not provide a comprehensive explanation for patterns of so-
ciality in octodontids and ctenomyids.
Specialization for subterranean life
The different degrees to which octodontids and ctenomyids
use subterranean burrows may influence social structure
independently of the energetic costs of burrowing. Among
truly subterranean rodents, group living is rare (Nevo
1979; Lacey 2000). It has been suggested that life under-
ground favors a solitary existence because the spatial limits
of a burrow system facilitate territory defense and, hence,
exclusion of conspecifics from an individual’s area of activ-
ity (Nevo 1979). In contrast, Alexander et al. (1991) have
proposed that the use of safe, expansible habitats such as
subterranean burrows facilitates sociality. Specifically, be-
cause burrow systems can be modified and may be used for
multiple generations, it may be advantageous for individu-
als to be philopatric and to form groups rather than to leave
the safety and known resource availability of their natal
burrow (Alexander et al. 1991).
Predictions regarding subterranean life and sociality
Although these hypotheses represent different levels of anal-
ysis (Sherman 1988), they yield strikingly different predic-
tions, which can be explored using available data for oc-
todontids and ctenomyids. Based on the territory defense
hypothesis, we would expect that, within the Octodonti-
dae, sociality should be negatively related to the degree of
specialization for life underground. Although all members
of the Ctenomyidae are considered truly subterranean (La-
cey et al. 2000), we would expect any interspecific variation
in the tendency for animals to be active above ground to
show a similar relationship with social structure. Between
families, the consistently greater specialization for subter-
ranean life displayed by ctenomyids leads us to predict that
sociality should be less prevalent in this family. Conversely,
the expansible burrow hypothesis predicts that sociality
should be positively related to the degree of specialization
for life underground. Accordingly, this hypothesis suggests
that sociality should be more prevalent among ctenomyids
than among octodontids.
Available evidence
Across families, the apparent rarity of social ctenomyids
appears to be consistent with the predictions of the territory
defense hypothesis. Data on behavioral variation with each
family are more difficult to interpret. The extent to which
different ctenomyid species are active above versus below
ground has not been quantified. Anecdotal accounts, how-
ever, suggest that the two reportedly social species,C. socia-
bilisandC. peruanus,spend more time at burrow entrances
than do members of at least some solitary species in this
family (Pearson 1959; Lacey et al. 1997). While intriguing,
these observations are inconclusive; because members of
these species do not typically leave their burrows or inter-
act above ground with residents of other burrow systems, it
is not clear whether greater time spent at the surface would
affect the proposed benefits of the territory defense hypoth-
esis. Data from octodontids are more problematic; the cu-
ruro is the octodontid that is most specialized for subterra-
nean life, yet burrow systems of this species are routinely
inhabited by multiple adults (Reig 1970; Begall et al. 1999;
Lacey, Ebensperger, and Wieczorek, unpublished data).
The rarity of social ctenomyids appears to contradict the
predictions of the expansible burrow hypothesis. Although
use of a protected nest or den site is associated with group
living in a number of vertebrate lineages (Andersson 1984;
Alexander et al. 1991), this relationship is not apparent
among octodontids or ctenomyids. More generally, the ab-
sence of social species in several lineages of subterranean
rodents (e.g., geomyids, mysospalacines, rhizomyines; Nevo
1979; Lacey 2000) suggests that this hypothesis does not
provide a comprehensive explanation for patterns of social
structure among semi- and fully subterranean rodents. Al-
though more detailed, quantitative studies are required to
determine whether social structure varies with the degree of
specialization for subterranean life in octodontids and cte-
nomyids, it is clear that other factors must contribute to
patterns of sociality among semisubterranean versus truly
subterranean species of rodents.Summary and Future DirectionsAs evident from this review, comparative studies of octo-
dontids and ctenomyids provide important opportunities to
explore the adaptive bases for variation in rodent societies.
In particular, the distinctive patterns of phyletic, ecological,
and behavioral diversity evident in these families offer mul-
tiple opportunities to explore relationships between cur-
rent environmental conditions and social structure. To date,
analyses of sociality in octodontids have emphasized intrin-
sic benefits to group living, while studies of ctenomyids
have focused on extrinsic constraints on natal dispersal that
lead to the formation of social groups. Although the num-
ber of species for which quantitative data are available is
small, neither soil conditions nor the spatial distribution
of food resources appear to explain the occurrence of soci-
ality in these animals. Similarly, no relationship is evident
between social structure and key life-history attributes of
these families.
The absence of a single, consistent ecological or life-
history predictor of group living suggests that multiple fac-
tors contribute to the social systems of octodontid and cte-
nomyid rodents. While resource distributions, burrowing414 Chapter Thirty-Four