in the past. However,
Gray (1926) finds that a fish egg derives its energy
throughout development largely from protein. He also
calls attention to the fact that a land animal must have
a considerable supply of water provided for the develop-
ing egg. This need for water may be related to the early
utilization of carbohydrate in the hen's egg.

Enviromnent. — The temperature of the environment
influences the temperature of animals living in it. A
poikilotherm tends to have a body temperature which is
the same as, or close to that of, the surroundings. At
very low or very high temperatures however, the tem-

FACTORS INFLUENCING BODY TEMPERATURE 53

perature of the body may differ from the surrounding
medium. For example, a dormant insect shows a tem-
perature which is practically the same as that of the air
about it, but an active insect lags behind as the air
temperature rises. When the air is very damp, body
temperature may be higher than that of air (Bachmet-
jew, 1901). Desert insects may show a body temperature
as much as 15 deg. C. below that of their surroundings.
Many animals in hot situations hide during the day or
during the dry, warm season and become active when the
direct rays of the sun are not present. In cool situa-
tions the opposite is often true. Some animals show
daily or seasonal rhythms in heat production and body
temperature. Even such a specialized homoiotherm as
a bank swallow may show considerable daily fluctuation
in body temperature (Stoner, 1926). Many homoio-
therms pass through long periods of rest, during which
they lose their powers of maintaining constant body tem-
perature.

Insulation. — Poikilotherms during cold periods are
obliged to resort to situations where they will not freeze.
They commonly burrow into the soil or into mud where
the decay of organic materials prevents freezing; they
hide away in springs, caves, under leaves, and in logs
and hollow trees. They seek insulation or a warm situa-
tion in their environment. The skins of reptiles protect
them well from loss of water, but not so well from loss
of heat. Insulation for conserving heat is generally not
present on the exteriors of animals, except among
homoiothermic vertebrates. Hair, feathers, and fat are
the materials employed and they show more or less adap-
tation for particular situations and localities. Animals in
the tropics are generally thin and those in frigid regions
commonly have a thick layer of subcutaneous adipose

54 HOMOIOTHERMISM

tissue. The thickest furry and feathery coats are found
in the arctic and antarctic. Well-insulated homoiotherms
are often obliged to exercise some care to prevent their
bodies from becoming overheated and many have de-
veloped special cooling mechanisms, which usually pro-
duce their effects by evaporating water and permitting
the ready loss of heat from the skin, lungs, or alimentary
tract. Most of the large animals in the tropics avoid
the direct rays of the sun.

Nervous Control.— The regulation of body tempera-
ture in man is largely under the control of the nervous
system, which contracts or expands appropriate blood
vessels so that liquid is sent to the skin or to the internal
organs, causes loss of liquid from the blood to the tissues,
stimulates glands to secrete water or substances which
facilitate the production of heat through chemical action,
and acts in other ways. A normal dog in an ice bath
shows a concentration of the blood, but a dog with a
transsected spinal cord does not. In mammals and birds
generally, temperature regulation is dominated by the
nervous system. In other vertebrates the nervous sys-
tem also exercises some control, but this control is not
complete enough to result in the maintenance of a con-
stant body temperature. In invertebrates the same type
of regulation appears to be present, but it takes the form
of slow acclimating changes which require days or weeks
for their completion (Behre, 1918; Hathaway, 1927).
Cooling mechanisms for the control of body temperatures
are present in insects, and probably in other terrestrial
arthropods, in birds, and in mammals. In homoiotherms
such mechanisms are concerned chiefly with peripheral
vasodilation and the evaporation of water from the skin,
lungs, or oral cavity. They are largely under the con-
trol of the nervous system.

FACTORS INFLUENCING BODY TEMPERATURE 55

Conclusions. — The loodj temperature of animals is
influenced by variations in metabolism, food, the transfer
of liquids, evaporation, insulation, glandular secretions,
the temperature of environment, and surface mass rela-
tions. The regulation of body temperature in homoio-
therms appears to differ from that in poikilotherms in
degree and somewhat in kind. In the former it is made
more rapid and effective by insulation, nervous control,
and better mechanisms for the chemical production of
heat and for cooling.

CHAPTEK VIII

THE MECHANISMS OF TEMPERATURE REGULATION IN
HOMOIOTHERMS

The preservation of life in a homoiothermal animal
requires the maintenance of a constant temperature,
though the external and internal conditions may vary-
greatly and may tend to warm or cool the body. A
variation of a few degrees in the internal temperature
of a higher mammal may result in death. Normally,
little variation occurs. Such constancy results from the
adjustment between characteristic temperature regulat-
ing mechanisms which are present in the homoiotherms :
heat is produced continuously and rapidly and is dissi-
pated at about the same rate. The living organism must
adjust heat production and heat loss in such fashion that
the temperature of the body cells is maintained near their
optimum. Such regulation is termed thermotaxis, or the
balance maintained between the two mechanisms:

1. Kegulation of the loss of heat. This is termed
physical heat regulation, or thermolysis.

2. Eegulation of the production of heat. This is the
chemical heat regulation, or ther mo genesis,

Thermogenesis. — The metabolism of the poikilo-
therms is influenced by temperature in such a way that
lower temperatures result in low rates of metabolism and
high temperatures in high rates. In general the Van't
Hoff law for chemical reactions is followed. This is

56

TEMPERATURE REGULATION IN HOMOIOTHERMS 57

evidently not true in the case of the homoiotherms.
Many experiments show that a fall in the temperature
of the environment of a warm-blooded animal induces
a higher rate of metabolism and with a rise in tempera-
ture the reverse seems to be true until the environmental
temperature reaches about 35 deg. C, but above this
temperature there is again an increase.

Thermogenesis operates in the body of a homoio-
therm to compensate for heat lost by radiation and con-
duction. Since the temperature of a homoiotherm is
usually above that of its environment, it will constantly
lose heat. The amount of heat lost will be determined
by the factors influencing radiation and conduction.
These have been described in Chapter IV. Thermoge-
nesis is thus in a sense a curative mechanism, since it
furnishes the means whereby heat is produced in the
body to compensate for that lost.

The energy which is required to maintain life proc-
esses at any given temperature and the additional heat
which is necessary with a fall in the temperature of the
surrounding medium are directly derived from metabolic
processes. Heat is liberated through oxidation, either
by voluntary muscular exercise, reflex muscular con-
tractions such as shivering, or by the transformation of
food. Oxidations take place in the tissues and the seat
of heat production is therefore in them. The greater
part of the body heat of a mammal is produced in the
muscles. Seventy to 80 per cent of the energy liberated
during a muscle contraction is in the form of heat. About
one-half the active tissues of ^n average vertebrate are
muscular and, as the muscles are by far the most active
of the tissues, a great amount of heat must be furnished
by them. Some of the glands, especially the liver, are
also important sources of heat.

58 HOMOIOTHERMISM

Supplemental to the production of heat by oxidation
during periods of falling temperatures, either in the
environment or in the blood itself, are certain physiolog-
ical adjustments on the part of the organism, some re-
tard heat loss and others hasten the dissipation of heat.
Among these are: (1) cutaneous vaso-constriction, which
decreases loss of heat through the skin; (2) reduction
in blood volume, which apparently permits the constric-
tion of peripheral blood vessels without excessive en-
gorgement of those on the interior; (3) behavior which
results in the aggregation of groups of individuals or
** huddling'' reflexes which in cold surroundings reduce
the area of exposed body surface, and thus check loss
of heat by radiation and conduction.

Thermolysis. — The mechanisms for regulation of heat
dissipation are mainly employed when there is danger
of overheating an animal's body. The dangers from
supraoptimum temperatures usually arise from within
the body rather than from the environment. A rise in
temperature either in the blood or skin results in the
heat center in the central nervous system stimulating
an increased heat dissipation which may be brought
about by the following physiological means: (1) hyper-
pnea, or panting, (2) cutaneous vaso-dilatation, (3)
evaporation of moisture from exposed moist surfaces,
(4) decreased muscular exertion, (5) maximum exposure
of body surfaces, as in outstretched animals during hot
weather.

Physical thermolysis is accomplished by radiation,
conduction, and water loss. It has been estimated that
at ordinary room temperatures about 75 per cent of the
heat produced by the human body is lost by radiation
and conduction and 25 per cent through the evaporation
of water from the surfaces of lung and skin. At higher

TEMPERATURE REGULATION IN HOMOIOTHERMS 59

surrounding temperatures this proportion changes, rela-
tively more heat escaping by evaporation and less by
radiation and conduction.

High temperatures usually cause animals to breathe
more deeply. Animals, such as the fur bearing mammals
which lack sweat glands, show the most marked hyper-
pnea. When a dog extends its tongue and pants, the
loss of water from its body is greatly increased. Ani-
mals subjected to sources of radiation from which they
absorb heat often show pronounced hyperpnea.

Blood volume is increased when the body tempera-
ture rises and this acts as a means of preventing vaso-
dilatation and also maintaining a sufficient supply of
water for sweat secretion.

