E
O. Wilson (1975)defines sociality as “the com-
bined properties and processes of social existence.”
Only a definition this broad could allow for the myr-
iad components that embody sociality proper, including:
(1) multiple individuals that overlap in space and time such
that a social group is formed, (2) the underlying ecological
and evolutionary factors promoting and maintaining group
living (e.g., Ebensperger 2001a), (3) interactions among
group members that impart both benefits and costs (social
behavior), and (4) the emergent properties of groups them-
selves, and in particular, their success relative to other
groups (D. S. Wilson 1975).
Unlike morphological traits, behavior does not leave fos-
sils. Thus inferences regarding the evolutionary origin of be-
havioral phenomena must be made indirectly, using a com-
bination of functional, phylogenetic, and comparative data
(Crespi and Choe 1997a). Alexander (1974) regarded the
revival of the comparative approach, along with the more
general acceptance that the structural and functional attri-
butes of organisms could be interpreted as products of se-
lection operating on individuals, as factors revolutionizing
the study of social behavior. Comparative analyses contrast
the expression of behavioral or morphological traits among
members of the same or of closely related species, relating
apparent differences in those traits to differences in the or-
ganisms’ environments (e.g., Crook 1970; Felsenstein 1985;
Lacey 2000). As such, comparative analyses may reveal con-
vergences in traits, or illuminate transitions by combining
presumed phylogenetic relationships with microevolution-
ary analyses based upon functional design, optimality, and
even the measurement of selection (Crespi and Choe 1997b).
The broadest comparative literature pertaining to soci-
ality exists for the social insects, where the application of
the comparative approach has provided many important
insights into the origins and maintenance of sociality (Ham-
ilton 1964; Wilson 1971; Lin and Michener 1972). Insight
into factors promoting reproductive skew (Vehrencamp
1979) within animal societies has also been gleaned from
more recent models integrating data from both vertebrate
and invertebrate societies (Vehrencamp 1983; Keller and
Reeve 1994; Reeve and Keller 1995; Sherman et al. 1995).
In this chapter, however, we adopt a taxon-centered ap-
proach to the comparative analysis of social behavior, in
agreement with Crespi and Choe’s (1997b) suggestion that
the causal factors underlying the origin and maintenance
of sociality, while overlapping, are unlikely to be universal
across taxa.
Of the rodent societies examined to date, the most com-
prehensive data in terms of combined taxonomic breadth,
adequate descriptions of spatial, life-history, and behav-
ioral relationships, and consideration of environmental cor-
relates, exist for the ground-dwelling squirrels (marmots,
prairie dogs, and ground squirrels proper). Indeed, in re-
viewing Murie and Michener’s (1984) volume “The Biology
of Ground-Dwelling Squirrels,” Barash (1985) suggested
that ground squirrels could be as important to understand-
ing vertebrate sociality as the giant squid motor axon had
been to the history of neurophysiology, given the ground
squirrels’ diurnal habits, wide geographic distribution, rel-
atively large number of species evidencing a diversity of so-
cial systems, and relative insensitivity to human observers.
In this chapter, we review comparative schemes examin-