The fact that most female primates (and many other mammals) live in groups is paradoxical, given that the presence of others presumably increases competition for foods and may, for some, reduce reproductive success. Competition for food resources is generally inferred from any of the following observations: (1) female dominance hierarchies within groups; (2) female aggression between groups; (3) increasing home-range size with increasing group size; (4) longer day-range length with increasing group size; and (5) lower reproductive rates in larger groups. Both female aggression (interference competition) and adjustments of ranging behavior to group size (exploitative competition) have been linked in the past to patterns of food distribution and abundance. Using data largely from the literature, this paper examines the covariance of female aggression and ranging behavior among 20 species of primates in an attempt to better explain the variation in female relationships within and between groups of primates. Results show that groups of females are aggressive toward other groups and that home-range size increases with increasing group size in most species. In addition, in those species with strong dominance hierarchies within groups, day-range length increases as a function of group size. However, in those species that do not have strong dominance hierarchies within groups, dayrange length does not increase as a function of group size. The implications of these results are presented in a model that suggests that intergroup competition is determined by food abundance, whereas intragroup competition is determined by food distribution. This model differs from earlier models in its explanation of the ecological conditions that influence female relationships within and between groups of primates. [Behav Ecol 1991;2:143–155]
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