Fellatio among male sanctuary-living chimpanzees during a period of social tension.

Same-sex sexual behaviour has been documented across the animal kingdom, and is thought to reflect and enhance dyadic cooperation and tolerance. For instance, same-sex fellatio — the reception of a partner’s penis into another’s mouth — has been reported in several mammalian species other than humans. Although same-sex sexual behaviour is observed in our close relatives, the chimpanzees, fellatio appears to be very rare — as yet there are no published reports clearly documenting its occurrence. At Chimfunshi Wildlife Orphanage in Zambia, we observed an instance of fellatio occurring during a post-conflict period between two adult male chimpanzees (born and mother-reared at the sanctuary) where one of the males was the victim. We discuss this event with respect to the putative functions of homosexual behaviour in great apes. Given its rarity in chimpanzees, this fellatio between adult males also highlights the apparent behavioural flexibility present in our close relatives