The evolution of multimale groups in Verreaux's sifaka, or how to test an evolutionary demographic model

Theoretical advances in the study of social evolution have highlighted the importance of studying group formation in conjunction with population dynamics. To address this need, a number of demographically explicit models have been developed. We parameterize such a model to study the evolution of multimale associations in Verreaux's sifaka (Propithecus verreauxi), a diurnal lemur living in small multimale-multifemale groups. We use long-term demographic data from a population in western Madagascar to estimate the fitness prospects of different male behavioral strategies. Our results indicate that dispersing male sifaka, rather than searching for a breeding territory of their own (floating), do better joining an established group as a subordinate with limited reproductive opportunities, mainly because they may later ascend to the dominant position. By contrast, resident males should not accept potential immigrants because we could not find any measurable benefit of living in a multimale group for them. There is thus a conflict over group membership between resident and floater males, and we suggest that floaters at least occasionally win this conflict because they have more to gain from joining than residents stand to lose by sharing their territory.