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AGGRESSION AMD FAMILIARITY AS FACTORS IN

MATE SELECTION IN Peromyscus pol ionotus

AND Peromyscus maniculatus

BY
DANIEL GEORGE WEBSTER

A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL

OF THE UNIVERSITY OF FLORIDA IN

PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA
1983

To my wife Carole and my daughter Danielle
and to my parents

ACKNOWLEDGEMENTS

Many individuals have generously contributed to this work, and
while these individuals deserve much of the credit for its completion,
any faults which may remain are my own. I thank Dr. D. A. Dewsbury for
his support and suggestions, for providing a critical scientific
atmosphere in which to conduct research, and for his example as a
scientist. I would also like to thank Drs. H. J. Brockmann, C.
VanHartesveldt, and W. B. Webb for their encouragement and support, and
for contributing of their time and knowledge. I particularly thank
Dr. M. E. Meyer whose consistant encouragement and support were a major
factor in my completion of the Graduate Program and the present work.
Several of my fellow students also contributed to this work. My
appreciation to D. Kim Sawrey, Bruce Ferguson, Larry Shapiro, Allen
Hodges, and Denis Baumgardner, for their support, encouragement and
critisms. I would especially like to thank Dean Williams for his time,
and for his patience in explaining some of the mysteries of electronics
to a neophyte. Our animal caretakers, T. C. Fryer and I. Washington,
also deserve my appreciation for the excellent care they have given my
animals. Last, but most of all, I would like to thank my wife Carole
who has supported me through the bad times and the good, and my daughter
Daniel le whose patience and understanding exceed her years.

This research was funded In part through NSF Grant BNS78-05173 to
Dr. D. A. Dewsbury.

in.

TABLE OF CONTENTS

PAGE

ACKNOWLEDGEMENTS Ml

ABSTRACT. v I

SECTION

I INTRODUCTION 1

Aggression as a Factor In Mate Selection 4

Fami I iarity as a Factor in Mate Selection 11

K i n Fam i I I ar i ty 12

Fam i I i ar Others 14

II GENERAL EXPERIMENTAL CONSIDERATIONS AND METHODOLOGY 18

Selection of Species 18

Approaches to the Study of Social Preference 19

General Experimental Information 22

General Methods 25

Subjects 25

Apparatus 26

Seminatural apparatus 26

Preference apparatus 29

Aggression apparatus 32

I I I EXPER I MENTS 35

Seminatural Experiments 35

I ntroduction 35

Subjects 36

Proced ure 36

Resu Its 40

Di scussion 61

Aggression and Fami I iarity Preference Tests 63

I n trod uct ion 63

Subjects 63

Proced ure 64

Results 67

Discussion "78

S ! b 1 1 ng Preference Tests 81

Introduction 81

Subjects 82

Proced ure 82

Resu Its 83

Discussion 90

GENERAL DISCUSSION 92

Aggressive Ability as a Factor in Preference 93

Ecology, Mating System, and Aggressive Ability 94

Peromyscus maniculatus 95

Peromyscus pol ionotus 101

Aggress i on and Mate Se I ect i on 1 06

Intraspecif ic aggression 106

Interspecific aggression 108

Ear I y breed i ng 1 09

Bruce effect 109

Heritable aggressive abi I ity 111

Familiarity as a Factor in Preference 113

Prior History 114

Ecology and Social System 115

Peromyscus maniculatus 115

Peromyscus pol ionotus 117

Kinship as a Factor in Preference 119

Inbreeding in Peromyscus maniculatus 120

Evidence for and against inbreeding 120

Ecological and social factors 121

Inbreeding in Peromyscus pol Ionotus 125

Evidence for and against inbreeding 125

Ecological and social factors 126

Evolution of Monogamy in Peromyscus Pol ionotus 129

S ummary 133

REFERENCES 1 37

BIOGRAPHICAL SKETCH 162

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

AGGRESSION AND FAMILIARITY AS FACTORS IN

MATE SELECTION IN Peromyscus pol ionotus

AND Peromyscus maniculatus

Daniel George Webster

April 1983

Chairman: Donald A. Dewsbury

Major Department: Psychology

Aggressive ability and familiarity were examined as factors in
the social preference and mate selection of males and females of the
monogamous species P*. pol ionotus. and the polygamous species P..
maniculatus. The aggressive behaviors and nesting behavior of £,.
pol ionotus were observed in a semi natural apparatus; factors assessed
were 1) familiarity based on cohabitation, and 2) aggressive ability
as determined through aggressive interactions. Groups observed were
composed of either two pairs of familiar opposi te-sexed individuals,
or two unfamiliar animals of each sex. Preferences of both species
were assessed in an automated preference apparatus. In addition to
the two factors assessed in the semi natural apparatus, the effects of
familiarity based on relatedness were assessed in the preference
apparatus. Measures recorded were the number and duration of visits

to stimulus animals. In the semi natural setting P. pol ionotus of
both sexes displayed aggression, and nested more frequently with the
more aggressive of two opposi te-sexed individuals. Males of this
species also exhibited a behavior, aggressive digging, that may
signal their aggressive status. Peromyscus pol ionotus and £,.
manicul atus of both sexes exhibited evidence of preference for more
assertive opposi te-sexed individuals (high rather than low tendency
to interact) in the preference apparatus. Peromyscus maniculatus of
both sexes also displayed preference for individuals of the opposite
sex that they had previously been housed with, but such familiarity
did not affect preference in P.. pol ionotus. The lack of a
significant effect of familiarity on preference in P. pol ionotus was
consistent with the nesting behavior of this species in the
seminatural apparatus. Differences in the responses of JL.
manicul atus and P*. pol ionotus to f ami I iar indi vidua I s may be based on
differences in the opportunities individuals of these species have to
use this factor in mate selection.

Peromyscus pol ionotus females demonstrated significant
preference for siblings over nonsiblings, and males tended to display
higher sibling than nonsibling scores. Inbreeding in Pj. pol ionotus
may permit individuals of this species to found populations in
isolated patches of favorable habitat. Lack of significant
preference by P. maniculatus for siblings or nonsiblings was
interpreted as due to competing preference responses in this species.

SECTION I
INTRODUCTION

The success of an organism In leaving a numerous
posterity is not measured only by the number of
its surviving offspring, but also by the quality
or probable success of these offspring. It is
therefore a matter of importance which particular
Individual of those available is to be their other
parent. (Fisher, 1958, p. 143)

Few factors are as important to an individual's reproductive

success as is the selection of a mate. Mate selection involves more

than simply the identification of potential partners as to species and

sex. In order for an individual to maximize its future representation

in the gene pool it must also select the "best" possible mates on the

basis of a variety of other considerations.

There has been much theoretical speculation on the proximate and

ultimate bases of mate selection and on the relative importance of

mate selection to different mating systems. In the last decade a

considerable effort has been made to collect empirical data on mate

selection, but large gaps still remain in the existing data which must

be filled before many important theoretical problems can be resolved.

Particularly lacking are data on male choice and data on mate

selection In mammalian species, especially those considered to be

monogamous. The lack of data on male choice probably stems In large

part from a traditional emphasis on female choice and male-male

competition (Bateman, 1948; Daly & Wilson, 1978; Trivers, 1972;

Williams, G. C, 1966) and the belief that male choice was either
nonexistent or negligible in the face of this competition. More
recently, however, it has been suggested that mate selection should be
of some consequence to males as well as females (e.g., Dewsbury,
1982c). The lack of data on monogamous mammalian species may be due In
part to two factors that make it somewhat difficult to observe these
species: 1) monogamy appears to be uncommon in mammals (Alexander,
1974; Crook, 1977; Kleiman, 1977; Orians, 1969) and 2) most mammals,
especially the smaller species, are largely nocturnal (Vaughan, 1978).

Study of the proximate factors involved in mate selection is
extremely important to the resolution of issues about the evolution and
relative importance of mate selection in various mammalian taxa.
Factors that have been proposed to be of major importance to mate
selection are of two general types: those related to the genotype of a
potential mate, such as genetic quality drivers, 1972, 1976; Zahavi,
1975), and relatedness (Maynard Smith, 1956); and those related to
resources, such as the ability to accrue resources drivers, 1976) and
parental investment (Eateman, 1948; Trivers, 1972). The relevance of
any particular factors as criteria in mate selection will vary among
species, between sexes, and across mating systems, as a function of
differences in a group of interrelated variables including ecological
factors (Borgia, 1979; Emlen & Oring, 1977; Halliday, 1978).
Differences in the degree to which Individuals of various species
utilize particular factors as criteria in mate selection would be
expected to reflect differences in the adaptiveness of those criteria
to mate selection in those species. Comparative studies of mate

selection or social preference in different species, therefore, can
provide an empirical basis for evaluation of hypotheses on the
importance of various factors to mate selection under different social
or mating systems. Such comparisons would be most effective when the
species compared were closely related (King, 1970) so that the species
do not differ in so many respects as to obscure the relationships of
interest.

Two factors of the sort that might be expected to be of broad
importance as criteria for mate selection across most species, but may
be expected to vary in importance among species, are the aggressive
ability of a potential mate, and that animal's degree of familiarity
with the Individual expressing choice. An individual of high
aggressive ability could be defined as one that is highly competent in
the performance of aggressive behavior (i.e., displays, threats, and
fights). Individuals of high aggressive ability would be expected to
perform we I I in competition for contested resources and defense of
mates and/or offspring. Familiarity can be of two types, these are 1)
familiarity gained through exposure to other unrelated individuals, and
2) familiarity with kin. In the first sense, familiarity may provide a
basis for evaluation of former mates or a means of discriminating
between two potential mates. In the second sense, familiarity may
provide a basis for avoidance of inbreeding, or as a yardstick for
comparison of potential mates (Bateson, 1978, 1980).

This study was designed to provide data on the importance of
aggression and familiarity as factors affecting mate preference in
males and females of two closely related species of muroid rodents with

different mating systems: the monogamous species Peromyscus pol ionotus
and the polygamous species Peromyscus maniculatus. In light of the
relative lack of data on mate selection in monogamous species the major
focus of this study is on L pol ionotus. The discussions that follow
provide a brief review of relevant theoretical factors in mate
selection, and a demonstration of the importance of aggression and
familiarity as factors in mate selection, with an emphasis on mammalian
species.

Aggression as a Factor in Mate Selection
Darwin (1859) recognized that some males could gain a reproductive
advantage over other males by defeating them in fights for females. It
has been suggested (Bateman, 1948) that the evolution of male-male
competition had its basis in differences in male-female strategies of
investment in gametes. Males are generally considered to invest little
energy in the production of gametes, whereas females invest a great
deal (Bateman, 1948; Orians, 1969; Stacey, 1982; Trivers, 1972).
Because of their larger investment females are valuable to males, and
the probability that females will obtain mates is high, but their
reproductive success will be limited by the number of gametes they can
produce. Males, however, because they invest little energy in the
production of individual gametes, can afford to produce large numbers
of gametes— with which they could potentially fertilize large numbers
of females. A male's reproductive success, therefore, may be greatly
influenced by the number of females he mates with — and males may be
expected to compete to fertilize females (see however, Dewsbury, 1982c;

Nakatsuru & Kramer, 1982). This argument was extended by Trivers
(1972), who restated It in a more general form based on the overall
relative level of parental investment of the two sexes, which he
hypothesized to determine the intensity of male-male competition in
species as well as the form of the mating system (see however, Kleiman
& Malcolm, 1981, p. 371; Wickler & Seibt, 1981). The level of
success achieved by a male in competitive mating may often depend upon
his ability to dominate other males. This notion has been examined in
a wealth of studies, recently reviewed by Dewsbury (1982b), on the
relationship between "dominance" and various aspects of reproduction.
A male's ability to conquer other males, however, also provides
females witn a basis by which to judge him against other males — a
basis for "female choice" (Darwin, 1874). Borgia (1979) has suggested
that a female's best indication of the relative overall genetic quality
of a male is provided through his aggressive interactions with other
males. Females choosing aggressive males, or allowing such males to
mate with them, may in effect be selecting "good genes" for their
offspring (Maynard Smith, 1956; Trivers, 1972). Selection for good
genes has also been suggested as a basis for the evolution of extra-
vagant sexually dimorphic characteristics (Fisher, 1930), as a basis
for lek behavior (Borgia, 1979), and as a basis for female choice in
Drosophi la (Partridge, 1980). The major obstacle to the use of
heritable factors as a basis for choice is the problem of "using up"
the genetic variance for a trait (Krebs & Davies, 1981; Maynard Smith,
1978). Several factors, however, have been suggested to act to
maintain genetic variance; these include 1) advantages for

heterozygotes (Borgia, 1979), 2) variation in the optimal genotype in
space and time (Krebs & Davies, 1981), 3) factors such as chronic
parasitism, that may result in cyclic changes in the optimal genotype
(Hamilton & Zuk, 1982), and 4) rate of mutation in polygenic characters
(Lande, 1976).

The northern elephant seal (Mirounga angustirostr i s) provides an
example of the importance of male aggressive ability in female choice
in a natural setting. During the breeding season males of this species
establish dominance hierachies which are maintained through threat and
combat. Dominant males guard groups of females from other males, and
account for the majority of first copulations (LeBeouf, 1974; LeBoeuf &
Peterson, 1969). Females help insure that they will be inseminated by
an aggressive dominant male by vocalizing loudly if a subordinate male
attempts to mount. The dominant male, alerted by the female, drives
off the subordinate male and copulates with the female (Cox & LeBoeuf,
1977). The authors note that females of many polygynous species might
be expected to incite male-male competition in this manner. Cox (1981)
has observed that female elephant seals are less likely to vocalize if
the male mounting them has just displayed dominance over another male
and suggested that females may thus select for males that frequently
display their aggressive ability. Cox hypothesized that in general "in
species where social status of males is correlated with their genetic
fitness, female choice is likely to be based on social signals which
are used in competition between males" (p. 197). Similar hypotheses
have been proposed by Borgia (1979) and Alexander (1975). Cox (1981)
has provided examples of several species in which females appear to use
male's aggressive signals toward each other as a basis for choice. The

list consists of a fairly diverse array of species including
territorial birds (Armstrong, 1973; Thorpe, 1961), tree frogs (Whitney
& Krebs, 1975a, 1975b), sticklebacks (Tinbergen, 1951), and a
lek-forming bird, the ruff Phi lomachus pugnax (Hogan-Warburg, 1966).

Male aggressive ability need not be heritable or manifest at the
time of mating to be an important factor in female choice. Male
differences in aggressive ability, for example, will be related to
differences in their ability to acquire and hold territories (Brown, J.
L., 1964). A female choosing a male with a superior territory is in
effect also choosing a male that has been able to first obtain that
territory in competition with other males, and further to maintain it
in the face of threats from other males.

Female preference for aggressive males has also been demonstrated
in laboratory choice tests. Sexually experienced female brown lemmings
(Lemmus tr imucronatus) in estrus were found, in olfactory choice tests,
to prefer dominant over subordinate males (Huck, Banks, & Wang, 1981).
Female preference was also predictive of male performance in later
dominance tests; estrous females again preferred dominant males while
diestrous females preferred subordinate males. The authors found that
dominant males had heavier testes and higher testosterone levels than
subordinates, and they hypothesized that female choice might be based
on differences in androgen dependent male odors. Estrous females were
also found to exhibit more copulation with dominant males in
tether-choice tests (Huck & Banks, 1982).

Costanzo and Renfrew (1977) studied the preference of sexually
experienced and naive female rats for dominant and subordinate male

odors. Ovariectomized sexually naive females displayed no preference
in hormonal ly induced estrus. Ovariectomized sexually experienced
females preferred dominant males when not injected with hormones, but
subordinate males when in hormonal ly induced estrus.

As discussed earlier a female may often invest relatively more than
a male in offspring, and a female's reproductive success may often be
more limited than a male's. Because of these factors females are
generally considered to be more choosey than males when selecting mates
(Bateman, 1948; Burley, 1977, 1981; Daly & Wilson, 1978; Trivers,
1972; Williams, G. C. , 1966) and the majority of studies of social or
mate preference have been studies of female choice. Male reproductive
success, however, depends on not only the number of mates males acquire
but the quality of these mates as well (Wade, 1979). Ralls (1976) has,
for example, suggested that larger females may often be better mothers.
Factors such as this might be considered trivial if males were capable
of inseminating an unl imited number of females. However, although
males appear to produce an enormous number of sperm and therefore to
be capable of inseminating an enormous number of females, these sperm
are emitted as ejaculates — of which a male may produce only limited
numbers (Dewsbury, 1982c; Nakatsuru & Kramer, 1982). Mate selection
may, therefore, often be of some consequence to males as well as
females (Dewsbury, 1982c).

The aggressive ability of females may be an important criterion for
male choice. Females with high aggressive ability may be more capable
than females with low ability in such behaviors as defense of young or
nest sites, or in competition for food items. Males of some species

have also demonstrated an ability to discriminate the aggressive status
of other individuals, and therefore the potential to use this factor in
mate selection. Male rats, for example, spend more time investigating
odor from a dominant male than from a subordinate (Krames, Cam, &
Bergman, 1969). Male mice also discriminate between the odors of
dominant and subordinate males (Carr, Martorano, & Krames, 1970) and
investigate areas marked by dominant males less than those marked by
subordinate males (Jones & Nowell, 1973). Both male and female
saddle-backed tamarins investigate dominant male scent marks more than
those of subordinate males (Epple, 1973, 1974).

Although the majority of preference studies to date have been
conducted with polygamous species, there is no reason to believe that
members of monogamous species should be any less adept at
discriminations based on differences in aggressive status, or that the
ability to make such discriminations should be any less useful to
members of monogamous species. Aggression serves many of the same
functions in monogamous species as in polygamous species. Although
male-male competition and aggression near the time of copulation may be
less important in monogamous than in many non-monogamous species,
aggression serves many functions at other times in an organism's
lifetime, and success in these other aggressive encounters will be just
as important to monogamous as to polygamous males. For example,
defense of resources critical to raising young, and in many cases to
initially attracting a mate, is hypothesized by some authors to be
universally displayed by males of monogamous species. Kleiman (1977)
states that "The male's territorial defense, which prevents the over-

utilization of necessary resources, is practiced by males of all
monogamous species" (p. 54). Kleiman (1977) has also indicated that
females of monogamous mammalian species may commonly be as involved as
males in territorial defense and contrasts this behavior with a lack
of territorial defense by non-monogamous females. This hypothesis is
similar to one previously proposed by Wilson (1975) as one of the
biasing ecological conditions for monogamy - that two adults are
required to defend valuable resources contained in the territory. If
monogamous females help to acquire and defend resources then monogamous
males may do well to choose aggressive females as mates.

