The Evolution of Eulemur Social Organization

Abstract

Recent comparative studies on the evolution of mammalian sociality came to opposite conclusions regarding the direction and drivers of evolutionary transitions in social organization, particularly concerning the evolution of pair-living among primates. Because the genus Eulemur is one of the few exceptional primate genera that contain both group- and pair-living species, eulemurs offer a rare opportunity to illuminate the processes driving social diversity among species with very similar life histories and ecologies. The aims of our study were therefore 1) to reconstruct the evolution of social organization in eulemurs with new demographic and phylogenetic data and 2) to evaluate several hypotheses about the evolution of pair-living with data from our long-term study of Eulemur rufifrons and published data on other eulemurs. Our review of published data confirmed that E. mongoz and E. rubriventer are pair-living, whereas the 10 other species of Eulemur live in groups with multiple adult males and females. Furthermore, pair-living evolved at least once among eulemurs from group-living ancestors. The available comparative evidence indicated that this evolutionary transition was probably not driven by male infanticide, social instability, or singular ecological factors. Instead, we propose that female competition has favored small female group size and that this effect may have been exacerbated by interspecific competition with sympatric congeners in the ancestors of the pair-living species. In conclusion, we confirmed that the evolution of pair-living from group-living ancestors among true lemurs is exceptional, but not unique, among mammals, and that commonly invoked hypotheses for the evolution of pair-living do not appear to be supported for this genus.