Introduction

Non-human primate socioecology and female social relationships

Female mountain gorillas (Gorilla gorilla beringei)are large, long-lived, slowly reproducing mammals whose diet consists mostly of vegetation that is abundant, evenly distributed, and high in structural carbohydrates. These few characteristics do much to explain the mountain gorilla social system, which we can only understand by placing long-term data on known individuals in comparative context. Variation in primate social systems depends largely on variation in life history tactics, predation risk, feeding competition, and conflicts and convergence of reproductive interest between the sexes (Sterck et al., 1997; Kappeler, 1999a). Ecological factors are especially important to females, because they have slower maximum reproductive rates and higher parental investment than males (Bradbury & Vehrencamp, 1977). Consequently, foraging efficiency is crucial for female reproductive success and feeding competition strongly influences female social relationships, whereas male reproductive success depends crucially on gaining access to fertile females (Bradbury & Vehrencamp, 1977; Wrangham, 1980; van Schaik, 1989). The need to minimize predation risk is probably the main reason why females of most diurnal species live in social groups (van Schaik, 1983, 1989; Janson, 1992; Kappeler, 1999a). Variation in diet and in food distribution is the main source of variation in social relationships among females because it determines the predominant mode and intensity of feeding competition within and between groups. Male reproductive competition can also influence relationships between females, and has complicated effects on group size and composition, male-female relationships, and the costs and benefits of dispersal for females (Figure 8.1) (van Schaik, 1989, 1996; Sterck et al., 1997; Nunn, 1999).