INTRODUCTION

Primates exhibit considerable diversity in their social systems (Smuts et al. 1987), a phenomenon that is thought to have evolved through the interaction of many factors. These include: (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); (3) demographic and life history variables (DeRousseau 1990; Ross 1998); and (4) phylogenetic constraints (Wilson 1975; DiFiore & Rendall 1994). Generally, tests of models of the effect of these variables on behavior have been made through broad comparisons of many taxa, usually across genera (Wrangham 1980; van Schaik 1989; DiFiore & Rendall 1994; Sterck et al. 1997). There have been fewer attempts to consider the influence of these factors on variability in social organization within and between closely related taxa, largely as a result of a dearth of species for which such data are available (but see Mitchell et al. 1991; Koenig et al. 1998; Boinski 1999; Barton 2000). In addition, most tests have focused intensively on a single class of traits, and their proposed influence on behavior (e.g. the influence of ecology on behavior, van Schaik 1989; but see Nunn & van Schaik 2000), rather than the role of all proposed factors. To date, no study has quantitatively examined the combined influence of ecology, habitat, demography, and phylogeny on behavior.