Introduction

Primates exhibit considerable diversity in their social systems (Smuts et al., 1987), which is thought to have evolved through an interaction of several factors including ecological variables, social factors, phylogeny, and demographic and life history variables. Attempts to understand how this variation in social organization evolves have focused primarily on (1) ecological variables, particularly predation pressure and the abundance and distribution of food (Alexander, 1974; Wrangham, 1979, 1980, 1987; van Schaik, 1983, 1989, 1996; Sterck et al., 1997), (2) social factors, primarily sexual selection and the potential risk of infanticide (Wrangham, 1979; Watts, 1989; van Schaik, 1996), and (3) phylogenetic inertia (DiFiore & Rendall, 1994). Gorillas provide a unique opportunity to reevaluate proposed models of ecological and social influences on social organization in African apes. Western lowland and mountain gorillas seem to differ dramatically in their habitats, resource availability, and foraging strategies. To what degree these differences are associated with differences in social organization is an intriguing question.

The genus Gorilla occurs in two widely separated forest habitats, one in western central Africa and one in eastern central Africa. Three subspecies of gorillas are generally recognized, western lowland (G. gorilla gorilla), eastern lowland (G. g. graueri), and mountain gorillas (G. g. beringei) (but see Doran & McNeilage, 1998 for reviewof taxonomic debate: Morell, 1994; Ruvolo et al., 1994; Butynski & Sarmiento, 1995). The three subspecies have not been studied equally. Most of our knowledge of gorilla behavior is based on the pioneering work of Dian Fossey (1974) and subsequent researchers on the well-studied mountain gorillas of Karisoke, Rwanda.