Kinship and coperation in the multi-level society of geladas (Theropithecus gelada)

Kin selection, where individuals aid close relatives thereby increasing their own fitness, has long been considered an essential force in the evolution of cooperation and the formation of kin-groups. However, much of what we know about the effects of relatedness on cooperation and group formation comes from relatively simple, single-level social systems. We know little about kinship works in more complex societies, thereby compromising our understanding of the role of relatedness in the evolution of these social systems. Here, I investigated the mechanisms underlying one such social system that has evolved across a range of species (from elephants to humans), the multi-level society, in which individuals within the same local population predictably separate or join one another in subgroups of varying size and composition. I used non-invasive genetic techniques in combination with longitudinal behavioral analyses to investigate the effect of relatedness on the behavior and association patterns of the multi-level social system of geladas (Theropithecus gelada). Overall, I found that geladas fit the typical mammalian pattern of female philopatry and male dispersal. Further analysis, however, revealed a striking difference between geladas and most mammals—females maintained close ties, as measured via association patterns, with relatives outside of their closed reproductive unit. Additionally, using data from three unit fissions, we found that large units split along familial lines, such that all mother-daughter pairs entered the same daughter unit after the fission event—leading to the tiered relatedness structure. While females likely increase their fitness by cooperating and forming groups with kin, the factors favoring the evolution of multi-male groups—where reproductive competitors tolerate one another—remains a mystery. I therefore examined the potential for cooperation among males within units—particularly in the context of reproductive sharing. I found that dominant and subordinate males within units engaged in mutually beneficial reproductive transactions. Dominant males conceded a small amount of reproduction subordinates, which effectively persuaded the subordinate to remain in and defend the unit. Taken together, these findings represent the first comprehensive genetic and behavioral analysis of the effect of kinship and cooperation in a complex, multi-level society