site, which may remain in the same location for multiple
years (Lacey, Ebensperger, and Wieczorek, unpublished
data). Within a group, all adult females show evidence of
reproduction, including pregnancy and lactation (Begall
et al. 1999; Lacey, Ebensperger, and Wieczorek, unpub-
lished data). At present, however, the relationship between
group size and female direct fitness is unknown, as is the
distribution of direct fitness among adult groupmates of the
same sex. Intergroup transfer by males has been detected,
as has natal philopatry by members of both sexes (Lacey,
Ebensperger, and Wieczorek, unpublished data). These ob-
servations suggest that groups are likely to contain a mix-
ture of close kin and unrelated, immigrant individuals.
Long-term field studies of cururos currently in progress at
Parque Nacional Fray Jorge and Santuario de la Naturaleza
Yerba Loca, Chile, promise to yield critical information
regarding philopatry, kinship, and social structure in this
species.
Like degus, cururos appear to cooperate to excavate bur-
rows and, frequently, multiple members of a social group
can be observed simultaneously expelling loose soil from
adjacent burrow entrances. Radiotelemetry studies indicate
that cururos are strictly diurnal (Urrejola et al. 2005); when
active during the daytime, the animals frequently alarm-call
in response to predators and other disturbances, which may
function to warn groupmates, including kin, of potential
threats. Although allonursing has not been reported for
captive cururos, the tendency for groupmates, including
multiple reproductive females, to share a single nest sug-
gests that this form of cooperation is possible for free-living
members of this species (Hayes 2000).
Example 3: Social structure in colonial tuco-tucos
Colonial tuco-tucos (C. sociabilis) are medium-sized (fe-
males: 200 g; males: 300 g), diurnal ctenomyids that are
endemic to an approximately 1500 km^2 area of Neuquén
Province, Argentina (fig. 34.4). Like cururos, colonial tuco-
tucos are almost exclusively subterranean. Although both
species feed on surface vegetation, they rarely emerge more
than one body length from their burrows when foraging or
expelling loose soil from their tunnels. Unlike cururos,
however, colonial tuco-tucos are habitat specialists; the lat-
ter species occurs primarily in mesic seeps and meadows at
elevations ranging from 800 to 1600 m. Dietary analyses
based on fecal samples collected during the spring and sum-
mer breeding season indicate that the animals consume pri-
marily grasses and sedges (Lacey and Wieczorek 2003); an-
ecdotal observations during the autumn and winter suggest
that the animals forage on these same types of vegetation
throughout the year.
Colonial tuco-tucos breed only once per year, with litters
of one to seven young born during September –November.
The length of gestation is not known for this species; esti-
mates for other ctenomyids range from 90 –110 days (Weir
1974; Busch et al. 2000), implying that C. sociabilismate
during June –August. Like degus and cururos, young colo-
nial tuco-tucos are precocial; neonates have pelage, their
eyes open within several days of birth, and they begin eat-
ing solid food at ca. 7–10 days of age. The young first be-
gin foraging for themselves on surface-growing vegetation
ca. 4 weeks after birth and, although they are weaned
shortly thereafter, they remain in the natal burrow for at
least an additional 1–2 months.
As their common name implies, colonial tuco-tucos are
social. Burrow systems are inhabited by up to six adult fe-
males, with ca. 45% of burrow systems also containing a
single adult male during the period when young are present
(Lacey and Wieczorek 2004). All residents of a burrow sys-
tem share a single nest site (Lacey et al. 1997). Groups form
due to natal philopatry by females; in contrast, all males
disperse from their natal burrow at the end of their juvenile
summer (Lacey and Wieczorek 2004). As a result, social
groups are composed of several closely related adult fe-
males, their dependent young, and, in some cases, an im-Social Structure in Octodontid and Ctenomyid Rodents 409Figure 34.4 Geographic distribution of Ctenomys sociabilis. Range data were
compiled from Pearson and Christie (1985) and from unpublished data collected
by E. A. Lacey. Inset: photo of C. sociabilis,taken by Eileen A. Lacey.