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On the evolution of Ape Social Systems

Despite theoretical advances in understanding the nature of social evolution (e.g. Hamilton, 1964; Trivers, 1972) a clear procedure for analysing the adaptive functions of social life has not yet emerged. In principle, attempts to explain the distribution and diversity of types of social group should refer to how the expected reproductive success of each individual is affected by its being social. In fact, however, the analysis of primate societies draws so much attention to groups that the reproductive rates of groups are sometimes treated as the consequences on which natural selection acts. This could be valid if natural selection (i.e. differential replication) of groups was at least partly responsible for the evolution of individual behavior patterns. It is clear, however, that group selection has effects only under extraordinary conditions, which include low rates of migration and/or high rates of group extinction (Maynard Smith, 1976). With empirical data now available for many species of primates it is certain that these conditions are not met: in some species social groups break up before having the opportunity to go extinct (e.g. Dunbar and Dunbar, 1975), while in those that persist for many generations at least half the individuals leave their natal groups at adolescence (e.g. Sugiyama, 1976; Packer, 1975; Harcourt et al., 1976; Pusey, in press; Hrdy, in press). This means that the genetic composition of a primate group changes continually, leaving no occasion for the selection of different types of group. The exclusion of group selection as a mechanism in primate evolution clarifies somewhat the logic underlying an analysis of their social systems. It is no longer sufficient to argue, for instance, that social evolution occurs because the group gives safety from predators, the group enables individuals to find more food or a stable hierarchy brings peace to the group. Such statements assume that different individuals in the group have common interests, but this will rarely be the case. In particular, the sexes are likely to have very different interests in the consequences of social life. If group selection has no systematic effects on social behavior ideas such as