An additional consideration, related to the possible function of
aggression as a factor in mate selection in monogamous species, is the
"early breeding" hypothesis proposed by Darwin (1874). According to
this theory the most healthy, more dominant members of a species will
come into breeding condition earlier in the season, and establish
territories earlier, than the less vigorous subordinate individuals.
The more dominant individuals of each sex should then choose each other
as the "better" mates in preference to subordinate individuals. Two
species whose behavior may support the occurrence of this form of
selection are the artic skua (Stercorar ius parasiticus,) and the
mourning dove (Zenaidura macroura) . Individuals generally pair for
life in the arctic skua (O'Donald, 1959). Darker males of this species
tend to breed earlier than birds of light or intermediate phenotype,
and females breeding for the first time are more successful with dark
males. Early breeding and large territory size are both correlated
with early hatching, and success of new pairs with dark males may be

related to male ability to hold large territories (Davis & O'Donald,
1976). Dominant male mourning doves prefer dominant females in
experiments with penned populations (Goforth & Baskett, 1971).
Dominant pairs breed earlier than subordinate pairs, and more offspring
of dominant pairs survive.

Based on theoretical considerations, and on the results of
research conducted to date, predictions can be made as to the relative
importance of aggression as a factor In mate selection in species with
polygamous and monogamous mating systems. Males that are above average
in their aggressive abilities should generally be preferred as mates by
females of monogamous and polygamous species. Females of monogamous
species, because they may form long-term pair bonds and consequences of
their choice may therefore be more long-term, might be expected to
display stronger preference than polygamous females. Males of
monogamous species might be expected to display preference for females
with higher aggressive ability because, among other considerations,
such females may be in breeding condition earlier and might be expected
to better share responsibility for territorial defense. Polygamous
males might also prefer females with higher aggressive ability because
such females may be better able to defend young or resources necessary
for raising young, but preference should not be as critical for these
males as for monogamous males.

Familiarity as a Factor In Mate Selection
Familiarity could be an important factor In mate selection In
several ways. It would be expected, for example, that early
familiarity with conspecifics should aid Individuals in discriminating

between members of their own and other species. Familiarity could
additionally be an important factor in recognition of kin, and
therefore a factor of importance in kin-selection and in avoidance of
inbreeding. It may also be of importance to animals of many species to
be capable of discriminating between unrelated strange and familiar
individuals. Two aspects of familiarity, recognition of kin and of
familiar others, will be discussed further.
Kin Fami I iarity

Bateson (1978, 1980) has hypothesized that early experience with
kin imprints individuals to aspects of both kin and species, and that
such kin familiarity is important to selection of mates of the
appropriate species and to avoidance of inbreeding at sexual maturity.
Avoidance of inbreeding is an important consideration in mate selection
because inbreeding often leads to inbreeding depression — a reduction
in the viability of inbred offspring and/or of their ability to
reproduce. Inbreeding depression has been documented in a variety of
species from ungulates (Ralls, Brugger, & Bal lou, 1979) to rodents
(Hill, 1974), birds (Bulmer, 1973), and Drosophi la (Maynard Smith,
1956). In many mammalian species inbreeding is avoided in part because
individuals of one sex emigrate from the natal group before sexual
maturity. This type of emigration has been observed in chimpanzees
(Pusey, 1980), lions (Bertram 1975, 1976), olive baboons (Packer,
1979), black-tailed prairie dogs (Hoogland, 1982), and a variety of
other species. Intergroup transfer of individuals in mammalian species
and the significance of this behavior to avoidance of inbreeding have
recently been discussed by Packer (1979). Emigration from natal groups

may be mediated by adult aggression in many species (e.g., langurs:
Sugiyama, 1965; elephant-shrews: Rathbun, 1979). However, female
(and male?) choice has also been suggested as a factor (Hoogland, 1982;
Wittenberger, 1981) since individuals should be selected to emigrate
from natal groups if 1) relatives refuse to mate with them and 2)
alternative mates are not readily available. In at least some rodent
species reproductive maturity may be inhibited by pheromones produced
by adults (Bediz & Whitsett, 1979; Drickamer, 1979; Lawton &
Whitsett, 1979; Lombardi & Whitsett, 1980). For young of such species
emigration from the natal group may provide the major opportunity for
reproduction.

Although It would appear that avoidance of inbreeding should
generally be the rule, certain circumstances may favor inbreeding. If
Inbreeding were not detrimental, individuals could increase their
inclusive fitness through mating with relatives (Bengtsson, 1978;
Maynard Smith, 1978). Female control of the sex of offspring in the
wasp Euodynerus foraminatus may be used to counterbalance the
detrimental effects of inbreeding and allow individuals of this
species to take advantage of the increase in relatedness resulting from
inbreeding (Cowan, 1979). Bengtsson (1978) hypothesized that it would
be adaptive for individuals to Inbreed if the costs of inbreeding were
lower than the costs that would be incurred in dispersal or in
competition for mates in the natal group. Costs incurred through
dispersal would include such factors as increased exposure to predators
and to unfavorable environmental conditions. Inbreeding may also at
times be suited to specialized environmental situations; as noted by

Mayer (1970) "An outbreeder may also be so well buffered that it
stagnates evol utionari ly. At the other end is the extreme inbreeder
which has found a lucky genotypic combination that permits it to
flourish in a specialized environmental situation ... (p. 245).
Shields (1982) compared the advantages and disadvantages of
outbreeding, inbreeding, and asexual reproduction and concluded that
inbreeding is often more advantageous than commonly assumed and should
be "expected to be common in organisms produced by stable lineage-
environment associations" (p. 274).

Although inbreeding may be adaptive under particular circumstances
individuals of most species should, given a choice, prefer to breed
with nonsiblings rather than siblings. Some support is lent to this
statement by the observation that the initiation of breeding in sibling
pairs is often delayed in comparison to the initiation of breeding in
nonsibling pairs (Batzli, Getz, & Hurley, 1977; Dewsbury, 1982a;
Hill, 1974; McGuire & Getz, 1981). The contribution of preference per
se to these findings is, however, difficult to assess. Animals in
these studies were not allowed a choice of mates and delayed
reproduction, or lack of reproduction, may result from several factors
(Dewsbury, 1982a) in addition to preference. It would be expected that
reproductively mature males and females of most species should prefer
to associate with nonsiblings rather than siblings, and that this
preference should be apparent in choice tests.
Fami I iar Others

Differences in familiarity need not only be defined in terms of
differences in relatedness, differences in familiarity may also be

defined in terms of differences in the amount and type of contact an
individual has had with others. Familiarity in this sense is an
important factor in many aspects of an animal's behavior, including
mate choice. Individuals should improve their reproductive success
by retaining mates that they have previously bred successfully with
and choosing unfamiliar others over mates with which breeding has
previously failed. Coulson (1966), for example, found kittiwakes
(Rissa tridactyla) were much more likely to change mates if breeding
had been unsuccessful; and red-billed gulls (Larus novaehol landiae
scopul inus) exhibit similar behavior (Mills, 1973). Although some
authors (e.g., Hal I iday, 1978) have indicated that mate choice based
on previous reproductive performance should only be of importance to
species which pair bond for more than one season, all that is
actually required is an ability to recognize previous mates. This
ability has been demonstrated in several species including rats
(Carr, Demesqui ta-Wander, Sachs, & Maconi, 1979; Carr, Hirsch, 2,
Balazs, 1980; Krames, Costanzo, & Carr, 1967), lemmings (Huck &
Banks, 1979) and prairie voles (Ward, Baumgardner, & Dewsbury, 1981).
The ability to recognize previous mates may also enhance reproduction
through allowing earlier breeding. This function of familiarity
has generally been stressed for monogamous species (e.g., Daly &
Wilson, 1978; Wilson, 1975).

The advantages of familiarity to mate selection, based on
reproductive performance or early breeding, are related to an
individuals previous breeding experience. Familiarity may, however,
also bias selection of mates by sexually inexperienced individuals.

Females may, for example, require a male to exhibit some evidence of
"commitment" to forming a pair-bond prior to copulation. The male
may fill this requirement by investing a large amount of his time in
the relationship, and thus preclude his finding another female
(Maynard Smith, 1977), and/or by demonstrating his ability to provide
resources (e.g., Nisbet, 1973). Evidence of commitment is likely to
be most important to members of monogamous species, especially those
in which individuals form prolonged or lifelong bonds, because many
individuals of these species may only choose a mate once in their
lifetime. Individuals of species that form prolonged pair bonds
might also be expected to be more "prepared" (Sel igman, 1970) to
recognize differences in familiarity, than would be individuals of
species that do not pair bond, if familiarity were important to the
maintenance of pair bonds.

Many authors have indicated that males of polygamous species
should mate with as many different individuals as possible (Adler,
1978; Bateman, 1948; Dawk ins, 1976; Williams, G. C., 1966; Zucker &
V/ade, 1968). Although as noted previously males may have a limited
capacity to mate and should therefore be somewhat selective when
allowed a choice of partners (Dewsbury, 1982c; Makatsuru & Kramer,
1982), it may still often be to a males advantage to obtain
additional matings if the opportunity is presented. Familiarity may
therefore be of importance (at least to polygamous males) in
identification of females a male has already mated with. However,
because mate infidelity could have serious consequences for
monogamously mated individuals (Grafen & Sibly, 1978; Trivers, 1972),

it is likely that individuals of monogamous species have been
selected to detect potential philanderers, and to select against such
individuals as mates (e.g., Erickson & Zenone, 1976). It may be
expected therefore that polygamous males would be more likely than
monogamous males to prefer novel over familiar partners. This
prediction is consistent with previous suggestions that males of
monogamous species should be less likely than males of polygamous
species to exhibit a "Coolidge effect" (Thomas & Birney, 1979;
Wilson, Kuehn, & Beach, 1963; but also see Dewsbury, 1981a,b).
As a general set of predictions it might be expected that
individuals of monogamous species would display greater preference for
familiar individuals than would individuals of polygamous species.
Because of differences in the consequences of choice it might also be
expected that females would display stronger preference than males
(e.g., Burley, 1981). Monogamous females would be expected to display
the strongest preference for familiar individuals. Monogamous males
should be expected to display some preference for familiar
individuals, as might also polygamous females (unless greater benefits
result from producing multiply sired litters). Polygamous males,
however, due to a greater possibility of increasing their reproductive
success through mating with more than one female, may be expected to
display some preference for novel individuals of the opposite sex.

SECTION I I
GENERAL EXPERIMENTAL CONSIDERATIONS AND METHODOLOGY

The discussions In this section provide a brief rationale for the
choice of the particular species and experimental procedures that were
followed In this study. This section provides general methodological
Information common to all experiments in this study, and a description
of the apparatus used in these experiments.

Selection of Species

One of the methods that may be particularly suited to exposing and
interpreting differences in social behavior among species is the
comparative approach (Dewsbury & Rethl Ingshafer, 1973; King, 1970).
Murold rodents are a group that Is particularly suited to the use of
the comparative method (Dewsbury, 1974, 1978). The two murold rodent
species that were chosen for comparison in this study, the monogamous
Peromyscus pol ionotus (oldfleld mouse) and polygamous Peromyscus
manlculatus (deer mouse), are both members of the manlculatus species
group of the subgenus Peromyscus. Because the majority of mammalian
species are considered to be non-monogamous (Alexander, 1974; Crook,
1977; Kleiman, 1977; Orians, 1969) and relatively little Information
is available about mate choice In monogamous mammalian species, It was
considered to be particularly important that one of the species
selected for comparison In the present study be a monogamous species.
Although monogamy has been suggested for several rodent species,

"except for P, pol ionotus. the data are circumstantial" (Foltz, 1981a,
p. 665) and are open to more than one interpretation. Data supportive
of monogamy in P_,. pol ionotus include the consistent finding that the
majority of reproductively mature individuals are captured as
heterosexual pairs (Blair, 1951; Foltz, 1981a; Rand & Host, 1942;
Smith, 1966), and behavioral (Blair, 1951) and electrophoretic (Foltz,
1981a) evidence that pairs form long-term reproductive associations.
Information about mate selection in this species was also considered to
be of importance, in addition to comparative considerations, because
one subspecies, the beach mouse (£, pol ionotus leucocepha.l us.) , is
presently considered endangered.

In contrast to £* pol ionotus. electrophoretic evidence indicates
polygamy for F\ maniculatus (Birdsall & Nash, 1973; Merrltt & Wu,
1975) and females may even, on occasion, raise young communally
(Hansen, 1957). Dewsbury (1981c) suggests that maniculatus may be even
more promiscuous than indicated by electrophoretic studies because
these studies do not consider the effect of factors such as
"differential fertilizing capacity" (Lanier, Estep, & Dewsbury, 1979)
that may affect estimates of the number of matings that have occurred.
Approaches to the Study of Social Preference
Ideally social behavior and social preference should be studied in
natural settings. Although this approach may be utilized successfully
with diurnal and highly visible species, it is often an impractical, or
nearly impossible, approach for many species. As an alternative
investigators have often turned to the study of populations in outdoor
(Agren, 1976; Boice, 1977; Boice & Adams, 1980; Gipps & Jewell,

1979; Jannett, 1980; Lidicker, 1980) or indoor (Bowen & Brooks, 1978;
Crowcroft & Rowe, 1963; Getz & Carter, 1980; Hill, 1977; Poole &
Morgan, 1976; Reimar & Petras, 1967; Thiessen & Maxwell, 1979;
Thomas & Birney, 1979) seminatural enclosures. While these enclosures
do not replicate natural conditions in many respects, they do allow the
investigator to approximate some aspects of the natural setting, and
allow a degree of control over experimental variables that is generally
not available in nature.

Even in a seminatural apparatus, however, interactions may often
be so complex that it is difficult to evaluate the effects of any
single variable on a particular behavior such as social preference.
This problem has led investigators to the use of even more controlled
situations, such as preference apparatus of various types, to evaluate
the role of various factors in social preference. In the typical
preference paradigm an animal (the "choice" animal) is allowed to
express preference by "choosing" between two or more alternative
stimuli. Behavioral measures of preference may include factors such as
the number of approaches, number of visits, duration of visits, time
spent huddling together, mating activity, and a variety of other
measures. Use of preference apparatus, in addition to allowing more
controlled investigation of particular factors (e.g., familiarity) than
may be available in seminatural apparatus, allows the experimenter to
control the degree of contact between choice animals and stimulus
animals. In a tether preference apparatus, for example, stimulus
animals are tethered in a fixed area while the choice animal is allowed
free access to the apparatus and may express preference through

proximity or contact behaviors, or under appropriate conditions, mating
behavior (Ward et a I., 1981; Huck & Banks, 1982; Webster, Williams, &
Dewsbury, 1982). An alternative method used in preference tests is to
place cellars on the stimulus animals, and then place these animals in
compartments with doorways of a size large enough to allow access by
choice animals, but too small for the collared animals to pass through
(Mainardi, Marsan, & Pasquali, 1965; McDonald & Forslund, 1978).
Direct contact between choice animals and stimulus animals may also be
prevented by simply constructing stimulus compartments or containers so
that they are not accessible by the choice animal (Agren & Meyerson,
1977; Carmichael, 1980; Carr, Wylie, & Loeb, 1970; Murphy, 1977;
Webster, Sawrey, Williams, & Dewsbury, 1982). Experimenters have also
opted at times to test preference for odors from stimulus animals
rather than using the animals themselves (Carr et al., 1980; Fass,
Guterman, & Stevens, 1978; Gilder & Slater, 1978; Huck & Banks, 1979,
1980; Krames et al., 1967; Ruddy, 1980), or to restrict choice cues
to olfactory cues by us<ng anesthetized stimulus animals (Landauer,
Banks, & Carter, 1977; Landauer, Seidenberg, & Santos, 1978; Murphy,
1980).

While preference apparatus offer the opportunity for greater
control over variables than do semi natural apparatus, the conditions
under which preference is assessed do not approximate natural
conditions as closely as do conditions in seminatural apparatus. With
preference apparatus, therefore, one may run a greater risk of
obtaining results that are misleading in respect to behavior under more
natural conditions. Social preferences may, for example, sometimes be

expressed less strongly in preference apparatus than they would be
In a more natural context; one might therefore be more likely to
falsely reject a factor as unimportant to social preference in these
tests. One way to minimize this problem is to first assess a species'
social behavior in more natural settings, such as in seminatural
apparatus, and select factors for preference experiments on the basis
of those results. Alternatively one might use results from seminatural
experiments in pert as a guide in interpretation of results from
preference experiments.

General Experimental Information

This study was designed to provide data on aggression and
familiarity as factors in the social preference of monogamous and
polygamous species. Partial data are available about the function of
these factors in the social preference of the representative species
chosen for this study, Pj. pol ionotus and £». manicul atus.

Available evidence indicates that aggression may be an important
factor in the social behavior of P,. manicul atusf that more aggressive
males may sire more offspring than less aggressive males, and that
differences in male aggressive ability may be important in female
choice in this species. In P_». maniculatus blandus Blair and Howard
(1944) found that, in experimental populations consisting of two
individuals of each sex, one male would generally establish dominance
over the other. The dominant male generally nested with both females
more frequently than did the subordinate, and the authors were able to
establish (throuah coat-color markers) that dominant males sired the

majority (19 of 21) of litters In their study. Dewsbury (1979, 1981c)
found that male dominance In E* maniculatus balrd.i was positively
related to copulatory behavior, and that dominant males not only
copulated more than subordinates, but that they also sired a larger
number of offspring (Dewsbury, 1981c). Eisenberg (1962) observed that
after the formation of dominance relationships between male E*.
maniculatus gambel i i , females of this species that had been paired with
subordinate males for two weeks prior to aggression tests generally
failed to remain with their subordinate male partners and nested
Instead with the dominant male.

Blair and Howard (1944) studied two subspecies of Em. pol Ionotus,
P. pol Ionotus albifrons and P. pol Ionotus Iftucocephal us, and found
little evidence of aggression against conspeclfics by Individuals of
either sex. In addition all four individuals (two males and two
females) In a group were frequently found nesting together. From these
observations the authors concluded that Pj. pol Ionotus were a very
social species. Field observations, however, do not support the notion
that adult E^ pol ionotus are highly social. Although £*. pol Ionotus
are commonly found in family groups composed of a male, a female, and
young (Blair, 1951; Foltz, 1979; Rand & Host, 1942; Smith, 1966;
personal observations), sexually mature Individuals of the same sex are
never (Smith, 1966), or very infrequently (Blair, 1951; Rand & Host,
1942) found together in the same nest. In addition Blair (1951)
observed wounding in some transient and immature Individuals and also
observed, In trap and release experiments, that adult females often
chased other females from nests. Smith (1967) has suggested that

females of this species "are normally dominant over their mates and
play a major role in the process of pair formation and maintenance of
the pair bond" (p. 236). JL. pol ionotus have also been observed to
exhibit aggression in some laboratory tests (Garten, 1976; Smith,
Garten, & Ramesy, 1975), but the conditions for these tests do not
allow evaluation of the function of aggression in a social context, or
as a factor in social preference.

Few data are available on the function of aggression and
familiarity in the social behavior of P, pol ionotus. or in the
social behavior of monogamous species in general. The first set of
experiments in this study was designed to provide such data. These
experiments were conducted in a seminatural apparatus that was designed
with artificial burrows. This design takes into consideration the
semifossorial habits of FV,. pol ionotus. and thereby allows an
approximation of natural conditions in this species.

The seminatural experiments with P*. pol ionotus were foi lowed by
preference experiments on both £*, pol ionotus and E*. maniculatus.
These experiments allowed preference based on aggressive ability and
familiarity to be assessed under the same conditions for both species,
and thus allowed a direct comparison of the relative value of these
factors in the social preference of these two species. The first set
of these experiments examines aggressive ability and familiarity with
unrelated individuals (based on previous contact) as factors in social
preference; the second experiment examines preference for siblings.

