Homosexual behavior in animals refers to the documented evidence of homosexual, bisexual and transgender behavior in non-human animals. Such behaviors include sex, courtship, affection, pair bonding, and parenting. A 1999 review by researcher Bruce Bagemihl shows that homosexual behavior, has been observed in close to 1500 species, ranging from primates to gut worms, and is well documented for 500 of them.[2][3] Animal sexual behavior takes many different forms, even within the same species. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied.[4] According to 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."[5] Current research indicates that various forms of same-sex sexual behavior are found throughout the animal kingdom. [6] A new review made in 2009 of existing research showed that Same-sex behavior is a nearly universal phenomenon in the animal kingdom, common across species.[7]
The natural existence of homosexuality in non-human animals is considered controversial by conservative religious groups who oppose LGBT social movements because these findings seem to point to the natural occurrence of homosexuality in humans.[1] It also counters the 'peccatum contra naturam' ('sin against nature') - after Thomas Aquinas - established since the Medieval Christianities.[8] Whether this has logical or ethical implications is also a source of debate, with some arguing that it is illogical to use animal behavior to justify what is or is not moral (see appeal to nature).[1][9][10]