While field studies of wild chimpanzees have investigated the proximate determinants of hunting success, little attention has been paid to the decision to hunt. We present evidence from Gombe National Park, Tanzania, showing that the social factors that most strongly influence the decision to hunt red colobus monkeys are the presence of female chimpanzees with anogenital sexual swellings in the foraging party, the number of adult and adolescent males in the party, and the total size of the foraging party. Of these, the presence of one or more swollen females was the best predictor of a decision to undertake hunts of red colobus groups at all but the smalles chimpanzee foraging party size. Two likely explanations for this pattern are discussed. First, swollen females may be a primary influence on male grouping patterns, which in turn promotes hunting. Second, this finding, together with previous research showing that male chimpanzees preferentially give meat to estrous females, suggests that male chimpanzee hunting performance may be under sexual selection. While nutritional and ecological factors may influence chimpanzee hunting patterns at times, chimpanzees appear to hunt red colobus at least partially to obtain meat for use as a social and reproductive tool.