General Methods
Subjects

Subjects for this study were 45 to 65 day old Individuals of two
species of muroid rodent, Peromyscus pol ionotus subqriseus and
Peromyscus maniculatus bairdf . The P_,_ pol ionotus were laboratory-bred
animals one to four generations removed from the wild. The parental
stock was obtained from two different subpopu lations in the Ocala
National Forest in Florida. The first group of these animals was
trapped in 1978 from road shoulders along State Road 316 between Salt
Springs and Eureka. Additional animals for breeding stock were trapped
in 1980 from road shoulders along U.S. Highway 19. These two
populations are from the same general area as that listed as population
25 by Selander, Smith, Suh, Johnson, and Gentry (1971). The method of
capture was similar to that detailed by Foltz (1979).

It is not possible to determine how many generations removed from
the wild the Pj. manicu latus were. This colony was founded at the
University of Florida with animals obtained from near East Lansing,
Michigan, in 1970, and additional wild stock has been added on several
occasions since.

Animals were housed In clear plastic cages measuring 48 x 27 x 13
cm or 29 x 19 x 13 cm with wood shavings as bedding. Purina laboratory
animal chow and water were provided ad lib. Prior to serving as
subjects all animals were maintained as litters. Peromyscus pol ionotus
I Itters were weaned at 22 or 23 days of age, E*. manicu latus were
weaned at 21 days of age. Animals that exhibited obvious physical

defects, such as extensive tail wounds or missing tails, were not
selected for study.

Animals of both species were maintained on a reversed 16L:8D
photoperiod. All adaptation and testing were conducted during the dark
portion of the photoperiod. With the exception of observations
conducted in the seminatural apparatus, which was in a separate room,
all studies and adaptation periods were conducted in the P*. pol iono.tus
colony room. Procedural details specific to particular studies are
described in the methods sections of those studies.
Apparatus
Seminatural apparatus

The seminatural apparatus was a large square Plexiglas arena 125
cm on a side and 46 cm deep. The sides of this arena were constructed
with 1/4 inch Plexiglas and the floor was constructed with 1/2 Inch
plywood and painted grey. The arena was partitioned, with four 85 cm
lengths of 1/4 inch Plexiglas, into a square central area that measured
85 cm on a side and four right angle triangular corner compartments
with sides of 85 cm, 61 cm, and 61 cm (See Figure 1).

Two nest boxes were attached to each corner compartment. Nest
boxes were constructed with sides of )/4 inch Plexiglas and 1/4 inch
plywood backs. They measured 1 0 x 9 x 8 cm and had hinged Plexiglas
lids to provide access for removal of animals and cleaning. A 3.2 cm
diameter hole cut in the front of each nest box provided access for the
animals. In each corner compartment two matching 3.2 cm diameter
holes, cut in the sides of the apparatus, 45.5 cm from the corner and
1.5 cm from the floor, provided access to the nest boxes. The front of

one of the nest boxes for each corner compartment was connected with
silicon cement directly to the side of the apparatus in line with one
of these holes. The other nest box in each corner compartment was
connected to the second opening by means of a 48 cm length of Tygon
polyethylene tubing with an internal diameter of 2.5 cm and an external
diameter of 3.2 cm, and thus formed an artificial burrow. Silicon
cement was used to attach one side of the nest box to the apparatus and
to connect the tubing to the openings for the nest box and the
apparatus.

Animals could gain access from the corner compartments to the
central area of the apparatus through 3.2 cm diameter holes centered on
and 1.5 cm from the bottom of the partition which formed the
compartment. A 2.5 cm hole 3 cm from the right angle corner and 2 cm
from the floor of the apparatus provided access for the drinking tube
of a water bottle.

The seminatural apparatus was in a room separate from the colony
room but maintained on a 16L:8D photopericd Identical to that
maintained in the colony room. The apparatus was illuminated In the
light phase of the photopericd by four 75-watt incandescent-white bulbs
and two 60-watt red bulbs, each suspended three feet above the floor of
the apparatus, and during the dark phase of the photopericd by the two
60-watt red bulbs alone.

Behavioral measures were recorded by means of a 20-channel
Esterl Ine-Angus event recorder. The behaviors exhibited by each group
of animals, during their four days In the seminatural apparatus, were
also recorded on videotape using a Hitachi CCTV low light television

camera, and a Panasonic time lapse VTR video tape recorder set on a 72
hour record mode.
Preference apparatus

The preference apparatus was a three chambered rectangular box
with a hinged lid; it was constructed of 1/4 Inch Plexiglas and
measured 44 x 21 .5 x 20 cm (Figure 2). The Inside measurements of the
two end chambers were 10 x 21.5 x 20 cm. These chambers were open to
the central area through a 7 x 7 cm opening. The end chambers were
designed to accommodate small removable "choice chambers" which
measured 10 x 8 x 6 cm. A "stimulus box" with an inside measurement of
8 x 7 x 8 cm was attached to each end chamber, and was open to It
through a 6 x 5 cm opening. When choice chambers were placed In the
end chambers, therefore, one end of the choice chamber was accessible
from the central area, while the other end was open to the stimulus
box. A hardware cloth screen installed In the opening to the stimulus
box and a second three-sided piece of hardware cloth which fit the
Inside of the stimulus box provided a "double screen" between the
choice chamber and the stimulus box.

A bank of three red-sensitive photocells (peak response at 735
nm), wired in a series behind each choice chamber, registered entries
to the chamber. The light sources for the photocells were 60-watt red
light bulbs placed 27 cm in front of the apparatus and directly in
front of the bank of photocells. Each photocell was attached to the
end of a tubular 4.3 cm piece cut from a 12 x 75 cm disposable plastic
culture tube. The outs Ides of these tubes were painted black; this in
effect columnated the light to the photocells. The photocells were

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situated such that In order to register a visit to the choice chamber,
an animal had to be completely Inside the chamber, and at least
partially In the half of the chamber closest to the stimulus animal.
Each bank of photocells was wired In a series with the coil circuit of
a 10,000 ohm, 24 VDC DPDT relay (see Figure 3). These relays were
powered through output from a variable power supply set for a
continuous output of 26 volts. One of the normally closed circuits of
each of these relays was wired into the pen circuit of an Ester line-
Angus event recorder; this provided a permanent record of entries into
each choice chamber. Output from another normally closed circuit of
these relays was used to control a second set of relays on a relay
rack. Two banks of Sodeco counters received input through the normally
open contacts of these relays. The first bank of counters was wired In
a series to pulse formers and recorded the number of visits to each
chamber regardless of visit duration. Input to the second bank of
counters was regulated by means of a recycling timer set to produce
pulses at 1/3 of a second. These counters recorded the total duration
of visits to the nearest 1/3 of a second. Session duration was
automatically controlled via another timer (not displayed) which
controlled the input to the recycling timer and counters.
Aggression apparatus

The "aggression arena" was constructed from a large 48.5 x 38
x 20 cm plastic cage. Two 3.2 cm diameter holes were cut in the two
longer sides of the cage centered 25.5 cm apart and 3.5 cm from the
bottom. Silicon cement was used to form a gasket around each hole.
Matching holes and gaskets were placed on one side of two 48 x 27 x 13

cm plastic cages. The larger cage and two smaller cages could then be
connected by 6.5 cm lengths of Tygon polyethylene tubing (Internal
diameter of 2.5 cm and external diameter of 3.2 cm) to form the
"aggression apparatus" (See Figure 4). During adaptation procedures
the aggression arena and smal ler cages were connected with unobstructed
lengths of tubing; a piece of metal screen in the center of each
length of tubing prevented animals from traveling through the tubes
during tests. A 1/4 Inch 53.5 x 43 cm Plexiglas lid was placed over
the large cage, and wire cage lids over the smaller cages, while
testing was conducted.

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48 cm

FIGURE 4 Aggression Apparatus

SECTION II I
EXPER I MENTS

Semi natural Experiments

This section is divided into three major subsections; the three
sets of experiments that comprise this study are each described within
separate subsections under the headings of Seminatural Experiments,
Aggression and Familiarity Preference Tests, and Sibling Preference
Tests. Each of these subsections begins with a brief introduction to
the experiments in that subsection, and specific information on
subjects and procedures for these experiments. This information is
followed by the results of these experiments and a brief discussion of
the results. The results of all three sets of experiments in this
study are discussed together, in the context of the ecology of £!_,.
pol ionotus and JL. maniculatus and theoretical considerations, in the
General Discussion section.
Introduction

This experiment was designed to provide information on aggression
and familiarity as factors in the social behavior end mate selection of
Pj. pol ionotus. (Similiar types of data already exist for P_j.
maniculatus: Blair & Howard, 1944; Dewsbury, 1979, 1981c: Eisenberg,
1962.) Although it is difficult to establish the relevance of factors
in social preference per se with seminatural observations, such data
can provide an indication of the functions a factor may serve in

nature, and can provide indications of whether a factor may be of

importance in social preference.

Subjects

Subjects were 40 male and 40 female JL. pol Ionotus. Pricr to
serving as subjects, animals were maintained as previously described In
the section on general methods. Subjects were selected using the
criteria described In the general methods, and the additional criterion
that the animals within any group had no common grandparents.
Procedure

In order to better separate and evaluate the roles of aggression
and familiarity in the social behavior of P. pol Ionotus animals were
observed under two different experimental conditions, the "paired"
condition and the "single" condition. Twenty animals of each sex were
assigned to either the single condition or the paired condition.
Animals in each condition were divided into 10 groups; two animals of
each sex were assigned to each group. Each of the 10 groups of animals
in each condition, single or paired, were treated separately. Animals
within each group were lightly anesthetized with ether and shaved In
one of the following four patterns: (1) band shaved around the neck;
(2) band shaved around the middle; (3) band shaved at the rear; (4)
no shaved area. Shaving was performed one day prior to beginning the
first experimental manipulation. Approximately equal numbers of males
and females received each shave pattern. Subjects under both the
single and paired conditions were exposed to a series of three
different experimental manipulations. These manipulations, in order,

and their durations were "nest building," 4 days; "seminatural
isolation," 3 days; and "seminatural interaction," 4 days.

During the four-day nest building period animals were housed in 48
x 27 x 13 cm plastic cages on San-i-cel bedding. Animals in the single
groups were housed individually; animals in the paired groups were
housed as two separate pairs of opposi te-sexed animals. Three 2-inch
square "Nestlets" (Ancare Corp.) were provided as nesting material in
each cage. The type of nest built was assessed just prior to the
beginning of the dark period on the next 4 consecutive days. Nests
were rated as one of three types: (0) no nest; (1) platform nest;
(2) covered nest.

The seminatural isolation period began in the first dark phase
which followed the nest building period. Animals were transferred from
the colony room to the room containing the seminatural apparatus
approximately 15 min after the beginning of the dark phase. Animals
from single groups were each placed individually in corner
compartments, with animals of the same sex in compartments diagonally
opposite each other. Animals from paired groups, which had been
maintained as pairs during nest building, were transferred to the
seminatural apparatus in the same paired relationship. Pairs were
placed in corner compartments of the apparatus diagonally opposite each
other. The opening from each corner compartment to the central area
was closed with a solid black rubber stopper so that animals were
restricted to the compartment in which they had been placed. The floor
of the central area and of the corner compartments had been covered
with San-i-cel to a depth of approximately 1.5 cm; each corner

compartment also contained three Nest lets, and food and water was

avai I able ad lib.

Animals were maintained in the corner compartments for 3 days.

Activity during this entire pericd was videotaped. Each pair of

animals in the paired groups was also observed for three alternate

10-minute periods during the dark phase on the day the animals were

Introduced, and on the following two days. On the first day,

observation was begun as soon as all animals had been placed In the

apparatus. On each of the fol lowing 2 days one of the pairs was

designated as the first pair to be observed, and the first period of

observation was begun when the members of that pair had emerged from

their burrow. The behavioral and aggressive measures that were

recorded were similar to categories described by All in and Banks (1968)

and Colvin (1973). Measures were recorded by means of a 20-channel

Esterl ine-Angus event recorder. The measures, and definitions of each,

were as fol lows:

Approach Scored when an animal came within one and
one-half body lengths of another while
oriented toward it.

Attack Scored when one animal lunged at or charged
another but did not pursue the other or
initiate vigorous biting behavior. This
behavior could be accompanied by a single
bite or attempts to bite.

Chase Scored when one animal pursued another.

Fight Scored when one animal's attack on another
escalated to vigorous biting behavior by
both individuals. Generally the initiator
would knock the other animal over, or roll
to one side with the other animal clenched
in Its jaws while shaking Its head, often
simultaneously clawing with the rear claws.
Fighting often resulted after one animal did
not retreat when attacked, but rather attempted
to defend Itself or at the end of a chase If
the pursuing animal caught the other.

Rough-and A very vigorous form of fighting; rough-and-
Tumble-Fight tumble fights were only scored when both
animals were tumbling end over end while
attempting to bite and claw each other.

Displacement Scored when one animal retreated upon
another animals approach.

Submissive Scored when one animal, upon approach or

attack by another, either rol led over on its
back, or reared back upon its hind legs with
its nose pointed up, and made no attempt to
defend itself. Both approach and submission
or attack and submission were scored for each
encounter.

Aggressive This was a very vigorous form of digging
Digging behavior much more Intense than the type of
digging these animals have been observed to
perform in an isolated test (Webster, Williams,
Owens, Geiger, and Dewsbury; 1981). Although
this behavior was not generally directed toward
an opponent, it was very similar to that described
by Al I In and Banks (1968).

Under both experimental conditions, single and paired, the
isolation period was followed by the seminatural interaction period.
Ten mln prior to the first dark phase In this period the rubber
stoppers were removed from the partitions between each corner
compartment and the central area. Behavioral and aggressive

interactions between animals, as defined above, were recorded during
the first hour of the dark phase for 4 consecutive days. Nesting
relationships were recorded each day, for the last 3 of these 4 days,
20 min prior to the beginning of the dark phase. An animal was defined
as having nested with another if it was found in the same nest with the
other, and videotape records verified that it had not switched nests
between the period extending from after the first 1/2 hr of the
preceding light phase to 1/2 hr prior to the nest check. Activity
during the entire isolation and interaction stages was videotaped.

The seminatural apparatus, including the artificial burrows
and nest boxes, was thoroughly cleaned with a solution of Sterigent (a
deodorant and disinfectant soap) before each group of animals was
introduced to the apparatus. Water bottles were also cleaned and
refilled, and fresh San-i-cel, food, and Nestlets were placed in the
apparatus.
Results

Peromyscus pol ionotus appeared to adapt very quickly to the
seminatural apparatus in general, and to the artificial burrows in
particular. All but five of the 40 individuals in the paired condition
entered and explored the artificial burrows within the first hour of
observation, and all except two pairs of the 20 pairs observed in the
paired condition had constructed at I eat a rudimentary nest in the
burrow by the end of their first day in the apparatus. Individuals in
the paired or single groups were only infrequently observed nesting in
the alternate nest box, although this box was frequently used for
feeding and as an escape when individuals were attacked. Aggressive

relationships between Individuals in each group appeared to remain
fairly stable over their four days together in the seminatural
apparatus. In all 10 single groups, and in seven of the 10 paired
groups, the individual with the highest total frequency of aggressive
behavior (sum of all attacks, chases, fights, and rough-and-tumble
fights) on day one, still exhibited the highest frequency of aggressive
behavior on day four. In the other three paired groups the Individual
that exhibited the highest frequency of aggressive behavior on day two
also exhibited the highest frequency on day four.

Each animal in a group could potentially Interact with twice as
many opposite-sexed individuals as same-sexed Individuals. To adjust
for this bias, the mean value of any measure of an animal's interaction
with both opposite-sexed individuals, rather than the total, was used
in analyses Involving opposite-sexed Individuals.

Several significant differences in aggression were apparent In
comparisons between males and females in both the paired and single
conditions. Males were more aggressive than females in a statistically
significant larger number of groups by all measures except the number
of rough-and-tumble fights (designated In tables as r&t fights; see
Table 1). Within the single condition males were the more aggressive
sex in a significantly larger number of groups by all measures except
the number of rough-and-tumble fights and the duration of
rough-and-tumble fights. Within the paired condition males were the
more aggressive sex In a significantly larger number of groups for the
measures of number of attacks, number of fights, duration of fights and
number of approaches. There was no measure, for either the paired or

Table 1

Comparison of the Number of Groups In Which
Males or Females Were More Aggressive

Total

(N=20)

Paired

(N=10)

Sing

e (N=10)

Measure

Mai e

Female

Male

Female

Male

Female

No. of attacks

1*#*

0**

No. of fights

2***

Duration of fights

2***

No. of chases

3**

Duration of chases

3**

No. of r&t fights

Duration of r&t fights

3**

No. of approaches

1***

0**

No. of displacements

All durations are In seconds.

Sign test 2-tail *£<.05 **£<.01 ***£<. 001

single condition, for which females were more aggressive than males in
a larger number of groups.

Males In general also exhibited higher total levels of aggression
than females by all measures (see Table 2). Males, in both single and
paired groups, exhibited a higher frequency of attacks, fights, chases,
displacements and approaches than females. They also exhibited longer
durations of chases and fights than females In both types of groups
(see Table 3). Although differences in the total amount of submissive
behavior exhibited by males and females across both groups were not
statistically significant (±=1.28, d_£=19, £>. 05), males had more
submissive behavior directed toward them than did females
(pa!red-±=2.71, d±=19, £<.05).

Because aggressive digging was not immediately recognized as a
possible correlate of aggression, it was not recorded for the three
Initial paired groups. It was recorded for all subsequent paired and
all single groups. No significant differences in the frequency,
duration, or mean duration of aggressive digging were apparent in
comparisons between paired and single animals. Males displayed higher
levels on all of these measures than did females, with the exception of
the comparison of the average duration of aggressive digging for single
animals. High-aggression males (those In each group with the highest
total frequency of aggressive behavior) displayed a higher frequency of
aggressive digging than low-aggression males (paired-±=2.30, d±=16,
£<.05). The difference between high and low-aggression animals In the
level of aggressive digging they displayed may be due In part to
differences In the response of these two classes of individuals to

Table 2

Comparison of Total Aggression V/ithin Groups by Males
and by Females In Both Paired and Single Conditions

Mai

Fema

Measure

Mean

(SE)

Mean

(SE)

No. of attacks

46.30

5.95

10.20

2.41

5.23****

No. of fights

14.55

2.24

3.05

1.07

4.23****

Duration of fights

23.00

3.96

4.60

1.46

4.31 ****

No. of chases

116.20

12.18

21.85

5.51

5.98****

Duration of chase

679.10

79.01

128.40

32.79

5.59****

No. of r&t fights

5.00

1.44

1.25

.42

2.60*

Duration of r&t fights

10.30

3.04

2.80

1.39

2.35*

No. of approaches

64.10

7.91

23.40

3.61

5.62****

No. of displacements

12.80

2.59

3.40

.90

3.43***

All durations are in seconds.
Paired ±- test d±=19 2-tail
****£<. 001

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without Interference.