When the external temperature is high or when
thermogenesis is great, as in muscular work, the cutane-
ous blood vessels dilate, those within the abdomen con-
strict, and a larger volume of warm blood is thus sent
to the surface to be cooled. The cooled blood is returned
to the interior to take up more heat and is again returned
to the skin. Thus a great flow of blood through the skin
is obtained and this results in a maximum discharge of
heat by radiation and conduction. On a cold day the
reverse takes place, so that the warm blood is restrained
from circulating too freely through the skin where more
heat would be lost. A rabbit depends chiefly on its vaso-
motor activity for temperature regulation, as it does
not possess a satisfactory water reserve. This is likely
true with other herbivores although no evidence seems
to be available. Certain drugs may paralyze the vaso-
motor mechanism. For example, an animal under the in-
fluence of alcohol feels warmer and loses more heat than
a normal individual.

Thermolysis is voluntary to a certain extent, for by

60 HOMOIOTHERMISM

artificially covering the body with clothing, by taking
advantage of shelter in homes, by warming the food and
drink taken into the body, and by heating the atmosphere
with which the body comes in contact, man protects him-
self against a too great loss of heat and decreases to a
large extent the necessity of relying upon the involun-
tary, and subconscious, means of temperature regulation.

Evaporation of Moisture. — When the temperature of
the body is like that of the environment of an animal, the
heat lost by radiation and conduction will be exactly bal-
anced by the heat absorbed. As the environment attains
a higher temperature than the body a means of heat
elimination by conduction or radiation become inade-
quate and thermolytic means are employed; i.e., the
evaporation of water from surface of the organism.

The quantity of heat lost by evaporation of moisture
depends upon several factors :

1. Humidity of circumjacent atmosphere.

2. Temperature gradient between body and environment.

3. Velocity of movement of circumjacent atmosphere.

4. Effective area of moist surface exposed.

5. Color of moist surface exposed.

The amount of moisture in the air affects its cooling
powers. If the air is saturated, it can obviously contain
no more moisture and consequently no water is lost by
the organism. Also moist air is a much better conductor
of heat than dry air and heat lost by conduction is at its
maximum in a humid atmosphere. The former factor
plays an important part when the surroundings are
warmer than the animal, the latter acts when the envi-
ronment is colder.

The temperature gradient between the surrounding
medium and the animal limits the amount of heat lost by
evaporation. The greater the gradient, the stronger the

TEMPERATURE REGULATION IN HOMOIOTHERMS 61

stimnlation of the meclianism for sweating. Currents of
air over tlie surface of an animal remove the water
vapor, bring dry air in contact with the skin, and thus
decrease the partial pressure of the gas next to the skin.
In a strong wind the partial pressure is at its minimum
in the layer of air next to the body. The drying effect
of winds is more or less independent of their tempera-
ture but is directly related to their water saturation
deficit.

The effective area of surface exposed to cooling de-
pends in great measure on the state of the superficial
blood vessels. Dilatation greatly increases the effective
surface ; constriction, the converse.

The effect of color is merely a matter of differential
absorption of radiant energy which produces local heat-
ing in proportion to the amount of energy absorbed and
so causes a more or less rapid evaporation. The black
moist nose of a dog is much cooler than a similar dry,
white area.

There are about 2 to 3 million sweat glands in the
entire skin of a man. The number apparently varies
considerably in different species of mammals; the dog,
for example, has none. The volume of secretion among
species which possess sweat glands is also variable and
there is a marked variation among the individuals of a
single species. In man it may vary from a minimum of
700 ml. per twenty-four hours to three times that amount.
Whether volume changes are due to changes in number
of active glands or to variations in the output of individ-
ual glands is not known. Adolph (1923) has shoi\Ti that
when an active body is exposed to high external temper-
atures the quantity of sweat is a linear function of the
'^effective" skin temperature. It appears therefore that
as the limit of heat dissipation by conduction and radia-

62 HOMOIOTHERMISM

tion is approached (at about 32 to 33 deg. C.) vaso-dila-
tation has reached its maximuin e:ffect and beyond that
impulses from the heat center can only accomplish an
increased heat loss from the body by augmenting sweat
secretion. All the skin changes resulting from excita-
tion of the heat center operate harmoniously to favor or
impede heat dissipation under normal conditions.

Water is very well fitted for the requirements of tem-
perature equalization and regulation in animals. Its
mobility permits it to circulate freely and rapidly in the
organism. Henderson (1913) attributes the fitness of
water for temperature equalization and regulation to
three qualities: (1) great specific heat, (2) large amount
of heat required for evaporation, and (3) good thermal
conductivity. The first promotes the storage of heat ; the
second permits very rapid elimination of heat when the
environmental temperature exceeds that of the body;
the third contributes to the rapid equalization of heat
within the immobile tissues of the body and minimizes
the possibility of injury from local overheating. The
cooling power of water is great. One liter of water re-
quires 580 kilogram calories for its vaporization. This
factor makes water of extreme importance in heat loss.

Water plays a significant role in the maintenance of
constant temperature in homoiotherms. It constitutes
about 70 per cent of the substance of an animal's body
and is one of the most abundant and available constitu-
ents of the environment. Aside from its function in the
regulation of temperature, it is fundamentally important
in the other physiological processes of the body. Water,
in consequence of its peculiar properties and abundance
in nature, is important in the regulation of the tempera-
ture on all parts of the earth, in both air and water, as
well as in organisms.

TEMPERATURE REGULATION IN HOMOIOTHERMS 63

The Thermal Nerve Center. — Since many means are
brought to play in organisms for the maintenance of a
constant body temperature and especially since a variety
of nerves — ^vasomotor, muscular, secretory — are in-
volved, it is apparent that a coordinating center is re-
quired. Experimental evidence obtained by Barbour
(1921) and others indicates that such a center exists in
the corpus striatum. The gain or loss of heat by an
organism depends upon many interwoven factors such
as blood-content, colloids, water-content, various ions
and metabolites, and other physiological factors. These
probably maintain a fair degree of constancy in the poi-
kilotherms. But temperature regulation must depend
upon something more. It occurs in those species which
have a highly developed nervous system and only when
in those particular species the nervous system has
reached a certain stage of development in each individ-
ual. Therefore one concludes that in the evolution of
the temperature regulation a new function has been
added and that the nervous system has gained control
of those physiological processes which affect the gain
and loss of heat in the organism. Barbour (1921) clearly
demonstrated that the application of heat or cold to the
anterior end of the corpus striatum by means of a small
metal tube through which water of various temperatures
was circulated, caused marked changes in the body tem-
perature of the animal. "When cold objects were applied
to the nerve center, the rectal temperature rose and
shivering and vaso-constriction of the skin were also
observed. If objects having a temperature above that
of the body were applied, a fall in rectal temperature
and muscular relaxation and dilatation of the skin blood
vessels occurred. These results have been confirmed by
several later investigators. Barbour (1921) states that

64 HOMOIOTHERMISM

the left "heat center" is more strongly developed than
the right in the rabbit. Barbour and Prince (1914)
found that local applications of heat to the heat center
caused diminution in the production of carbon dioxide
and in the consumption of oxygen. Cooling the center
reversed these effects.

Effects of Decerebration and Spinal Cord Sections on
Thermoregulation. — The importance of the brain on heat
regulation has been demonstrated by the removal of the
cerebrum. Eogers (1919) has shown that the ability to
regulate body temperature is lost in decerebrate pigeons
when the optic thalamus has been injured. Freund and
Strosmann (Barbour, 1921) sectioned the cord at
various levels and found that, while normal rab-
bits withstood environmental temperatures of 6 to 31
deg. C. with no appreciable change in body temperature,
those with the cord sectioned were poorly regulated.
Eabbits which had the dorsal cord sectioned, could regu-
late their temperature fairly well between 18 and 31 deg.
C, but were ineffective beyond those limits. In rabbits
with the cervical cord sectioned a poikilothermal condi-
tion obtained in environments varying from 19 to 33
deg. C. Freund (Barbour, 1921) has found more recently
that if in addition to the dorsal cord operation a double
vagotomy just below the diaphragm is performed an
animal becomes poikilothermic to essentially the same
extent as those with the cervical cord transsected. He
concludes that this is likely due to the severance of the
sympathetic fibers.

Garrelon and Langlos (Barbour, 1921) suggested a
"polypnea center." In corroboration of that idea Nico-
laides and Dontas (Barbour, 1921) have found that heat
polypnea cannot occur if the medulla is separated from

TEMPERATURE REGULATION IN HOMOIOTHERMS 65

the brain, altliongli the ordinary respiratory movements
are not disturbed.

It would seem, therefore, that, although there may
be some grounds for believing that the nervous control
of heat regulation is widely distributed, the evidence
points to the conclusion that the temperature regulation
of the mammals is chiefly coordinated at the base of the
brain in the corpus striatum.

