migrant adult male. Most dispersal occurs within the local
population; simulation analyses based on observed demo-
graphic patterns indicate that, on average, the male and fe-
male(s) in a burrow system share a common maternal an-
cestor within the last five to seven generations (Lacey and
Wieczorek 2004), suggesting that even putative reproduc-
tive partners may be relatively close kin. To date, efforts to
determine parentage within social groups have been unsuc-
cessful due to very low levels of genetic variation among
conspecifics (Lacey 2000).
All females in a group produce offspring, although the
per capita number of young reared to weaning declines sig-
nificantly as group size increases (Lacey 2004). At present,
it is not known if this decrease in direct fitness is equitably
distributed among the females resident in a burrow system.
Adults of both sexes contribute to the care of young (Soares
2004). Studies of free-living animals indicate that the per-
centage of time that the nest is unattended (i.e., no adult
present) is significantly less for multiadult groups than for
lone females (Izquierdo 2005). Studies of captive animals
indicate that presence of an adult significantly increases the
probability that juveniles will remain in the nest (Soares
2004). Collectively, these data suggest that one benefit of
group living may be the increased retention of young in the
relative safety of the shared nest. As with degus, allonursing
by female colonial tuco-tucos has been observed for cap-
tive animals (Lacey, unpublished data), although this form
of cooperation has not yet been documented in the field.
Other potential forms of cooperation among colonial tuco-
tucos include shared participation in burrow excavation
and alarm-calling to alert groupmates to the presence of
predators. All adults excavate burrows and call in response
to raptors or other threats (Lacey, unpublished data); as
with degus and cururos, sharing of these activities may rep-
resent an adaptive benefit to group living.
In summary, the social systems of the octodontids and
ctenomyids considered here are similar in that all are group
living and communally nesting. While the degree of special-
ization for subterranean life varies among species, all use
underground burrows and, hence, the excavation of new
tunnels represents a significant component of their behav-
ior. In each species, groups contain multiple adult females,
all of whom are thought to produce offspring. In contrast,
one of the most striking differences between the social sys-
tems of these animals is the number of males per burrow
system; while groups of colonial tuco-tucos never contain
more than one adult male, multimale groups occur regu-
larly in cururos, and, apparently, in some populations of
degus. Accordingly, patterns of natal philopatry vary be-
tween species. Specifically, while philopatry by both sexes
has been reported for cururos, only female colonial tuco-
tucos remain in their natal burrows. The habitats in which
the animals occur also vary markedly, with cururos span-
ning much of the range of environmental conditions occu-
pied by degus and colonial tuco-tucos. Thus, these species
provide ample opportunity to explore the adaptive bases
for variation in several fundamental aspects of rodent social
structure.Adaptive Bases for SocialityGiven that the octodontids and ctenomyids examined to
date are social, it is not surprising that behavioral studies of
these families have focused on the adaptive bases for group
living. The conceptual and methodological approaches that
have been employed, however, are varied, with surprisingly
little overlap among the adaptive hypotheses examined for
each species. To help forge a more comprehensive picture of
social structure in these animals, we briefly review the vari-
ety of research strategies applied to these species and the
resulting insights into the ecological causes and fitness con-
sequences of group living. We then consider several more
general attributes of octodontid and ctenomyid rodents that
may contribute to observed patterns of sociality in these an-
imals. A primary objective of this discussion is to draw at-
tention to adaptive patterns not evident from studies of in-
dividual species in these families.Conceptual approaches
Numerous adaptive benefits have been proposed to explain
the occurrence of group living and communal nesting in ro-
dents (Hayes 2000; Ebensperger 2001a). While the selec-
tive factors identified by these explanations represent mul-
tiple adaptive hypotheses, they are frequently divided into
the following two categories: (1) benefits of philopatry that
are intrinsic to social groups, and (2) constraints on natal
dispersal that are extrinsic to social groups (Solomon and
Getz 1997; Hatchwell and Komdeur 2000; Hayes 2000).
These categories are not mutually exclusive; both types of
factors may contribute to sociality in a given species (Lacey
and Sherman, chap. 21 this volume), and the same variable
may simultaneously represent a benefit of philopatry and a
constraint on dispersal. Although there is increasing aware-
ness that these arguments represent two sides of the same
coin (Mumme 1997; Lacey and Sherman 1997), the dis-
tinction between benefits of philopatry and constraints on
dispersal remains entrenched in the literature and is evident
in the conceptual and methodological approaches applied
to studies of the adaptive bases of group living in octodon-
tid and ctenomyid rodents.410 Chapter Thirty-Four