T

he octodontidae and ctenomyidaeoffer
numerous intriguing opportunities to explore the
evolution of rodent societies. Although closely re-
lated, these hystricognath families vary markedly with re-
gard to patterns of phyletic, geographic, and ecological di-
versification (Mares and Ojeda 1981; Contreras et al. 1987;
Reig et al. 1990; Redford and Eisenberg 1992). Corre-
spondingly, the animals are behaviorally diverse, with each
family including solitary as well as social species (Weir 1974;
Redford and Eisenberg 1992; Muñoz-Pedreros 1992). Col-
lectively, these attributes suggest that octodontids and cte-
nomyids provide a rich array of possibilities for compara-
tive studies of rodent social structure.
Given their potential to yield new insights into the evo-
lution of rodent social systems, it is disappointing that so
little behavioral research has been conducted on octodon-
tids and ctenomyids. To date, information on social struc-
ture has been obtained for fewer than 10% of the species in
these families, making quantitative analyses of the correlates
of social structure difficult. Of those species that have been
the subjects of behavioral research, none has been studied as
extensively as “model” taxa such as voles (Tamarin 1985;
Tamarin et al. 1990), African mole-rats (Sherman et al.
1991; Faulkes and Bennett, chap. 36 this volume), or ground
squirrels (Murie and Michener 1984; Barash 1989; Hoog-
land 1995; Yensen and Sherman 2003; Hare and Murie,
chap. 29 this volume), and much remains to be learned re-
garding the behavior of even the best-known octodontids
and ctenomyids.
In this chapter, we review the behavioral biology of the
Octodontidae and Ctenomyidae, with emphasis on the vari-

ation in social structure found among these animals. We be-

gin by comparing patterns of taxonomic, ecological, and

morphological diversification in these families, which, de-

spite their close phylogenetic affinity, have followed quite

different evolutionary trajectories. To illustrate how these

animals can contribute to our understanding of rodent so-

cieties, we present detailed information regarding the so-

cial systems of three members of these families, much of it

drawn from our own research programs in rodent behav-

ioral biology. Using this framework, we review the adaptive

hypotheses that have been proposed to explain social struc-

ture in these animals. We also consider how several of the

distinctive life history characteristics of these families may

influence their social behavior. Based upon the available

data, we suggest that comparative studies of these taxa will

substantially improve our understanding of the evolution-

ary bases for sociality and communal breeding in rodents.

We conclude by identifying several directions for future re-

search in rodent social behavior, for which octodontids

and ctenomyids represent particularly appropriate study

subjects.

Patterns of Evolutionary Diversification

The Octodontidae (degus, cururos, viscacha rats) and Cte-

nomyidae (tuco-tucos) are caviomorph rodents in the Sub-

order Hystricognathi (Woods 1993). Together with the

Abrocomidae (chinchilla rats), Capromyidae (huitas), Echi-

myidae (spiny rats) and Myocastoridae (coipu), these ani-

mals comprise the superfamily Octodontoidea (Nedbal

Chapter 34Social Structure in Octodontid and

Ctenomyid Rodents

Eileen A. Lacey and Luis A. Ebensperger