Chacma baboon mating markets: competitor suppression mediates the potential for intersexual exchange

Contrary to the expectations of biological market theory, in species where sexual coercion is effective males often exchange resources or services with females for the opportunity to mate. This suggests that an ability to control mating partners does not preclude the need for their cooperation. We argue that this is because, in many systems, female resistance to coercion can precipitate intermale competition and, as such, a male's mating strategy may often be better served by securing female compliance through affiliative rather than agonistic means. Based on this reasoning, we predicted that the need for males to exchange resources/services for mating access with females will be absent only when intermale power differentials are such that dominant individuals can secure uncontested access to receptive females. Accordingly, data from our long-term study site revealed no support for a biological markets model of intersexual mating exchange in chacma baboons, where the mating monopolies of alpha males are near absolute. Specifically, we found that males groomed females substantially less than they were groomed by them, their propensity to groom was poorly described by measures of female fertility, male rank, and the operational sex ratio, and their mating success was not associated with their grooming effort. We further predicted that an additional consequence of the degree of competitor suppression seen in chacma would be a reversal in the direction of intersexual trade of services for mating. We found that female grooming was positively associated with the probability that they would successfully initiate a copulation. Our study strongly suggests that it is variance in competitor suppression, not partner control, that mediates the potential for and direction of intersexual cooperative exchange. Copyright 2010, Oxford University Press.