List of mammals displaying homosexual behavior

For these mammals, there is documented evidence of homosexual behavior of one or more of the following kinds: sexual behavior, courtship, affection, pair bonding, or parenting.
Bruce Bagemihl writes that the presence of same-sex sexual behavior was not officially observed on a large scale until the 1990s due to possible observer bias caused by social attitudes towards LGBT people, which made homosexuality in animals a taboo subject. He 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 realise 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.
Animal 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 sexual activity, has been documented in about 500 species as of 1999, ranging from primates to gut worms. Homosexuality in animals is controversial with some social conservatives because it asserts the naturalness of homosexuality in humans, while others counter that it has no implications and is nonsensical to equate animal behavior to morality. On the other hand, social liberals and many gay people believe homosexuality is natural, and therefore find the existence of homosexual sex in animals unsurprising. Animal preference and motivation is always inferred from behavior. Thus homosexual behavior has been given a number of terms over the years. Modern research applies the term "homosexuality" to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex.
This a list of some mammals that had been recorded engaging in homosexual behavior, which is part of a larger list of animals displaying homosexual behavior including birds, insects, fish, etc.