that provide the basis for comparative studies of this phe-
nomenon. One axis that is often used to characterize so-
cial species is the reproductive structure of groups (Brown
1987; Keller and Reeve 1994). Specifically, societies are
frequently divided into those in which all group members
reproduce (i.e., egalitarian, low-skew, or plural-breeding
groups) versus those in which reproduction is limited to
a single member of each sex (i.e., despotic, high-skew, or
singular-breeding groups; Vehrencamp 1982; Brown 1987;
Clutton-Brock 1998a, 1998b; Reeve et al. 1998). Although
the differences between these breeding structures are likely
continuous rather than dichotomous (Sherman et al. 1995;
Lacey and Sherman, 2005), distinguishing between plural-
and singular-breeding groups provides a useful heuristic
that has important implications for patterns of coopera-
tion and conflict among group members, as well as for the
occurrence of behavioral and morphological specializations
among conspecifics (Vehrencamp 1982; Sherman et al.
1995; Emlen 1996; Lacey and Sherman, 2005).
A second axis that often is used to characterize social
species is the kin structure of groups. Kinship within groups
typically arises due to natal philopatry (Ims 1989; Koenig
et al. 1992; Jarvis et al. 1994; Emlen 1995; Nunes, chap. 13
this volume) and hence, kin structure often can be inferred
from data indicating which animals remain in their natal
group. In general, natal philopatry among mammals is fe-
male biased (Greenwood 1983; Dobson 1983; Brody and
Armitage 1985; Ims 1990; Hoogland 1995; Solomon
2003); unlike many group-living birds (Koenig and Dick-
inson 2005), social mammals are rarely characterized by
exclusively male natal philopatry (Solomon 2003; Nunes,
chap. 13 this volume). Because natal philopatry by females
predominates, groups in many mammal species consist pri-
marily of female kin (e.g., Michener 1983a; table 1). As a
result, females in these species should receive greater indi-
rect fitness benefits from assisting group mates (Hamilton
1964; Emlen 1997; Reeve 1998) and, accordingly, nep-
otism (Sherman 1977; Hoogland 1995; Holmes and Ma-
teo, chap. 19 this volume) and cooperation should be more
prevalent among females. In contrast, in species in which
both sexes remain in the natal area and groups consist of
multiple adult females and males, kinship and cooperation
should be more equitably distributed between the sexes.
Clearly, the process by which a group forms has impor-
tant implications for kin structure and hence, the fitness
consequences of sociality. Among rodents, natal philopatry
appears to be the predominant mode of group formation
(Solomon 2003). Although groups in some mammal species
form when unrelated individuals aggregate to avoid preda-
tors (e.g., Thomson’s and Grant’s gazelle; FitzGibbon 1990)
or to gain access to critical resources (e.g., river otters;
Blundell et al. 2002; elephant seals; Le Boeuf and Laws
1994), we know of no social rodents in which groups do
not arise primarily due to natal philopatry. The prevalence
of philopatry has two important implications for rodent so-
cial structure. First, most rodent groups are composed pri-
marily, if not exclusively, of close kin. Second, because na-
tal philopatry is typically female biased, multifemale groups
should be more common than multimale groups. Indeed,
rodent groups containing multiple adult males appear to be
rare (Nutt 2003), while groups containing multiple adult
females are common (Solomon 2003; Nunes, chap. 13 this
volume).Philopatry and breeding structure
Among social rodents, philopatry and breeding structure ap-
pear to be closely related. Specifically, while plural-breeding
groups tend to be characterized by female-only natal philo-
patry, singular-breeding groups frequently include philo-
patric animals of both sexes. Among social species of
rodents for which appropriate data are available, this as-
sociation between breeding structure and pattern of philo-
patry is significant (N23 species, G17.3, P0.0001;
table 21.1). A similar, significant relationship between
breeding structure and philopatry is obtained when anal-
yses are restricted to a single species per genus to mini-
mize the potentially confounding effects of shared evolu-
tionary history (N11 species, Fisher’s Exact P0.0002;
table 21.1). Two apparent exceptions to this pattern are
the California mouse (Peromyscus californicus), which is
singular breeding with male natal philopatry (Ribble 1991;
1992), and the montane vole (Microtus montanus), which
is also singular breeding but, at high densities, is character-
ized by female natal philopatry (Jannett 1978).
For three genera (Microtus, Cryptomys, Marmota), data
on group structure and dispersal patterns are available for
4 species (table 21.1). All Cryptomysstudied to date are
singular breeding, and both sexes are philopatric. Microtus
and Marmotaexhibit considerable interspecific variation in
social structure but, in general, covariation between breed-
ing structure and philopatry within each genus is the same
as that evident among genera. This variation among closely
related species suggests that these aspects of social structure
reflect species- or even population-level variation in envi-
ronmental conditions, rather than constraints imposed by
shared phylogenetic history (Reeve and Sherman 2001).
Comparative studies of these four genera should be partic-
ularly informative regarding the ecological factors favoring
a given dispersal pattern and breeding structure.
Why do patterns of philopatry and breeding structure
covary? Ecological conditions play a significant role in de-
termining which individuals remain in their natal area (Em-
len 1982; Chepko-Sade and Halpin 1987; Koenig et al.The Ecology of Sociality in Rodents 245