The naturalness of homosexuality in non-human animals is considered controversial by conservative religious groups who oppose LGBT social movements because it points to the naturalness of homosexuality in humans. 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).
Animal preference and motivation is always inferred from behaviour. In wild animals, researchers will as a rule not be able to map the entire life of an individual, and must infer from frequency of single observations of behaviour. The correct usage of the term homosexual is that an animal exhibits homosexual behaviour, however this article conforms to the usage by modern research applying the term homosexuality to all sexual behaviour (copulation, genital stimulation, mating games and sexual display behaviour) between animals of the same sex. In most instances, it is presumed that the homosexual behaviour is but part of the animals overall sexual behavioural repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans, but cases of clear homosexual preference and exclusive homosexual pairs are known (see examples for details).
Some researchers believe it to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Joan Roughgarden, Bruce Bagemihl, Thierry Lodé and Paul Vasey suggest the social function of sex (both homosexual and heterosexual) is not necessarily connected to dominance, but serves to strengthen alliances and social ties within a flock. Others have argued that social organization theory is inadequate because it cannot account for some homosexual behaviors, for example, penguin species where same-sex individuals mate for life and refuse to pair with females when given the chance. While reports on many such mating scenarios are still only anecdotal, a growing body of scientific work confirms that permanent homosexuality occurs in species with permanent pair bonds, but also in non-monogamous species like sheep.
One report on sheep cited below states:
In fact, apparent homosexual individuals are known from all of the traditional domestic species.
If the pair are a male and a female, hybrid offspring can result if the two species are related. However, this offspring may not be able to breed itself. The mule, for example (horse/donkey cross) is normally sterile, whilst the liger (lion/tiger cross) is sometimes fertile. Novosibirsk zoo director Rostislav Shilo says on the liger (born in his zoo); “It’s just that the lion and the tiger live in neighboring caves in the Novosibirsk zoo, and got used to each other. It’s practically impossible in the wild.”. Cross species sex in the wild has been observed between several species, among them blue tit and great tit, Chimpanzees and olive baboons and Amazon River Dolphin and the tucuxi dolphin.
An estimated one-quarter of all black swans pairings are homosexual and they steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. More of their cygnets survive to adulthood than those of different-sex pairs possibly due to their superior ability to defend large portions of land.
Mallards form male-female pairs only until the female lays eggs, at which time she is left by the male. The clutch is 8–13 eggs, which are incubated for 27–28 days to hatching with 50–60 days to fledging. The ducklings are precocial, and can swim and feed themselves on insects as soon as they hatch, although they stay near the female for protection. Young ducklings are not naturally waterproof and rely on the mother to provide waterproofing. Mallards also have rates of male-male sexual activity that are unusually high for birds. In some cases, as many as 19% of pairs in a Mallard population are male-male homosexual
Zoos in Japan and Germany have also documented gay male penguin couples. The couples have been shown to build nests together and use a stone to replace an egg in the nest. Researchers at Rikkyo University in Tokyo found 20 gay pairs at 16 major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to break up the gay male couples by importing female penguins from Sweden and separating their male couples, but they were unsuccessful. The zoo director stated the relationships were too strong between the gay couples. German gay groups protested at this attempt to break up the male-male couples.
Courtship, mounting, and full anal penetration between bulls has been noted to occur among American Bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behaviour, to "ensure the return of the buffalo in the coming season." Also, mounting of one female by another is common among cattle. Inter-sexual bison also occur. The Lakota refer to them as pte winkte —pte meaning bison and winkte designating two-spirit— thereby drawing an explicit parallel between transgender in animals and people.
Janet Mann, Georgetown University professor of biology and psychology, argues that the strong personal behaviour among male dolphin calves is about bond formation and benefits the species in an evolutionary context. She cites studies showing that these dolphins later in life as adults are in a sense bisexual, and the male bonds forged earlier in life work together for protection as well as locating females to reproduce with.
Researchers have treated homosexual behavior in animals as a taboo subject perhaps from innocent confusion or even from a fear of "being ridiculed by their colleagues." A case of overlooking behaviour is noted by Bruce Bagemihl describing mating giraffes where nine out of ten pairings occur between males.
Every male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only "revolving around" dominance, competition or greetings.
Both male and female lions have been seen to interact homosexually. Male lions pair-bond for a number of days and initiate homosexual activity with affectionate nuzzling and caressing, leading to mounting and thrusting. About 8% of mountings have been observed to occur with other males, while female pairings are held to be fairly common in captivity but have not been observed in the wild.
It should be noted that some view this study to be flawed in that the determination of homosexuality within the sheep (sample population of 27 for the study) was to have animals who were unable to mount female ewes placed in a cage with two stanchioned males and two unstanchioned females (i.e. the males could not move or struggle while the females could). Given the aggressive nature of sheep copulation, the uneven treatment of males and females, many see this as simply evidence that the sheep in question were unable to be aggressive enough to mount females. The results were situational sexuality, unlike the bonds seen in human homosexuality.
The scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol, a form of estrogen which is believed to facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes."
"The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "...naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis." As noted prior, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. It should also be noted that the results of this study have not been confirmed by other studies.
The female Spotted Hyena has a unique urinary-genital system, closely resembling the penis of the male. The family structure is matriarchal and dominance relationships with strong sexual elements are routinely observed between related females.
Early naturalists thought hyenas were hermaphrodites or commonly practiced homosexuality, largely due to the female spotted hyena's unique urogenital system which looks more like a penis than a vagina. According to early writings such as Ovid's Metamorphoses and the Physiologus, the hyena continually changed its sex and nature from male to female and back again. In Paedagogus, Clement of Alexandria noted that the hyena (along with the hare) was "quite obsessed with sexual intercourse." Many Europeans associated the hyena with sexual deformity, prostitution, deviant sexual behavior and even witchcraft. Research has shown that "in contrast to most other female mammals, female Crocuta are male-like in appearance, larger than males, and substantially more aggressive," they have "been masculinized without being defeminized.”
This unique genitalia and aggressive behavior in the female hyena has led to the understanding that more aggressive females are better able to compete for resources including food and mating partners. Research has shown that "elevated levels of testosterone in utero" contributes to extra aggressiveness from their mother and both males and females mount members of the same sex, who in turn are possibly acting more submissive because of lower levels of testosterone from their mothers.
Male homosexuality has been inferred in several species of dragonflies (the order Odonata). The cloacal pinchers of male damselflies and dragonflies inflict characteristic head damage to females during sex. A survey of 11 species of damsel and dragonflies has revealed such mating damages in 20 to 80 % of the males too, indicating a fairly high occurrence of sexual coupling between males.
Lizards that perform the courtship ritual have greater fertility than those kept in isolation due to an increase in hormones triggered by the sexual behaviors. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success.
From an Evolutionary standpoint, these females are passing their full genetic code to all of their offspring (rather than the 50% of genes that would be passed in sexual reproduction). Certain species of gecko also reproduce by parthenogenesis.