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Homosexual behavior has been reported in all of the Great Apes, for both males and females, adults and juveniles, in captivity and in the wild, although exclusive engagement in homosexual behavior may be unique to humans. Homosexual behavior, same-sex genital contacts or manipulation, likely functions in several contexts in the Great Apes, including: dominance, tension regulation, reconciliation, and social bonding. Homosexual contacts in primates can include ventro-dorsal and ventro-ventral mounting, genito-genital (g-g) rubbing, manual-genital contact, and oral-genital contact, and may also be accompanied by penile erection and ejaculation, as well as female orgasm.
Homosexual behavior is likely universal in human populations. It is more common, or more commonly acknowledged, in human societies in which it is not perceived of as an abnormal or aberrant behavior.
Homosexual behavior has been observed in mammalian and avian species and appears to be particularly common among marine mammals.
Male homosexuality and maternal immune responsivity to the Y-linked protein NLGN4Y, , PNAS, 2017/12/11, (2017)
Female Genito-genital Rubbing in a Group of Captive Chimpanzees, , Volume 25, Issue 2, p.477 - 488, (2004)
Homosexual behavior in wild Sumatran orangutans (Pongo pygmaeus abelii)., , Am J Primatol, 11/2001, Volume 55, Issue 3, p.177-81, (2001)
Biological Exuberance: Animal Homosexuality and Natural Diversity, , New York, p.751, (1999)
Homosexual behavior in primates: A review of evidence and theory, , Volume 16, Issue 2, p.173 - 204, (1995)