Intrinsic benefits of group living in degus
Studies of degus have emphasized the intrinsic benefits of
group living and communal nesting. For example, it has
been hypothesized that increased protection from preda-
tors represents a primary benefit of sociality in this species
(Ebensperger and Wallem 2002). Above ground, degus are
preyed on by numerous species, including foxes and several
types of raptors (Jaksic et al. 1981; 1993) and individuals
appear to adjust their behavior in response to predator risk
(Lagos et al. 1995; Vásquez et al. 2002; Ebensperger and
Hurtado 2005a). In particular, animals that are active in
sparsely vegetated areas form larger groups and spend more
time vigilant than do animals in areas with more substan-
tive vegetative cover (Ebensperger and Wallem 2002; Vás-
quez et al. 2002). In open habitats, degus in larger groups
are able to detect predators at a greater distance than are
degus in small groups (Vásquez 1997; Ebensperger and
Wallem 2002), suggesting that group living reduces preda-
tion risk in this type of environment.
It has also been proposed that group living may confer en-
ergetic benefits on individuals that nest together (e.g., Eben-
sperger and Wallem 2002). Captive degus have been shown
to reduce their energetic expenditure by huddling with con-
specifics (Canals et al. 1989); this benefit may extend to nat-
ural habitats in which individuals are expected to experi-
ence greater extremes in ambient temperature. Group living
may also reduce the energetic costs of burrow construction
(Ebensperger and Bozinovic 2000a). Laboratory studies in-
dicate that while degus housed in groups do not spend less
time burrowing than solitary animals, group-living individ-
uals coordinate their digging activities and, per capita,
move more soil than do solitary diggers (Ebensperger and
Bozinovic 2000a). As a result, cooperative burrowing may
lead to a reduction in the energy required per individual to
move a given amount of soil. Burrow excavation, however,
represents a relatively limited portion (ca. 1–2%; Ebens-
perger, unpublished data) of an animal’s activity budget,
leading to questions concerning the ability of this apparent
energetic savings to maintain group living in this species
(Ebensperger and Wallem 2002).
Other adaptive benefits proposed to explain sociality in
this species include the defense of critical food resources.
For at least some degu populations, the abundance and
quality of food appear to vary both spatially and temporally
(Jaksic and Fuentes 1980; Holmgren et al. 2000). If, as has
been argued for group-living bathyergid mole-rats (Jarvis
et al. 1994; Bennett and Faulkes 2000), cooperative for-
aging increases the animals’ ability to locate or to defend
patchily distributed resources, then this may represent an-
other benefit to group living in degus. Relationships between
food resource distributions, social structure, and individual
fitness have not been quantified for this species, however,
and thus this hypothesis remains largely untested for degus.Extrinsic constraints on dispersal in tuco-tucos
In contrast to studies of degus, studies of colonial tuco-
tucos have focused on ecological factors that constrain na-
tal dispersal, thereby leading to the formation of social
groups. A primary objective of these studies has been to de-
termine if group living in C. sociabilisis associated with the
same ecological factors thought to favor sociality in Afri-
can mole-rats (family Bathyergidae). Sociality among bath-
yergid rodents is thought to reflect the combined effects of
hard-to-excavate soils, patchily distributed food resources,
and limited and unpredictable rainfall (Jarvis et al. 1994;
Lacey and Sherman 1997; Lacey and Sherman, chap. 21
this volume). If these variables represent general ecological
correlates of sociality among burrow-dwelling rodents,
then the same factors should be associated with group liv-
ing in other, phylogenetically and biogeographically inde-
pendent taxa.
To test the generality of this hypothesis, Lacey and Wiec-
zorek (2003) compared patterns of resource use by the co-
lonial tuco-tuco and its solitary congener, the Patagonian
tuco-tuco (C. haigi;Lacey et al. 1997, 1998). Both spe-
cies occur in the Limay Valley of southwestern Argentina,
where populations of heterospecifics are separated only by
the width of the Río Limay (ca. 500 m). The habitat in the
valley consists primarily of arid, steppe grassland that is
punctuated at irregular intervals by mesic patches known as
mallines. Comparative analyses of habitat use have revealed
that while the solitary C. haigioccurs in steppe and mallín
portions of the habitat, the group-living C. sociabilisis re-
stricted to the borders of mallín patches. Soil humidity and
the distribution of critical food resources do not differ be-
tween habitat types, but steppe soils are significantly more
difficult to penetrate than are mallín soils (Lacey and Wiec-
zorek 2003). Although direct comparisons of the energetic
costs of burrow excavation in the two habitat types have
not been completed, the data on soil penetrability suggest
that burrowing is more difficult in habitats occupied by
the solitary C. haigi,leading Lacey and Wieczorek (2003)
to conclude that adaptive hypotheses proposed to explain
group living in African mole-rats do not account for social-
ity in colonial tuco-tucos. Instead, they propose that greater
habitat specialization byC. sociabilisincreases the difficulty
of dispersing from one habitat patch to another, thereby fa-
voring natal philopatry and group living. Thus, ecologi-
cal constraints on dispersal rather than intrinsic benefits of
group living appear to favor sociality in this species and, ac-
cordingly, the direct fitness of lone females is greater than
that of group-living animals (Lacey 2004).Social Structure in Octodontid and Ctenomyid Rodents 411