Communal nesting is a fundamental component of many animal societies. Because the fitness consequences of this behavior vary with the relatedness among nest mates, understanding the kin structure of communally nesting groups is critical to understanding why such groups form. Observations of captive degus (Octodon degus) indicate that multiple females nest together, even when supplied with several nest boxes. To determine whether free-living degus also engage in communal nesting, we used radiotelemetry to monitor spatial relationships among adult females in a population of O. degus in central Chile. These analyses revealed that females formed stable associations of > 2-4 individuals, all of whom shared the same nest site at night. During the daytime, spatial overlap and frequency of social interactions were greatest among co-nesting females, suggesting that nesting associations represent distinct social units. To assess kinship among co-nesting females, we examined genotypic variation in our study animals at six microsatellite loci. These analyses indicated that mean pairwise relatedness among members of a nesting association (r=0.25) was significantly greater than that among randomly selected females (r=-0.03). Thus, communally nesting groups of degus are composed of female kin, making it possible for indirect as well as direct fitness benefits to contribute to sociality in this species.

I analyse and summarize the empirical evidence in mammals supporting alternative benefits that individuals may accrue when committing nonparental infanticide. Nonparental infanticide may provide the perpetrator with nutritional benefits, increased access to limited resources, increased reproductive opportunities, or it may prevent misdirecting parental care to unrelated offspring. The possibility that infanticide is either a neutral or maladaptive behaviour also is considered. I devote the second half of this article to reviewing potential mechanisms that individuals may use to prevent infanticide. These counterstrategies include the early termination of pregnancy, direct aggression by the mother against intruders, the formation of coalitions for group defence, the avoidance of infanticidal conspecifics, female promiscuity, and territoriality. I evaluate the support for each benefit and counterstrategy across different groups of mammals and make suggestions for future research.

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Ebensperger L. A. 2001. A review of the evolutionary causes of rodent group-living. Acta Theriologica 46: 115-144.I analyze and summarize the empirical evidence supporting alternative hypotheses posed to explain the evolution of rodent group-living. Eight hypotheses are considered: two rely on net fitness benefits to individuals, five rely on ecological and life-history constraints, and one uses elements of both. I expose the logic behind each hypothesis, identify its key predictions, examine how the available evidence on rodent socioecology supports or rejects its predictions, and identify some priorities for future research. I show that empirical support for most hypotheses is meager due to a lack of relevant studies. Also, empirical support for a particular hypothesis, when it exists, comes from studies of the same species used to formulate the original hypothesis. Two exceptions are the hypothesis that individual rodents live in groups to reduce their predation risk and the hypothesis that group-living was adopted by individuals to reduce their cost of thermoregulation. Finally, most hypotheses have been examined without regard to competing hypotheses and often in a restricted taxonomic context. This is clearly an unfortunate situation given that most competing hypotheses are not mutually exclusive. I suggest that in the future comparative approaches should be used. These studies should examine simultaneously the relevance of different benefits and constraints hypothesized to explain the evolution of rodent sociality.

Studying the causes and reproductive consequences of social variation can provide insight into the evolutionary basis of sociality. Individuals are expected to behave adaptively to maximize reproductive success, but reproductive outcomes can also depend on group structure. Degus (Octodon degus) are plurally breeding rodents, in which females allonurse indiscriminately. However, communal rearing does not appear to enhance female reproductive success, and larger group sizes are correlated with decreasing per capita pup production. To further investigate mechanisms underlying these patterns, we asked how differences in sex, season and average group reproductive success are related to degu association networks. We hypothesized that if reproductive differences mirror social relationships, then females (core group members) should show stronger and more stable associations than males, and female association strength should be strongest during lactation. We also hypothesized that, at the group level, social cohesion would increase reproductive output, while social conflict would decrease it. Females did have higher association strength and more preferred partners than males, but only during lactation, when overall female associations increased. Females also had more stable preferred social partnerships between seasons. A measure of social cohesion (average association strength) was not related to per capita pup production of female group members, but potential social conflict (heterogeneity of association strengths) was negatively related to per capita pup production of female group members. Our results highlight temporal and multilevel patterns of social structure that may reflect reproductive costs and benefits to females.