Competition over food and predation pressure plays an important role in shaping the social system of group-living primates (Wrangham, 1980, 1987; van Schaik, 1983, 1989; van Schaik and van Hooff, 1983; Terborgh and Janson, 1986; Dunbar, 1988). When a high risk of predation forces primates to live in cohesive groups, within-group scramble competition may stimulate females of folivorous primates to develop individualistic and egalitarian ranking systems (van Schaik, 1989; Barton et al., 1996; Sterck et al., 1997). When the risk of predation is low, females no longer form cohesive groups and tend to disperse to forage alone. Both food distribution and male mating strategy may influence female grouping patterns.
Gorillas have a folivorous diet and an individualistic ranking system (Fossey and Harcourt, 1977; Stewart and Harcourt, 1987). By contrast, chimpanzees have a fission–fusion social system based on individual foraging, and their party size varies with fruit abundance and availability (Goodall, 1968; Nishida, 1970; Wrangham, 1980). Male chimpanzees tend to associate with each other and to show territorial behavior against other groups of males (Goodall et al., 1979; Nishida et al., 1985). The various mating patterns (promiscuous, possessive or consort) adopted by males may influence the size and composition of temporary parties (Tutin, 1979; Goodall, 1986). These observations seem to support previous arguments that food distribution and male mating strategy effect social relationships among females (Wrangham, 1987; Dunbar, 1988; van Schaik, 1989).