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File:LesbianBitches.JPG
Two female domestic dogs engaging in sex play.

Homosexual behavior in animals is same-sex behavior, homosexual or bisexual, among non-human species. Such behaviors include sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairings. Homosexual behavior has been observed in close to 1,500 species, ranging from primates to gut worms, and is well documented for 500 of them.[1][2] The sexual behavior of non-human animals 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.[3] 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."[4]

Research indicates that various forms of same-sex sexual behavior are found throughout the animal kingdom.[5] 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.[6] Homosexual behavior is best known from social species. According to geneticist Simon Levay in 1996, "Although homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity."[7] One species in which exclusive homosexual orientation occurs, however, is that of domesticated sheep (Ovis aries).[8][9] "About 10% of rams (males) refuse to mate with ewes (females) but do readily mate with other rams."[9]

The observation of homosexual behavior in animals can be seen as both an argument for and against the acceptance of homosexuality in humans, and has been used especially against the claim that it is a peccatum contra naturam ('sin against nature').[1] For instance, homosexuality in animals was cited in the United States Supreme Court's decision in Lawrence v. Texas which struck down the sodomy laws of 14 states.[10]

Applying the term homosexual to animals[edit]

The term homosexual was coined by Karl-Maria Kertbeny in 1868 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 species other than humans.[12] Thus homosexual behavior has been given a number of terms over the years. When describing animals, the term homosexual is preferred over gay, lesbian, and other terms currently in use, as these are seen as even more bound to human homosexuality.[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 or even same-sex sexual behavior; however, this article conforms to the usage by modern research,[13][14][15][16][17] 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 homosexual preference and exclusive homosexual pairs are known.[18]

Research[edit]

A majority of the research available concerning homosexual behavior in animals lacks specification between animals that exclusively exhibit same-sex tendencies and those that participate in heterosexual and homosexual mating activities interchangeably. This lack of distinction has led to differing opinions and conflicting interpretations of collected data amongst scientists and researchers. For instance, Dr. Bruce Bagemihl, author of the book Biological Exuberence: Animal Homosexuality and Natural Diversity, emphasizes that there is no anatomical or endocrinological differences between exclusively homosexual and exclusively heterosexual animal pairs.[19] However, if the definition of "homosexual behavior" is made to include animals that participate in both same-sex and opposite-sex mating activities, hormonal differences have been documented among key sex hormones, such as testosterone and estradiol, when compared to those who participate solely in heterosexual mating.[20]

Many of the animals used in studies of homosexuality do not appear to spontaneously exhibit these tendencies often in the wild. Such behavior is often elicited and exaggerated by the researcher during experimentation through the destruction of a portion of brain tissue, or by exposing the animal to high levels of steroid hormones prenatally.[21] Information gathered from these studies is limited when applied to spontaneously occurring same-sex behavior in animals outside of the laboratory.[21]

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,[22] innocent confusion, or even from a fear of "being ridiculed by their colleagues."[23] 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."[24] They also noted "Not every sexual act has a reproductive function ... that's true of humans and non-humans."[24] 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? in 2007, speculated that the true extent of the phenomenon may be much larger than was then 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.[23]

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.[25]

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[26] 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.[27][28] While reports on many such mating scenarios are still only anecdotal, a growing body of scientific work confirms that permanent homosexuality occurs not only in species with permanent pair bonds,[17] 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...[29]

In fact, apparent homosexual individuals are known from all of the traditional domestic species, from sheep, cattle and horses to cats, dogs and budgerigars.[1]

Physiological basis[edit]

A definite physiological explanation or reason for homosexual activity in animal species has not been agreed upon by researchers in the field. Numerous scholars are of the opinion that varying levels (either higher or lower) of the sex hormones in the animal,[30] in addition to the size of the animal's gonads,[20] play a direct role in the sexual behavior and preference exhibited by that animal. Others firmly argue no evidence to support these claims exists when comparing animals of a specific species exhibiting homosexual behavior exclusively and those that don't. Ultimately, empirical support from comprehensive endocrinological studies exist for both interpretations.[30][31] Researchers found no evidence of differences in the measurements of the gonads, or the levels of the sex hormones of exclusively homosexual western gulls and ring-billed gulls.[32] However, when analyzing these differences in bisexual rams, males were found to have lower levels of testosterone and estradiol in their blood, as well as smaller gonads than their heterosexual counterpart.

