valuable opportunity to determine whether these patterns
of philopatry represent quantitatively different endpoints
along a continuum of dispersal options or qualitatively dif-
ferent routes to sociality that produce distinct patterns of
kinship and reproductive success. Temporal variation in
social structure has been reported for some group-living
rodents, but these examples appear to reflect primarily
changes in group size, rather than group structure. For ex-
ample, prairie voles may occur as lone females, male-female
pairs, or singular-breeding groups composed of multiple
adults of both sexes (Getz et al. 1993; McGuire and Getz
1998). Similarly, white-footed mice (P. leucopus) and deer
mice (P. maniculatus) may occur as lone females or plural-
breeding multifemale groups (Wolff 1994b). None of these
species, however, appears to switch from plural-breeding
groups with female-only philopatry to singular-breeding
groups with philopatry by both sexes, suggesting that these
differences in kin and breeding structure reflect qualita-
tively different responses to selective pressures favoring
group living.
Conceptual Approaches to Sociality
The adaptive bases for group living have long puzzled evo-
lutionary biologists, particularly given that individuals ap-
pear to incur two unavoidable fitness costs (increased com-
petition, increased exposure to pathogens) but receive no
automatic fitness benefits when living with conspecifics
(Alexander 1974; Brown and Brown 1996; Krause and Rux-
ton 2002). Consequently, studies of social species have tra-
ditionally focused on identifying benefits intrinsic to group
living, such as increased predator protection, increased ac-
cess to limited resources, and improved foraging via co-
operation among group mates (Alexander 1974). Because
these analyses have frequently relied upon mean group- or
population-level estimates of adaptive benefits (Safran et al.,
in press), they have not always considered the kin structures
of groups or individual variation in the adaptive conse-
quences of sociality.
In contrast, studies of cooperatively breeding vertebrates
have emphasized the individual-level fitness benefits of re-
maining in the natal group (Emlen 1982, 1991; Sherman
et al. 1995; Koenig and Dickinson 2004). Given the preva-
lence of natal philopatry among group-living rodents, this
conceptual approach has been widely applied to these ani-
mals (Jarvis et al. 1994; Solomon 2003; Lacey and Wiec-
zorek 2004; Hare and Murie, chap. 29 this volume, Ar-
mitage, chap. 30 this volume, and Lacey and Ebensperger,
chap 34 this volume). On the one hand, ecological con-
straints models of philopatry assert that groups will form
when physical or social conditions raise the costs of disper-
sal to the point that, on average, individuals achieve greater
fitness by remaining in their natal group than by dispersing
and attempting to breed elsewhere (Emlen 1982; Koenig
et al. 1992). Factors frequently identified as potential con-
straints on dispersal include the availability of suitable habi-
tat, potential mates, and resources required for successful
reproduction (Emlen 1982; Solomon 2003). On the other
hand, benefits of philopatry models (Stacey and Ligon 1991)
argue that it is the intrinsic fitness benefits of living with
conspecifics that lead individuals to remain in their natal
area. Such benefits include increased predator detection,
cooperative foraging, and the inheritance of breeding sites
— a list that closely parallels the “classic” benefits of group
living identified by Alexander (1974).
It is now generally accepted that ecological constraints
and benefits of philopatry models represent complementary
approaches to the same basic problem, namely identify-
ing the net costs and benefits associated with remaining at
home rather than dispersing (Lacey and Sherman 1997;
Mumme 1997; fig. 21.1). Despite the emergence of this
more synthetic conceptual framework, a dichotomous ap-
proach to studies of natal philopatry and the formation of
social groups remains common (Safran et al., in press). In
part, the persistence of this dichotomy reflects the difficulty
of identifying the adaptive consequences of complex behav-
ioral traits such as philopatry. To simplify this task, many
studies focus on individual causal factors that, alone, may
act primarily as constraints on dispersal or benefits of philo-
patry. Clearly, one challenge for future studies of sociality is
to integrate these approaches into a comprehensive concep-
tual framework for assessing the fitness consequences of na-
tal philopatry and group living.Fitness Consequences of Sociality in RodentsQuantifying the direct fitness consequences of sociality may
yield critical insights into the nature of the selective factors
favoring group living. Individuals may gain indirect fitness
benefits from their interactions with group mates but, typi-
cally, these benefits accrue only after groups form (Emlen
1991; Lacey and Sherman 1997), suggesting that the adap-
tive consequences of group living per se are better under-
stood by considering the direct fitness tradeoffs associated
with living and breeding alone versus within a group. At
one extreme, in species in which direct fitness increases with
group size, sociality is probably maintained by a net bal-
ance toward intrinsic benefits associated with group living
(Hoogland and Sherman 1976; Brown and Brown 1996;
Safran et al., in press). At the other extreme, in species inThe Ecology of Sociality in Rodents 247