The Temperature Sense. — The sensations of heat and
cold which an animal experiences from time to time are
caused by changes in the temperature of the skin. In-
numerable nerve endings are distributed over the skin
and these, when stimulated, evoke sensations of heat
or cold. Each responds to but one type of stimulus, i.e.,
a warm object applied to a heat nerve ending elicits a
sensation of warmth. It has little or no effect upon a
cold spot. The reverse is also true. The sensitivity of
the human skin to temperature changes is very acute.

It is an interesting correlation that the main channel
of heat elimination is through the skin, that the skin is
the only part of the body directly exposed to the environ-
ment, and that the sldn has widely distributed through
its outer layer the temperature sense organs. The skin
is the first part of an animaPs body to respond to tem-
perature variation in the environment. The physiologic
stimulus to the thermic end-organs is the passage of heat
through the skin from the interior to the surrounding
medium. Impulses are sent to the heat center. Thus the
heat center anticipates changes in actual blood tempera-
ture by compensatory reactions to changes in the skin
temperature. Consequently if radiation and conduction
are continuous and uniform, the end organs adapt them-
selves to the temperature of the surrounding medium.

66 HOMOIOTHERMISM

If there is a sudden rise in the external temperature
caused by some factor which diminishes the loss of heat,
the temperature of the skin will rise, the end organs will
be stimulated, and the sensation of warmth is developed.
Conversely a fall in temperature stimulates the recep-
tors for cold. The afferent impulses in either case stimu-
late the heat center to the proper activity resulting in a
regulation through heat dissipation or conservation.
Since changes in skin temperature would eventually in-
fluence the general body temperature if not compensated
for, the temperature sense organ undoubtedly serves a
useful purpose in temperature regulation.

Fever. — One of the most striking aspects of many
pathological disturbances is a rise in body temperature,
a condition known as fever. This increase in heat may
be the consequence of a greater production than can be
lost through the normal channels, or of a lack of regula-
tion of the loss of heat. Most investigators hold that,
although there may be a slight increase in the production
of heat in fevers, this is too small to account for a rise
of several degrees in the body temperature ; in fact, many
physiologists now believe that the rise in basal metabo-
lism during fever is to be regarded as a result of the
rise in temperature, rather than as a cause.

Fevers are apparently secondary factors in infec-
tions. Toxins act to increase the affinity of protoplasmic
colloids for water with the result that the blood volume
is reduced. This effect is assumed to be localized mainly
at the body surface and leads to a cooling of the periph-
ery and the sending of afferent impulses to the heat
center. Thus the thermolytic mechanism is brought into
play, without regard to the actual heat equilibrium.

The Endocrines and Temperature Regulation. — The
endocrines unquestionably have an influence on the tem-

TEiMPERATURE REGULATION IN HOMOIOTHERMS 67

perature regulation of homoiotherms. The thyroid has
marked effects upon the oxidative processes and also
influences heat production. Mansfield and Pap (1920)
emphasized the relationship between heat regulation and
the control of the thyroids over oxidation. It has been
observed also from clinical practice that thyroidectom-
ized animals regulate their temperatures with difficulty.
The more pronounced disturbances of heat regulation
seem to occur most commonly and most strikingly in con-
nection with diseases of the thyroid gland. In myxedema
the body temperature often drops far below normal. In
exophthalmic goiter, hyperthermia is commonly reported.
Structural variations in the thyroid which vary accord-
ing to environmental temperatures are described by Bar-
bour (1921).

The influence of insulin upon body temperature has
been observed by Cassidy, Dworkin, and Finney (1926).
These investigators studied the relation of sugar me-
tabolism to hibernation in mammals and found that if
insulin was injected and the blood sugar thus decreased,
the shivering reflexes were absent when the animal was
subjected to cold.

Erection of hairs, ruffling of feathers, constriction of
peripheral blood vessels, and increase of blood sugar
have been mentioned as responses of homoiotherms to
cold temperatures. The investigations of Cannon,
Querido, Britton, and Bright (1927) indicate that adrena-
lin brings about such responses and furnishes a means
for protecting the organism against cooling. They found
that by producing a heat debt in animals an increased
activity of the medullary portion of the adrenal gland
was induced and that this led to an extra output of
adrenin, which in turn hastened combustion. They also
believe that heat is produced in the body without shiver-

68 HOMOIOTHERMISM

ing as a result of the direct increase of metabolism by
adrenin and that this compensates for heat loss.

Some observations reported by early workers indi-
cate that the pituitary decreases in size during hiberna-
tion but enlarges again and becomes more vascular when
an animal awakens. Easmussen (1921) however believes
that changes in the pituitary are not a cause of hiberna-
tion. He bases his conclusion on experiments with a
large number of woodchucks. The pituitary hormone
exerts a marked influence on the water balance of
the body (Eowntree, 1922) and therefore would aifect
the regulation of bodily temperature. Falta (1923)
states that hypophysial dystrophy is always accompanied
by abnormally low body temperatures. The hypophysis
apparently promotes temperature regulation on account
of its influence on the nervous system.

One may conclude from the observations made in the
study of endocrinology that hormones doubtless play an
important role in the control and maintenance of the body
temperature. Since little is known of the reciprocal ac-
tion of hormones, the exact part they play is still some-
what hypothetical. Future investigation will likely add
much to knowledge of their mode of action and this is a
promising field for study.

CHAPTER IX
GROWTH AND LONGEVITY

The growth of any organism is, of course, the result
of many interdigitating factors. An animal to increase
in size must have food to supply building material. It
must have a body temperature which permits anabolic
reactions to proceed at such a rate that they exceed the
destructive changes which are essential for living from
day to day. It must be free from parasites or diseases
which sap its vitality. Growth also depends on a con-
tinual balance between an animal and its environment.
Food must not only contain material but must furnish
a proper balance of calories, amino acids, and vitamins.
Environmental temperature should not only fall within
the limits where metabolism and growth may occur, but,
to permit an animal to really flourish, should vary so
that growth receives healthful stimulation. Growth is
a complex phenomenon which through metabolic wasting
and waxing results in a net increment in size.

Rate of Growth. — The rate of growth of animals is
usually progressively slower with increasing age (Brody,
1927). There may be seasonal fluctuations or variations
associated with progression through the life cycle, but
in general the metabolic rate gradually runs down. Some
animals grow rather slowly and at a rather constant rate,
and others increase largely during intermittent periods
of building.

Rubner (1924) in his admirable studies on metabolism

69

70

HOMOIOTHERMISM

and growth has computed the times required for various
vertebrates to double their weights during early growth
and gives the figures shown in Table II. The gestation
period of the dog and cat are 56 and 63 days; the com-
parable development of the pike requires 574 days. The
rate of growth of a dog is comparable to that of a pike
after it is dependent on its own resources for the manu-
facture of food; i.e., after it has ceased to depend on
food supplied by its mother. At 16 deg. C. a mammal
requires 800 calories per kilogram of body weight per
day; a fish at the same temperature needs only 30.8 kg.-
cal. per day. A pike is slower but for the same amount
of growth requires about the same amount of energy. A
fish weighing 1.75 grams needs 39 calories in 24 hours ;
a mouse of the same size needs 997 calories in 24 hours.
The mouse consumes 25 times as much as the fish.

TABLE II

Time Required for Animals to Double their Weights,
AS Determined by Rubner (1924)

Cat

Dog

Pike

Doubling

Weight,
Grams

Time,
Days

Energy
Needed

Weight,
Grams

Time,
Days

Energy
Needed

Weight,
Grams

Time,
Days

Energy
Needed

Birth weight
1
2
3
4
5

87

174

348

696

1392

0

7.5

12.0

22.0

53.0

0
3235

3975
5295
9587

225

450

900

1800

3600

0
10.0
12.5
22.5
40.0

0
2983
3068
4010
6180

70

140

280

560

1120

2240

0
274
303
336
613
1179

0
3267
2350
2773
3598
5162

Kubner (1924a) has made further studies which re-
late especially to amphibians and reptiles and makes a
general comparison between homoiotherms and poikilo-
therms. Growth begins with a considerable amount of
water in body cells and a variable colloid content. As
growth progresses, water gradually decreases. Hor-

GROWTH AND LONGEVITY 71

mones and endocrine secretions play an important role,
especially in liomoiotherms. Larger animals in general
require less energy than smaller, on account of surface-
mass relations. Fishes have little physical regulation of
body temperature because they live in water. Amphib-
ians commonly regulate their body temperature through
their lungs and skin. The growth of homoiotherms and
poikilotherms is similar. Energy requirements depend
on size and rate of growth, and rate of growth in many
cases depends on the temperatures at which metabolic
reactions take place.

