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Homosexual behavior in animals

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Roy and Silo, two New York Central Park Zoo male Chinstrap Penguins similar to those pictured, became internationally known when they coupled and later were given an egg that needed hatching and care, which they successfully did.[1]

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]

Applying the term "homosexual" to animals

The term homosexual was coined in Prussia in 1869 to describe same-sex sexual attraction and sexual behavior in humans.[11] 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 other species than humans.[12] Thus homosexual behavior has been given a number of terms over the years. The term "homosexual" is preferred over "gay", "lesbian" and other terms currently in use, as these are seen as even more bound to the human condition.[13]

Animal preference and motivation is always inferred from behavior. 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 behavior. The correct usage of the term homosexual is that an animal exhibits homosexual behavior, however this article conforms to the usage by modern research[13][14][15][16] 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,[16] but cases of clear homosexual preference and exclusive homosexual pairs are known (see examples for details).

Research on homosexual behavior in animals

The presence of same-sex sexual behavior was not 'officially' observed on a large scale until recent times, possibly due to observer bias caused by social attitudes to same-sex sexual behavior,[17] innocent confusion, or even from a fear of "being ridiculed by their colleagues."[18] Georgetown University biologist Janet Mann states "Scientists who study the topic are often accused of trying to forward an agenda, and their work can come under greater scrutiny than that of their colleagues who study other topics.[19] They also noted "Not every sexual act has a reproductive function ... that's true of humans and non-humans."[19] It appears to be widespread amongst social birds and mammals, particularly the sea mammals and the primates. The true extent of homosexuality in animals is not known. While studies have demonstrated homosexual behavior in a number of species, Petter Bøckman, the scientific advisor of the exhibition Against Nature? speculates that the true extent of the phenomenon may be much larger than currently recognized:

No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue.[18]

Two male giraffes in Kenya.

An example of overlooking homosexual behavior 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.[18]

Some researchers believe this behavior 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[20] 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.[21] 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:

Approximately 8% of rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes...[22]

In fact, apparent homosexual individuals are known from all of the traditional domestic species, from sheep, cattle and horses to cats, dogs and budgerigars.[23]

Cross species sex

Main article: Animal sexual behaviour § Cross species sex

Although a commonly held conception is that animals' sexuality is instinctive almost to the point of being mechanistic, research regularly records that many animals are sexual opportunists, and may show an interest in partners that are not of their own, or related, species.[24] This is more visible in domesticated species, as domestication commonly selects for increased breeding rate (and so an accelerated breeding cycle has commonly arisen in domesticated species over the centuries), and also because these species are easier to witness by humans. Cross-species sex has been observed in the wild and investigations describe productive and non-productive inter-species mating as a "natural occurrence".[25][26] Most observations, however, are from animals in captivity, including those kept in zoos.

If the pair are a male and a female, and if the two species are related, hybrid offspring can result, however, the offspring themselves may not be able to breed. The mule, for example (a horse/donkey cross) is normally sterile, whilst the liger (a lion/tiger cross) is sometimes fertile. Novosibirsk zoo director Rostislav Shilo says of 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.”.[citation needed] Cross species sex in the wild has been observed between several species, among them blue tit and great tit, Chimpanzee and olive baboon, and between Amazon River Dolphin and the tucuxi dolphin.[27], as well as a reported attempt at copulation with a king penguin by a fur seal.[28]

Some selected species and groups

See also: List of animals displaying homosexual behavior

Birds

Black swans

The Black Swan, Cygnus atratus is a large waterbird which breeds mainly in the southeast and southwest regions of Australia.

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.[29] 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. The same reasoning has been applied to male flamingo pair raising chicks.[30]

Gulls

Studies have shown that 10 to 15 percent of female western gulls in some populations in the wild exhibit homosexual behavior.[31]

Mallards

Two male Mallards, Anas platyrhynchos

Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population.[32]

Penguins

In early February 2004 the New York Times reported that a male pair of chinstrap penguins in the Central Park Zoo in New York City were partnered and even successfully hatched a female chick from an egg.[1] A children's book, titled And Tango Makes Three, was written about the real-life Roy and Silo and their chick, Tango. Tango, a female, later was found in a pairing with another female. Other penguins in New York have also been reported to be forming same-sex pairs.[33] [34]

Zoos in Japan and Germany have also documented homosexual male penguin couples.[21] 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 homosexual pairs at 16 major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to encourage reproduction of the endangered species by importing female penguins from Sweden and separating the male couples, but this was unsuccessful. The zoo director stated the relationships were too strong between the homosexual couples.[35] German gay groups protested at this attempt to break up the male-male couples [36] even though the zoo's intention was explicitly to see whether the penguins were truly homosexual (rather than just intensely bonding due to a lack of female penguins), and not to prevent them from being so [37]. A gay male couple, together for six years at the San Francisco Zoo split when the male of a couple in the next burrow died and the female sought a new mate.[38]

Mammals

Amazon Dolphin

The Amazon River dolphin or boto has been reported to form up in bands of 3-5 individuals enjoying group sex.[39] The groups usually comprise young males and sometimes one or two females. Sex is performed in non-reproductive ways, using snout, flippers and general rubbing, without regards to gender.[39] They will sometimes perform homosexual penetration of the blowhole, the only known example of nasal sex.[40] The males will sometimes also perform sex with tucuxi males, a small porpoise.

American Bison

The American Bison is a bovine mammal which commonly displays homosexual behavior.

