For many species, a critical component of an animal's environment is its social setting, specifically whether an animal lives alone or with conspecifics. Living in a group may confer benefits associated with cooperation but almost invariably comes with costs resulting from conflict and competition. In contrast, living alone may reduce costly interactions with conspecifics but likely requires greater effort when caring for young, finding food, or avoiding predators. My research investigates the adaptive (fitness) and physiological consequences of group living using the social system of the colonial tuco-tuco (Ctenomys sociabilis), a subterranean rodent endemic to southwestern Argentina. Unlike most species in the genus Ctenomys, colonial tuco-tucos are social with burrow systems shared by 2 - 6 females and, sometimes, one male. However, not all females live in groups. About one-third of the burrow systems in the population are comprised of females that have dispersed to live alone, providing a rare opportunity to explore the effects of naturally occurring social variation on the fitness and physiology of individuals living in groups and alone.

Previous studies of C. sociabilis have shown that living alone is associated with significant differences in survival and direct fitness compared to remaining in the natal group. Because social setting may also substantially impact proximate factors such as individual physiology, I examined the effects of intraspecific differences in social setting on measures of baseline glucocorticoid (GC) levels. GCs are adrenal steroids critical to maintaining homeostasis. As such, GCs provide an appropriate gauge of the physiological response to social and physical stressors. By combining data on GC variation in free-living animals with experimental manipulation of housing conditions for captive individuals, I tested the hypothesis that living and breeding alone are associated with increased GC levels in this species. I also investigated how group size and composition correlate with GCs in group-living individuals. These resulting data indicate the social environment is an important determinant of baseline GC levels and that these effects vary with group composition. Collectively, these analyses yield important new insights into physiological consequences of sociality.

In colonial tuco-tucos, variation in the social environment also includes whether or not an adult male is present in the communal nest of this plural-breeding rodent. Due to high mortality, males are not found in all burrow systems. When a male is present in a colony, the time spent in the nest with pre-weaned offspring does not differ than that of females. Male assistance with young is rare for mammals, and for this part of the study, I tested two major hypotheses explaining male next attendance - the parental effort hypothesis and the mating effort hypothesis. Combining evidence from the field and lab, I found that male nest attendance did not increase the number or growth of young, nor future access to the females in the burrow system. However, the presence of an adult male increased the survival of male - but not female - pups. This result, along with observations in the lab indicating that adult males affected the pubescent onset of aggressive interactions between male pups, suggests that they may play a critical role in the development of male offspring, thereby affecting survival.