Mammals may differ among themselves in rate of
growth. Brody (1928) has recently made careful studies
of several domestic animals and compares them with
man. ' * The length of the juvenile period in man is about
10 years (4 to 14 years). This relatively enormous
length of the juvenile period appears to be the most dis-
tinguished feature of the growth curve of man. ... In
the curve of man, the major inflection (at puberty) oc-
curs when the body weight is, roughly, two-thirds of the
mature weight ; in other animals it occurs when the body
weight is, roughly, one-third of the mature weight. . . .
There are no radical quantitative or qualitative differ-
ences between the growth curves of man and other ani-
mals during the phase of growth following puberty. . . .
If the percentage rate of growth for a given group of
children is relatively low during earlier ages, then there
is usually an acceleration between 12 and 15 years; if
it is high, there is no acceleration. The pubertal accel-
eration, when present, appears to be in the nature of a
compensatory growth for earlier deficiency. ... It
should be said that growth in length takes place at an
approximately constant time rate when growth in weight
takes place at a constant percentage rate. This is evi-

72 HOMOIOTHERMISM

dent from geometrical considerations when growth in
length is strictly terminal. It is also evident from physi-
ological considerations; constant percentage rates of
growth in volume and constant time rate of terminal
growth both imply that the physiological environment
with respect to the growth-limiting process remains con-
stant." MacDowell, Allen, and MacDowell (1927) have
compared the growth rate of the monse, guinea pig, and
chick. They conclude that ** growth of different animals
may be compared more accurately if, instead of either
birth age or conception age, embryo age is used."

It is difficult to compare the growth of homoiotherms
and poikilotherms and often difficult to compare that of
animals in the same class or order. A guinea pig or a
snake is able to shift for itself soon after birth ; a robin
or a child has a long youthful period during which it is
nourished and protected by its parents.

As has been stated, some types of animals grow at a
rather constant rate ; others grow in spurts ; and others
grow rapidly during youth and then more slowly. A
painted turtle {Chrysemys belli) doubles its weight and
length during its second year of life, but after twelve
years increases at one-thirtieth the rate for the first two
years (Pearse, 1923; Pearse, Lepkovsky, and Hintze,
1925a). The sea turtle, Caretta caretta, at hatching
weighs 20 grams; at 43^4 months, 800 grams; and at 3
years, 19 kilograms (Parker, 1926). The Pacific striped
bass grows 150 per cent in length during its second year,
but during its twelfth year of life, only two per cent
(Scofield, 1928). A carp may weigh 500 grams when it
is two years of age and then gain 750 grams between
the beginning of May and the middle of August, when it
may attain a weight of 1250 grams (Piitter, 1909). How-
ever, a fish living in natural conditions often does not

GROWTH AND LONGEVITY 73

grow to the full extent of its capabilities. A particular
species which occurs in two lakes only a few miles apart
may be uniformly of small size in one lake and large in
the other (Pearse, 1918; Pearse and Achtenberg, 1920).
In nature, animals in particular habitats may never be
able to exercise their full capacity to grow. Peterson
(1918) found that in one locality off the coast of Den-
mark the plaice did not grow for two-thirds of a year,
but if individuals were transferred to another locality
they grew at normal rates. Sundstroem (1922) kept
mice in an ^* artificial tropical" environment where there
was little circulation of air and where other conditions
were monotonous. He found that they did not grow as
well as mice reared where conditions were variable.

Methods of Growth. — While in general percentage
additions become less and less as an animal grows, there
are usually more or less intermittent periods of rapid
and slow growth which are characteristic of species and
of individuals. Young tissues grow fastest and perhaps
old animals grow more slowly because they come to con-
tain a considerable bulk of inert, differentiated, and com-
paratively stable tissues. *' Accompanying differentia-
tion there is, therefore, a decrease in the activities of
metabolism'' (Beer, 1924). An animal may, however,
show an increased rate of metabolism during degrowth,
or decrease in size.

Kobertson (1923) in his comprehensive work on
growth observes that most animals increase in size by
spurts with periods of slower growth between. He be-
lieves this is due to an autocatalyzed process. He found
that there were usually three growth cycles. Eobertson
believes that the rate of growth reactions depends on
(1) the specific reaction velocity of the process, (2) the
concentration of autocatylist, (3) the concentration of

74 HOMOIOTHERMISM

raw food materials, and (4) the rate of nuclear synthesis.
He characterizes the process which takes place at the
slowest rate — and therefore limits all others — as *Hhe
master reaction." From his point of view, growth may-
be slowed up by the exhaustion of antocatylist or food,
or by the accumulation of products.

Brody (1927) has made extensive studies of growth.
** Growth curves consist, in all cases, of two major seg-
ments. The first major segment is, in the case of higher
animals and plants, made up in turn of several (probably
five) shorter segments during each of which growth takes
place at a constant percentage rate. The transitions be-
tween the successive stages are abrupt, the abruptness
being of the same order as that of metamorphosis in cold-
blooded animals. . . . The junction between the two
major segments occurs at puberty in animals and flower-
ing in plants. . . . The two major segments of the in-
flection are not symmetrical about the major inflection.
The slope of the segment following the inflection is al-
ways less than the slope of the segment preceding the
inflection. The major inflection does not occur in the
center of the growth curve.'' At the beginning, growth
may be 100 to 200 per cent per day. The body weight
may be doubled in from 7 to 17 hours. Two months after
conception the increase in man is about 8 per cent per
day. **Dr. Stockard called attention to the fact that the
peak in the mortality curve of the chick at 5 days is the
counterpart of the peak in the prenatal mortality curve
in man at 3 months. This is the junction between the
embryonic period (formation of organs) and the fetal
period (enlargement of body and organs)." The nature
of the growth in the two periods is quite different, hence
it is not strange to find a high mortality at the time of
transition from one to the other.

GROWTH AND LONGEVITY 75

Growth and Temperature of Environment.— The

growth of poikilo therms and of young homoio therms is
limited by the temperature of the environment. A young
bird or mammal must be kept warm in a nest, marsupial
pouch, or uterus if development is to follow a normal
course. Yet such development does not necessarily need
to take place at temperatures Avhich are near those of
the bodies of the parents. An egg may mature in the
body of a hibernating frog and an embryo may continue
to develop in a hibernating mammal. After leaving the
body of the parent a frog's egg may rest in water be-
neath ice and develop normally as the temperature rises.
The eggs of various reptiles are commonly buried where
they are heated by the rays of the sun, as on warm hill-
sides, or where they may be heated by decaying organic
matter, as in rotting logs. A hen's egg may remain cool
for some time before development begins. It can later
endure for short periods temperatures which are below
the body temperature of the hen and still develop into a
normal chick.

Homoiotherms in gaining the ability to maintain a
constant temperature through metabolic activity and
other means have also become able to provide a favor-
able temperature for the development of their offspring.
The eggs of frogs developing at 8 deg. C. require 30 days
to reach the same stage which they will reach in 8 days
at 21 deg. C. (Barthelemy and Bonnet, 1926).

Determinate Growth. — Some animals grow less and
less as time goes on but appear to continue indefinitely,
others cease to grow when they reach maturity. In gen-
eral indeterminate growth is more characteristic of
poikilotherms than of homoiotherms. The prolonged and
more or less indeterminate growth which occurs among
fishes, amphibians, and reptiles is characteristic of

76 HOMOIOTHIJRMISM

species or individuals which have long lives. Among
homoiotherms there is usually a rather sharp demarka-
tion between youth and maturity.

The reasons why animals cease to grow are not cer-
tainly known, hut several theories have been advanced.
Continued growth is perhaps normal for all living things
but it is stopped in some cases by other factors. Di:ffer-
entiation may result in the production of such a bulk of
inert tissues that growth is no longer possible (Kobert-
son, 1923). Growth-stopping substances may increase
with age (Beer, 1924). These may be waste products,
hormones, or other substances which inhibit cell activity.
Metabolic gradients may be concerned (Child, 1915).
Eegions where a high rate of metabolism obtains domi-
nate those regions of lower rate. When a part gets too
far from the dominant region for effective control,
growth ceases. There may be a lack of growth-promot-
ing substances, such as available food or autocatylist
(Kobertson, 1923).

Growth in homoiotherms is apparently not funda-
mentally different from that in poikilotherms, but the
adjustments associated with a high degree of somatic
differentiation, the prolonged care of young by parents,
and the maintenance of constant temperature have appar-
ently led to determine growth. The body of a homoio-
therm has, through differentiation, come to have a
greater segregation of the various functions of its life
cycle, and the growth period is rather sharply differen-
tiated from the reproductive period.

Longevity. — Protozoa, as individuals, may live for a
few hours, days, or months. Invertebrates exist as indi-
viduals for a few months or perhaps even for several
years. Anemones have been kept alive for more than
60 years and corals have been known to live for 20 to

GROWTH AND LONGEVITY 77

30 years, but the animals having the longest lives are
vertebrates, and these have also been most successful
in invading the most variable habitats on land and in
fresh water. The arthropods and vertebrates are the
dominants on land, where the rarity of the atmosphere
makes it possible to move swiftly and the constant pres-
ence of an abundance of oxygen permits a rapid rate
of living. The arthropods have never as individuals at-
tained constant body temperature, though a few species
of ants and bees as colonies maintain a rather high tem-
perature. If the arthropods possessed long life and the
ability to maintain constant temperature, they would per-
haps excel the vertebrates.

