overlap with that of her father. Therefore, male but not fe-
male mice should disperse, to reduce the likelihood of in-
breeding. Clutton-Brock (1989a) observed that in the few
mammalian species in which natal dispersal is female bi-
ased, the reproductive tenure of males in the natal area or
natal group is consistently greater than the age at which fe-
males begin reproducing.
The dispersal patterns observed by Wolff (1992) in
white-footed mice have also been documented in other ro-
dent species. Young disperse in response to the presence
of the opposite-sex parent in root voles (Microtus oecono-
mus;Gundersen and Andreassen 1998) and Richardson’s
ground squirrels (Spermophilus richardsonii;Michener and
Michener 1977). Moreover, in Belding’s ground squirrels
(S. beldingi) and a variety of other mammalian species, all
young males emigrate from the natal area regardless of eco-
logical or social conditions, suggesting that competition for
mates or environmental resources is not driving natal dis-
persal in these species (Holekamp 1984, 1986; Smale et al.
1997).
Waser et al. (1986) postulated that if inbreeding avoid-
ance were an evolutionary cause of dispersal, then in po-
lygynous species females rather than males should disperse.
They reasoned that because the maximum reproductive out-
put of females is more limited than that of males, inbreed-
ing depression in offspring is more costly to the fitness of
females than males. Thus females should benefit more by
dispersing to find unrelated mates. However, Lehmann and
Perrin (2003) proposed that inbreeding avoidance is a vi-
able ultimate explanation for male-biased natal dispersal in
polygynous species if female choice is taken into account.
That is, because inbreeding depression can be costly for fe-
males, natural selection should favor females who can dis-
criminate between closely related males and more distantly
related or unrelated males, and who reject as mates males
who will sire inbred offspring. Lehmann and Perrin (2003)
further suggested that the choosiness of females for unre-
lated males should be influenced by inbreeding load and the
costs of finding unrelated mates. Thus according to Leh-
mann and Perrin (2003), inbreeding avoidance can explain
sex-biased dispersal in polygynous species, whereas the di-
rection of sex-bias can be explained by female mate choice.
The occurrence of kin recognition abilities and female mate
choice in a variety of rodent species (e.g., Sherman 1976;
Holmes 1994, 1995; Mateo and Johnston 2000a) provide
some empirical support for this idea.
Competition for mates
Dobson (1982) suggested that competition for mates may
have acted synergistically with inbreeding avoidance in
shaping male-biased dispersal behavior in rodents and other
mammals. Because most mammalian mating systems are
polygynous, competition for mates tends to be more intense
among males than females. Thus Dobson (1982) suggested
that males might benefit more than females from dispersing
to areas with less competition for mates. Hence, dispersal
may have evolved to reduce inbreeding, and competition
for mates may have made dispersal more functionally adap-
tive for males than females in species with polygynous mat-
ing systems. Dobson (1982) in fact observed that males are
the predominant dispersers in polygynous species, whereas
both males and females tend to disperse in species with mo-
nogamous mating systems. However, Dobson (1979) also
observed that male California ground squirrels (S. beecheyi)
still emigrate from the natal area when adult males have
been removed and competition is likely to be low. The spe-
cific importance of competition for mates as a driving force
in the evolution of sex-biased dispersal remains to be eluci-
dated. However, competition for mates does appear to have
a distinct influence on some aspects of male dispersal. For
example, male Columbian ground squirrels (S. columbi-
anus) tend to settle in areas where the ratio of males to fe-
males is low, and consequently competition for mates might
be low (Wiggett and Boag 1993b).Competition for environmental resources
Competition for food and nest sites has been suggested to
be an important ultimate cause of dispersal in female mam-
mals. The energetic requirements of females increase dra-
matically during gestation and lactation, making food a vital
resource for reproduction. Moreover, infanticide by conspe-
cifics is common among mammalian species, and may be
adaptively beneficial to individuals by reducing intraspecific
competition that their own offspring will face in the fu-
ture (Hrdy 1979; Wolff 1993a; Ebensperger and Blumstein,
chap. 27, this volume). Establishing and defending territo-
ries from infanticidal conspecifics is one defense mecha-
nism that females use to protect their young (Wolff 1993a;
Solomon and Keane, chap. 4, this volume). Thus space in
which to establish a territory is also a critical resource for
reproduction in females. In a food supplementation ex-
periment with California ground squirrels, Dobson (1979)
observed that females were more likely to immigrate into
the supplemented colony than unsupplemented colonies;
however, males were equally likely to settle in both sup-
plemented and unsupplemented colonies. By contrast, in a
food provisioning experiment with Belding’s ground squir-
rels, Nunes et al. (1997) observed that females were more
likely to disperse away from areas receiving extra food,
whereas emigration and immigration by males were not af-
fected by food provisioning. They also observed that expe-
rienced adult females tended to shift their territories so that
they were closer to food boxes, causing an increase in lo-
cal population density. They further suggested that this in-152 Chapter Thirteen