animals may be more affected by variation in surface habi-
tats than are ctenomyids, suggesting that habitat structure
may have played a more conspicuous role in the evolution-
ary diversification of octodontids.
Morphological diversity
Associated with the general ecological differences outlined
in the previous section are differences in the range of mor-
photypes evident within each family. In keeping with their
shared specialization for subterranean habitats, all cteno-
myids exhibit similar morphological features associated
with life in underground burrows, including reduced eyes
and ear pinnae, shortened limbs, and generally squat, tub-
ular bodies (Stein 2000). Among octodontids, the fully
subterranean Spalacopusshares a number of the general
morphological traits identified for ctenomyids. In contrast,
members of the semisubterranean genera (e.g., Octodon,
Octodontomys,and Tympanoctomys) exhibit a markedly
different body structure, characterized by large eyes and
ear pinnae and, often, relatively enlarged hind feet. While
all ctenomyids possess comparatively short, unornamented
tails, tail structure among octodontids varies; although the
tails of the more subterranean genera (Spalacopus, Aconae-
mys) resemble those of ctenomyids, the tails of the more
surface-active genera (e.g., Octodon, Octomys) are often
characterized by pronounced brushes or plumes at the dis-
tal end. Additionally, octodontids display marked diversity
in the structure of the masticatory apparatus, which may be
linked to the variety of habitats and diets used by these ani-
mals (Olivares et al. 2004). In short, while all ctenomyids
are characterized by the same basic body plan, the octodon-
tids can be divided into two distinct morphological types:
the subterranean specialists (Spalacopusand Aconaemys)
and the burrow-dwelling but surface-active genera (Octo-
don, Octodontomys, Tympanoctomys, Octomys).
Behavioral diversity
Few octodontids or ctenomyids have been the subjects of
detailed behavioral research. At present, quantitative data
regarding social organization are available for only two
species (20%) of octodontids and four species (ca. 8%) of
ctenomyids. Anecdotal accounts provide intriguing hints
as to the social structures of a number of other species in
each family, although these reports require verification
through field studies of marked individuals. In addition,
both families contain multiple species for which no infor-
mation regarding social behavior is available (tables 34.1
and 34.2).
Despite the paucity of behavioral studies of these ro-
dents, data obtained to date suggest that patterns of social
structure differ markedly between octodontids and cteno-
myids. In particular, while sociality (defined as group liv-
ing) appears to be quite common among octodontids, it is
thought to be rare among ctenomyids. Although data for
many octodontids are largely anecdotal, comparisons of
the behavior of poorly known species with that of species
which have been studied in some detail (e.g., Octodon de-
gus, Spalacopus cyanus) suggest that some form of group
living likely occurs in at least half the members of this fam-
ily (table 34.1). While even anecdotal data are lacking for
several species (e.g., Aconaemys sagei, Octomys mimax), at
present the only species specifically identified as solitary is
the red viscacha rat, Tympanoctomys barrerae.
In contrast, even anecdotal suggestions of sociality are
limited to less than 10% of ctenomyid species (Reig et al.
1990; Lacey 2000). The only species for which quantita-
tive evidence of sociality has been obtained is the colonial
tuco-tuco, Ctenomys sociabilis(Lacey et al. 1997; Lacey
2004). In comparison, solitary burrow use has been dem-
onstrated for at least eleven species (table 34.1). These data
suggest that the ratio of social to solitary species is con-
siderably smaller for ctenomyids than it is for octodon-
tids. While we suspect that these values will change as stud-
ies of the behavioral biology of these animals increase, it
seems unlikely that the prevalence of sociality among cte-
nomyids will come to equal, let alone exceed, that among
octodontids.Case Studies of Social StructureGiven the apparently pronounced difference in the preva-
lence of group living in octodontids versus ctenomyids, an
obvious question that must be addressed when comparing
the social biology of these taxa is: why is sociality so much
more common among octodontids? As a first step toward
answering this question, we review the behavioral and eco-
logical data available for those octodontids and ctenomyids
whose social systems have been studied in some detail: de-
gus, cururos, and colonial tuco-tucos. As this list suggests,
behavioral studies of these animals have focused on social
species, with few comparative data available for solitary
taxa. Consequently, it is not yet possible to exploit the full
range of behavioral variation in these families to explore
the adaptive bases for differences in social structure. The in-
formation provided here, much of which is drawn from our
own ongoing studies of these species, reveals intriguing par-
allels as well as striking differences between the social sys-
tems of octodontid and ctenomyid rodents. These contrasts
form the basis for the remaining sections of this chapter,
which explore the evolution of variation in social structure
among these animals.Social Structure in Octodontid and Ctenomyid Rodents 405