Fishes have been known to live for more than a hun-
dred years (Giinther, 1880). The giant Japanese sala-
mander has lived 52 years; Amphiuma, 27; Siren, 25;
Hyla, 20; Eana, 16; and Bufo, 13 (Fowler, 1925). Many
reptiles live to great ages. Ditmars (1902) estimated
the ages of certain giant tortoises at 350 and 400 years.
Other types of tortoises have been known to live for
from 60 to 143 years (Babcock, 1928). Many birds and
mammals live to great ages. Among vertebrates gen-
erally, larger species of animals live longer than smaller
types of comparable structure. Animals which grow
quickly to a rather definite size are usually comparatively
short-lived. Mammals which live long lives commonly
breed less frequently than smaller, short-lived types.

Ecologically, longevity is related to the balance of
life in any locality or habitat. It is correlated with food
supply, favorable seasons for feeding and growth, avail-
able space in which to live, time required to reach matur-
ity, and other factors. By acquiring ability to live long,
vertebrates are able to have the lives of parent and
offspring overlap and thus heritages, such as particular ^

78 HOMOIOTHERMISM

homes or customs, which are not part of the somatic
equipment of each individual, may be passed on from
generation to generation to a greater degree than among
short-lived animals.

Long life and the ability to maintain a constant tem-
perature have given birds and mammals great advan-
tages over other animals. They live without any breaks
in their activities and thus have better opportunities for
gaining and profiting by experience. Continuity of liv-
ing at a constant temperature and a high rate of metabo-
lism have probably thus been the chief factors which
have made it possible for them to become wiser.

CHAPTER X
PERIODIC FLUCTUATIONS IN BODY TEMPERATURE

Homoiotlierms may show periodic fluctuations, di-
urnal or seasonal, in body temperature while in a state
of normal health. They may also show variations in
body temperature which are independent of extracorpo-
ral or periodically recurring influences during certain
phases of their life cycle. No intrinsic periodic fluctua-
tions are apparent in the poikilotherms, obviously be-
cause the environmental temperature is the determining
factor.

Diurnal Variation. — The diurnal variation in the
normal body temperature of animals has been little
studied. In man its usual range is about 1 deg. C. The
maximum temperature usually occurs about 4 to 6 p. m.
and the minimum about 4 to 6 a. m. With these diurnal
changes in temperature are associated parallel oscilla-
tions in the rate of metabolism, as shown by the elimi-
nation of carbon dioxide. While such variation may be
partially accounted for by the movement and tonus of the
muscles occurring during the waking hours, an inherent
cycle apparently exists. Benedict's (1904) observations
made on night workers showed that adjustment to change
in the daily routine was incomplete even after a period
of years. Gibson (1905) found that his diurnal metabolic
variations maintained a parallelism with the daylight
and darkness while he traveled around the world. It

79

80 HOMOIOTHERMISM

seems that while a diurnal variation occurs in hirds, it
can readily be changed by changing the time of activity.
Owls and other nocturnal birds show their maximum
temperatures in the night.

It is an interesting speculative consideration that
maximum and minimum temperature oscillations in the
bodies of animals coincide with those of the sea. The
maximum temperature of the sea is late in the afternoon
while the minimum is late in the night, near dawn. The
normal variation in shallow waters is similarly about 1
deg. C. It appears possible that there is in the body tem-
perature of the homoiotherms an indication of the tem-
perature of the sea at the time when their ancestors mi-
grated from the water to the land. If this is true, a
homoiotherm, although it at present maintains a constant
temperature much above that of its surroundings, has
mechanisms for keeping pace with temperatures to which
the ancestors were accustomed or perhaps to those which
they found advantageous in their original marine envi-
ronment.

Age. — ^At birth the temperature of the homoiotherms
varies with the species. Some are more affected by en-
vironmental temperature than others. Young marsu-
pials, such as opossums, with their undeveloped nervous
system, have little resistance to temperature changes in
the surroundings. On the other hand the young of the
guinea pig will soon after birth maintain its body within
a fraction of a degree of that of the mother. A human
infant has a slightly higher temperature at birth than
its mother. During childhood the body temperature
gradually falls to that of the adult. In old age the tem-
perature rises as a rule and attains a maximum at about
eighty years. The young of certain birds are practically
poikilothermic at hatching (Kendeigh and Baldwin,

PERIODIC FLUCTUATIONS IN BODY TEMPERATURE 81

1928) but gradually become homoiotbermic as they grow
older.

Hibernation. — Hibernation is a resting state during
which animals exist in a more or less torpid condition.
It is a response usually associated with cold, in contrast
to other states of inactivity such as aestivation, which
is a response to warm, dry periods, and to inactivity
which occurs at certain periods in the life cycle inde-
pendent of temperature variations, such as the pupal
stage of many insects.

Hibernation is a term generally applied to a torpid
state which occurs concomitantly with low temperatures
and is widely distributed through various groups of the
animal kingdom. Protozoans, rotifers, annelids, and
copepods commonly rest within cysts; snails close the
mouths of their shells and remain dormant ; certain fishes
inclose themselves in cocoons of mud and slime; many
poikilothermal vertebrates burrow in order to remain
torpid through cold seasons. Among the homoiothermal
animals only a few representatives of the mammals hi-
bernate ; namely, certain insectivores, bats, rodents, and
carnivores. There are no hibernating birds, but many
of them evade unfavorable temperature conditions by
migrating.

Whether hibernation is directly induced by low tem-
peratures or not is still a controversial question. Many
investigators believe that it is more likely a response to
lack of food and that temperature is only a coincident
factor. Animals like squirrels prepare for hibernation
either by storing up food, which they consume at inter-
vals when they awake, or by fattening before they become
dormant. Pembrey (1895) has shoA\Ti that there is an
extraordinary low respiratory quotient in hibernating
mammals. This means that there is a conversion of fat

82 HOMOIOTHERMISM

into carbohydrates. The fact that a hibernating marmot
increases in weight during hibernation indicates that
oxygen is actually retained. A marmot's respiratory
quotient under such conditions is about 0.30.

In a study of hibernation in the ground squirrel, Citel-
his tridecemlineatus (Mitchell), Johnson (1928) showed
that during the period of torpidity the body temperature
fell as low as 0 to 2 deg. C, which was within one degree
of the surrounding temperature. During the period of
hibernation a ground-squirrel sometimes loses 40 per
cent of its body weight. Johnson also observed that the
respirations dropped from 100-200 per minute in active
animals to 1-4 in torpid animals. The rate of heart
beat likewise ranged from 200 to 350 per minute in the
active animals but decreased to a minimum of 5 beats
per minute in hibernating individuals. These observa-
tions indicate a very low metabolism during hibernation
and a close approximation to the poikilothermal condi-
tion during the cold seasons.

Hibernation is a condition apparently fairly well dis-
tributed through a variety of mammals. Martin (1903)
observed Echidna in hibernation, during which it took
neither food nor water and showed a body temperature
which followed that of its surroundings within 0.5 deg. C.
Animals remained in a torpid state for as much as 4
months. The production of heat in Echidna was propor-
tional to the difference in temperature between animal
and environment.

Mammals are descended from poikilothermal animals.
Some have reached a less perfectly homoiothermic state
than others and perhaps have some imperfections in their
heat regulating mechanism. These have made a virtue
of necessity by becoming hibernators. They cannot sus-
tain the temperature at the level required for a continu-

PERIODIC FLUCTUATIONS IN BODY TEMPERATURE 83

ance of everyday life, so tliey relapse into somnolence
and inactivity. They pass into a state of imperfect
homoiothermism, which is very economical of energy and
during scarcity of food may permit them to withstand
unfavorable conditions in their environments. While
torpid they slowly burn away the energy sources of their
bodies to keep up a modicum of animal heat, thus com-
pensating for the inevitable loss. Hibernation is a con-
dition which apparently obtains in certain groups
throughout the course of mammalian evolution. It seems
to be a persistent, adaptive reaction of survival value.

CHAPTEE XI

BEHAVIOR AND BODY TEMPERATURE

Behavior is the term used to characterize the move-
ments which animals make in response to stimuli. In
general behavior shows adaptation in that it is so cor-
related with usual environmental changes that it helps
an animal to iind food, escape dangers, and produce off-
spring. By behaving in accordance with its hereditary
pattern an animal usually continues to live. Considering
the animal kingdom as a whole, behavior becomes more
complex and nervous control becomes of increasing im-
portance. The protozoa depend on cell responses;
sponges have tissues which act as responding units;
coelenterates have their behavioristic responses con-
trolled to some extent by a rather loosely coordinated
nerve net ; worms show increasing domination of activi-
ties by polarized neurones which make up the ganglionic,
segmental type of nervous system, which is so character-
istic of annelids and reaches its climax in arthropods;
the tubular central nervous system of chordates readily
lends itself to the building up of controlling centers which
produce general responses in particular types of organs,
rather than in particular bodily segments (Parker, 1919).
JYet in the most complex and highly coordinated of ani-
mals there are certain types of behavior which remain
primitive. Even in man, where a specialized brain has
made it possible for intelligence to serve as the finest

84

BEHAVIOR AND BODY TEMPERATURE 85

tool for making adaptive responses, there are activities
which are largely or wholly controlled by rather primi-
tive nerve nets or by glandular secretions and other
chemical substances.