Additional studies pertaining to hormone involvement in homosexual behavior indicate that when administering treatments of testosterone and estradiol to female heterosexual animals, the elevated hormone levels increase the likelihood of homosexual behavior. Additionally, boosting the levels of sex hormones during an animal's pregnancy appears to increase the likelihood of it birthing a homosexual offspring.[30]

Genetic basis[edit]

Researchers found that disabling the fucose mutarotase (FucM) gene in laboratory mice – which influences the levels of estrogen to which the brain is exposed – caused the female mice to behave as if they were male as they grew up. "The mutant female mouse underwent a slightly altered developmental programme in the brain to resemble the male brain in terms of sexual preference" said Professor Chankyu Park of the Korea Advanced Institute of Science and Technology in Daejon, South Korea, who led the research. His most recent findings have been published in the BMC Genetics journal on July 7, 2010.[33][34] Another study found that by manipulating a gene in fruit flies (Drosophila), homosexual behavior appeared to have been induced. However, in addition to homosexual behavior, several abnormal behaviors were also exhibited apparently due to this mutation.[35]

Neurobiological basis[edit]

In March 2011, research shows that serotonin is involved in the mechanism of sexual orientation of mice.[36][37] A study conducted on fruit flies found that inhibiting the dopamine neurotransmitter inhibited lab-induced homosexual behavior.[38]

Some selected species and groups[edit]

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Birds[edit]

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Black swans[edit]

File:Black Swans.jpg
Swans, Cygnus atratus

An estimated one-quarter of all black swans pairings are of homosexual males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs.[39][40] 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 pairs raising chicks.[41][42]

Gulls[edit]

Studies have shown that 10 to 15 percent of female western gulls in some populations in the wild exhibit homosexual behavior.[43]

Ibises[edit]

Research has shown that the environmental pollutant methylmercury can increase the prevalence of homosexual behavior in male American White Ibis. The study involved exposing chicks in varying dosages to the chemical and measuring the degree of homosexual behavior in adulthood. The results discovered was that as the dosage was increased the likelihood of homosexual behavior also increased. The endocrine blocking feature of mercury has been suggested as a possible cause of sexual disruption in other bird species.[44][45]

Mallards[edit]

File:Males Anas platyrhynchos 2 .jpg
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.[1]

Penguins[edit]

Template:Further2 Penguins have been observed to engage in homosexual behaviour since at least as early as 1911. George Murray Levick, who documented this behaviour in Adélie Penguins at Cape Adare, described it as "depraved". The report was considered too shocking for public release at the time, and was suppressed. The only copies that were made available privately to researchers were translated into Greek, to prevent this knowledge becoming more widely known. The report was unearthed only a century later, and published in Polar Record in June 2012.[46]

In early February 2004 the New York Times reported that Roy and Silo, a male pair of chinstrap penguins in the Central Park Zoo in New York City had successfully hatched and fostered a female chick from a fertile egg they had been given to incubate.[10] Other penguins in New York zoos have also been reported to have formed same-sex pairs.[47][48]

Zoos in Japan and Germany have also documented homosexual male penguin couples.[27][28] The couples have been shown to build nests together and use a stone as a substitute for an egg. Researchers at Rikkyo University in Tokyo found 20 homosexual pairs at 16 major aquariums and zoos in Japan.

The Bremerhaven Zoo in Germany attempted to encourage reproduction of endangered Humbolt penguins by importing females from Sweden and separating three male pairs, but this was unsuccessful. The zoo's director said that the relationships were "too strong" between the homosexual pairs.[49] German gay groups protested at this attempt to break up the male-male pairs[50] but the zoo's director was reported as saying "We don't know whether the three male pairs are really homosexual or whether they have just bonded because of a shortage of females... nobody here wants to forcibly separate homosexual couples."[51]

A pair of male Magellanic penguins who had shared a burrow for six years at the San Francisco Zoo and raised a surrogate chick, split when the male of a pair in the next burrow died and the female sought a new mate.[52]

Buddy and Pedro, a pair of male African Penguins, were separated by the Toronto Zoo to mate with female penguins.[53][54] Buddy has since paired off with a female.[54]

Vultures[edit]

