Kinship and Dominance Rank Influence the Strength of Social Bonds in Female Geladas (Theropithecus gelada)

In many primates, close social relationships are associated with lower stress, better health, and increased life span. However, individuals do not form bonds indiscriminately; rather, they focus on a few primary partners. This suggests that the identity of the partner may be as important as the bond itself. Although dominance and kinship have repeatedly emerged as salient predictors of female relationships, most of this research comes from species with multimale, multifemale groups and strict dominance hierarchies. Further, kinship was typically determined based on either behavior or on known mother-daughter relationships alone. To understand the generality of previous findings, we use behavioral and genetic sampling to examine whether dominance rank and/or genetic relatedness mediate female social bonds in geladas (Theropithecus gelada) living in the Simien Mountains National Park, Ethiopia. First, we found that, even though females in the same unit are closely related, female geladas still preferentially bond with the closest of these relatives. Second, females that were close kin formed the strongest bonds with females of similar rank to themselves. Finally, rank disparity predicted grooming rates but did not predict whether females were nearest neighbors. This suggests that, in contrast with data from other cercopithecines, spatial proximity among females may be less indicative of strong social bonds for geladas, a species that routinely exhibits a high degree of spatial overlap with extra-unit individuals. Together, these results highlight the importance of combining genetic data with detailed behavioral observations to help us understand how individuals choose and interact with social partners.