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opposite-sex individuals in both the paired and single conditions (see
Tables 4 and 5). Males in both single and paired groups directed more
fights and rough-and-tumble fights, and longer durations of these
behaviors, against other males than against females. Females in both
single and paired groups directed more attacks toward same-sexed than
opposite-sexed Individuals. Males and females in both types of groups
directed more chases toward same-sex than opposite-sex Individuals.

Although many of the differences In aggression between sexes were
significant, and many significant differences were also found in the
level of aggression directed at same versus opposite-sexed Individuals,
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paired conditions were very similar (means and standard errors were
presented in Table 3). Animals in the two conditions displayed
significant differences on only two of the aggressive measures: paired
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Individuals and a greater number of approaches to same-sexed
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within each sex yielded significant differences for females only (means
and standard errors were presented in Tables 4 and 5). Paired females
exhibited a greater number of approaches to same-sex individuals and a
greater number of displacements of same-sex individuals than did single
females (±=2.57, d±=38, £<.02; and ±=2.08, d±=38, £<.05,
respectively) .

The various measures of aggression recorded tended to be
correlated with each other. The total frequency of attacks, chases,
fights, and rough-and-tumble fights were correlated within each sex for
both paired and single groups, as were the duration of chases, fights,
and rough-and-tumble fights. The frequency of approaches was
correlated with the number of chases for males and females in both
conditions and with the number of attacks for paired males and females
and single males (see Table 6). The amount of submissive behavior
directed toward single females was correlated with the frequency and
duration of fights (Pearson correlation, £=.724 and £=.798,
respectively, p.<.001) and the frequency and duration of rough-and-
tumble fights (Pearson correlation, e=-704 and .800, respectively,
£<.001). Submission was also correlated with the total frequency of
approaches for paired females and single males (Pearson correlation,
£=.444 and £=.515, respectively, £<.05). Frequency of aggressive
digging was correlated with the total frequency of aggressive behavior
(combined frequencies of attacks, fights, chases, and rough-and-tumble
fights) for paired males and females (£=.638, £<.05, and £=.845, £<.001
respectively) and single males (£=.697, £<.001). Duration of

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aggressive digging was correlated with total frequency of aggressive
behavior for single males (£=.527, £<.05) and paired females (£=.845,
£<.001). An animal's weight did not appear to be an important factor
In aggressive interactions. Only the correlation between weight and
the frequency of rough-and-tumble fights in single females was
statistically significant (£=.378, £<.05).

For paired groups comparisons were also made between the level of
aggressive behavior that occurred while pairs were confined to the
corner compartments and the level after access to the entire apparatus
was al lowed. Very few aggressive interactions of any type (chases,
fights, attacks, rough-and-tumble fights) were observed between pair
members during the period pairs were confined to the corner
compartments (Mean total frequency of aggressive interaction per paired
individual. 20, range = 0 - 4.0). Therefore only the total frequency
of aggressive behavior, rather than the frequency of behavior in each
category, was used for comparison.

The total frequency of all aggressive Interactions (attacks,
fights, rough-and-tumble fights, and chases) between pair members while
confined to the corner compartments was not useful In predicting
frequency of later aggressive interactions with pair members
(males:£=.034, p>.05; females: £=.087, £>.05) or total frequency of
aggressive interactions (males: £=.015, £>.05; females: £=.118,
£>.05).

Pairing did, however, have some effects on later levels of
aggression. Males in paired groups were less aggressive to the females
with which they had previously been paired than to females with which

they had not been paired by the measures of frequency and duration of
chases (one-tal I paired-±=2.18 and 2.22, respectively, d±=19, £<.05).
Males also exhibited a higher frequency of approach to females with
which they had been paired (one-tail paired-±=1 .78, d_£=19, £<.05).
None of these comparisons were significant for females (Number and
duration of chases, ±=.38 and .76 respectively, approach; t=1 .08,
d±=19, £>.05).

A problem arises when one attempts to compare different classes of
animals as to their levels of aggressive interactions with one another.
The problem is that the total amount of contact between different
classes of animals may vary. For example, if females tend to avoid
other animals, but males do not, Individuals would have more
opportunities to be aggressive to males than to females. A difference,
therefore, in the level of aggression an individual expresses toward
Individuals of one class versus another may reflect a true difference
in the frequency of aggression, or a difference in the frequency of
access to individuals of the two classes. One method of gaining a
clearer understanding of the level of aggression, and differences in it
between classes of individuals, Is to construct a scale or index for
comparisons which accounts for differences In frequency of contact.

An "aggressive Index" was calculated for this study by dividing
the total frequency of all aggressive encounters initiated by an animal
(frequency of attacks, chases, fights, and rough-and-tumble fights)
toward any other class of Individuals by the total number of contacts
Initiated by that animal toward that class of individuals (total
aggressive encounters plus the frequency of approaches and

displacements). Although there may be qualitative differences in
approaches which elicit displacement or submission and those which do
not (for example an aggressive individual may signal its status through
adopting a particular posture), no means was available in the present
study to detect these cues. Therefore displacements or approaches with
submission were classed as contacts without aggression for purposes of
constructing the index.

No significant differences were apparent between paired and single
groups, or in comparisons between males or females of these groups, in
the total frequency of contacts or the frequency of contact with
same-sexed or opposi te-sexed individuals (see Talbe 7). Single females
did, however, have a significantly higher overall aggressive index
(frequency of all aggressive behaviors divided by frequency of all
contacts) and a higher aggressive index against opposi te-sexed
individuals than paired females.

The total frequency of contact was significantly higher for males
than for females in the paired and single conditions. Although the
overall aggressive index was significantly higher for paired males than
paired females, the difference between single males and females was not
significant (see Table 8).

Paired males and females and single males all displayed a
significantly higher frequency of contact with same-sexed than
opposi te-sexed individuals. Whereas both paired males and females
displayed a significantly higher aggressive index against same-sexed
than against opposi te-sexed individuals, this comparison was not
significant for single males or females (see Table 9). Paired males,

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but not paired females, exhibited a lower aggressive index against pair
members than against opposi te-sexed non-pair member (two-tail t-test,
d_f=19, males and females respectively: t=2.25, £<.05; ±=1.41, £>.05).

The frequency of nesting arrangements, for days on which complete
nest data were available, is presented in Table 10. It is of interest
that two animals of the same sex nested together only once without an
opposi te-sexed animal also present. Animals nested as two opposite-
sexed pairs en over 1/3 of the days for which data were available and
almost another 1/4 of the nesting arrangements observed included one
opposi te-sexed pair.

Only data on nesting behavior for days on which nesting
relationships were known for all individuals in a group were analyzed
statistically. Nest data were available for all but one paired and one
single group. Three days of nest data (the total possible) were
available for two of these nine paired, and seven of these nine single
groups. Two days of data were available for six paired and two single
groups, and only one day of data was available for one of the paired
groups. The mean number of days of data available for nine paired and
nine single groups were 2.11 days and 2.78 days, respectively.

The analyses of nesting behavior presented in the tables are based
on the "IV 1 Icoxon-test" (Siegel, 1956). Analysis of nesting behavior by
this test in the present study may give more "weight" to observations
from groups for which more days of data are available. Significant
comparisons in the tables that were not also significant by the "sign-
test" (Siegel, 1956) are noted in text. Animals generally nested with
opposi te-sexed rather than same-sexed animals. Differences in nesting

Table 10
Frequency of a I I Possible Nesting Arrangements

Paired

Single

Total

Groups

A I 1 four together

Two opposite-sex pairs

Two males, one female

Two females, one male

Two males

Two f ema 1 es

One opposite-sex pair

None together

Only days on which all animals could be accounted for are included
(Total number of days=44).

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frequency with same and opposi te-sexed animals were significant for all
comparisons except paired animals in general, and paired females (see Table
11). (Ey the sign test the comparison for all females was also non-
significant, x=10). In general animals also nested with high-aggression
rather than low-aggression animals of the opposite sex (see Table 12). This
was true for animals in paired groups and single groups, and for males and
females. The finding of non-significance for comparisons within paired and
single groups, except single females, may be due to the small number of non-
tied observations available for comparison. (By the sign test comparisons
for paired animals and males in general were also non-significant, x=2 for
both comparisons). Total frequency of aggression was correlated with
frequency of nesting with opposi te-sexed individuals for paired males and
single males (Pearson correlation, £=.710, £<.001 and £=.519, £<.05
respectively) but not for paired or single females (r=.066 and r=.21 9
respectively, £>.05).

Animals did not nest more frequently with familiar individuals.
Comparisons based on all paired animals, males, or females were all
non-significant (Wilcoxon, N=number of non-tied observations: all
animals, N=24, z=1.24; males, N=10, £=1.22; females,
N=14, z=.60). Neither the number of days pairs had built nests
together during the nest building stage, nor the average type of nest
built, was correlated with nesting frequency with pair members (Pearson
correlation, nest days, r=.395; nest type, r_=.250).
Discussion

These seminatural observations provide evidence that aggression is
likely to be an important factor in social interactions in £*.

pol ionotus. As predicted of the behavior of monogamous species
(Kleiman, 1977) frequent aggression was displayed by both males and
females, and the majority of aggressive behavior was directed against
same-sexed Individuals. The suggestion that females of this species
are normally dominant over males (Smith, 1967) was not supported by the
results of the present study. Males displayed much higher levels of
aggression than did females, and females generally exhibited very low
levels of aggression toward males. In addition, high-aggression males
more frequently performed a behavior, aggressive digging, that could
function to display their aggressive status.

Although the frequencies of aggressive behaviors In paired and
single groups were very similar, the aggressive index Indicated that
individuals in paired and single groups behaved differently.
Individuals in single groups did not appear to discriminate between the
targets of their aggression as well as paired individuals did.
Overall, the differences in aggressive behavior between paired and
single groups may indicate a tendency for reduced aggression toward
opposite-sexed Individuals, especially pair members, In paired
Individuals. This Is particularly true of paired males, which
displayed both reduced total frequencies of aggression and a lower
aggressive Index, against females with which they had been paired than
against those with which they had not been paired.

Superior aggressive ability would appear to provide some social
benefits for individuals, as both males and females nested more
frequently with high-aggression rather than low-aggression Individuals
of the opposite sex. On the other hand familiarity, although it

appeared to reduce aggression between pair members, did not have a
significant effect on nesting behavior. It would appear from these
observations that, In P^ pol Ionotus. aggressive ability may be a more
potent factor In social preference than In familiarity. However,
because It is unlikely that Individuals under the present conditions
were always able to control who nested with them, it is probably best
to use caution In interpreting these results.

Aggression and Familiarity Preference Tests
Introduction

The results of the semi natural experiments indicated that
aggression may be an important factor in social preference in Pj,
pol ionotus. but cast some doubt on the importance of familiarity In the
social preference of this species. This experiment was designed to
test preference based on aggression and familiarity In JL. pol ionotus
and E*. manlculatus in a more controlled manner, through the use of a
preference apparatus.

Subjects

Subjects were Individuals of two species of muroid rodents, P_*.
pol ionotus and JL. maniculatus. A total of 40 animals of each species
served as subjects for aggressive tests and aggression preference
tests. Prior to serving as subjects, animals were maintained as
described in the general methods section. Within each species these
animals were each assigned to one of 10 groups, with two animals of
each sex per group. In addition to the criteria described In the
general methods section, no individual within each group could be

related by more than two common grandparents to any other animal in the
group.

Following the aggression tests and aggression preference tests the
animals described above also served as "stimulus" animals for
familiarity preference tests, while an additional 40 animals, 20 of
each sex of each species, served as "choice" animals for these tests.
Procedure

Procedures were identical for each experimental group. Animals
for each group were separated from litter mates and individually housed
in 48 x 27 x 13 cm clear plastic cages. Peromyscus man icu I atus were
moved from their colony room to the £*. pol ionotus colony room. On the
following day, within the first 1/2 of the dark phase of the
photoperiod, animals were lightly anesthetized with ether and marked
for identification by shaving them in one of two patterns: either (1)
a band was shaved from around the neck area, or (2) a band was shaved
from around the middle of the animals. One animal of each sex was
shaved in each pattern. Animals were placed in 48 x 27 x 13 cm plastic
cages modified (as previously described under aggression apparatus) for
aggression testing.

All adaptation and testing were conducted during the dark portion
of the photoperiod. Animals were adapted to the preference apparatus
on the two days following the marking procedure. On the first of these
two days animals were adapted to the procedure that would be used when
they served as "choice" animals.

Adaptation to "choice" procedures was as fol lows: the animal was
placed in the start box for 5 min, followed by 1 hour free in the

apparatus without other animals present. Animals that did not exit the
start box within 1 1/2 mln after the door was lifted were gently
prodded with the eraser end of a pencil, often simply lifting the lid
of the start box slightly provided sufficient stimulus for the animal
to leave the box. The same procedure was fol lowed during tests.

The day following adaptation to choice procedures, animals were
adapted to "stimulus" conditions. Adaptation for stimulus animals
consisted of being placed In the stimulus boxes at either end of the
preference apparatus for 1 hour. Animals which were tested together as
stimulus animals for experimental tests were also adapted together.
During adaptation of stimulus animals for the aggression preference
tests an opposi te-sexed "pretest" animal was allowed free in the
apparatus and its visits to either chamber recorded. This period was
designated as the pretest period. Each pretest choice animal had been
adapted to the apparatus previously, and served In several pretests
with animals of the opposite sex.

Following adaptation to the preference apparatus animals were
adapted to the "aggression apparatus" for 2 hours on each of the next
three consecutive days. For these adaptation periods the animals home
cage was connected to the aggression arena by means of lengths of Tygon
tubing inserted into the holes cut in the sides of the home cage and
Into the sides of the aggression arena. Animals were restricted to the
arena for the first 40 mln of the 2 hr period by means of #6 black
rubber stoppers inserted In the ends of the connecting tubes. The
stoppers were removed for the remainder of the period so that the
animal had access to both the arena and its home cage.

Aggression tests were conducted on 3 consecutive days following
adaptation to the aggression apparatus. Males and females were
observed. Tests were conducted by placing the two shaved animals of
the same sex into the aggression arena and observing them for 40 min.
Behavior during this period was categorized as approach, displacement,
aggressive, or submissive. Frequency of all aggressive behaviors
(total of all attacks, chases, and fights) was Included under one
category because aggressive behaviors other than attacks were extremely
infrequent.

Aggressive preference tests were conducted on the 2 days following
aggression tests. Two animals of one sex, that had been tested
together in the aggression tests, each served once as choice animals.
The two animals of the opposite sex, that had been tested together on
the aggression test, served as stimulus animals for both tests. Tests
were arranged by placing one of the same-sexed pair of stimulus animals
In each of the boxes at the ends of the preference apparatus 15 min
prior to the beginning of the test, and the choice animal in the start
box 10 min later. Tests were initiated by raising the door to the
start box, and thereby allowing the choice animal access to the
apparatus. Test duration was 1 hr, timed from when the choice animal
exited the start box. Order of testing was counterbalanced for sexes
across days. Although the stage of estrus was not controlled for In
these tests, smears were taken for each female after she had served as
a stimulus animal and on the following day.

On the day after the conclusion of aggression tests for a group,
the members of the group were each housed in a 29 x 19 x 13 cm plastic

cage with an Individual of the opposite sex. These "new" opposite-
sexed individuals had been adapted on the previous day to the apparatus
under the procedures described for choice animals. These animals each
served once as choice animals in the familiarity preference tests that
followed. Animals that had been paired for the aggression preference
tests also served together as stimulus animals for the familiarity
preference tests (The two same-sexed stimulus animals for each of these
tests were two Individuals that had been partners In tests for
aggression). This arrangement produced four tests for each group, two
tests for each same-sexed set of stimulus animals.

Smears were obtained from the females of each group prior to the
onset of the dark period on the seventh day after animals had been
paired. Tests were conducted with stimulus females only when both of
these females exhibited smears consisting of at least 15% leucocytes.
This type of smear would normally indicate a nonreceptive state.
Females that displayed sperm on the smear were tested after they
displayed this type of smear. Choice females were tested individually
if they displayed smears with at least 15% leucocytes. Test procedures
for choice and stimulus animals were as described previously.
Results

Aggression Tests. The level of aggression (attacks, chases,
fights, and rough-and-tumble fights) displayed in the aggression tests
were very low. No aggressive behaviors were displayed in four of the
ten groups by females of either species. Peromyscus pp| ionptus. males
did not display aggression In two groups, while P^ m.a,nicu latus males
did not display aggression In three groups. The comparison of total

values (for all three tests) for the four behavioral categories
recorded are displayed by sex for each species in Table 13. Only the
comparison of E_l pol ionotus males and females on frequency of
aggressive behavior was significant, with males displaying a higher
frequency of aggressive behaviors.

Because the frequency of aggression in these tests was very low
it was difficult to determine which animal of a pair was actually the
"most aggressive". Instead, the total frequency of approaches and
aggressive behaviors was used to provide an indication of which animal
of a pair might be more aggressive. This total frequency score,
although not an aggression score per se, does allow a comparison of the
tendency to initiate Interactions or "assert iveness" of the two
individuals In a pair. It would seem reasonable to expect that the
less timid of two Individuals under these test conditions might also be
more likely to exhibit more aggression under other conditions. The
individual of a pair of animals in aggression tests that displayed the
greatest tendency to initiate interactions will be termed the
"high- interaction" individual, while the Individual that displayed the
lower tendency to interact will be termed the "low- interact I on"
Individual .

Aggression Preference. Table 14 presents data on the preferences
of animals of each sex for high or low- Interact I on Individuals of the
opposite-sex. No preference for high- interact ion animals of the
opposite sex was displayed by ELm. pol ionotus of either sex. Perpmyscus
maniculatus males, however, did display significantly longer durations
of visits with high- interaction females than low- Interaction females

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High- interaction P. maniculatus females and low- Interact ion P.
maniculatus males both displayed significantly longer durations of
visits with high rather than low- interact ion animals of the opposite
sex (means for high- interaction females with high and low- Interact ion
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Although the stage of estrus for females in aggression tests was
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male preference for high- interaction and low- Interact ion females were
made for these tests in which both stimulus individuals were In
diestrus (see Table 15). The only significant finding was a preference
by Ej. maniculatus males for high- Interact ion females by the measure of
duration of visits. This is also the only comparison which had been
significant when data from all females was included. Comparisons could
not be performed within the non-diestrous condition as there were no
cases for P_». pol ionotus in which both stimulus females were
non-dietrous, and only two such cases for £». manicu latus females.

Comparisons of male preference for dlestrous versus non-diestrous
high- interact ion females and diestrous versus non-diestrous low-
interaction females are presented In Table 16. Peromyscus pol Ionotus
males displayed more visits to non-diestrous high- interact ion females
than to diestrous high- interaction females, and Pj_ manicu I atus males

Table 15

Comparison of Male Preference for

High- Interact ion and Low- I nteraction

Females in Diestrus

High Low

Interaction Interaction

70.86

11 .41

76.00

16.18

.29

618.14

152.07

419.60

115.14

1.33

11.22

2.75

8.47

2.54

1.12

Measure Mean (SE) Mean (SE)

P. pol ionotus

No. of visits

Total duration of visits

Mean duration of visits

E*. maniculatus

No. of visits

Total duration of visits

Mean duration of visits

217

.00

187.73

17.50

3.85

1.07

1291

.28

510.14

114.94

61.94

2.43*

.55

48.03

6.32

2.36

1.37

All durations are in seconds.