The activities of poikilotherms, and of a few homoio-
therms, are limited by temperature and other factors
in the environment and they may therefore be restricted
to particular times of day or to particular seasons of
the year. A hibernating animal is not wholly inactive.
Certain of its physiological processes continue at a slow
rate, but its behavioristic responses are absent. A poi-
kilotherm therefore is not able to have continuous rela-
tions with environment by receiving sensory stimuli and
responding in a more or less adaptive way through the
activation of effectors. During enforced periods of
quiescence it has a chance to forget lessons learned dur-
ing previous experiences in its environment. Homoio-
therms, with their separate pulmonary and systemic cir-
culations and effective mechanisms for temperature regu-
lation, permit their nervous systems to continue to func-
tion from day to day at a high level of activity. A
homoiotherm, except for short periods of rest from fa-
tigue, lives a continuously active life.

In general metabolism increases in rate with in-
creased temperature, but every animal has an optimum
range where it is most active and efficient and above
which its activities may be abnormal. For example, an
amoeba shows its maximum rate of locomotion at 21.5
deg. C. and above or below that temperature moves more
slowly (Schwitalla, 1925). Ants crawl faster at higher
temperatures (Andrews, 1927). Some animals are ad-
justed to high temperatures and some to low. In ther-
mally stratified lakes and oceans particular species of
fishes and plankton organisms are found regularly to

86 HOMOIOTHERMISM

inhabit certain levels where the temperature range fa-
vors their metabolic activities. Some poikilothermic
animals are very quick to perceive variations away from
their optimum and quickly respond in such a way as to
remain in favorable temperatures. This is true of her-
ring (Powers, 1923), migrating salmon (Ward, 1921),
various plankton animals (Pearse, 1926), animals in
deserts (Buxton, 1923), and bats which live in caves
(Halm, 1908).

The behavior of many poikilotherms changes mark-
edly with variations in temperature. A frog, for ex-
ample, at temperatures above 10 deg. C. is usually
strongly positive in its responses to light, but if the tem-
perature falls below that, the frog becomes negative and
if given an opportunity will burrow or crawl under some
object, and at temperatures near 0 deg. C. becomes more
or less torpid (Torelle, 1903). Sudden changes in tem-
perature will cause a reversal of the usual responses of
various animals to light (Mast, 1911). Such responses
are usually adaptive. Among organisms which are nega-
tive to light at low temperatures Washburn (1926) men-
tions swarm spores, certain protozoa, an amphipod, and
certain aquatic insects ; among those which are positive
at lower temperatures, certain copepods and annelid
larvae. When planarians are subjected to increasing
temperatures, between 23 and 26 deg. C. they become
quite active and are positive to light ; between 26 and 38
deg. C. they continue to crawl actively but become nega-
tive to light ; and at 38 to 39 deg. C. the crawling becomes
extremely active and is accompanied by violent twisting
and turnings. Perhaps the sensations experienced are
increasingly unpleasant.

A homoiotherm which is insulated with feathers or
hair to prevent the loss of heat and has mechanisms to

BEHAVIOR AND BODY TEMPERATURE 87

check overheating can live a continuously active life vnth
its body at the optimum temperature for metabolism.
When the environment varies it may experience sensa-
tions of pain or discomfort to a greater degree than is
possible among poikilotherms, but such suffering is the
penalty for living continuously with full perception of
stimuli. It is perhaps more than compensated for by the
opportunity for mental development that continual living
gives. Because they are able to live continually at an
optimum rate birds and mammals are quicker, better
coordinated, and more astute than reptiles, amphibians,
or fishes.

A poikilotherm must seek an environment where a
constant temperature obtains or confine its activities to
favorable times. A constant environment is the ideal.
A homoio therm can not only remain active at all times
but is able also to enjoy the intellectual luxury of seeking
thermal variation in its environment. It thus gains a
greater variety of experiences and these react to produce
a more experienced and a more capable controlling nerv-
ous mechanism. The behavior of a homoiotherm there-
fore proceeds on a higher level than that of a sloAvly-
responding, fitfully-living poikilotherm.

A few colonial insects have attained to the advantages
which go with continuous living at temperatures approach-
ing their optima. Honey bees gather in a close cluster,
within a hive or hollow tree where they are protected
somewhat from cold by wood insulation and there main-
tain a comparatively high temperature by metabolic ac-
tivity. When their nest cavity cools, they beat their
wings and raise the temperature, largely through muscu-
lar activity. The temperature of a cluster inside a hive
may remain as high as 20 to 30 deg. C, while the tem-
peratures outside are varying between 9 and 11 deg. C.

88 HOMOIOTHERMISM

(Gates, 1914). Ant societies are not so well organized
for maintaining high temperatures but some species build
their mounds in such a way that they receive a maximum
amount of energy from the sun. Tight roofs and inclosed
air spaces help to maintain higher temperatures at night.
Ants carry their eggs and young to the galleries within
their nests where temperatures are most favorable. Dur-
ing prolonged cloudy and cool weather, however, they
must remain quite inactive (Andrews, 1927).

As an individual, no poikilotherm is able to maintain
a high, constant temperature in a varying environment.
Homoiotherms, while attaining the ability to live continu-
ously at a high rate, also gained the capacity to experi-
ence more varied sensations and to develop a better and
higher degree of nervous control, consciousness, and in-
tellect. Adaptation to life in variable land habitats has,
through the development of a constant optimum bodily
temperature, made intellectual life possible. The intel-
lect gives endless opportunity for experiencing, remem-
bering, foreseeing, planning, and surviving. It also allows
the highest types of intellectuals to have leisure which
is not spent altogether in thinking about surviving, but
gives time for practice, amusement, and thought concern-
ing better expedients for living, better ways of perform-
ing conventional social activities, the improvement of
culture, and the extension of knowledge.

CHAPTER XII
riTNESS OF TEMPERATURE CONSTANCY

In his '* Lemons sur les phenomenes de la vie" Claude
Bernard concluded that '*all the vital mechanisms, varied
as they are, have only one object, that of preserving con-
stant the condition of life in the internal environment,"
the blood — a very sagacious statement which has received
much support from recent investigations. The evolution
of the higher forms of life has been accompanied by a
gradually increasing independence of environment. This
has been achieved by the creation and maintenance within
the bodies of such animals of a suitable environment for
their constituent cells. The means by which animals
have reached this state of independence is through in-
crease in the cooperative functioning of their diverse
tissues. This evolution has passed through four signifi-
cant stages, which received their impetus from two evo-
lutionary events — migration from salt water to fresh
water and migration from water to land.

A properly balanced nutrient level was early estab-
lished in the Metazoa about their body cells. By means
of this the higher Metazoa acquired a considerable degree
of independence in relation to fluctuations in their supply
of nutrients. The sea offers^ a considerable degree of
stability as an animal habitat. This is especially true
in respect to its food resources. A nutrient level or
reserve was not as essential for the maintenance of an

90 HOMOIOTHERMISM

individual marine animal as for that of one on land,
where the environmental conditions varied so greatly.
That terrestrial animals have achieved the ability to
maintain a rather uniform nutrient level is exemplified
in the constancy of the composition of their blood, even
in extreme environmental fluctuations. The blood of the
higher forms has become of constant reaction. Thus
fluctuations in the hydrogen-ion concentration of the en-
vironment do not noticeably affect that of the pericel-
lular media. This is accomplished by the presence of
'^buffers" in the blood and the coordinated activity of
the respiratory center and the kidney.

The more complex animals have also attained a re-
sistance to, or independence of, variations in the salinity
and osmotic pressure in the environment. In the sea,
with its miiformity of salt content, there was little need
for independence. The lower Metazoa contain almost
exactly the same proportion and concentration of salts
as the sea water which they inhabit. In the higher
Metazoa the salinity and osmotic pressure of the blood
may differ greatly from that of the surrounding medium.
Internal constancy is maintained by the selective elimi-
nation of certain constituents by the kidneys. Macal-
lum (1904) maintained that the saline composition of the
blood of higher vertebrates closely resembles the prob-
able composition of sea water at the time when the proto-
vertebrate types first acquired a kidney. The latest stage
through which certain animals have passed in becoming
independent of environmental fluctuations through the
attainment of a constant internal environment has been
the maintenance of constant body temperature. In
aquatic habitats, especially in the sea, the variations in
temperatures are slight as compared to those in the air.
Furthermore the range of variation in water is usually

FITNESS OF TEMPERATURE CONSTANCY 91

within that tolerated by protoplasm, but the temperature
of the air may vary much beyond such limits. Homoio-
thermism has been attained by the cooperation of nerv-
ous, metabolic, and vascular activities. Through it opti-
mum conditions are ensured for the processes that must
go on for the maintenance of the individual.

