ual’s lifetime, such as one or more rounds of reproduc-
tion (e.g., Wilson 1975; Jennions and Macdonald 1994).
Spatially, members of a group are expected to show consid-
erable overlap, including, in some cases, sharing of a nest or
den site (Andersson 1984; Jennions and Macdonald 1994;
Lacey 2000). Behaviorally, interactions within groups are
expected to differ markedly from those between groups,
with the former being more likely to include affiliative, co-
operative, and nepotistic activities (Hoogland 1995; Sol-
omon and French 1997; Blumstein and Armitage 1997a;
Nevo, chap. 25 this volume). Neither criterion provides an
absolute indicator of social structure (Krause and Ruxton
2002) but, taken together, information regarding spatial
and social relationships provides a reasonable means of
identifying group-living taxa.
One reason that it is difficult to provide a precise defini-
tion of sociality is that “solitary” and “social” are not dis-
crete alternatives but, rather, endpoints along a continuum
of spatial and social interactions among conspecifics (Jen-
nions and Macdonald 1994; Sherman et al. 1995; Lacey
2000; Krause and Ruxton 2002; Lacey and Sherman,
2005). Numerous intermediate patterns of spatial overlap
and social cohesion are expected and, indeed, many rodent
species appear to fall somewhere between these extremes.
For example, in multiple species of ground-dwelling sci-
urids, females overlap spatially with one another, although
each maintains an area of exclusive use that includes her
nursery burrow (Michener 1983a; Sherman 1980a, 1981a;
Hoogland 2003a and chap. 37 this volume; Hare and Mu-
rie, chap. 29 this volume; Yensen and Sherman 2003). Be-
cause females do not share burrows or nests, they do not
exhibit the same type of sociality found in naked mole-rats
or prairie voles (Microtus ochrogaster;Sherman et al. 1991;
Bennett and Faulkes 2000; Solomon and Getz 1997), but
neither are they solitary like woodchucks (Marmota monax;
Barash 1989; Armitage 2000), pocket gophers (Thomomys
spp.; Nevo 1979), or blind mole-rats (Spalax ehrenbergi;
Nevo, chap. 25 this volume). Thus rather than struggling
to achieve a single, comprehensive definition of sociality,
it seems more appropriate to identify criteria that are rele-
vant to the specific conceptual issues and taxa under study
(Krause and Ruxton 2002; Lacey and Sherman, 2005).
Distribution of sociality in rodents
Group living is widespread within the Rodentia, occurring
in at least 70 species representing 39 genera and 18 fami-
lies, including 4 subfamilies of murids. Undoubtedly, this
list underestimates the actual occurrence of group living in
rodents. While social structure is relatively well documented
in some lineages such as bathyergid mole-rats (Sherman
et al. 1991; Bennett and Faulkes 2000; Faulkes and Ben-
nett, chap. 36 this volume) and ground-dwelling sciurids
(Murie and Michener 1984; Barash 1989; Armitage 2000
and chap. 30 this volume; Hoogland 2003a and chap. 37
this volume; Hare and Murie, chap. 26 this volume), the so-
cial systems of many other rodent taxa (e.g., echimyid spiny
rats, thryonomyid cane rats) are unknown and, hence, many
examples of group living may be unreported. One objective
of this chapter is to stimulate research on the social behav-
ior of these poorly known taxa.
Despite the paucity of behavioral data for many spe-
cies, it is clear that sociality is not evenly distributed among
rodent families. For example, while group living occurs in
the majority of bathyergid mole-rats (Bennett and Faulkes
2000), sociality has never been reported among geomyid
pocket gophers (Lacey 2000). Similarly, group living ap-
pears to be widespread in the family Octodontidae, but
is rare among members of the sister family Ctenomyidae
(Lacey and Ebensperger, chap. 34 this volume). More gen-
erally, sociality is particularly prevalent among hystricog-
nath rodents (Burda 1990; Bennett and Faulkes 2000); 72%
of families in the suborder Hystricognathi include at least
one social species, versus 46% of families in the suborder
Sciurognathi.^1 While these figures represent only crude esti-
mates, these apparent biases in the taxonomic distribution
of group living are intriguing and warrant further investi-
gation. Multiple factors may contribute to the prevalence of
sociality in some rodent lineages, including the production
of precocial young (Burda 1990), the risk of predation on
highly visible, diurnal animals (Jarman 1974), and the use
of safe, expansible burrows (Alexander et al. 1991). None
of these hypotheses has been rigorously tested, but a single
causal explanation seems unlikely. For example, even among
subterranean hystricognaths — species that share the ten-
dency to produce precocial young and to live in under-
ground burrows — the prevalence of sociality varies mark-
edly among families (Lacey and Ebensperger, chap. 34 this
volume). As a critical first step toward identifying the selec-
tive pressures that have favored group living in some rodent
lineages, a better understanding of the nature of rodent so-
ciality is needed, and thus we begin with an overview of
group structure in these animals.Characterizing social groups
Sociality occurs in myriad forms. Underlying this diversity,
however, are several general elements of group structure244 Chapter Twenty-One
- We have used the taxonomy of Wilson and Reeder (1993), which recog-
nizes two suborders of rodents. A version of this reference (Wilson and Reeder,
- contains a substantially revised taxonomy that includes five suborders of
rodents. In this revised taxonomic scheme, the Hystricognathi remain largely in-
tact as the new Suborder Hystricomorpha, with the four other suborders pulled
from the Sciurognathi.