There has been much debate over the role of inbreeding avoidance versus the roles of competition for mates and resources as primary causes of primate dispersal. The first half of this paper evaluates the primate literature in relation to the underlying assumptions of the inbreeding avoidance hypothesis and reviews the variety of behaviors, including dispersal, that limit inbreeding in primates. The underlying assumptions of the inbreeding avoidance hypothesis hold for most primates: (1) inbreeding is evolutionarily costly; (2) the costs of inbreeding outweigh its benefits; and, (3) there exist a variety of behaviors that limit inbreeding: behavioral avoidance of inbreeding, extragroup copulation, reproductive suppression, and dispersal. Thus, inbreeding avoidance cannot be relegated to a minor cause of primate dispersal on theoretical grounds. However, the costs of dispersal are highly variable, taking on condition-specific values for individual organisms. The high variability of dispersal costs among individuals may explain why several mechanisms of inbreeding avoidance can be present within a single primate species. The second half of the paper is an empirical assessment of the concordance between dispersal data gleaned from the primatological literature and the predictions of each hypothesis for dispersal: inbreeding avoidance, competition for mates/mate selection, and competition for resources. We find that while most cases of dispersal are consistent with the predictions of the competition hypothesis (for mates and resources), there are several species for which the dispersal data are better explained by the inbreeding avoidance hypothesis. Thus, the variability of dispersal patterns evident within and among primate species results from a complex mix of these three major causes, the existence of non-dispersal mechanisms for inbreeding avoidance, and the variability of dispersal costs.