Homoiothermism apparently had its origin at the
time when the great dinosaurs were becoming extinct
and the newer but wiser mammals were rising. Variable
climates were associated with, and perhaps stimulated,
the development of a more precisely regulated internal
temperature. The aridity of the earth at the end of the
Paleozoic had its influence upon the terrestrial tj^es,
especially upon the more progressive of them. The strife
between various types for food and water likely led to a
more intensified living — a higher rate of metabolism —
which implied a higher and more stable temperature.
Then with increasing cold a premium was put upon such
animals as could maintain their activity beyond the limits
of the shortening summers. This was accomplished by
the development of a mechanism whereby a relatively
constant temperature could be maintained within the or-
ganism regardless of external conditions.

Thus it appears that the evolution of the bird and
mammal, particularly the latter, was permitted by the
concurrence of two factors — aridity and cold. Homoio-
thermism was not suddenly attained. It was a relatively
slow process, just as was the emergence from aquatio
habitats. Indications of imperfectly adjusted mechan-
isms are still to be seen in existing animals. An investi-
gation of a comparative nature was conducted by Martin
(1903), who states that Echidna is the lowest in the scale
of warm-blooded animals. The attempt of this animal
to attain homoiotherism fails to the extent of 10 deg. C.

92 HOMOIOTHERMISM

when the environment varies from 5 to 35 deg. C. Dur-
ing cold weather Echidna abandons all attempts at
homoiothermism and hibernates for four months with
practically no difference between its body temperature
and that of the surroundings. At high temperatures
Echidna does not increase the number or depth of its
respirations. It possesses no sweat glands and exhibits
no evidence of ability to vary loss of heat by vaso-motor
adjustment in response to changes in external tem-
peratures.

Ornithorhynchus, which is also a primitive homoio-
therm, is distinctly above Echidna, for, although its body
temperature is low, it is fairly constant. This animal
possesses numerous sweat glands upon the soft snout
and frill but has none elsewhere on the body. Martin
found that carbon dioxide production varied with the
external temperature and this indicates that the animal
can modify heat-loss as well as heat-production. Orni-
thorhynchus does not increase the rate of its respirations
at high temperatures. Marsupials, however, show evi-
dence of utilizing variation in heat-loss to a greater ex-
tent than it does, but less than higher mammals. Their
respiratory rates are sometimes slightly increased at
high temperatures.

Variation in production of heat is the ancestral
method of regulating body temperature. Through de-
veloping a mechanism by means of which an organism
may vary the production of heat in accordance with heat
lost, animals have overcome the one great disadvantage
of cold-bloodedness — activity is no longer dependent
upon external temperature. At this stage a homoiotherm
has also increased its ability to remain active at low tem-
peratures. Later, by developing a mechanism control-
ling loss of heat, it increases its range in the direction

FITNESS OF TEMPERATURE CONSTANCY 93

of high temperatures. Thus the higher mammal main-
tains its body temperature without direct reference to
muscular activity, and metabolism may always proceed
under optimum conditions.

In homoiotherms is seen the culmination of changes
which began in the early communal life of Metazoa for
the apparent advantage to be obtained from a relative
independence from a fluctuating environment. Perhaps
the word ** advantage" implies a teleological explana-
tion, but another view is equally plausible — that, through
the agency of selection, evolution has been directed along
the lines of modifications in the internal physiological
environment of the organism as much as in morphologi-
cal changes. Animals apparently become more special-
ized, or morphologically adapted, to particular environ-
ments when conditions are most stable. When conditions
lack stability or are rapidly changing, evolution of mor-
phological characters acquires no momentum. On the
other hand the variations which lead to successive lib-
erations from the influences of a varying environment
would be of the greatest survival value in the environ-
ment of greatest instability. Such variations have pro-
ceeded along several lines, one of which has terminated
in regulation at a constant temperature level.

For the organism as a whole the most favorable tem-
perature is that which is conducive to the harmonious
interaction of all its manifold activities. The attainment
of a constant temperature by the homoiotherms has
given them two advantages. (1) A temperature which
is usually above that of the environment in which an
animal lives permits a greater expenditure of energy.
The velocities of chemical reactions going on in an
animal's body are proportional to the ratio existing
between the chemical force and the chemical resistance.

94 HOMOIOTHERMISM

Chemical force is a function of free energy. Rise in
temperature greatly diminishes chemical resistance.
Temperature, therefore, is a very important factor in
determining the velocity of reactions going on in a living
organism. The homoiotherms manifest a rapidity of
adjustment to external conditions which is far above that
shown by the poikilotherms. In addition to the effects
of a higher temperature on chemical reactions there is
also an advantage on physical processes as in the dimi-
nution of the internal friction of liquids such as the
blood, the hastening of diffusion, and the increasing of
the velocity of conduction of nerve impulses. The influ-
ence of temperature on general metabolism has already
been discussed. (2) A constant temperature level, which
assures the organism that its physiological processes
shall continue in a state of equilibrium, which is opti-
mum for the processes involved, is the second advantage.
A constant internal temperature thus protects the organ-
ism against environmental irregularity.

It is common knowledge that in homoiotherms physi-
ological processes take place most readily at body tem-
perature. The value of a constant temperature level is
readily seen in the case of enzymotic activity. The en-
zymes are effective only through a very well defined
range of temperatures. Moreover, their maximum rate
of reaction takes place at optimum temperatures , which
are very near those of the bodies of homoiotherms.
While many chemical reactions show acceleration with
increased temperature, enzymes do not absolutely follow
this rule. They increase their activity from 0 deg. C. to
about 40 deg. C, but beyond that show a gradual decrease.
At 60 deg. C. enzymes are destroyed. The enzymes of the
poikilotherms have optima like those of homoiotherms.
Early workers regarded the optimum as a mystery and

FITNESS OF TEMPERATURE CONSTANCY 95

it was used as an indication of ^^ vitalism.'' It is now
known that with enzyme action, as with certain other
chemical reactions, a rise in temperature quickens cataly-
sis, in accordance with the temperature coefficient of
chemical processes generally, but also renders an enzyme
less stable, so that it breaks down more and more rapidly
as the temperature rises. The temperature at which the
maximum rate is shown is therefore that at which the
acceleration due to temperature is in greatest excess over
the simultaneous destruction of the enzyme, and is known
as the optimum. Thus the fitness of the constant tem-
perature level of homoiotherms can be readily appre-
ciated.

Heat is not to be regarded as the cause of body proc-
esses; it is only one of the conditions. The extent to
which heat may influence such processes is relative and
varies with the quantity available in kinetic form. The
constancy of temperature maintained at the particular
level represented in the homoiotherms permits the con-
tinuity of the physiological processes of animals at their
maximum effectiveness, independent of environmental
caprice — and therein lies its fitness!

CHAPTER XIII
MAN AS A HOMOIOTHERMIC ANIMAL

Man is a mammal and shows greater similarities to
anthropoids and other primates than those animals do
to other orders of mammals. On the other hand, he
shows certain rather striking differences from other pri-
mates, but these are largely in degree rather than in
kind. His growth in length takes place at a fairly con-
stant time rate when his increase in weight takes place
at a constant percentage rate. This indicates that *Hhe
physiological environment with respect to growth-limit-
ing process remains constant" (Brody, 1928). The blood
of man is the fluid medium which bathes active living
cells, and is remarkably constant in its composition. It
contains buffers, rather definite proportions of nutrients
and wastes, and various mechanisms for maintaining
uniformity. By adding or removing insulation in the
form of clothing and by frequenting dwellings man is
able during most of his life to live in conditions which
are uniform and on the whole conducive to his physio-
logical and mental efficiency, and to his peace of mind.

Stability. — In all habitats and among all groups of
the animal kingdom animals seek stability. An animal
has better chances of surviving if the environment in
which it lives is dependable. Some animals attain this
end by living in an environment which is unvarying.
Others live in variable environments and vary with them.

96

MAN AS A HOMOIOTHERMIC ANIMAL 97

Homoiotherms as a whole have become more or less in-
dependent of environment by attaining the ability to
maintain a constant and comparatively high body tem-
perature. Man, more than any other animal, is able to
live to his full capacity at all times. For this privilege
the constancy of the environment of his cells, tissues,
organs, and body is largely responsible.

It is ^* human nature'' to long for stability. A man
believes that if he could cease to worry about making a
living and surviving, living better than his fellows (or
being a success), his state after his present life, and
other vexatious matters, he would be perfectly happy
and contented. But would he? There is a certain satis-
faction in overcoming difficulties and in enduring hard-
ships. An easy, contented, assured existence must be
monotonous. Is continual struggle better than mo-
notony?

