Same-sex love triangles in female monkeys: Intra-sexual mate competition between female Japanese macaques for female sexual partners

While most reports of intra-sexual mate selection occur in the reproductive context of heterosexual interactions, this study aims to explore the behavioral mechanisms and evolutionary significance of a few rare cases of intra-sexual competition during triadic female homosexual relationships in a non-human primate species. Focusing on a population of Japanese macaques known for their routine female homosexual activity, we provide the first detailed quantitative description of three instances of female homosexual triads. These triads are characterized as the spatio-temporal overlap between two dyadic but non-exclusive female homosexual relationships, with the pivot female switching her sexual attention towards a new mate, while her initial sexual partner persists in sexually soliciting the pivot female and tries to chase the female competitor away. We documented the intra-sexual competition that may result from differing levels of sexual attraction among three female protagonists involved in each triad. We presented morphological, behavioral, and socio-demographic evidence in support of the sexual nature of such interactions, which is consistent with the “bisexual preference hypothesis”. We speculated on the proximate causes of this unusual phenomenon.