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 behavior, to "ensure the return of the buffalo in the coming season."[41] 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.[41]

Bonobo and other apes

The Bonobo displays the highest rate of homosexual activity in any animal, being a fully bisexual species.[42][43]

The Bonobo, which has a matriarchal society (unusual amongst apes), is a fully bisexual species -- both males and females engage in heterosexual and homosexual behavior, being noted for female-female homosexuality in particular. About 60% of all sexual activity in this species is between two or more females. While the homosexual bonding system in Bonobos represent the highest frequency of homosexuality known in any species, homosexuality has been reported for all great apes (a group which includes humans), as well as a number of other primate species.[42][43][44][45][46][47][48][49][50] Dutch primatologist Frans de Waal on observing and filming bonobos noted that there were two reasons to believe sexual activity is the bonobo's answer to avoiding conflict.

Anything that arouses the interest of more than one bonobo at a time, not just food tends to result in sexual contact. If two bonobos approach a cardboard box thrown into their enclosure, they will briefly mount each other before playing with the box. Such situations lead to squabbles in most other species. But bonobos are quite tolerant, perhaps because they use sex to divert attention and to diffuse tension.

Bonobo sex often occurs in aggressive contexts totally unrelated to food. A jealous male might chase another away from a female, after which the two males reunite and engage in scrotal rubbing. Or after a female hits a juvenile, the latter's mother may lunge at the aggressor, an action that is immediately followed by genital rubbing between the two adults.[51]

Bottlenose Dolphins

Bottlenose Dolphin males have been observed working in pairs or larger groups to follow and/or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. The same pairs/groups have also been observed engaging in ardent sexual play with each other.

Janet Mann, Georgetown University professor of biology and psychology, argues that the strong personal behavior among male dolphin calves is about bond formation and benefits the species in an evolutionary context.[31] 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.

Elephants

African and Asiatic males will engage in same-sex bonding and mounting. Such encounters are often associated with affectionate interactions, such as kissing, trunk intertwining, and placing trunks in each other's mouths. Male elephants, who often live apart from the general flock, often form "companionships", consisting of an older individual and one or sometimes two younger, attendant males with sexual behavior being an important part of the social dynamic. Unlike heterosexual relations, which are always of a fleeting nature, the relationships between males may last for years. The encounters are analogous to heterosexual bouts, one male often extending his trunk along the other's back and pushing forward with his tusks to signify his intention to mount. Same-sex relations are common and frequent in both sexes, with Asiatic elephants in captivity devoting roughly 45% of sexual encounters to same-sex activity.[52]

Giraffes

Male giraffes have been observed to engage in remarkably high frequencies of homosexual behavior. After aggressive "necking", it is common for two males giraffes to caress and court each other, leading up to mounting and climax. Such interactions between males have been found to be more frequent than heterosexual coupling.[53] In one study, up to 94% of observed mounting incidents took place between two males. The proportion of same sex activities varied between 30 and 75%, and at any given time one in twenty males were engaged in non-combative necking behavior with another male. Only 1% of same-sex mounting incidents occurred between females.[54]

Japanese Macaque

With the Japanese Macaque, also known as the "snow monkey", same-sex relations are frequent, though rates vary between troupes. Females will form "consortships" characterized by affectionate social and sexual activities. In some troops up to one quarter of the females will form such bonds, which will vary in duration from a few days to a few weeks. Often, strong and lasting friendships will result from such pairings. Males also have same-sex relations, typically with multiple partners of the same age. Affectionate and playful activities are associated with such relations.[55]

Lions

A group of male lions at the Melbourne zoo.

Both male and female lions have been seen to interact homosexually.[56][57] 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. Pairings between females are held to be fairly common in captivity but have not been observed in the wild.

Polecat

European polecats Mustela putorius were found to engage homosexuality in non-sibling animals. Deliberate exclusive homosexuality with mounting and anal penetration in this solitary species apparently serves no adaptive function[58].

Sheep

An October 2003 study by Dr. Charles E. Roselli et al. (Oregon Health and Science University) states that homosexuality in male sheep (found in 8% of rams) is associated with a region in the rams' brains which the authors call the "ovine Sexually Dimorphic Nucleus" (oSDN) which is half the size of the corresponding region in heterosexual male sheep.[22]

A pair of St. Croix ewes.

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."[22] 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 Merck Manual of Veterinary Medicine appears to consider homosexuality among sheep as a routine occurrence and an issue to be dealt with as a problem of animal husbandry.[59]

Spotted Hyena

The Spotted Hyena is a moderately large, terrestrial carnivore native to Africa.

The family structure of the female Spotted Hyena is matriarchal, and dominance relationships with strong sexual elements are routinely observed between related females. Due largely to the female spotted hyena's unique urogenital system, which looks more like a penis than a vagina, early naturalists thought hyenas were hemaphroditic males who commonly practiced homosexuality.[60] Early writings such as Ovid's Metamorphoses and the Physiologus suggested that 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.

The reality behind the confusing reports is the sexually aggressive behavior between the females, including mounting between females. 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,"[61] and they have "been masculinized without being defeminized.”[60]

Study of 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.[62][60] Research has shown that "elevated levels of testosterone in utero"[63] contribute to extra aggressiveness; both males and females mount members of the same sex,[64][63] who in turn are possibly acting more submissive because of lower levels of testosterone in utero.[61]

Others

Dragonflies

The head of Darner Dragonfly (Basiaeschna janata).

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[65][66] has revealed such mating damages in 20 to 80 % of the males too, indicating a fairly high occurrence of sexual coupling between males.

Fruit flies

Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males.[67] The genetic basis of animal homosexuality has been studied in the fly Drosophila melanogaster.[68] Here, multiple genes have been identified that can cause homosexual courtship and mating.[69] These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains.[70][71] These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.[72][73]

Lizards

Whiptail lizard (Teiidae genus) females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard.[74] Females engage in sexual behavior to stimulate ovulation, with their behavior following their hormonal cycles; during low levels of estrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high estrogen levels assume "feminine" sexual roles.

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.[75]

See also

References

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