Overriding reproductive constraints: Hormonal profiles fail to predict intrasexual competition

Abstract

Abstract Intrasexual competition among females has long been overshadowed by the focus on that in males. Apart from the studies on Callitrichidae, mechanisms beyond reproductive suppression are virtually uninvestigated. However, that females compete for breeding is likely ubiquitous across polygynous mating primates, where males are likely the limited resource for reproducing and/or offspring raring. As the northernmost distributed Asian colobine, golden snub-nosed monkey retained its ancestral one-male, multi-female unit, but evolved a multi-level society through social aggregation. As a result, the operational sex ratio (the ratio of sexually receptive males to sexually receptive females, hereafter OSR) is expected to be near one as numerous bachelor males shadow the breeding band for mating opportunities. Nevertheless, intrasexual competition was intense, at least in the two study units. The females copulated primarily with the residential males. Hormonal evidence indicates that they were receptive for only 70 days every two years, with only 1 to 3 ovulations. Finally, within units, 50% to 100% of their receptive periods overlapped with one another. Surprisingly, not a single case of aggression or reproductive suppression was observed. By the end of the breeding season, all receptive females showed signs of pregnancy, suggesting successful conceptions. Because staying in stable large units confers a competitive edge over resources through dominance, evolution might have provided strategies for avoiding costly mating competition within units. Further behavioral investigation tentatively reveals two such strategies: coordinating solicitation frequency through turn-taking and/or migrating to units where the intensity of intrasexual competition is low. Our future studies will address the nature and fitness consequences of them.