The evolution of female social relationships in nonhuman primates

Abstract

Considerable interspecifc variation in female social relationships occurs in gregarious primates, par- ticularly with regard to agonism and cooperation be- tween females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food dis- tribution. According to the current ``ecological model'', predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and be- tween groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has suc- cessfully explained differences in the occurrence of for- mal submission signals, decided dominance relation- ships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incor- porate the effects of infanticide risk and habitat satura- tion. We review evidence in support of the ecological model and test the power of alternative models that in- voke between-group competition, forced female phi- lopatry, demographic female recruitment, male inter- ventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a ``socioecological model'' based on the combined im- portance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these re- main to be explored in detail.