Climate. — Climate is a potent factor in limiting or
enforcing natural abilities. In regions where conditions
of life are severe men may expend most of their energies
in living, but where there is an abundance of food and
an easy life is possible, culture may reach a high level
of development. Probably the most important climatic
factors which influence the attainments of human soci-
ties are temperature, humidity, and wind (Barbour,
1921). On the wind-swept arctic barrens there are no
great cities, no universities or factories. The same is
true of the Sahara Desert and of the Amazon Basin. The
human race is not at its best where the climate is monot-
onous. It makes no difference whether the monotony is
cold and bleak, hot and dry, or hot and wet.

Caucasians and perhaps other races are at their best
in the temperate regions. Frequent changes in tempera-
ture, humidity, and wind appear to have a tonic effect

98 HOMOIOTHERMISM

on the human race and man does his best work where
such variations obtain (Huntington, 1915). Thus,
though man strives in general to make liis physiologic,
psychic, and economic conditions for living as uniform
and dependable as possible, he does not reach his great-
est degree of accomplishment in monotony.

Frigid and Tropical Regions. — The frigid regions of
the earth produce little on land, but the sea has long
been exploited for its food resources. The tropics have
not been the seat of much human achievement. There
are perhaps several reasons for this, but two are note-
worthy: (1) hot, monotonous climate and (2) disease.
The humid tropics, though often producing an abundance
of plant and animal life, have never been the environ-
ment for the highest types of civilization. The charac-
teristic racial stocks in tropical countries are Negroids,
Malays, Mongolians, and Semitics. With the clearing
up of tropical diseases, Caucasians are dominating tropi-
cal countries more and more, but Caucasians as a rule
do not flourish in the tropics. Perhaps it will be best
for the world to leave the tropics as the home of com-
paratively primitive, simple people who will work for
low wages and furnish raw materials for the more aggres-
sive and progressive peoples in temperate zones. There
is no question but that the dark-skinned races are more
healthful than Caucasians in warm countries.

There are certainly racial peculiarities which make
some types of men better adapted to certain climates
than others. White men who go from temperate to trop-
ical countries show reduced basal metabolism (Knipping,
1925; Fleming, 1925). Native inhabitants of the tropics
have a lower blood pressure than people who have re-
cently come from temperate regions (Cadbury, 1923).
There are fundamental differences in metabolism be-

MAN AS A HOMOIOTHERMIC ANIMAL 99

tween blacks and whites (Woodruff, 1916). In compar-
able areas of skin the former have more sweat glands
than the latter. The basal metabolism of the white
natives in tropical Brazil was found by Almeida (1919,
1920) to be 24 per cent low^er than in the inhabitants of
temperate zones. ** Their habitual intensity of heat pro-
duction is a factor additional to the law (Rubner-Eictet)
of surface area, and its lowering is regarded as an adap-
tation to their environment, and an advantage in their
struggle against high air temperatures." The same
holds true for the colored inhabitants of Brazil. On the
other hand Eijkman (1921) found in the Dutch East In-
dies that the basal metabolism of blacks and whites was
about the same as of those in temperate climates. In
reviewing these papers Halliburton notes that white men
in warm dry climates usually have a high color and profit
by exercise, while those in warm moist climates are
often pale and take little exercise. There is need for
more knowledge concerning the effects of tropical cli-
mates on the various races of men.

Life Indoors. — As civilization progresses man has less
contact with pioneer conditions of life ; his environment
and his daily, annual, and perennial life become more
and more monotonous. He must therefore take thought
for the morrow and, while insuring stability in his liv-
ing, also retain the variety which is the spice and leaven
of life. A dwelling should not furnish such an artificial
environment that it is monotonous or abnormal. Colds
are incubated in steam-heated houses where there is in-
sufficient moisture. Tuberculosis flourishes among those
who live indoors in dust and darkness. The great mass
of the Chinese people is docile, unprogressive, hopeless,
and stolid largely on account of monotony — sameness
in diet, social contacts, and outlook for the future. Man

100 HOMOIOTHERMISM

must attain stability, but, equally, he must avoid
monotony.

Conclusions. — Man today proudly dominates the
earth. He has attained to his present position through
a long struggle up from ancestors in the dim past. Gill-
breathers in the water attained some ability to live on
land, developed lungs, and took up terrestrial life. At
first land vertebrates lacked proper insulation. Doubt-
less many of them lost the water from their bodies and
died. But as time went on tough skins developed which
held water in. Later these skins grew specialized feath-
ers and hairs which served to conserve bodily heat as
well as water. Then constant temperatures at optimum
levels for metabolism became possible and were devel-
oped through various controlling mechanisms. Given
a constant bodily temperature, which permitted peren-
nial activity, and a highly efficient brain, man has been
able to excel all competitors by inventions which make
him more independent of environment than any other
animal. Yet, he is not, and probably never will be,
wholly emancipated. He must still be more or less
** natural.''

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1 12 HOMOIOTHERMISM

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INDEX

A

Acclimatization 25

Adaptation to land life 15

Adjustment 51, 77, 94

Age 38, 76, 77, 80

Air sacs of birds 12

Ants 85, 88

Arboreal life 12

Atmosphere 5

B

Bank swallow 12

Behavior 84

Birds 3, 11

Blood 3, 51, 90, 96, 99

Body temperature 50, 55, 79

constancy 34, 79

fluctuations 55, 79, 80, 86

of homoiotherms 30

of poikilotherms 20, 28

regulation of 32, 56

Brain, evolution of 12

C

Carbon-dioxide

in relation to climate 7

in relation to respiration 42

Chlorophyll 9

Climate 6, 97

Clothing 35, 96

Coelenterata ., 23

Color 23

Conduction 19

Conservation of energy 9

Constancy of body temperature 34, 89

Cooling mechanisms 54

Cynodontia 13

115

116 INDEX

D

Decerebration 64

Desiccation 47

Digestion in poikilotherms 45

Dinosaurs 10, 13, 14, 91

Duckbill 30, 92

E

Earth 4

temperature of 7

Echidna ."0, 82, 92

Embryology 52

Endocrines 46, 51, 66

Environment • 28, 52

independence of 1, 89

Enzymes 46, 94

Eurythermal animals 26

Evaporation 22, 60, 97

Evolution of homoiotherms 10, 15, 91

of seed plants 15

P

Feathers 11

Fever 66

Fitness of temperature constancy 89

Foods 43, 52, 77, 89

Frigid regions 98

G

Gases of primitive atmosphere 5

Glands 51, 66

Growth 69, 73, 75

H

Hair 13

Heart beat, influence of temperature 21

Heat centers

in turtle 48

in mammals 54, 63

Heat production 2, 56

INDEX 117

Hibernation 81

Homes 34

Homoiothermism

advantages of 94

origin 90

Homoiotherms

ability to ignore environment 1

responses to temperature changes 30

Hormones 46, 51, 66

Hot springs 27

House wrens 12

Humidity 22, 60, 97

I

Independence of environment 1, 90

Indoor life 99

Insulation 2, 33, 53, 67, 86

L

Lactation 13

Land animals 3

Longevity 69, 76

M

Mammals, evolution of 13

Man 96, 100

Marsupials 13, 14, 82

Metabolism 1, 2, 50, 85

age, effect of 38

desiccation, effect of 47

hibernating period 46

nervous control of 48

of homoiotherms 37

of poikilotherms ." 37

temperature, effect of 39

Migrations 25, 32

Moisture 22, 60

Monotremes 13, 14, 82

Morphological adjustments 32

118 INDEX

N

Nervous control of body temperature 54, 63, 84

Nutrition 43, 52, 89

P

Pigmentation 23

Planaria

behavior 86

tolerance to extreme temperatures 26

Planetesimal hypothesis 4

Poikilotherms

acclimatization 25

behavior 85

body temperature of 17, 20, 28

energy requirements 2, 69

growth 69

hibernation 81

metabolism 50

tolerance to temperature changes 26

Protozoa, encystment 25

Pseudoseuchia 11

R

Radiation 7, 8, 22

Eeptilia 11, 16

Eesistance to extreme temperatures 33

Eespiration 41, 42

Ehythms in metabolism 49

S

Seed plants, evolution 15

Sloth 31

Solar radiation 9

Sources of heat 7

Spinal cord sections 64

Spiny ant-eater 30, 82, 92

Stability 3, 89, 93, 96

Starvation 45

Stenothermal animals 26

Surface area 19, 52, 60

INDEX 119

T
Temperature

body, factors influencing 18

body, nervous control of 54, 63, 84

fluctuations 80

metabolism, influence on 50

of modem earth 6

of primitive earth 14

Temperature gradients 20, 60

Temperature regulation 35, 56, 94

Temperature sense 65

Thermal gradients 20, 60

Thermogenesis 56

Thermolysis 56, 58

Thermotaxis 56

Tolerance to temperature changes 25, 26

Torpidity 25

Tropical oceans 2

Tropics 98

Tunnies 10

V
Vertebrates, origin of 10

W

Warm springs 2

Water 22, 60, 62, 97

Water vapor in regulation of climate 7

Wind 21, 60, 97