Same-sex sexual partner preference in hormonally and neurologically unmanipulated animals

Proximate and ultimate biological theories for understanding sexual behavior predict that sexual dimorphism in sexual partner preference should be ubiquitous in the animal kingdom. A review of the literature found evidence for same-sex sexual partner preference in a small number of species (female pukekos, cows, domestic rams, female Uganda kobs, female Japanese macaques). Thus, theoretical predictions concerning the development and evolution of sexual partner preference appear to hold true except for a handful of exceptional species. Why individuals in some animal species exhibit same-sex sexual partner preference remains the object of debate. At a proximate level, domestic rams that exhibit same-sex sexual partner preference have been shown to differ in certain aspects of their neurobiology and physiology from rams that do not exhibit such a preference. It remains unclear, however, as to whether these differences are produced by sex-atypical perinatal exposure to androgens and their estrogenic metabolites. At an ultimate level, numerous functional hypotheses for same-sex sexual partner preference have been tested in female Japanese macaques but have failed to receive support. Understanding why same-sex sexual partner preference evolves in some species may involve abandoning a strictly functional perspective and, instead, approaching the issue from the perspective of each species' unique evolutionary history.