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visited non-dlestrous low- interact ion females more frequently than
diestrous low- Interact I on females. All other comparisons of male
preference based on stage of estrus were non-significant. The stage of
estrus did not significantly affect female preference for high or
low- interact ion males of either species (see Table 17).

Fami I iarity Preference. Male and female P. pol ionotus did not
display preference for familiar individuals over unfamiliar Individuals
of the opposite sex by any measure, although the comparison of the
total duration of visits did approach significance for females
(one-tall paired-±, di=19, ±=1.47, £=.054). Peromyscus maniculatus
males and females displayed significantly more visits to familiar than
to unfamiliar individuals of the opposite sex. Females also displayed
longer durations of visits to familiar males than to unfamiliar males
(see Table 18), and a greater number of females exhibited more visits
to familiar than to unfamiliar males (one-tail sign test, N=18, 21=5,

H<.05).

Because stimulus animals in familiarity preference tests had
previously been evaluated In aggression tests, the "attractiveness" of
these animals In familiarity preference tests could also be evaluated
on the basis of their tendency to interact with other animals. These
comparisons are presented in Table 19. Peromyscus pol ionotus females
displayed significantly more visits to high- interaction than to
low- Interact ion males, and the difference In the number of females that
spent longer durations with high- interact ion than low- Interact ion males
was also significant (one-tai I sign test, N=20, x=5, £<.05).

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Discussion

Whereas individuals observed in the seminatural apparatus had
exhibited appreciable levels of aggressive behavior, levels of
aggression displayed by individuals in the aggression apparatus were
very low. This finding was somewhat unexpected. The object of the
procedures followed in the present study had been to condition
Individuals to treat the aggression arena as an extension of the home
cage. In previous observations Individuals housed in cages Identical
to those used to form the aggression arena had displayed fairly high
levels of aggression to Intruders after one to two weeks of residency
(unpublished observations). In the present study Individuals were
tested for aggresslvity in the arena on the seventh through ninth days
of residency in the home cage. Although individuals did not have
access to the home cage during aggression tests, they had been allowed
to travel freely between the home cage and aggression arena during
adaptation. In addition, during aggression tests the home cages and
the aggression arena were attached in a manner that allowed a fairly
free flow of air between them. Many Individuals were observed to spend
long periods sniffing and gnawing at the entrances to their home cages.
Individuals therefore appeared to recognize their home cage as opposed
to a strange cage (unfortunately these observations were not
quantified). During aggression testing, however, individuals did not
behave as residents in the aggression arena, nor did they defend the
entrances to their home cage. Animals In these tests were
simultaneously exposed to olfactory cues from both their home cage and
that of their opponent. These test conditions may have led to

conflicting "fight" and "flight" responses (See Hinde, 1966), and
thereby resulted in low aggression scores.

It is of interest that individuals In the seminatural apparatus
displayed high levels of aggression toward one another Immediately upon
being allowed access to the central arena, even though none of these
Individuals had previous exposure to this area, and had been in
residence In the connected home areas only three days. The observation
of higher levels of aggression under the seminatural conditions than
under aggress ion- test conditions may be due in part to the fact that
individuals in the seminatural apparatus were exposed to opposi te-sexed
individuals during tests while individuals in the aggression apparatus
were not (Barnett, Evans & Stoddart, 1968; Brain, Benton, & Bolton,
1978; deCantazaro, 1981; Flannel ly & Lore, 1977; O'Donnell,
Blanchard, & Blanchard, 1981). Exposure to females has been
demonstrated to Increase male-male aggression In Ejl rngpiculatus balrdl
(Termen, 1982; Dewsbury, Personal communication). Exposure to
opposi te-sexed Individuals was, however, not required to elicit
aggression in the previously mentioned tests of resident £*. pollonotus
In aggression-arena-sized cages; and males in the seminatural
single-housed condition were observed in several tests to Initiate high
levels of attacks and chases prior to exposure to females.

Preferences for high-Interaction (and presumably more aggressive)
individuals were not as strong as might have been predicted from the
results of the seminatural experiments in the present study, or from
the results of previous studies (e.g., Blair & Howard, 1944;
Eisenburg, 1962). Although in the majority of comparisons (over 80?)

scores on the measures of preference for high- interaction individuals
were higher than those for low- interact ion individuals, most of the
differences displayed between high and low- interact I on individuals were
non-significant. This may be a reflection of the low levels of
aggression displayed In these tests. Previous Investigators (e.g.
Huck et a I., 1981) have hypothesized that the differences In odors
displayed by dominant and subordinate animals are mediated by
differences in the physiological changes induced In these Individuals
through aggressive encounters. The levels of aggression in the present
tests may not have been high enough to fully induce the physiological
changes necessary for clear-cut discriminations on the part of choice
animals.

Although levels of aggression were low in aggression tests, females
of both species, and E*. manlculatus males, displayed significant
preferences for the more "assertive" individual of the opposite sex
under some preference test conditions. Significant preferences were
not displayed for low- interact I on individuals of either sex in either
species. Under the assumption that Individuals that are more assertive
would also normally be more aggressive, these findings are consistent
with the hypothesis proposed earlier that Individuals of these species
should prefer aggressive opposite-sexed individuals, and also with the
nesting behavior exhibited by EU. pol ionotus In the semlnatural
apparatus.

In familiarity preference tests JL_ manlculatus of both sexes
displayed preference for familiar Individuals of the opposite sex,
while Ex pol ionotus did not display such preference. Although the

lack of preference displayed by £. pol ionotus In the familiarity
preference tests Is consistent with observations made In the
seminatural study, results of the familiarity preference tests are
contrary to predictions made earlier as to the behavior of these two
species.

Sibling Preference Tests
Introduction

Individuals of most rodent species will be exposed to siblings
during development. Such exposure may, of itself or in conjunction
with genetic factors, influence mate selection (Grau, 1982; Halpin &
Hoffman, 1982; Smith, 1966). The general consensus holds that, except
under special circumstances, Individuals should prefer to breed with
non-relatives (Daly & Wilson, 1978; Dewsbury, 1982a; Krebs & Davles,
1981; WIttenberger, 1981). Rasmussen (1970), however, has suggested
that Inbreeding may be fairly extensive In some species of Peromyscus.
particularly JPj. maniculatus (Rasmussen, 1964); and Howard (1949) has
proposed that inbreeding may account for as much as 10 percent of
breeding in P. maniculatus. Smith (1966) has suggested that "a
considerable amount of Inbreeding" (p. 50) also occurs In Pj.
pol ionotus.

Not all Investigators agree with these proposals however.
Selander (1970) has questioned the genetic basis for Rasmussen' s (1964)
conclusions regarding P_». maniculatus. and other Investigators
(Dewsbury, 1982a; Hill, 1974) have demonstrated suppressed
reproduction for sibling matings In this species. Foltz (1981b) has
also questioned Smith's (1966) proposal of high levels of Inbreeding In

EL. pol ionotus. The present study was designed to evaluate preference

for sibl Ings in P^ pol ionotus aM EL manlculatus.

Subjects

Both FLl pol Ionotus and EL manlculatus served as subjects for
sibling preference tests. Subjects were selected from 12 litters of
each species that contained at least two animals of each sex. Two
animals of each sex were selected from these litters, under criteria in
the general methods section, and maintained together throughout the
experimental procedure. Litters selected for groups were unrelated by
more than one common grandparent. Two I itters of each species
comprised an experimental group.
Procedure

Individuals were ear-punched for identification, and EL
manlculatus I itters were moved to the EL pol ionotus colony room. On
the following day animals were adapted to the preference apparatus,
without other individuals present, as both stimulus animals and choice
animals.

Preference tests were of two types, same-sex tests and opposite-
sex tests. In same-sex tests choice animals had a choice between a
same-sexed sibling stimulus animal and a same-sexed nonslbling. In
opposite-sex tests animals had a choice between two opposlte-sexed
individuals, one a sibling and one a nonslbling. Each animal in a
group served twice as a stimulus animal and once as a choice animal for
each type of test. In an opposite-sex test, for example, males 2 and 4
would serve as stimulus animals for choice females 2 (one of the 2 male
siblings) and 4 (a 4 male sibling). They would each also serve as

choice animals with stimulus females 1 (a 2 male sibling) and 3 (a 4
male sib I Ing).

Preference tests were conducted on the two days following
adaptation. All tests of one type (i.e. opposite-sex tests) and half
of the tests of the other type were conducted on the same day. Testing
was completed on the second day. The order of test type across days
was counterbalanced. The number and duration of visits to each chamber
were recorded on each test and for the adaptation period. Vaginal
smears were obtained for each female before the beginning of the dark
cycle on each test day.
Results

Peromyscus pol ionotus females demonstrated a preference for
siblings over nonsibllngs (see Table 20). Sibling males were visited
significantly more frequently than nonsibl Ing males, and a
significantly larger number of females spent longer durations with
sibling rather than nonsibl ing males (sign test, N=24, x.=6, 2-tai I
£<.05). They also spen+ significantly longer durations with sibling
rather than nonsibl ing females. None of the comparisons for siblings
versus nonsibl ings were significant for P*. pol Ionotus males or for P.^.
manlculatus of either sex (see Tables 20 and 21). It Is, however, of
Interest that scores for £». pol Ionotus males on tests with opposlte-
sexed animals mirror those of JL. pol ionotus females (higher scores for
siblings), while for all but one measure (average duration of visits
for females) P.,. manlculatus display higher scores for opposl te-sexed
nonsib I Ings.

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Table 22

Comparison of Male Preference for
Sibling and Nonsibling Females in Diestrus

Measure

Nons

ibl inq

Mean

(SE)

Mean

(SE)

90.83

777.06

28.66

24.40

173.36

13.16

95.33

904.81

35.11

21 .10

259.62

15.51

.33
.41
.94

P. pol ionotus

No. of visits

Total duration of visits

Mean duration of visits

Ej. maniculatus

No. of visits 18.21 4.45 14.64 3.22 1.36

Total duration of visits 1048.67 323.90 1448.26 367.33 .64

Mean duration of visits 87.96 35.79 201.42 84.01 1.12

All durations are in seconds.

±-test P. pol ionotus dl=1 1 P. maniculatus df=14 2- tail

£>.05 for all comparisons

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m o co
r- vo o

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p*. k\ o p- co t

p* k\ <7* — r- t
in \o o o r- i

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cn cn

CD —

in —

C JO

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J0 c

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c +- +■

— o > > — —

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E Z

As with the aggression preference tests comparisons were made of
male preference for siblings and nonsiblings in diestrus and of male
preference for diestrous versus non-dlestrous siblings and nonsiblings.
None of these comparisons were significant for either fL. pol ionotus or
fL. manlculatus males (see Tables 22 and 23). Comparisons of the
preferences of females in different stages of estrus did, however,
yield some significant differences. Although P^ po| lonotus females
exhibited no significant differences In preferences based on stage of
estrus, P. manlculatus females did display significantly greater
preference for nonslbling males, by the measures of total and average
durations of visits, when in a non-diestrous condition over a diestrous
condition (see Tables 24 and 25).

The possibility of litter effects on preference scores was also
evaluated by conducting a one-way analysis of variance on the
"difference scores" between preference scores for siblings and
nonsiblings. Significant litter effects were found for the average
duration of visits by JL. pol lonotus males to females (F( 1 1 ,12)=3.77,
p_<.05). and for the total duration of visits by fL_ pol lonotus females
to males (F(1 1 ,12)=3 .08, £><.05).
Discussion

Whi le Peromyscus pol ionotus males general ly displayed higher
scores on preference measures for siblings than for nonsiblings, these
comparisons were not significant. EL. pol Ionotus females, however, did
display significant preferences for siblings over nonsiblings. The
results of this study, therefore, may offer some support for proposals
of high levels of Inbreeding In P. pol ionotus (Smith, 1966). The

observation that scores for preference by IL. maniculatus males and
females were generally in the opposite direction from those for E*
pol ionotus Is of interest. This difference may Indicate a greater
preference for siblings by Em. pol Ionotus than by P. maniculatus.
It is also of interest that non-diestrous FL. maniculatus females
displayed a greater preference than diestrous females for nonsibl ing
males, since females in a non-diestrous condition would be more likely
to be receptive. These results are consistent with previous studies
demonstrating suppression of reproduction In sibling matings of E*.
maniculatus (Dewsbury, 1982a; Hill, 1974), and the Implicit conclusion
from these studies that inbreeding should be avoided by members of this
species.

SECTION IV
GENERAL DISCUSSION

In the general discussion the observations of the present study
are examined in light of previous research, and interpretations are
suggested for these observations that are consistent with the ecology
and mating systems of Pj. maniculatus and P*. pol ionotus. The general
discussion section is divided into six subsections. The first of these
subsections examines aggressive ability as a factor in preference.
This subsection begins with a fairly extensive overview of the ecology
of Pj. maniculatus and P_». pol ionotus. and also presents evidence for
the role of aggression in the ecology of these two species. The
ecological information presented in this subsection serves as a
background for discussions which fellow in all subsequent subsections.
The discussion of the ecology of these species is followed by a
discussion of factors that may provide an adaptive basis for selection
of aggressive mates in these species.

The second subsection deals with familiarity as a factor in
preference and with the effect of prior breeding experience on
preference for familiar individuals, and includes a discussion of the
opportunities that may be available for P_». maniculatus and P,.
pol ionotus to utilize familiarity as a basis for mate selection. This
subsection is followed by a subsection on the related topic of kinship
as a factor in preference. The subsection on kinship discusses

evidence for and against Inbreeding In JL. manlculatus and EU.

pol ionotusr and the ecological and social factors that may affect

preference for siblings as mates In these species. This subsection is

followed by a subsection on the evolution of monogamy In E*

pol ionotus, and the summary for this study.

Aggressive Ability as a Factor In Preference

Many of the observations In the present study were consistent with
the general hypothesis that aggressive ability may be Important In the
social behavior and mate selection of monogamous and polygamous
species. They were also consistent with the specific predictions that
aggression should be an important component of the social behavior of
E* pol ionotus, and that P. pol ionotus and E*. maniculatus of both
sexes should prefer mates with high aggressive ability. In the
seminatural experiments aggression was routinely displayed by both
male and female E*. pol Ionotus. and both males and females of this
species nested more frequently with the more aggressive of the two
opposlte-sexed Individuals. Female E*. pol Ionotus also displayed
preference for the more assertive of two males In familiarity
preference tests; and preference for more assertive individuals was
displayed by E*. maniculatus males and high- Interaction P. manlculatus
females In aggression preference tests.

The preferences exhibited by these species may be mediated through
advantages In reproduction gained by individuals that choose mates with
good aggressive ability over those that choose mates with poor
aggressive ability. Some of the possible advantages accrued by
individuals that choose mates with high aggressive ability have been

briefly discussed In previous sections. The present discussion will

focus on the advantages that might be gained through such choice by

Individuals of the two species of interest in this study, P_».

pol ionotus and P^. manicu latus, In the context of the ecology and

mating system of these two species.

Ecology, Mating System, and Aggressive Ability

Many ecological variables will play a role In determining to what
extent aggression may be adaptive for individuals of any particular
species, and thus also the Importance of aggressive ability as a
quality in mates. The ecology of a species may often be as major a
factor as its mating system In determining the Importance of aggressive
ability to reproductive success in that species. Among the more
general ecological problems with which Individuals of a species must
cope with in order to reproduce successfully, and one that also affects
the mating system of a species, Is the availability and defensibi I ity
of resources necessary to reproduction (Brown, J. L., 1975;
Clutton-Brock & Harvey, 1978; Emlen & Oring, 1977; Hal I Iday, 1978;
Orians, 1969). Aggressive ability may play an Important role In the
acquisition and defense of these resources in monogamous and
non-monogamous species. Individuals that choose mates high In
aggressive ability may be Insuring that adequate resources will be
available for themselves and their offspring; choosing such mates
should therefore be more adaptive than choosing mates with low
aggressive abi I ity.

Peromyscus manlculatus

Peromyscus maniculatus occurs In a wide variety of habitats over
most of the North American continent (Baker, 1968; Hamilton, 1943;
Hooper, 1968). This species will accept a wide variety of food items
(Cogshall, 1928; Martel I & Macaulay, 1981; Williams 0., 1955) and
nest sites (Blair, 1940; Hamilton W. J., 1943; Howard, 1949), and
the distribution of the various subspecies in the environment may be
governed more by different behavioral responses to different habitat
types than by any absolute differences in nesting requirements or food
preference (Dice, 1922; Harris, 1952; Wecker, 1963). The subspecies
of Ej. maniculatus may for the most part be divided Into two general
types, those adapted forand occupying openlands such as fields, and
those adapted for and occupying woodlands and brush lands (Baker, 1968).
Peromyscus maniculatus bairdi, the subspecies observed In the present
study, Is generally found in "open fields, sand beaches, and arable
land of the prairie states..." (Hamilton W. J., 1943, p. 270).

Peromyscus maniculatus bairdi generally live in overlapping home
ranges (Blair, 1940). As is typical of other Peromyscus (Stickel,
1968), each home range contains several nests and refuge holes, and
mice may change nests frequently (Blair, 1940; Howard, 1949). In
addition to utilizing several types of naturally occurring nest sites
(Blair, 1940; Howard, 1949) individuals of this subspecies are capable
of constructing their own burrows (Houtcooper, 1972). Peromyscus
maniculatus bairdi is therefore more likely to be restricted to
particular locations within Its preferred habitat by considerations
such as the availability of sufficient food than by availability of

nest sites. This Interpretation receives support from observations by
Howard (1949) which indicate that Increasing the number of nests
available In an area does not increase the number of resident breeding
mice In that area; and by observations which indicate that Pj_ nh.
bairdl cache food for winter use (Hamilton W. J., 1943; Howard,
1949), and that the availability of food may limit the number of
resident breeding adults in other subspecies of FL. maniculatus
(Fordham, 1972; Gashwiler, 1979; Taitt, 1981).

The habit of caching food and the large winter aggregations
observed in this subspecies (Howard, 1949, 1951) are probably
adaptations for winter survival. Energy requirements for these mice in
the winter months are high (Howard, 1951) and populations In some
localities may suffer severe winter mortality (Blair, 1940; Howard,
1949). These mice do exhibit an ability to become torpid at low
temperatures. In severely cold weather, however, individual mice may
not be able to become torpid without freezing. Utilization of cached
food allows individuals to maintain high metabolic rates (and therefore
a high body temperature), and huddling in winter aggregations both
reduces body heat loss and allows Individuals to become torpid (Howard,
1951). Winter aggregations generally appear to consist of a breeding
pair and at least one previous litter, additional conspecific adults of
both sexes, and sometimes Individuals of other species (Howard, 1949).
Peromyscus maniculatus bairdi have general ly not been observed to breed
In the winter (Blair, 1940; Howard, 1949), although a few individuals
may sometimes be capable of breeding through the winter In
favorable microhabltats (e.g., Cornshocks: Linduska, 1942).

