are available. Descriptions of juvenile C. talarumindicate
that at birth the eyes are closed and young lack a full pelage
(Weir 1974; Cutrera, pers. comm.). In contrast, neonatal
C. peruanushave open eyes, are fully furred, and are ca-
pable of leaving the nest and feeding on green vegetation al-
most immediately after birth. Juvenile C. sociabilisappear
to fall somewhere in between; the young are born with a
short but complete pelage, the eyes open within a few days
of birth, and young begin leaving the nest 48 –72 hours
after birth (Soares 2004). While this sample is minimal, it is
intriguing to note that the two species with more precocial
young are reportedly social (Lacey et al. 1997; Pearson
1959; Lacey 2000), while the species with the least pre-
cocial young is solitary (Pearson et al. 1968).
Comparable data for octodontids are even more lim-
ited and thus, at present, analyses of precociality and so-
cial structure within this family are not possible. Given that
sociality, including communal nesting, occurs in numerous
nonhystricognath species (Solomon and Getz 1997; Hayes
2000; chapters 24, 30, and 31 this volume), we expect that
production of precocial young is not a primary selective
factor favoring group living in rodents. Future studies of
hystricognath rodents, however, should help to clarify rela-
tionships between precociality and group living by provid-
ing additional data regarding neonate ontogeny and degree
of sociality.
Fossorial lifestyle
All ctenomyids and the majority of octodontids are fos-
sorial, meaning that individuals exhibit morphological or
other specializations for digging (table 34.1; Nevo 1979;
Reig et al. 1990; Lacey et al. 2000). Energetic costs of
burrow excavation have been implicated in the evolution
of sociality in at least one other hystricognath lineage,
the bathyergid mole-rats. Specifically, for animals living in
difficult-to-excavate soils (i.e., high energetic costs of exca-
vation), individuals may live in groups and dig coopera-
tively in order to obtain access to patchily distributed but
locally abundant food resources (Lovegrove and Wissell
1988; Lovegrove 1991; Jarvis et al. 1994; Faulkes and Ben-
nett, chap. 36 this volume). Under this scenario, the costs of
burrow excavation may vary as a function of the physical
properties of soil (Lovegrove 1989; Ebensperger and Bozi-
novic 2000b), the unpredictable distribution of food or
other critical resources (Jarvis et al. 1994), or both.
Predicted relationships between burrowing
costs and sociality
Among species with similar degrees of specialization for
subterranean life (i.e., similar reliance on burrowing to ac-
crue resources), a positive relationship should be evident
between sociality and the energetic costs of burrow excava-
tion. Thus within the Octodontidae and Ctenomyidae, so-
ciality should occur in species that occupy particularly dif-
ficult (e.g., hard, dry) soils or in species that must burrow
particularly long distances to reach critical resources. Com-
parisons between families are potentially confounded by
differences in degree to which animals are specialized for
digging. Given, however, that total burrowing effort should
be greater for taxa that are more specialized for excavation,
we would expect the effects of soil conditions and resource
distributions to be more pronounced among ctenomyids.Available evidence
Relationships between soil conditions, resource distribu-
tions, and group living have not been examined for octo-
dontids. Within ctenomyids, however, group living does
not appear to reflect more difficult-to-excavate soils or more
sparsely distributed food resources. Comparative studies of
the colonial tuco-tuco and its syntopic congener, the Pata-
gonian tuco-tuco (C. haigi), have revealed no differences in
food resource distributions for populations of these species
in the Limay Valley of southwestern Argentina (Lacey and
Wieczorek 2003). Further, these analyses indicate that it is
the solitary C. haigithat occurs in harder, presumably more
costly-to-excavate soils (Lacey and Wieczorek 2003). Al-
though the energetics of tunnel excavation have not been
examined directly, the available data suggest that burrow-
ing costs due to soil conditions or resource distributions are
unlikely to explain sociality in ctenomyids.
More generally, sociality may be influenced by the rela-
tive extent to which a species engages in digging and bur-
row construction. A phylogenetically controlled analysis
of twenty-six New World hystricognath species revealed
that apparent energetic costs of burrow excavation provide
a better predictor of social group size than do measures
of predation risk or parental investment (Ebensperger and
Cofré 2001). Because this analysis included burrowing as
well as nonburrowing species, it is likely that this outcome
reflects the distinction between species that routinely dig
tunnels or nests and those that rarely, if ever, excavate sub-
terranean structures. Applying this interpretation to octo-
dontids and ctenomyids, we would expect sociality to be
more prevalent among the latter, since these are the animals
that should be most affected by soil conditions or other fac-
tors that increase burrowing costs. Again, the quantitative
data needed to test this hypothesis within octodontids are
lacking. The rarity of social ctenomyids, however, suggests
that simply engaging more frequently or more extensively
in burrow excavation is not sufficient to favor group living.
Thus, while we cannot exclude the possibility that energetic
costs of burrow excavation contribute to sociality in at least
some of the species considered in this review, these costs doSocial Structure in Octodontid and Ctenomyid Rodents 413