Male homosexual behavior in a free-ranging all-male group of Japanese macaques at minoo, Japan.

We documented nine male homosexual consortships within three different male-male dyads in a free-ranging all-male group of Japanese macaques (Macaca fuscata), at Minoo, Japan. A total of 63 male-male mounts were observed during these consortships. Male homosexual interactions shared most of the behavioral components that have been reported to characterize heterosexual and female homosexual consortships in this species. Convergent behavioral data, including analysis of male-male solicitations, mounting postures, body orientations, inter-mount activities, and third-party male intrusions supported the conclusion that male-male consortships are a sexual phenomenon. We discussed a series of proximate and ultimate hypotheses that purport to account for the occurrence of male homosexual behavior in all-male groups of primates, including humans. This first report of male homosexual interactions in an all-male group of Japanese macaques contributes to the growing database used to provide insights into the developmental processes, causal mechanisms, adaptive significance, and phylogenetic pathways of same-sex sexual behavior.