If food availability is the primary factor determining the
suitability of a particular area (within the preferred habitat type)
for breeding in P__ m__ bairdi . one might expect that it would also
affect the social organization and mating system of this species.
Because £__, nix. bairdi appears to uti I ize a wide variety of different
food items, the Importance of which may vary seasonally (Houtcooper,
1978), it is likely that defense of the food supply per se Is not
economically feasible. Peromyscus maniculatus bairdi appear to have
opted Instead for a strategy of limiting the number of breeding
individuals in or near their home ranges. This Is accomplished in part
by adults aggressively limiting juvenile settlement in their home range
(Ayer & Whitsett, 1980; Enders, 1978; Whitsett, Gray, & Bediz, 1979).
In some subspecies of E*. maniculatus juvenile settlement appears to be
restricted largely as a result of male aggression toward juveniles
(Fairbairn, 1977; Healey, 1967; Metzgar, 1979, 1980; Mlhok, 1979;
Sad lei r, 1965); while for other subspecies (Including ___. n__ ba.lr.d_D
female aggression might be as important as or even more Important than,
male aggression In limiting juvenile settlement (Ayer _ Whitsett, 1980;
Enders, 1978; Fordham, 1971; Taltt, 1981; Whitsett et al., 1979).
As In other subspecies of P__. maniculatus (Fairbairn, 1978; Healey,
1967; Llewellyn, 1980), male aggression In P. m. bairdi appears to
be under hormonal control (Whitsett et al., 1979). The level of
aggression in male £_. m__ bairdi may therefore follow a seasonal
pattern of changes as described for other subspecies of P__. maniculatus
(Healey, 1967; Llewellyn, 1980; Metzgar, 1979; Sadlelr, 1965).

The general seasonal pattern of changes in the level of aggression
for male Pj. maniculatus. as described by Healey (1967), consists of a
spring increase In male aggression (which is correlated with an
increase In testicular size) to a peak through the major breeding
season, followed by a drop in the level of aggression to negligible
levels at the time fall aggregations occur. This seasonal pattern of
changes In the level of male aggression Is reflected In seasonal
changes In social organization. At least some subspecies of Em.
maniculatus have been observed to form large non-aggressive non-
breeding winter aggregations (Dice & Howard, 1951; Howard, 1949, 1951;
Metzgar, 1979). Although sufficient observations are not available to
permit evaluation of the generality of the tendency of JL. maniculatus
to form large winter aggregations, most subspecies of P*. maniculatus
do appear to exhibit a spring dispersal period prior to the major
breeding season. The shift in behavior and social organization that
occurs at this time have probably been best described by Healey (1967)
for Pj. nh. austerus. According to Healey a greater number of mice
survive the winter than are compatible on the breeding territories.
Animals which were resident in the overwintering areas, and their
offspring, are most likely to be dominant In and therefore most likely
to settle in these areas, while subordinate animals disperse.
Established residents aggressively exclude new settlers, while
aggressive Interactions between neighboring residents are reduced. In
p. m. austerus mutual avoidance between same-sexed resident adults
may lead to mutal ly exclusive home ranges within sexes, but overlapping

home ranges between sexes (Healey, 1967; Sadleir, 1965). Other

authors (e.g., Metzgar, 1979), however, have observed a pattern In this

subspecies that is exhibited by several other subspecies of Ex

maniculatus including Ex m*. bairdi — same-sexed home ranges

may overlap, but appear to overlap much less extensively than home

ranges for opposite-sexed individuals (Blair, 1940, 1942, 1943; Howard,

1949; Morris, 1955). The general pattern observed In these studies

consists of large resident-male home ranges that overlap each other

slightly, while each also extensively overlaps several smaller

resident- female home ranges. In at least some subspecies residents may

also tolerate same-sexed non-breeding subordinates within their home

range (Metzgar, 1979, 1980).

The breeding system of Ex maniculatus appears to be somewhat

labile. Although males generally appear to form breeding relationships

with a few adult females within their home range (Blair, 1958; Howard,

1949; Mihok, 1979; Terman, 1961), adults have been observed in

combinations that included more than one breeding individual of either

sex (Blair, 1958; Howard, 1949), and members of this species have also

been observed to form long lasting and apparently exclusive breeding

pairs under some conditions (Blair, 1958; Howard, 1949). Metzgar

(1979) has proposed a general system to explain the lability apparent

in Peromyscus breeding systems.

In the proposed system, the home ranges of breeding
males overlap those of adult females broadly and the
two classes occur together far more frequently than by
chance alone. Within a sex, breeding adults are evenly
dispersed but considerable home range overlap may occur
depending on density and home range size. The large
home range of a breeding male usually includes all or
parts of several adult female home ranges. Female
ranges may be overlapped by ranges of several breeding

100

males, especially when male-male overlap is extensive.
However, even with extensive overlap, each breeding
male might spend most of his time with a particular
female (Garson, 1975). Furthermore, under some
conditions (low densities and small home ranges), this
generally loose male-female association might be
expressed as enduring male-female pairs. (p. 142)

The particular breeding relationships exhibited by a given

population of E* maniculatus may be largely determined by food

availability, as food availability appears to influence population

density and home range overlap in this species, and may also result in

an alteration in the sex ratio of the breeding population. A more

abundant food supply in an area may result in an Increased density of

adult mice in that area (Fordham, 1972; Gashwiler, 1979; Taitt,

1981), and a contraction of home range for breeding individuals of both

sexes (Taitt, 1981). Although Taitt (1981) noted an increase in the

number of both sexes in an area supplied with additional food; Fordham

(1972) observed an increase in the number of females and the proportion

of females breeding, but no increase in the number of males. A

reasonable explanation for this difference was proposed by Taitt (1981)

who noted that while she had provided additional food to a winter

population, Fordham (1972) had provided additional food during the

breeding season. The aggressive resident males in Fordham' s study may

have prevented recruitment of additional males, but not females.

Abundant food in the breeding season may, therefore, result in a female

biased breeding population. A skewed sex ratio may in turn have an

effect on the breeding associations exhibited by EL. EL. bairdl .

Howard (1949) has noted that "in areas where the sex ratio was not

equal, as many as three females lived in the same nest box with one

101

male, and as many as three males lived In one nest box with one female"
(p. 14).

As noted earlier, although fL. maniculatus do not appear to
defend feeding areas per se, resident animals may prevent
over-utilization of food supplies within their home ranges by
aggressively limiting the settlement of potential breeders in these
areas. The presence of an adequate food supply within a home range may
increase the probability of winter survival for residents and their
offspring. The observations reviewed in the preceeding discussions
indicate that the availability and distribution of food items may have
major consequences not only on the distribution and survival of these
mice, but on the social behavior and breeding relationships exhibited
by them as we I I .
Peromyscus polionotus

In light of several aspects of the ecology of P_t pol ionotgs,
choosing mates with high aggressive ability may be adaptive for members
of this species. One ecological factor of primary importance to this
species is the availability of suitable nest sites. Because they
construct their own burrows (Hayne, 1936; Smith, 1966; Smith & Criss,
1967) Ej. pol ionotus are not restricted by the availability of
naturally occuring nests per se. This species does, however, require a
fairly narrow range of soil conditions for nest construction, and In
habitat undisturbed by man is probably restricted to areas in early
successional stages (Golley, Gentry, Caldwell, & Davenport, 1965), and
sandy beach areas. According to Smith (1966) abundance of £j.
pol ionotus "is correlated with soil type, amount of soil drainage

102

(Table 2), type and amount of vegetation. All of the habitats occupied
by this species are characterized by sparse vegetation and relatively
well-drained or recently plowed soils ..." (p. 11). Well-drained fine
sand soils were preferred for burrowing, and mice "never constructed
burrows in hard soils where digging was difficult, nor in areas where
the hardpan was close to the surface of the ground" (p. 13). Few mice
were observed in heavily forested areas, or in areas with dense
vegetation (Rand & Host, 1942; Smith, 1966; Personal observations),
and the number of mice nesting in a given area appeared to be
negatively correlated with the density of vegetation (Rand & Host,
1942; Smith, 1966). Individual mice construct several burrows in
close proximity (Rand & Host, 1942; Smith, 1966) within a fairly well
defined home range (Blair, 1951; Davenport, 1964) and defend these
burrows against intruders (Blair, 1951). It is likely that in addition
to defending burrows within their home range, individuals also
aggressively exclude other potential settlers from suitable nest areas
near their burrows. This suggestion is supported by the observation
that often the burrows within a given area al I appeared to have been
constructed by a single individual, or pair of individuals (Rand &
Host, 1942; Smith, 1966), and by observations in the present study
that animals attacked and chased one another in all areas of the
semi natural apparatus, and not just in the area that actually contained
their burrow. Although P*. pol ionotus defend burrows within their home
range (Blair, 1951), they do not appear to defend their entire home
range (Davenport, 1964). The apparent lack of defense of home range by

103

E*. DQl ionotus may be a reflection of the fact that this species
utilizes a wide variety of food items (Gentry & Smith, 1968; Smith,
1966) that are probably not economically defendable.

Although multiple burrows may simply serve a survival function as
refuges from predators, the maintenance of several burrows is more
likely to act in some manner to maximize reproductive success in E*.
pol ionotus. Blair (1951) and Rand and Host (1942) observed that P*.
pol ionotus change nests frequently; this frequent change of nest site
could be a strategy to reduce the level of parasitic Infestation of
offspring. Ej. pol ionotus have also been observed to utilize
unoccupied burrows as food caches (Blair, 1951; Rand & Host, 1942;
Smith, 1966). Some evidence exists that populations of this species
may at times be food limited (Smith, 1971; Smith & Blessing, 1969);
cached food might provide a food reserve that would allow parents and
offspring to survive and/or reproduce during such periods.
Alternatively cached food could contain nutrients that , although not
critical for survival, might be critical for reproduction. Food caches
may therefore allow breeding in seasons during which it would not
otherwise be adaptive. Evidence supportive of these hypotheses are
Smith's (1966) observations that food-deprived individuals of this
species become torpid at very cool temperatures while non-food-deprived
individuals do not, and observations of improved reproductive
performance in pairs fed acorns (a frequently cached food item)
parasitized by beetle larvae. Abundant cached food may allow
individuals to maintain a level of activity compatible with breeding,
even in cooler temperatures, and high-quality food caches (e. g.

104

acorns containing beetle larvae) may provide nutrients that would
otherwise not be available in winter months.

An additional benefit that may derive from the maintenance of
multiple burrows has been discussed by Foltz (1979). Upon the birth of
a new litter older offspring may move into a nearby burrcw with their
father. The behavior of moving to nearby burrows may allow offspring
to extend the period during which they may take advantage of parental
resources and care. Offspring that receive extended care may be better
able to compete for food and to establish burrows when they disperse
than offspring that do not receive such care. This may be of particular
importance in £* pol ionotus because dispersing individuals appear to
suffer very high mortality (Smith, 1966, 1968).

The maintenance of multiple burrows by E* pol ionotus appears, as
a result of the factors discussed, to be a highly adaptive strategy for
members of this species. The facts that E* pol ionotus are restricted
to very specific types of habitat, and that individual mice maintain
several burrows in an area, probably act as major factors that limit
the size of the breeding populations in this species. If nest sites
are a limited and critical resource for Ej. pol ionotus. one would
expect individuals of this species to compete for them, and to defend
them from conspeci f ics.

According to consensus polygyny should be generally be
advantageous to males (Brown J. L., 1975; Daly & Wilson, 1978;
Dawkins, 1976; Orians, 1969; Verner, 1964), and males should
therefore attempt to control limited resources In a manner that allows
them to acquire additional mates. Because nest sites appear to be a

105

limited and defendable resource for R*. pol ionotus. males of this
species might be capable of gaining multiple mates by controlling areas
suitable for nesting (see Verner & Willson, 1966, p. 145). This
species has, however, consistently been found to exhibit a monogamous
mating system (Blair, 1951; Foltz, 1979, 1981; Rand & Host, 1942;
Smith, 1966).

According to Em I en and Oring (1977) the two preconditions for
a species to exhibit a polygamous mating system are that (1)
individuals must be potentially capable of economically defending
multiple mates, or resources critical to gaining multiple mates, and
(2) individuals must be able to utilize that potential. Several
factors may prevent R«. pol ionotus males from fulfilling these two
preconditions for polygamy. Among the more important of these factors
are the distribution of nest sites and food items, and it is likely
that monogamy in £* pol ionotus rests upon considerations related to
both of these factors.

Avai labi I ity of nests. As discussed previously the number of
suitable nest sites for fL. pol ionotus in any given area may be very
limited. It is possible that nest sites for this species may be so
limited as to impose monogamy because two adults are required for their
defense (See Wilson, 1975, p. 330). Alternatively a male may be
capable of defending a sufficient number of burrows to maintain a
single female and her litter, but nest sites may be too dispersed for a
male to defend a sufficient number of burrows to maintain two females.
Although the maintenance of multiple burrows appears to be important in
terms of the ecology of JL. pol ionotus. in light of the available data

106

it is difficult to evaluate how many burrows a pair might actually
require to maximize its reproductive success. While the availability
or distribution of nest sites alone might explain monogamy in Pj.
pol ionotus, it is most likely to be of importance in conjunction with
other factors. One might expect for example that if food items were
abundant year-round, burrows would not be used as food caches, and
males might be more capable of acquiring additional females.

Availability of food items. If suitable habitat for burrowing
was abundant a male might be capable of defending enough burrows to
maintain several females. However, even if burrows were abundant,
females themselves may be too widely dispersed to defend if food items
are sparse (CI utton-Brock & Harvey, 1977, 1978; Eisenberg, 1977), or
if food items are so widely dispersed around nest areas as to make it
highly unlikely that a female could raise a litter without assistance.
Under these conditions females may require males to display some
evidence of commitment (Maynard Smith, 1977) prior to mating with them,
or may impose monogamy on males by aggressively preventing other
females access to their mate (Wittenberger, 1979, 1981; Wittenberger &
Til son, 1980).

Aggression and Mate Selection
Intraspecif ic aggression

Peromyscus maniculatus. As discussed in preceeding sections
aggression is an important component of the social behavior of
Peromyscus maniculatus. and an individual's aggressive ability probably
plays a major role in determining if that individual will be capable of
establishing and maintaining itself as a breeding member of the

107

population. In laboratory studies dominant males of this species have
been found to be more successful than subordinate males in nesting with
females (Blair & Howard, 1944; Eisenberg, 1962), copulating with
females (Dewsbury, 1979; 1981c), and siring offspring (Blair & Howard,
1944; Dewsbury, 1981c). Dewsbury (1981c) has also observed that
females tend to approach and solicit dominant males more frequently
than subordinate males. Although female approaches were confounded
with copulatory behavior and male availability because females "tended
to approach and solicit males with which they were copulating and which
were nearby" and "this was generally the dominant male" (Dewsbury,
1981, p. 892); these observations may indicate female preference for
dominant males in this species. The observation in the present study,
of a preference by males and females for the more assertive of two
opposite-sexed individuals, is consistent with the hypothesis that
individuals of this species may prefer aggressive individuals as mates.

Peromyscus pol ionotus. Although some investigators (e.g. Blair &
Howard, 1944) have observed only very low levels of aggression in P*.
pol ionotuSp other investigators (Garten, 1976; Smith, Garten & Ramsey,
1975) have observed higher levels; and these investigators have
proposed that aggression in Pj. pol ionotus is genetically based, and
that it may play an important role in both the social behavior and
social organization of this species. Information presented in the
preceeding discussions suggested that aggressive ability may serve
important functions in i ntraspecif ic competition in P. pol ionotus. and
may therefore be an important factor in mate selection in this species.
The results of the present study support the hypotheses that aggressive

108

ability may be important in the social behavior and mate preferences
exhibited by this species. In the seminatural experiments Individuals
of both sexes displayed aggressive behavior, the majority of which was
directed against same-sexed individuals. This is consistent with the
type of behavior Kleiman (1977) has proposed as indicative of monogamy.
In addition members of both sexes nested more frequently with the more
aggressive of two opposite-sexed individuals, and members of both sexes
displayed significant preference for more assertive individuals of the
opposite sex in preference tests.
Interspecific aggression

Because individuals of most species appear to compete with members
of other species for at least some resources, aggressive ability could
function in interspecific as well as intraspeci f ic interactions.
Peromyscus maniculatus appears to compete with other species In many
parts of its range (Drickamer, 1978; Holbrook, 1978; Kritzman, 1974;
Redfield, Krebs, & Taitt, 1977), and P. pol ionotus often appears to
compete with Mus musculus for food (Briese & Smith, 1973; Caldwell &
Gentry, 1966a). Aggressive competition for resources is also common in
Microtus; for example: M*. agrestes appears to compete with Mj.
arval is for resources (Dienske, 1979), while both of these species may
at times be in competition with Clethr ionomys glared us (DeJonge,
1979); Mj. longicaudus appears to be excluded from its preferred
habitat by M. montanus in some localities (Randall, 1978); and Nk
pennsy I van icus and M. ochrogaster may also compete In some areas
(Miller, 1969). Individuals in most rodent populations are probably
exposed to at least a moderate level of interspecific competition for

109

resources, and individuals with high aggressive ability would be
expected to fare better in this competition than individuals of low
aggressive ability, and thus to be preferred as mates. Interspecific
competition could, therefore, function to maintain choice for
aggressive mates in many species — including those that may not
exhibit high levels of i ntraspeci f ic aggression.
Farlv breeding

Earlier breeding (Darwin, 1874) is probably not a consideration in
the preference displayed for aggressive mates by Pj_ po| jonotus, but
may be of some importance to IL. maniculatus. It is unlikely that
early breeding is an important factor to P_«. pol ionotus. Although
Ej. pol ionotus display breeding peaks and declines they do not appear
to be seasonal breeders (Davenport, 1964; Smith, 1966). Therefore,
although some seasons may be better for breeding than others (Smith &
McGinnis, 1968), an individual can not really get an "earlier start" in
breeding than others in the population.

Peromyscus maniculatus on the other hand do appear to be seasonal
breeders, and also form large winter aggregations that may contain
unrelated adults (Howard, 1949). More-aggressive individuals might be
capable of initiating breeding earlier than less-aggressive individuals
after dispersal from winter aggregations when competition for resources
is likely to be intense (Healey, 1967; Taitt, 1981).
Bruce effect

Exposure of females to unfamiliar males may result in pregnancy
blockage (Bruce, 1959; 1960). This phenomenon is known as the "Bruce

110

effect", and several explanations have been offered for it. Many of
these hypotheses have been reviewed by Schwagmeyer (1979), who has
proposed that "females are, in effect, selecting one mate In preference
to another when pregnancy blockage occurs. One would therefore predict
that the Bruce effect would be limited to circumstances in which the
benefits from mating with the new male outweigh any cost of the delay
in parturition or physiological effects involved" (p. 934).

