Homosexual behavior in animals

Various non-human animal species exhibit behavior that can be interpreted as homosexual or bisexual, often referred to as same-sex sexual behavior (SSSB) by scientists. This may include same-sex sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Various forms of this are found among a variety of vertebrate and arthropod taxonomic classes. 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.

Scientists observe same-sex sexual behavior in animals in different degrees and forms among different species and clades. A 2019 paper states that it has been observed in over 1,500 species. Although same-sex interactions involving genital contact have been reported in many animal species, they are routinely manifested in only a few, including humans. Other than humans, the only known species to exhibit exclusive homosexual orientation is the domesticated sheep (Ovis aries), involving about 10% of males. The motivations for and implications of these behaviors are often lensed through anthropocentric thinking; Bruce Bagemihl states that any hypothesis is "necessarily an account of human interpretations of these phenomena".

Proposed causes for same-sex sexual behavior vary across species. Theories include mistaken identity (especially for arthropods), sexually antagonistic selection, balancing selection, practice of behaviors needed for reproduction, expression of social dominance or submission, and social bonding. Genetic, hormonal, and neurological variations as a basis for individual behavioral differences within species have been proposed, and same-sex sexual behavior has been induced in laboratory animals by these means.

The term homosexual was coined by the Hungarian writer and campaigner Karl Maria Kertbeny in 1868 to describe same-sex sexual attraction and sexual behavior in humans. Its use in animal studies has been controversial for two main reasons: animal sexuality and motivating factors have been and remain poorly understood, and the term has strong cultural implications in western society that are irrelevant for species other than humans. Thus homosexual behavior has been given a number of terms over the years. According to Bagemihl, when describing animals, the term homosexual is preferred over gay, lesbian, and other terms currently in use, as these are seen as even more bound to human homosexuality.

Bailey et al. says:

Homosexual: in animals, this has been used to refer to same-sex behavior that is not sexual in character (e.g. 'homosexual tandem running' in termites), same-sex courtship or copulatory behavior occurring over a short period of time (e.g. 'homosexual mounting' in cockroaches and rams) or long-term pair bonds between same-sex partners that might involve any combination of courting, copulating, parenting and affectional behaviors (e.g. 'homosexual pair bonds' in gulls). In humans, the term is used to describe individual sexual behaviors as well as long-term relationships, but in some usages connotes a gay or lesbian social identity. Scientific writing would benefit from reserving this anthropomorphic term for humans and not using it to describe behavior in other animals, because of its deeply rooted context in human society.

Animal preference and motivation is always inferred from behavior. In wild animals, researchers will not be able to map the entire life of an individual, and must infer from frequency of single observations of behavior. The correct usage of the term homosexual is that an animal exhibits homosexual behavior or even same-sex sexual behavior; however, this article conforms to the usage by modern research, applying the term homosexuality to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex. In most instances, it is presumed that the homosexual behavior is but part of the animal's overall sexual behavioral repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans.^{[page needed]}

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 Psychological 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.