Male homosexual behavior among different age groups in captive stump-tailed macaque (Macaca arctoides): socio-sexual or sexually motivated?

The present study aims to determine whether homosexual behavior in stump-tailed macaque males of different age groups were socio-sexual or sexually motivated. Male homosexual encounters were divided into two categories for analysis; manual genital stimulation and actual mounting. Manual genital stimulation in adult males occurred bidirectional, in the context of social affiliations. There was no significant difference between dominant and subordinate partners in terms of genital stimulation performed or received. Mounting was in double foot-clasp position similar to heterosexual encounter. We identified three types of mounts. All types of mounts occurred unidirectional among adult males and mostly occurred in aggressive context. Unique homosexual encounters was observed between α-male and β-male where mounting involved anal intromission terminated by ejaculation and occurred only during the mating season after the heterosexual copulation between β-male and estrous female, which was known to be sexually motivated. We examined several socio-sexual explanations for male homosexual mounting, including (1) dominance-assertion (2) tension-regulation (3) reconciliation and (4) practice for heterosexual copulation. Homosexual encounters between adult males interpreted: a strict relationship between dominant individuals mounting subordinate individuals, reconciliation attempted by the subordinate; and the mechanism employed to regulated aggression and reestablished social bond among adult males. Mounting among immature males occurred bidirectional. Five evidences indicated the homosexual mounting between immature males as socio-sexual behavior. The evidences based on: (a) the mating season (b) the same day heterosexual encounter (c) assistance provided by adult male to immature males (d) occurrence in the context of play (e) immature males in homosexual mounting and learning heterosexual mounting with adult females. The male-male homosexuality adds a new dimension in sexual behavior of stump-tailed macaque.Â