This argument has recently been extended by Huck, Soltis, and
Coopersmith (1982). These investigators observed that dominant male
house mice (Mus musculus) significantly reduced the survival of strange
pups, whereas subordinate males did not, and that dominant males would
copulate with a female after killing her litter. Previous
investigators (Hrdy, 1979; Labov, 1980; Mai lory & Brooks, 1980) had
suggested that, in species in which males committed Infanticide, it
would be less costly for females to block pregnancy and mate with a
strange male than to lose the litter later through infanticide.
Following these suggestions, their own observations, and the
observation by Huck (1982) that dominant males are more effective in
initiating pregnancy blockage; Huck et a I . ( 1 982 ) proposed that it is
most advantageous for females to display pregnancy blocks when
confronted with dominant rather than subordinate males because they
face the greatest risk of infanticide by these males. As stated by
Schwagmeyer (1979), by displaying pregnancy blocks females are in
effect displaying a preference to mate with aggressive males; and the
Bruce effect would be adaptive in this context as long as the costs of
postponing the current litter were outweighed by the benefits derived

Ill

from mating with a male of high aggressive ability. The Bruce effect,
like the inciting behavior of female elephant seals (Cox, 1981; Cox &
LeBeouf, 1977), may be a means by which a female ensures that she mates
with males with high aggressive ability.
Heritable aggressive ability

The benefits of selecting mates with high aggressive ability that
have been discussed to this point are not dependent upon any component
of an animal's aggressive behavior being heritable. The expression of
many of the components of aggressive behavior has, however, been
demonstrated to be at least partial ly under genetic control (Scott,
1966; Scott & Frederickson, 1951; Simon, 1979). The components of
aggression may be divided into two major categories: components
related to the tendency for an animal to display aggression, or its
"aggressiveness" , and components related to the ability of an animal
to effectively perform aggressive behaviors. The validity of this
division is supported by research reviewed and discussed by Scott
(1966), and by Scott and Frederickson (1951). As stated by Scott
(1966) "Heredity produces important differences in fighting behavior
between mouse strains, some being more easily excited to fight than
others and some strains being more capable of winning than others" (p.
691).

Although an animal's tendency toward aggression and its ability to
perform aggressive behavior effectively may be separable, they would be
expected to be highly positively correlated. One would expect that it
would not be adaptive for an animal to be highly prone to engage In
aggressive encounters If it stood little chance of winning those

112

encounters; nor on the other hand would it be expected to be adaptive
for an animal to be completely unprovokable if it had high aggressive
ability. Because fights involve potential costs as well as potential
benefits (Maynard Smith, 1976; Maynard Smith & Price, 1973),
individuals should assess opponents carefully, and fight only when they
have a reasonable expectation of winning (CI utton-Brock & Albon, 1979;
Clutton-Brock, Albon, Gibson & Guinness, 1979). Individuals that most
frequently display aggressive behavior, therefore, should normally also
be those individuals that have the highest aggressive ability. If the
display of aggression is an indication of an individual's aggressive
ability, then these displays may be used to evaluate that individual's
potential as a mate (Cox, 1981; Cox & LeBeouf, 1977). While, as
previously noted, many of the benefits that may accrue to individuals
that chose mates with high aggressive ability do not depend on that
ability being heritable, choosing aggressive mates should be even more
adaptive if any of the components of aggressive ability are heritable.
Individuals selecting aggressive mates would gain not only immediate
benefits, such as better territory or defense of nest site, but "good
genes" (Maynard Smith, 1956; Trivers, 1972) as well. (However see,
Krebs & Davies, 1981; Maynard Smith, 1978, p. 170, 171; Parker,
1979, p. 146; for limits on the use of heritable factors as a basis
for choice). Selection of mates with high aggressive ability may be
adaptive even when such individuals do not hold the best available
resources, if these individuals are more attractive as mates, and
aggressive ability is heritable. This combination of conditions has

113

been suggested to act to lower the polygny threshold in species (the
"sexy son" hypothesis: Heisler, 1981; Weatherhead & Robertson, 1979).

Some observations of Peromyscus maniculatus bairdi and P.
pol ionotus suggest that the display of aggression, or displays related
to aggressive ability, may provide a basis for mate selection in these
species. Dewsbury's (1981b) observation that FL. maniculatus females
approach and solicit dominant males more frequently than subordinates
in two-male copu I atory tests, although open to other intrepretations,
is also consistent with this suggestion. In the present study
high-aggression JL. pol ionotus males displayed aggressive digging more
frequently than low-aggression males, and may act to prevent the
display of this behavior by low-aggression males. The areas around
nest sites of this species often exhibit evidence of frequent digging
behavior. Although members of this species do not appear to exhibit
territorial behavior (Davenport, 1964) they do defend nest burrows
(Blair, 1951), and most likely also some of the area around burrows.
The "incipient burrows" and other evidence of digging near nest burrows
may provide an indication to individuals that an area Is occupied; and
aggressive digging may be a means of displaying an ability to construct
and defend burrows.

Familiarity as a Factor in Preference
The results of the present study were not consistent with the
hypothesis that members of monogamous species should display greater
preference for familiar than unfamiliar individuals, nor with the
hypothesis that the effects of familiarity on the choice of members of
polygamous species should be negligible. In familiarity preference

114

tests individuals of both sexes of the polygamous species Peromyscus
maniculatus displayed preference for familiar individuals of the
opposite sex, while individuals of the monogamous species Peromyscus
pol ionotus did not display preference. In addition, Et pol ionotus did
not display preference for nesting with familiar individuals of the
opposite sex in the semi natural experiments. Two types of factors may
have influenced the behavior of _P. maniculatus and £*. pol Ionotus in
familiarity preference tests; these are 1) factors related to the
prior history of the animals tested and 2) factors related to the
ecology and social system of these particular species.
Prior History

The ability to recognize differences in the familiarity of
individual odors has been demonstrated in a wide variety of mammalian
(Brown, R.E., 1979; Halpin, 1980) and non-mammalian (Hal pf n, 1980)
species; and evidence exists that individuals of many rodent species
may exhibit preferences based on this ability (Brown, R.E., 1979). Of
particular relevance to the present discussion are observations
suggesting that an individual's preference for familiar or unfamiliar
conspecifics may be determined in part by whether that individual
previously received monogamous or polygamous mating experience. While
monogamously mated female rats appear to prefer the odor of a familiar
over a novel male, polygamous I y mated females display no preference,
polygamously mated males prefer novel females, and monogamously mated
males may either display no preference or preference for novel females
depending on test conditions (Carr, Krames, & Costanzo, 1970; Carr et
al., 1979; Carr et al., 1980; Krames et al., 1967). The general

115

tendency in these studies appears to be greater preference for familiar
individuals after monogamous mating experience and greater preference
for novel individuals after polygamous mating experience. Dewsbury
(1979) has also observed a higher probability of mating In fL. nu
bairdi in tests in which individuals were familiar (had previous
monogamous mating experience) than in tests in which mates were
unf ami I iar.

Prior to familiarity preference tests animals in the present study
were housed with a single opposite-sexed conspecific and therefore,
outside the possibility of having mated with siblings, would have had
only monogamous mating opportunities prior to these tests. While
pretest housing conditions may in part explain the preference for
familiar individuals displayed by JL. maniculatus. they can not explain
the lack of preference displayed by fL. pol ionotus. The lack of
preference displayed by fL. pol ionotus in the familiarity preference
tests, although consistent with observations of the nesting behavior of
this species in the seminatural study, is inconsistent with predictions
based on the mating system of this species. These differences in the
preference of P. maniculatus and P_». pol ionotus may be explained by
differences in the ecology and social organization of these two
species.

Ecology and Social System
Peromyscus maniculatus

As noted previously breeding fL. eu. bairdi appear to maintain
overlapping home ranges (Blair, 1940). Although breeding adults appear
to aggressively limit settlement of strange juveniles in their home

116

range (Ayer & Whittsett, 1980; Enders, 1978, Whitsett et al., 1979)
little overt aggression appears to occur between members of established
populations (Hill, 1977; Terman, 1961, 1974). Lack of aggression
between established residents In a population is not uncommon, and may
be a result of the "dear enemy" phenomenon as described by Wilson
(1975): "A territorial neighbor is not ordinarily a threat. It should
pay to recognize him as an individual, to agree mutually upon the joint
boundary, and to waste as little energy as possible in hostile
exchanges thereafter" (p. 273). This effect appears to occur in
at least some (Healey, 1967), but not all (Vestal & He I lack, 1978)
subspecies of £*. manicu latus.

The dear enemy phenomenon may be a factor in juvenile dispersal
and settlement in Eju maniculatus. Blair (1958) observed that the
majority of juveniles in a population of E_l maniculatus in Texas
dispersed either very short distances from the natal site or not at
all; and 38 percent of the females and 28 percent of the males in a
population of R*. el. bairdi in Michigan did not disperse from their
natal home range to breed (Dice & Howard, 1951). Healey (1967) has
suggested that for £,. maniculatus "an animal's chances of breeding are
severely limited when it moves any distance from its birth place" (p.
388). These observations suggest that a juvenile has the greatest
probability of breeding successfully if it remains on or near the
parental home range. In light of the aggression directed toward
unfamiliar juveniles by adults (Ayer & Whitsett, 1980; Enders, 1978;
Healey, 1967; Sadleir, 1965) one of the more successful strategies for

117

juvenile £». man leu latus may be to breed with its neighbors rather than
to attempt to disperse and breed with unfamiliar conspecif ics.

Early breeding may be an additional factor acting to increase the
adaptive value of selecting familiar mates in this species. As Is
typical of other Peromyscus (Terman, 1968), E*. maniculatus tends to
remain in a particular area once it has established itself as a
breeding resident of that area. The apparent sedentary nature of
residents, and the overlapping home ranges they exhibit (Blair, 1940;
Howard, 1949; Mihok, 1979; Morris, 1955), probably result in each
resident breeding within a fairly well defined group of familiar
conspecif ics. Individuals within an area aggregate in the winter and
disperse from these aggregations to breed in the spring (Howard, 1949;
Metzgar, 1979). Individuals that have become familiar In overwintering
aggregations or through prior breeding may be able to pair more
quickly, and establish themselves as breeders earlier in the spring
than unfamiliar Individuals.
Peromyscus pol ionotus

Peromyscus pol ionotus are sedentary once they become resident in
an area (Blair, 1951; Smith, 1966, 1968), are not seasonal breeders
(Davenport, 1964; Smith, 1966; Smith & McGinnis, 1968), and exhibit a
monogamous mating system in which they form long-term reproductive
pairs (Blair, 1951; Foltz, 1979, 1981). In addition, members of this
species are generally fairly short lived (Dapson, 1972). As a result
of these factors the opportunities for members of this species to
utilize familiarity as a factor in mate selection during its lifetime
may be somewhat limited. Two situations that could occur in which

118

familiarity may be a factor In mate selection in this species would be
in the initial selection of a mate upon attaining reproductive
maturity, or in the selection of a new mate upon the death or
incapacitation of the current mate. These possibilities will be
examined in the following discussion.

As in L maniculatus. L*. pol ionotus exhibit overlapping home
ranges that do not appear to be defended against breeding adult
conspecifics (Davenport, 1965). It Is likely that the majority of
aggressive interactions in IL. pol ionotus. other than possibly
aggression incidental to foraging, are centered around the defense of
nest sites (Blair, 1951). In light of the specific nesting
requirements of this species, nest site defense would probably
effectively limit juvenile recruitment into the resident breeding
population since it is unlikely that a juvenile could obtain nest sites
in competition with adult conspecifics. A juvenile could breed near
the natal home range if it were able to attract a neighboring resident
animal as a mate. Opportunities for a juvenile to enter a breeding
relationship with a familiar resident are, however, likely to be very
limited. Established residents may be exposed to the choice of
juvenile mates only in the event of the reproductive failure or death
of their existing mates. Juveniles, however, would generally be
expected to perform more poorly in competition with residents than
would an adult mate. In addition a juvenile's reproductive performance
is unproven, and at least for females generally below that of adults
(Caldwell & Gentry, 1965b; Smith, 1966; Williams, Gol ley, & Carmon,
1965). The best choice for a resident adult after the loss of a mate,

119

therefore, would probably be to mate with a neighboring resident
rather than a juvenile; or barring the availability of a neighboring
resident to mate with a transient adult.

Although small groups of young sexually immature R*. pol ionotus
have been observed in the winter (Smith, 1966), it is unlikely that
familiarity from relationships in these groups has a general effect on
mate selection in this species. These groups comprised less than three
percent of the social groups observed by Smith (1966), and such groups
were never observed by Rand and Host (1942). Smith (1968) has
suggested that these groups are composed of I itter mates that are
overwintering in parental burrows. Several observations lend support
to this suggestion. First, adult defense of burrows (Blair, 1951)
makes it unlikely that unrelated individuals would be tolerated in
burrows. In addition, evidence reviewed by Foltz (1979) suggests that
parental males may move with litters to burrows near the natal burrow.
Finally, the largest of these groups observed by Smith (1966) was
composed of six individuals, which is within the range of litter sizes
reported for P. pol ionotus (Laffoday, 1957; Smith, 1966; Williams,
Gol ley & Carmon, 1965) .

Kinship as a Factor in Preference

Although inbreeding may be advantageous under certain conditions
(Bengtsson, 1978; Cowan, 1979; Maynard Smith, 1978), the generally
detrimental effects of inbreeding, such as inbreeding depression, have
likely led to the evolution of mechanisms to avoid inbreeding in most
species (Bixler, 1981). Evidence available for the two species of
interest in the present study, P_,_ maniculatus and P_l pol ionotus,

120

suggests both high and low levels of Inbreeding for these species. In

the following discussion evidence for and against inbreeding In these

two species, and the relevant observations from the present study, will

be discussed in the context of the ecology and social behavior of these

species.

Inbreeding in Peromyscus Maniculatus

Evidence for and against inbreeding

Howard (1949) noted that in the population of P.^. nu. bairdi he
he studied "There seemed to be no bar to the mating of close relatives,
and parent-offspring matings and pairing between sibs occurred when
conditions were such that these related mice happened to be together at
the time when they became sexually active" (p. 15). Howard (1949)
estimated that up to 10 percent of the matings in the population he
observed were inbred. Rasmussen (1964) has also suggested a high level
of inbreeding for £*. nu. graci I is based on the observation of a
shortage of heterozygotes in the population he observed. Foltz (1979,
1981b) has, however, criticized both the Howard (1949) and Rasmussen
(1964) studies on the basis of methodological flaws. This criticism
seems well founded on the basis of evidence cited by Foltz (1979), and
by observations that reproductive performance in sibling matings in
this species is below that for nonsibling matings (Dewsbury, 1982;
Hill, 1974). Although differences in the preference of P. maniculatus
for siblings and nonsiblings in the present study were statistically
non-significant, scores in the majority of comparisons were higher for
nonsibl ings than for sibl ings; and P_*. maniculatus females that were
not in diestrus displayed significantly greater preference for

121

nonsibling males than did females in diestrus. These observations are
consistent with the hypothesis that E*. maniculatus should prefer
nonsib lings as mates over siblings.
Ecological and social factors

Because JL. DU. bairdi may exhibit inbreeding depression (Dewsbury,
1982a; HIM, 1974) individuals of this species would be expected to
display stronger preference for nonsib lings than was exhibited in the
present study. The small differences in preference exhibited in the
present study might be explained i f P*. m*. bairdi. although attracted
to nonsiblings as mates, are also attracted to siblings on other bases.

It might be adaptive for siblings to be attracted to one another
if by remaining together they were able to increase direct benefits to
themselves (e. g., through increased survival), or if such behavior
resulted in an increase in their inclusive fitness (Hamilton, W. D.,
1964, a, b). At least two factors in the behavior of L. Dk bairdi
make it probable that they will maintain extensive contact with
relatives, including siblings, and thereby provide opportunities for
kin selected behavior in this species. First, P*. nu bairdi tend to
disperse short distances from their birthplace before establishing a
home range (Dice & Howard, 1951; Howard, 1949). It is likely
therefore that many of the residents in a population will have settled
near relatives. In addition, because winter aggregations may Include
nearby residents (Howard, 1949), it is likely that at least some of
these relatives are likely to overwinter together.

Participation in winter aggregations may greatly increase an
individual's probability of surviving until the spring breeding season

122

(Howard, 1951). The ability to remain in aggregations through the
winter appears to depend largely on the availability of adequate food
to maintain the aggregated mice (Howard, 1951). Although mice could
venture out of aggregations to forage, this behavior would be somewhat
self-defeating. Food items will likely be scarce in winter months and
difficult to find, and increased exposure to cold during foraging will
result in increased heat loss, making it necessary to forage even more
intensely to gather sufficient food to maintain body temperature at an
adequate level. The amount of food cached near an aggregation of ILl
DL. bairdi may therefore determine how successful ly members of that
aggregation survive the winter.

Although information is not available as to what factors determine
where aggregations are formed, the nucleus of these aggregations is
generally a parental pair and one of their litters (Howard, 1949). It
is likely that parents cache food near the time of the birth of their
last fall litter, and that the combination of this small aggregation
and food cache may often attract additional neighboring individuals.
Because many of these individuals may be related to the family group,
inclusive fitness may be increased by allowing them to join the
aggregation and utilize the food cache. Two additional factors,
however, may be of importance in this context. First, at least up to a
point, Increasing the size of an aggregation may benefit all of its
members because a larger group can more easily maintain a higher
temperature. This may in part explain the observation that individuals
of other species are sometimes allowed in aggregations (Howard, 1949).
Second, laboratory observations (Rice, 1972; Terman, 1974) suggest

123

that an existing food cache may act as a stimulus for hoarding behavior
in Ea su bairdi. If individuals joining an aggregation also add to
the food cache they may in effect "pay their own way" as members of the
aggregation. The behaviors exhibited by at least some of the
individuals in aggregations, therefore, may be mutual istic.

Whether the behavior of an individual in an aggregation is viewed
as mutual istic, or kin selected (or both), will depend on the costs of
that behavior to the individual, and upon who receives the benefits of
that behavior. The behavior of Individuals of other species in
aggregations of E* maniculatus. for example, may be mutual istic. The
costs and benefits to the resident parental pair, however, are much
more complex. Costs Incurred by these animals in caching food include
expenditure of time and energy, and possibly increased exposure to
predators. An additional possible cost of aggregation is related to
the fact that abundant food In the spring may a I low pairs to breed
early (Gashwiler, 1979; Taitt, 1981). If food caches are severely
depleted during winter aggregation, spring breeding may be delayed for
the parental pair. In return for these costs the parental pair provide
benefits to themselves, their offspring, and possibly neighboring
siblings. Kin selection may therefore act as one of the factors that
maintain winter aggregations in £». nu. bairdi, and may therefore be
one of the factors that act to maintain attraction to siblings outside
of a breeding context. Although this hypothesis is untested, the
relatedness of individuals within aggregations could be assessed
through electrophoretic and trap-retrap studies.

