Gibbons are unusual among primates in that they form socially monogamous family groups. As illustrated by the epigraph of this chapter the extent to which social monogamy entails mating monogamy has long been a subject of debate. In any case monogamous social systems are rare among mammals and have generated considerable theoretical interest (Table 7-1). Here I am mostly interested in the evolutionary determinants of social monogamy, which I define as a stable association between one male, one female, and affiliated young that share a common home range or territory. Although mating monogamy, or exclusive sexual access, is not a necessary corollary of social monogamy, primate social structure1 (the size and composition of social groups) is closely tied to primate social organization (the patterns of interactions between group members) because it determines the availability of social partners (Eisenberg et al. 1972; Terborgh and Janson 1986). This is especially true in two-adult groups, in which most of an adult’s time is spent in proximity to a single member of the opposite sex. Given the constraints that a two-adult social structure

places on the reproductive options of socially monogamous pair mates, it follows that an important first step in understanding the evolution of gibbon social organization is examining ecological factors that limit group size.