relative safety and familiarity of their birth areas, expose
themselves to risk, and seek out and settle in new homes?
Second, why is natal dispersal biased toward young males?
Three explanatory hypotheses are generally proposed to
explain the adaptive function of mammalian natal disper-
sal: young animals disperse (1) to avoid inbreeding, or in
response to competition for (2) mates or (3) environmental
resources (Greenwood 1980; Pusey 1987; Pusey and Wolf
1996). These hypotheses also offer possible explanations
for the direction of sex bias in mammalian natal dispersal.
The selection pressures that shape behavior can vary among
groups of animals, and it is possible that dispersal behavior
can benefit animals in different ways. Thus it is not likely
that a single hypothesis can explain dispersal in all species.
It is possible that in some cases more than one selective pres-
sure shaped the evolution of dispersal behavior (Dobson
and Jones 1985). Moreover, although there is a strong male
bias in mammalian natal dispersal, when young females
emigrate from the natal area they may do so for different
adaptive reasons than do young males (e.g., Dobson 1979;
Nunes et al. 1997; Gundersen and Andreassen 1998).
Inbreeding avoidance
Studies of rodents suggest that avoidance of inbreeding is
an important factor in the evolution of dispersal behavior.
That is, individuals leave their natal sites to find breed-
ing areas or groups with potential mates who are unrelated
to them. Inbreeding depression is a well-documented phe-
nomenon. Although some degree of inbreeding can benefit
animals by maintaining gene complexes adapted to local en-
vironmental conditions, consanguineous matings can detri-
mentally affect offspring by reducing heterozygosity and in-
creasing expression of recessive lethal alleles.
Animals have a variety of behaviors for avoiding matings
with close relatives (Blouin and Blouin 1989). In a study of
black-tailed prairie dogs (Cynomys ludovicianus), Hoog-
land (1982) observed that females are less likely to become
estrus when their fathers live nearby, and behaviorally avoid
mating with close male relatives. Hoogland suggested that
in addition to these mechanisms, natal dispersal by male
prairie dogs also prevents inbreeding.
Moore and Ali (1984), however, suggested that animals
rely on behaviors that are not as potentially risky as disper-
sal to avoid inbreeding. Because most mammals have po-
lygynous mating systems, variance in reproductive success
tends to be substantially greater for males than females.
That is, males can potentially have more mating partners
and father many more young than females. Thus inbreed-
ing depression associated with consanguineous mating is
likely to have a greater negative impact on the reproduc-
tive success of females than males, and reproductive fe-
males should avoid choosing male relatives as mates. In
turn, males should disperse from their natal areas so as to
increase the likelihood of encountering females willing to
mate with them. According to Moore and Ali (1984), in-
breeding avoidance is not a cause of natal dispersal, but
rather natal dispersal is a consequence of females not se-
lecting male relatives as mates.
In a study of white-footed mice (Peromyscus leucopus),
Keane (1990b) observed that matings between siblings
yielded smaller litters and offspring than did matings be-
tween mice with a lower coefficient of relatedness, and sug-
gested that this was a result of inbreeding depression. He
also observed that female mice discriminated between the
odors of males based on their relatedness, and suggested
that females might use relatedness as a criterion for select-
ing a mate. Keane (1990a, 1990b) suggested, as proposed
by Moore and Ali (1984), that inbreeding avoidance is fa-
cilitated by the behavior of female mice, and that male mice
must leave the natal area to gain access to receptive mates.
By contrast, Wolff (1992, 1993b) suggested that in-
breeding avoidance was the main cause of natal dispersal in
both male and female white-footed mice. He conducted re-
moval experiments and observed that young mice dispersed
at higher rates when the opposite-sex parent remained in
the area. Moreover, young mice remaining in the natal area
along with the opposite-sex parent were less likely to be-
come reproductively mature than were young whose oppo-
site-sex parent had been removed. This finding is consistent
with the idea that young mice avoid mating with close rela-
tives, and disperse to increase their chances of encountering
mates to whom they are not closely related. Wolff (1992)
also observed that the presence of same-sex parents did not
influence dispersal by male mice, which does not support the
hypothesis that competition with same-sex relatives is the
motivating force for natal dispersal in white-footed mice.
In earlier work, Wolff et al. (1988) found that natal dis-
persal by white-footed mice decreases as population density
increases, and presumably competition for environmental
resources also increases. Wolff et al. (1988) surmised that
resource competition is not a driving force behind natal dis-
persal in this species, and suggested that competition for
resources may hinder dispersal by causing young mice to re-
main in the natal area when fewer possible home sites over-
all are available for settlement.
Wolff (1994a) further suggested that the male bias in na-
tal dispersal may arise from the fact that in most mammal-
ian species the reproductive life span is ordinarily longer
in females than males. Thus the reproductive tenure of a
young male who remains in his natal area is likely to over-
lap with that of his mother, whereas the reproductive life-
span of a young female in her natal area is not as likely toDispersal and Philopatry 151