Numerous studies have provided support for the AFDH
in various mole-rat species (e.g., Spinks et al. 1999; Hazell
et al. 2000; Spinks, Bennett, and Jarvis 2000; Spinks, Jarvis,
and Bennett 2000; Moteno and Bennett 2002). However,
Burda et al. (2000) questioned the validity of this hypothe-
sis. They suggested that the AFDH explains large group size
in mole-rats inhabiting arid regions, but does not explain
philopatry. Instead, they propose that extensive philopatry
in mole-rats results from a monogamous mating system and
high coefficient of relatedness among individuals. Coopera-
tive breeding occurs in social mole-rat colonies, with non-
breeding individuals contributing to the care of offspring
produced by a breeding pair. If the degree of relatedness
is high among individuals in a colony, nonbreeders can
enhance their inclusive fitness by caring for related young.
Burda et al. (2000) also suggested that monogamy has led
to cooperative breeding and high degrees of philopatry in
other species, such as prairie voles (e.g., Solomon 1991).
Proximate causes of philopatry and delayed dispersal
Social and physical barriers to dispersal
Social barriers to dispersal can promote facultative philo-
patry or delayed dispersal. For example, in many species of
rodents, rates of dispersal decline sharply as population
density increases and the availability of suitable habitat in
which to settle declines (see Wolff 1994a). Under crowded
conditions, residents of an area typically act aggressively to-
ward prospective immigrants to deter them from settling.
By contrast, dispersal rates are often high when popula-
tion density is low and habitat is available for settlement.
Physical barriers to dispersal can also increase tenure in the
natal area. For example, dispersal is opportunistic in mole-
rat species inhabiting arid regions, occurring when rain-
softened ground facilitates digging and encounters with
unfamiliar individuals or different groups (e.g., Spinks,
Bennett, and Jarvis 2000; Spinks, Jarvis, and Bennett 2000).
Moreover, Dobson et al. (1998) observed high rates of phi-
lopatry and short dispersal distances in a small lagomorph,
the plateau pika (Ochotona curzoniae) in areas with heavy
predation, and suggested that potential predation risks as-
sociated with dispersing through unfamiliar habitat might
influence dispersal distance and tendencies for philopatry in
small mammals.
Inheritance of territories and absence of parents
As earlier noted, adult females of some species leave their
maternal territories, allowing their young to occupy them
and remain philopatric. Also, as noted previously, the ab-
sence of the opposite-sex parent may prompt young to re-
main in the natal area (Michener and Michener 1977; Wolff
1992; Gundersen and Andreassen 1998).
Consequences of philopatry
Cooperation among kin living in proximity to each other
may be a driving force behind philopatry or a consequence
of it (see the preceding). Regardless of which is respon-
sible for the other, there is typically a close association be-
tween philopatry and cooperation. Remaining in proximity
to close kin throughout life provides the opportunity for co-
operative interactions and the evolution of nepotistic be-
havior, which can increase the inclusive fitness of individu-
als (Sherman 1977, 1981a). Altruistic interactions depend
on the ability to discriminate between closely related and
more distantly related or unrelated individuals (Perrin and
Lehmann 2001; Lehmann and Perrin 2002), and this abil-
ity has been documented in some rodents (e.g., Holmes
1994, 1995; Mateo and Johnston 2000a; Holmes and Ma-
teo, chap. 19, this volume).
The behavior of female Belding’s ground squirrels dur-
ing the breeding period illustrates ways in which philopatry
can facilitate cooperation (Sherman 1980a). Infanticide is
common in Belding’s ground squirrels (fig. 13.2), as it is in
most mammals (Hrdy 1979; Sherman 1981b). Mother and
daughter squirrels tend to set up maternal territories near
each other and act cooperatively in defending the territories
against infanticidal conspecifics. Early in the breeding sea-
son, much of the squirrels’ habitat is covered with snow, and
space in which to establish maternal territories is limited
(fig. 13.2). During gestation, rates of aggressive behavior are
high among female squirrels as they compete with each
other for space in which to set up a territory. Rates of ag-
gressive behavior are significantly higher among adult fe-
males that are establishing a territory next to a yearling
daughter than among females with no yearling daughters
(fig. 13.3). This increased aggression among females with a
daughter nearby raises the possibility that the behavior of
adult females may enhance their daughters’ chances of ac-
quiring a maternal territory. Adult females with territories
near yearling daughters also exhibit more intense vigilant
behavior than do adult females with no yearling daughters
(fig. 13.4), which may reflect helping their daughters to de-
fend their maternal territories.
Cooperation is also observed in other rodent species. For
example, female black-tailed prairie dogs communally nest
with close female relatives (Hoogland 1995). Moreover,
female meadow voles, common mole-rats (Cryptomys hot-
tentotus), Damaraland mole-rats (Cryptomys damarensis),
and naked mole-rats (Heterocephalus glaber) remain in the
natal area in a nonreproductive state and aid in rearing
young of close relatives (e.g., Solomon 1991; Spinks et al.
1999; Burda et al. 2000; Molteno and Bennett 2002).
Armitage (1981 and chap. 30, this volume) suggested
that philopatry allows for the evolution of complex social158 Chapter Thirteen