Naturwissenschaften (2004) 91:391–395 DOI 10.1007/s00114-004-0545-5

Abstract Communal nesting is a fundamental compo-nent of many animal societies. Because the fitness con-sequences 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 to-gether, even when supplied with several nest boxes. To determine whether free-living degus also engage in com-munal nesting, we used radiotelemetry to monitor spatial relationships among adult females in a population of O. degus in central Chile. These analyses revealed that fe-males 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 interac-tions 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 micro-satellite loci. These analyses indicated that mean pair-wise relatedness among members of a nesting associa-tion (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