For these animals, there is documented evidence of homosexual behavior of one or more of the following kinds: sex, courtship, affection, pair bonding, or parenting, as noted in researcher and author Bruce Bagemihl's 1999 book Biological Exuberance: Animal Homosexuality and Natural Diversity. Bagemihl writes that the presence of same-sex sexual behavior was not "officially" observed on a large scale until the 1990s due to observer bias caused by social attitudes towards nonheterosexual people, making the homosexual theme taboo. Bagemihl devotes three chapters, "Two Hundred Years at Looking at Homosexual Wildlife", "Explaining (Away) Animal Homosexuality" and "Not For Breeding Only" in his 1999 book Biological Exuberance to the "documentation of systematic prejudices" where he notes "the present ignorance of biology lies precisely in its single-minded attempt to find reproductive (or other) 'explanations' for homosexuality, transgender, and non-procreative and alternative heterosexualities." Petter Bøckman, academic adviser for the Against Nature? exhibit, stated "[M]any researchers have described homosexuality as something altogether different from sex. They must realize that animals can have sex with who they will, when they will and without consideration to a researcher's ethical principles." Homosexual behavior is found amongst social birds and mammals, particularly the sea mammals and the primates. In 1986, it was even discovered amongst insects when butterfly scientist W.J. Tennent observed four male Mazarine blues competing for the attention of another male in Morocco. Sexual behavior takes many different forms, even within the same species and the motivations for and implications of their behaviors have yet to be fully understood. Bagemihl's research shows that homosexual behavior, not necessarily sex, has been documented in about five hundred species as of 1999, ranging from primates to gut worms. Homosexuality in animals is seen as controversial by social conservatives because it asserts the naturalness of homosexuality in humans, while others counter that it has no implications and is nonsensical to equate natural animal behaviors to morality. Sexual preference and motivation is always inferred from behavior. Thus homosexual behavior has been given a number of terms over the years. The correct usage of the term homosexual is that an animal exhibits homosexual 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 October 2023, biologists reported studies of mammals (over 1,500 different species) that found same-sex behavior (not necessarily related to human orientation) may help improve social stability by reducing conflict within the groups studied.

Mammals

Birds

Homosexual behaviour is demonstrated by 120 known species of birds. While an uptick in research on bird homosexuality – and animal homosexuality in general – has been coming out in recent years, it's common for some authors to labour in articulating the view any root cause or function of bird homosexuality is poorly understood. Hypotheses contrived in an attempt to explain the behaviour – homosexuality in birds – typically diverge from one another, further exacerbating an apparent perception in the concerned scientific communities knowledge's quest to realise bird homosexuality remains elusive as an objective. Some authors posit the behaviours are a result of any bird demonstrably homosexual being less inclined to rear young, while other authors posit the timing of emergence from the egg is a factor. In 1977, The New York Times also reported on a colony of lesbian seagulls "off [the] coast of California."

Fish

Reptiles

The all-female Whiptail lizard species Aspidoscelis neomexicanus (center), which reproduces via parthenogenesis, is shown flanked by two sexual species having males, A. inornatus (left) and A. tigris (right). Research has shown that simulated mating behavior increases fertility for Aspidoscelis neomexicanus. One female lies on top of another, playing the role of the male, the lizard that was on bottom has larger eggs, in most cases. The lizards switch off this role each mating season.

Amphibians

Insects

Male homosexuality has been inferred in several species of dragonflies. A survey of damsel and dragonflies reveals characteristic cloacal pincher mating damage in 20–80 percent of the males, indicating a fairly high occurrence of sexual coupling between males.

Other invertebrates