W

hether conspecifics live aloneor in
groups has important implications for numerous
aspects of behavior, including the nature and
intensity of cooperative as well as competitive interactions
(Brown 1987; Koenig et al. 1992; Lacey and Sherman 1997;
Hayes 2000; Hoogland, chap. 37 this volume). As a result,
determining why groups occur is a central goal of many
studies of rodent behavior. The selective pressures favor-
ing sociality in rodents, however, are not well understood.
Although numerous benefits of group living have been
proposed (Alexander 1974; Hoogland and Sherman 1976;
Emlen 1984; Solomon and Getz 1997; Blumstein and Ar-
mitage 1999; Danchin and Wagner 1997), only a handful
of studies of free-living rodents have rigorously tested these
hypotheses, and even fewer have systematically explored
the effects of multiple selective factors on social structure
(Ebensperger 2001a; Ebensperger and Cofré 2001). To fa-
cilitate understanding of this important aspect of social or-
ganization, we review the occurrence of group living among
rodents, including processes of group formation and the
associated patterns of kin structure. We then consider the
conceptual approaches that have been employed to explain
these phenomena. Using studies of social, subterranean ro-
dents as a starting point, we develop a general conceptual
model of the ecological factors thought to promote group
living. Our hope is that the resulting integrative framework
for exploring ecological correlates of sociality will stimulate
future empirical studies of this key component of rodent
societies.

Group Living and Sociality

Sociality is typically defined as group living (Alexander

1974; Lee 1994). As this statement implies, the tendency

for conspecifics to live in groups provides the foundation

for many of the elaborate forms of social interaction ob-

served among animals (Alexander 1974; Lacey and Sher-

man 1997). This definition, however, is deceptively simple

given that groups may vary dramatically in size, structure,

and degree of cohesion (Krause and Ruxton 2002; Safran

et al., in press). Within species, the tendency to form groups

may differ among populations in response to ecological

conditions (Nevo et al. 1992; Jarvis et al. 1994; Spinks et al.

2000a, 2000b; Nevo, chap. 25 this volume; Macdonald

et al., chap. 33 this volume), and individuals may shift be-

tween a solitary and a social existence during the course of

their lifetime, including from one round of reproduction to

the next (Wolff 1994b; Solomon and Getz 1997). Thus it

is not possible to characterize species or even individuals as

solitary versus social without considering both the environ-

ments in which they occur and the timing of their behav-

ior relative to key life-history events such as natal dispersal,

mating, and parental care.

In practice, social groups often are identified on the ba-

sis of the spatial and social interactions among conspecifics

that occur during the breeding period. Although groups

may form only briefly (e.g., in response to the presence of

a predator), most definitions of sociality emphasize inter-

actions that persist for a significant portion of an individ-

Chapter 21The Ecology of Sociality in Rodents

Eileen A. Lacey and Paul W. Sherman