124

The tendency toward philopatry in P^ maniculatus may also lead
to opportunities to increase inclusive fitness through cooperative
breeding. Communal litters, and apparently cooperative care of these
litters, have been observed in populations of P*. maniculatus in Texas
(Blair, 1958), Colorado (Hansen, 1957), and Michigan (Pj. m^ bajrdi;
Howard, 1949). Many of the hypotheses on the effects of ecological and
kinship factors on cooperative breeding have recently been reviewed
(Koenig & Pitelka, 1981) and will not be discussed in detail here. The
generally accepted hypothesis is "that habitat saturation provides the
primary impetus for philopatry, and through it for evolution of group
territoriality and cooperative breeding. . ." (Emlen, 1982, p. 32). As
described by Emlen (1982) "As population numbers increase, suitable
habitat becomes filled or 'saturated'. Unoccupied terri tori ties are
rare, and territory turnovers are few. As the intensity of competition
for space increases, fewer and fewer individuals are able to establish
themselves on quality territories. The option of breeding
independently becomes increasingly limited" (p. 32). An additional
factor that may mediate the occurrence of cooperative breeding in Pt
maniculatus is the sex ratic. Howard (1949) has noted that breeding
combinations with more than one individual of either sex occur in areas
where the sex ratio is not equal. The relatedness of the individuals
in these breeding groups is unknown. A reasonable hypothesis, in light
of the social organization and behavior of this species, is that under
conditions of high population density and unequal sex ration, same-sexed
siblings of the "surplus" sex may find it more adaptive to establish

125

themselves in breeding groups with a member of the opposite sex than to
attempt to gain resident breeding status on their own.
Inbreeding in Peromyscus Pol ionotus
Evidence for and against inbreeding

Smith (1966) and Smith, Carmon, and Gentry (1972) have presented
evidence that fLu pol ionotus may be highly inbred. This finding
appears to be at odds with evidence that reproductive performance in P_«.
pol ionotus is positively correlated with genie heterozygosity (Smith et
al., 1975). Smith et al . (1975) have suggested that the level of
inbreeding in this species is linked in an adaptive manner to
population density, level of aggression, and dispersal. These
investigators suggest that at low and increasing population densities
levels of aggression and dispersion will also be low, while levels of
inbreeding will be high. When population density rises individuals
will begin to outbreed more, and produce more-aggressive heterozygous
offspring that are better suited to competition in the population or
during dispersal. Smith (1968) has also suggested that opposite-sexed
siblings display a tendency to disperse together, and has observed that
the females in these sibling pairs may often be pregnant.

Foltz (1979, 1981) has suggested alternative interpretations for
many of the observations presented by these investigators as evidence
of inbreeding in fL. pol ionotus. These interpretations are however
only presented as alternatives, and Foltz (1979, 1981) was not able to
exclude the possibility of high levels of inbreeding In this species.
In light of Smith's (1966) observation of a preference by P_t.
pol ionotus females for sibling males over nonsibling males Foltz (1981)

126

suggested a need for additional research on the mating preferences of

P. pol ionotus. The present study indicates that females R,.

pol ionotus. as suggested by Smith (1966), display a preference for

siblings. Males of this species also tend to display higher sibling

than nonsibling scores on preference measures, although their

comparisons were nonsignificant.

Ecological and social factors

The apparent tendency toward inbreeding in Rj. pol ionotus (Smith,
1966, 1968; Smith, Carmon & Gentry, 1972) may be mediated by the very
specific habitat requirements of this species, and the probability that
individuals must disperse long distances to find new patches of
favorable environment. Shields (1982) has suggested that "if
conditions existed that favored relatively faithful transmission of
parental genomes, then inbreeding could be favored over both asexual ity
and outbreeding (p. 264).... Owing to its flexibility and capacity to
transmit successful parental genomes with maximum fidelity, inbreeding
is expected to be common in organisms produced by stable
lineage-environment associations" (p. 274). The very specific habitat
requirements of R*. pol ionotus may result in such a stable
lineage-environment association in this species. Because habitat
requirements for this species are so specific (Rand & Host, 1942;
Smith, 1966, 1968), offspring are likely to be most successful If they
breed in areas in which conditions vary little from those of their
birth place.

127

The patch iness of suitable environment for P^. po| jonotus may also
predispose this species to inbreeding through selective pressures
similar to those that hae been proposed to operate on Microtus
pennsylvanicus (Batzli et al., 1977; Getz, 1978). In reference to the
breeding habits of M. pennsylvanicus Batzli et al. (1977) note that
"Microtus pennsylvanicus . . . occupies smaller patches of moist meadow
or marsh. Under these circumstances, strange mates may not always be
available, and it would be disadvantageous if siblings could not breed
with one another. ... If ii. pennsylvanicus must continual ly locate
and repopulate isolated patches, the offspring of the founder(s) must
mate in order to assure success." (p. 590)

Similarly, for P^. pol ionotus. Smith (1968) noted that

these mice are found characteristically in habitats of
early stages of primary or secondary succession (Golley
et al., 1965). For this reason it is likely that large
distances between suitable habitat exist, and with time
succession makes the habitat unsuitable for the mice.
The pine forest habitats which they are associated with
on the mainland are fire subclimaxes (Laessle 1958a,
1958b; Smith, 1966) and utilization of available habitat
might require certain individuals to disperse long
distances to find recently burned areas, (p. 49)

Smith (1968), as noted previously in this discussion, has also observed

a tendency for opposi te-sexed sibling pairs of FL. po| ionotus in

breeding condition to disperse together.

Bateson (1978, 1979) has suggested that animals learn

characteristics of parents and siblings, and then use this information

to choose individuals that are only slightly different from kin as

mates. Gilder and Slater (1978) have observed behavior in mice that

appears to conform to this rule, and a similar rule of thumb may

provide a basis for the apparently cyclic preference for siblings

128

observed by Smith et al. (1972). The cyclic changes in sibling
preference that may occur in this species could be generated by the
rule "choose siblings as mates if they are not ±02 similar". For El
pol ionotus this may mean "prefer siblings as mates as long as your
parents were not the product of a sibling mating".

Garten (1976) has observed a positive correlation between
aggression and genie heterozygosity in E*. pol ionotus. and (Garten,
1977) between genie heterozygosity and exploratory behavior. As
offspring from nonsibling matings are more heterozygous than offspring
from sibling matings, offspring from nonsibling matings should be more
likely to disperse. In light of the previous discussion on the
patchiness of the environment for this species, it may be adaptive for
them to disperse to new breeding habitat as sibling pairs (as observed
by Smith, 1966) and for offspring of these pairs to breed with
siblings. Smith et al. (1975) have observed low levels of
heterozygosity In £j. pol ionotus populations at low and early stages
of increase in population density, which may indicate that individuals
at this stage of population growth may in fact be inbreeding. The
offspring of the second generation in the new habitat, however, being
the offspring of inbred parents, would be expected to choose
nonsiblings as mates. As the population density peaked many of the
offspring from these outbred matings would be expected to disperse with
siblings and renew the cycle. At the stages of late population rise
and early decline then, the level of heterozygosity in the population
would be expected to be relatively high, as observed by Smith et al.
(1975).

129

Evolution of Monogamy in Peromyscus Pol ionotus
Peromyscus pol lonotus is morphologically more similar to the
"prairie forms" of JL. maniculatus (e.g. Pj. ULl bairdi or P. m.
pal lescens) than to the "forest forms" of this species (Hooper, 1968),
and most likely originated from one of the prairie forms of P_j.
maniculatus during the Pleistocene interglacial stages (Blair, 1950).
As discussed previously, although Ejl pol ionotus and P. maniculatus
are closely related, and display similarities in a number of aspects
ranging from habitat preference and morphology to behavior, they
exhibit large differences in social organization and mating system.
The evolutionary divergence in the social behavior and mating systems
of R*. pol ionotus and the prairie forms of fL. maniculatus. such as
_P. m. bairdi, may be explained through examination of the differences
in the amount of and distribution of suitable habitat for these taxa,
and differences in climate in the present day distribution of these
taxa. Blair (1950) and Smith (1966) have described the factors that
apparently have led to the existing distribution of Ej. pol ionotus. and
Its separation from the parental species P_«. manicu latus.

The present geographic relationships of these two
species can be explained if we assume continuous
distribution of maniculatus across the coastal
plain in Pleistocene time. This distribution
possibly, but not necessarity, might have been
only in a narrow strip along the Gulf beaches.
With encroachment of the Gulf on the land during
Pleistocene inter-glacial stages, there was the
opportunity for a part of this population to be
isolated in Florida, for parts of Florida projected
as islands during these periods (see Cooke, 1939).
The postulated coastal-plain population of
manicu latus disappeared eastward of Texas,
effectively isolating the Florida population.
(Blair, 1950, p. 266)

130

Certain soil characteristics appeared to be
important n limiting the distribution of mice. In
relatively undisturbed habitats, the mice occurred
primarily on fine sand.... Deposits of sorted sands
have been laid down in several ways in Florida
(Laessle, 1958b). Wind was an important agent along
the beach dunes. The action of water was important
along the flood plains of large rivers, the shore-
lines of lakes and islands, and submerged offshore
bars. Al I areas above the current water level were
at one time part of the Florida shoreline. As the
water level fell during glacial periods, numerous
deposits of fine textured sand were gradually exposed.
Their continuity was later destroyed by erosion (Alt
and Brooks, 1965). These deposits and their
associated vegetation are frequently widely spaced
with the intervening habitat unsuitable for the old-
field mouse. These interrupted sand deposits are
ecological islands for this species. (Smith, 1966,
p. 13-15)

The climate in Florida would have been much cooler during the
Pleistocene glaciations than at present, and may have been at least
somewhat similar to conditions under which fL. nu. bairdi exists
presently. Assuming that the Pleistocene prairie forms of P_*.
maniculatus would share many characteristics with present day prairie
forms of P.*. maniculatus, one may hypothesize that many of the
characteristics of the species ancestral to Pj. pol iopotus may
presently be exhibited by fL_ nk bairdi. Among these characteristics
would be the ability to construct shallow burrows (Houtcooper, 1972),
tendency to form winter aggregations (Howard, 1949) and to cache food
for winter use (Hamilton, W. J., 1943; Howard, 1949), and possibly a
predisposition under some conditions to form exclusive reproductive
pairs (Howard, 1949).

Food caches and winter aggregations may have been as adaptive
for the ancestral stock P_,_ pol ionotus was derived from as they appear
to be for £* nu. bairdi today (Howard, 1951). As the glaciers

131

retreated and the climate warmed, however, the function of these habits
may have changed. • The loose sand soils available may have allowed the
ancestral P_«. pol ionotus to construct deeper burrows than its
predecessors. This factor, and a warmer climate, would have allowed
the ancestral species to maintain a favorable temperature in nest
burrows year-round, and would probably have resulted in lower
mortality. The advent of more constant nest conditions would have in
turn reduced the necessity for large winter aggregations as it would be
more likely that a family unit (parents and offspring) could maintain
an adequate nest temperature alone. More constant nest burrow
conditions and warmer climate would also act to reduce the need for
large food caches for winter survival. Food caches may, however, also
serve another function. Peromyscus maniculatus. although normal ly a
seasonal breeder, is capable of breeding through the winter if adequate
food is available (Linduska, 1942; Taitt, 1981). Conditions of more
moderate and stable temperature, along with an increased probability of
an adequate winter food supply, are likely to have increased the
possibility for successful year-round breeding In fL. pol ionotus. The
capacity of winter breeding would further act to limit winter
aggregations to immediate family, because It would be more adaptive to
use food caches to produce additional offspring, than to use these
resources to increase Inclusive fitness through supporting more distant
rel atives.

The possibility of breeding continuously, in conjunction with
ecological factors, may have provided a basis for the establishment of
monogamy as the predominant mating behavior in Pj. pol ionotus. The

132

major ecological factors of importance to monogamy in this species, as
discussed previously, appear to be the availability and distribution of
nest sites and food items. Constructing deep nest burrows limits
choice of breeding areas; and caching of food suggests that food may
be only seasonally abundant, with more food available than necessary
for survival and breeding in warmer months, and a reduced food supply
in winter months. Distribution and availability of nest sites and food
items may act, as discussed earlier, to limit possibilities for
polygamous matings by males. Constructing deep nest burrows and
provisioning food caches, however, provide a stable breeding
environment for E*. pol ionotus. A longer breeding season would allow
females to produce more offspring. Through investment in burrows and
food caches, males may have been able to increase the number of
offspring they produced by pairing with a single female, to above that
they would have expected by mating polygamous ly. Although this shift
in the behavior of ancestral £* pol ionotus males could be interpreted
as "investment in offspring" in a very broad sense, these behaviors do
not really go beyond those presently practiced by polygamous Pj. nu
bairdi males, who also maintain nests and cache food that may be used
by mates and offspring. The major shift that occurred in individuals
of the ancestral species may rather be interpreted as a shift in
emphasis from behavior resulting in increased inclusive fitness through
benefits to distant relatives, to a limitation of these same benefits
to offspring and to increased productivity by male-female pairs. This,
of course, does not preclude the possibility that improved male care of
offspring could have been a factor that added to the adaptive value of

133

exclusive breeding relationships in E*. pol ionotus. but suggests that
such behavior may not have been necessary for the evolution of monogamy
in this species.

Summary

The present study is consistent with the hypothesis that
aggressive ability may serve as a basis in mate selection for both
sexes in monogamous as well as non-monogamous species. In a
seminatural setting aggressive interactions occurred frequently between
members of both sexes of the monogamous species E*. pol ionotus, and
individuals within groups appeared to form stable aggressive
relationships. Males of this species exhibited a behavior, aggressive
digging, that may function to signal their aggressive status to
females. Individuals of this species of both sexes nested more
frequently with opposite-sexed individuals that exhibited high rather
than low aggressive ability. Male and female E*. pol ionotus. and male
and female E*. maniculatus. also exhibited evidence of preference for
more assertive opposite-sexed individuals (high rather than low
tendency to interact) when tested in a preference apparatus.

Preference for individuals of high aggressive ability appears
to be adaptive in terms of the ecology and social system of these two
species. In E*. pol ionotus high aggressive ability may insure that
an individual is able to obtain limited nest sites and food and defend
them against conspeci f ics. Female aggression could also be a factor
acting to maintain monogamy in this species (see: Kleiman, 1977;
Whittenberger, 1979, 1981; Whittenberger & Tilson, 1980). Females of
this species, however, do not appear to be dominant over males (see

134

Smith, 1966). Peromvscus maniculatus of both sexes appear to utilize
aggressive ability to limit settlement of juveniles on their home range
(Ayer & Whitsett, 1980; Enders, 1978; Fordham, 1971; Taitt, 1981;
Whitsett et al., 1979). Aggressive ability is also important in
male-male competition in E*. maniculatus (Blair & Howard, 1944;
Dewsbury, 1981c). Although in order for it to be adaptive to choose
mates with high aggressive ability it Is not necessary for aggressive
ability to be heritable, the adaptiveness of such choice would be
expected to increase if components of this ability were heritable.

In preference tests familiarity appeared to be an important
factor to individuals of both sexes of the polygamous species E*.
maniculatus. but of little consequence to individuals of either sex of
the monogamous E*. pol ionotus. The lack of significant preference for
familiar individuals by E*. pol ionotus in preference tests was
consistent with observations of the nesting behavior of this species in
the seminatural apparatus. Although the preferences displayed by Et.
maniculatus could be a result of housing conditions prior to
familiarity tests (Cam, Krames & Costanzo, 1970; Cam et al., 1979;
Cam et a I., 1980; Krames et a I., 1967), these conditions do not
appear to provide an explanation for the lack of preference displayed
by E*. pol ionotus. Differences in the responses of P. maniculatus and
E*. pol ionotus to familiar individuals in preference tests may be based
on differences in the opportunities individuals of these two species
have to make use of this factor in mate selection. As a result of
factors of ecology, social behavior, and breeding system, these
opportunities may be much more limited for £j, pol ionotus than for Ex

135

maniculatus.. Familiarity may, however, aid in maintaining pair bonds
in P. pol ionotus through reducing aggression, as familiarity did
appear to reduce aggression between familiar opposite-sexed individuals
in seminatural experiments.

Although only E* pol ionotus females demonstrated a significant
preference for siblings over nonsiblings, males of this species also
tended to display higher sibling than nonsibling scores in preference
tests. This finding is consistent with the observation by Smith (1966)
that female E* pol ionotus appear to prefer siblings as mates over
nonsiblings. Peromyscus maniculatus of both sexes displayed only
nonsignif icantly higher scores for nonsiblings than for siblings in
preference tests.

Inbreeding in Ex pol ionotus may be an adaptive strategy that
permits individuals of this species to found populations in Isolated
patches of favorable habitat. A similar strategy has previously been
proposed for Microtus pennsy I vanicus (Batzli et al., 1977; Getz,
1978). The lack of significant preference for nonsiblings demonstrated
by Ej, maniculatus. a polygamous species, may be due to competing
preference responses in this species. Although P^ maniculatus appear
to avoid breeding with siblings (Hill, 1974; Dewsbury, 1982a),
Individuals may also be attracted to relatives through a preference for
mating on or near their natal home range, and through opportunities to
increase their inclusive fitness through interactions with relatives.

The shift from polygamy to monogamy in ancestral E*. pol ionotus
may have occurred as a result of a shift from an emphasis on
aggressively limiting settlement on home ranges to defense of the
nest site, concomitant with a shift away from increasing inclusive

136

fitness through aid to distant relatives to increasing personal
fitness through limiting aid to a single mate.

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BIOGRAPHICAL SKETCH

I was a member of the "baby boom," born to George E. and
Mary L. Webster on May 5, 1948. I started my life in Oshkosh,
Wisconsin. Three moves later the family was settled in my parents
present home in Rothschild, Wisconsin. I completed high school in
neighboring Schofield at D. C. Everest (home of the "Evergreens") in
1966, and continued my education for another two years at the Marathon
Campus of the University of Wisconsin, in Wausau, Wisconsin. Toward
the end of 1968 I met Carole ... and followed her south to New
Orleans, where we were married in July of 1969. In August of 1969 I
became a member of the U. S. Air Force and served four years as an
oral surgery technician at Keesler A.F.B., Biloxi, Mississippi. Our
daughter, Danielle, was born in 1972 just prior to our leaving the
service and rejoining the student population. I completed the
requirements for my B.S. at the University of Wisconsin in Madison,
Wisconsin, in 1976, and we headed back south to the University of
Florida where I completed the requirements for the M.S. in 1979.

162

I certify that I have read this study and that In my
opinion It conforms to acceptable standards of scholarly
presentation and is fully adequate, In scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Dr. Donald A. Dewsbury, Chairman
Professor of Psychology

I certify that I have read this study and that In my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation fcr the degree of Doctor of Philosophy.

kfAdiim.

Dr. Merle E. Meyer
Professor of Psychology

'/ ■>,,yj,av^/-

Dr. Wi Ise B. Webb

Graduate Research Professor of Psychology

I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality,
as a dissertation for the degree of Doctor of Philosophy.

Dr. Carol Van Hartesveldt
Associate Professor of Psychology

;C^ (1

Dr. H. Jane Brockmann
Associate Professor of Zoology

This dissertation was submitted to the Graduate Faculty of
the Department of Psychology in the College of Liberal Arts
and Sciences and to the Graduate Council, and was accepted
as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.

April 1983 Dean for Graduate Studies

and Research

UNIVERSITY OF FLORIDA

3 1262 08553 5945

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