as has been observed in Columbian ground squirrels (Har-
ris and Murie 1984). Demand for these resources typically
exceeds supply, they are energetically expensive to con-
struct, and they are vital to survival and reproductive suc-
cess. Thus increased inclusive fitness may occur when these
resources are relinquished to offspring. By contrast, Lambin
(1997) suggested that territorial bequeathal is unlikely to be
adaptive when inherited resources are available elsewhere
or are not critical for survival and reproduction, as is the
case in a variety of vole species. Lambin (1997) found no
data supporting territorial bequeathal in Townsend’s voles.
Similarly, McGuire et al. (1993) observed that young prai-
rie voles rarely remain in the natal site and become repro-
ductive after their parents disappear from the area.
Third, adults may disperse to increase the quality of their
territory with regard to food resources or mates. In a study
of Eurasian red squirrels, Lurz et al. (1997) observed that
breeding dispersal was highest among females in unstable,
low-quality habitat. Only females with a continuously avail-
able supply of food resources were faithful to their breeding
territories. Adult male squirrels also engaged in breeding
dispersal. However, their movements tracked those of adult
females. Thus whereas adult female squirrels dispersed to
increase their access to food resources, males dispersed to
maintain access to potential mates.
Philopatry
Ultimate causes of philopatry
It has been suggested that most female mammals are philo-
patric because the costs of dispersal and benefits of philo-
patry are greater for females than males. Mammalian re-
productive strategies typically involve investing a great deal
of care in a small number of young. Because embryonic and
fetal development occur internally in mammals, males can
abandon females after mating, to optimize their reproduc-
tive success by seeking additional mating opportunities. By
contrast, females optimize reproductive success by maxi-
mizing care of young to enhance the probability that they
will survive to reach reproductive maturity. The costs in fe-
males of caring for young include not only lactation, which
can be energetically expensive, but also the costs of defend-
ing young against predators and conspecifics. Residing in
familiar terrain may help females acquire or defend food re-
sources and defend territories. Moreover, females may ben-
efit by remaining in the natal site near female relatives, who
are not likely to attempt infanticide.
Wolff (1994a) proposed an alternative hypothesis for fe-
male philopatry, which is predicated on the idea that in-
breeding avoidance is an important driving force behind
natal dispersal. He suggested that because of the intense
competition for mates among polygynous males, they typi-
cally have shorter reproductive longevity than females. Thus
by the time a young female reaches reproductive maturity,
the probability is low that her father will still be in the area
and siring offspring, and she need not disperse to avoid po-
tentially mating with him. By contrast, males are likely to
reach reproductive maturity while their mothers are present
in the natal area, and disperse to avoid potential inbreed-
ing (see the preceding). Wolff (1994a) also suggested that in
polygynous species, males often wander over large areas in
search of mates. Thus even if a young female remains in her
natal area, there is a high probability that she will encounter
unrelated males as they wander through her home area.
Perrin and Lehmann (2001) suggested that female mate
choice facilitated by kin recognition might have contrib-
uted to the evolution of female philopatry. According to this
idea, because inbreeding depression is potentially costly
to the fitness of females, natural selection favored females
who could recognize closely related males and who rejected
them as mates. This choosiness by females allowed them
to avoid inbreeding without emigrating from the natal area.
However, it is also possible that kin recognition is a conse-
quence rather than cause of philopatry, and might have been
favored by natural selection because it facilitates coopera-
tive interactions among kin.
Habitat constraints may be an underlying cause of philo-
patry in some species (Solomon 2003). This possibility has
been extensively examined in African mole-rats, subterra-
nean rodents native to sub-Saharan Africa (Spinks et al.
1999; Hazell et al. 2000; Spinks, Bennett, and Jarvis 2000,
Spinks, Jarvis, and Bennett 2000; Molteno and Bennett
2002; Faulkes and Bennett, chap. 36, this volume). Mole-rat
species range from solitary to highly social, and the social
species predominate in arid regions where food resources
are patchy. Mole-rats subsist in large part on geophytyic
plants, and randomly dig to locate patches of these plants.
The amount of energy an individual expends to locate a
food patch can thus be substantial. Moreover, digging is
limited to brief periods of rainfall during which the ground
is softened. Philopatry and delayed dispersal are common
among social mole-rats. The aridity food-distribution hy-
pothesis (AFDH) has been proposed to explain the high de-
gree of philopatry and sociality in various species of mole-
rats. According to this hypothesis, foraging is risky and
energetically expensive for individuals, and a solitary indi-
vidual may perish before locating a food patch. By remain-
ing philopatric and associating with conspecifics, mole-rats
can act cooperatively to find food patches, thereby increas-
ing their chances of survival.Dispersal and Philopatry 157