Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Research indicates that various forms of this are found throughout the animal kingdom. As of 1999, about 500 species, ranging from primates to gut worms, have been documented engaging in same-sex behaviors. According to the organizers of the 2006 Against Nature? exhibit, it has been observed in 1,500 species.
According to Bruce Bagemihl, "the animal kingdom [does] it with much greater sexual diversity – including homosexual, bisexual and nonreproductive sex – than the scientific community and society at large have previously been willing to accept". Bagemihl adds, however, that this is "necessarily an account of human interpretations of these phenomena".Simon LeVay introduced the further caveat that "[a]lthough homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity." One species in which exclusive homosexual orientation occurs, however, is that of domesticated sheep (Ovis aries). "About 10% of rams (males) refuse to mate with ewes (females) but do readily mate with other rams."
The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from social species. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied.
The observation of homosexual behavior in animals can be seen as both an argument for and against the acceptance of homosexuality in humans, and has been used especially against the claim that it is a peccatum contra naturam ("sin against nature"). For instance, homosexuality in animals was cited by the American Psychiatric Association and other groups in their amici curiae brief to the United States Supreme Court in Lawrence v. Texas, which ultimately struck down the sodomy laws of 14 states.