In 1998 two male Griffon vultures named Dashik and Yehuda, at the Jerusalem Biblical Zoo, engaged in "open and energetic sex" and built a nest. The keepers provided the couple with an artificial egg, which the two parents took turns incubating; and 45 days later, the zoo replaced the egg with a baby vulture. The two male vultures raised the chick together.[55] A few years later, however, Yehuda became interested in a female vulture that was brought into the aviary. Dashik became depressed, and was eventually moved to the zoological research garden at Tel Aviv University where he too set up a nest with a female vulture.[56]

Two homosexual male vultures at the Allwetter Zoo in Muenster built a nest together, although they were picked on and often had their nest materials stolen by other vultures. They were eventually separated to try to promote breeding by placing one of them with female vultures, despite the protests of German homosexual groups.[57]

Pigeons[edit]

Both male and female pigeons sometimes exhibit homosexual behavior. In addition to sexual behavior, same-sex pigeon pairs will build nests, and hens will lay (infertile) eggs and attempt to incubate them.

Mammals[edit]

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Amazon Dolphin[edit]

The Amazon river dolphin or boto has been reported to form up in bands of 3–5 individuals enjoying group sex. The groups usually comprise young males and sometimes one or two females. Sex is often performed in non-reproductive ways, using snout, flippers and genital rubbing, without regard to gender.[58] In captivity, they have been observed to sometimes perform homosexual and heterosexual penetration of the blowhole, a hole homologous with the nostril of other mammals, making this the only known example of nasal sex in the animal kingdom.[58][59] The males will sometimes also perform sex with males from the tucuxi species, a type of small porpoise.[58]

American Bison[edit]

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File:American bison k5680-1.jpg
The American Bison is a bovine mammal which 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."[60]Template:Dead link Also, mounting of one female by another (known as "bulling") is extremely common among cattle. The behaviour is hormone driven and synchronizes with the emergence of estrus (heat), particularly in the presence of a bull.

Bonobo and other apes[edit]

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Bonobos, which have a matriarchal society, unusual among apes, are a fully bisexual species—both males and females engage in heterosexual and homosexual behavior, being noted for female-female homosexuality in particular. Roughly 60% of all bonobo sexual activity occurs between two or more females. While the homosexual bonding system in Bonobos represents 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.[61][62][63][64][65][66][67][68][69] 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 defuse 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.[70]

Bottlenose dolphins[edit]

Template:Duplication Template:See also Dolphins of several species engage in homosexual acts, though it is best studied in the bottlenose dolphins.[1] Sexual encounters between females take the shape of "beak-genital propulsion", where one female inserts her beak in the genital opening of the other while swimming gently forward.[71] Between males, homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting the penis in the others genital slit and sometimes anus.[72]

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.[73] 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. Confrontations between flocks of bottlenose dolphins and the related species Atlantic spotted dolphin will sometimes lead to cross-species homosexual behaviour between the males rather than combat.[74]

Elephants[edit]

Template:Further2 African and Asian 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 herd, 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.[75]

Giraffes[edit]

Template:Further2 Male giraffes have been observed to engage in remarkably high frequencies of homosexual behavior. After aggressive "necking", it is common for two male 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.[76] 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.[77]

Monkeys[edit]

Among monkeysTemplate:Clarify, Lionel Tiger and Robin Fox conducted a study on how Depo-Provera contraceptives lead to decreased male attraction to females.[78]

Japanese macaque[edit]

Template:Further2 With the Japanese macaque, also known as the "snow monkey", same-sex relations are frequent, though rates vary between troops. Females will form "consortships" characterized by affectionate social and sexual activities. In some troops up to one quarter of the females form such bonds, which vary in duration from a few days to a few weeks. Often, strong and lasting friendships 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.[79]

Lions[edit]

Template:Main Both male and female lions have been seen to interact homosexually.[80][81] 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[edit]

European polecats Mustela putorius were found to engage homosexually with non-sibling animals. Exclusive homosexuality with mounting and anal penetration in this solitary species serves no apparent adaptive function.[82]

Sheep[edit]

Template:Duplication Template:Further2 Ovis aries has attracted much attention due to the fact that some rams seem to have an exclusive homosexual orientation.[8]

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.[29]

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."[29] As noted before, 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.[83]

Spotted Hyena[edit]

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File:Spotted hyena2.jpg
The Spotted Hyena is a moderately large, terrestrial carnivore native to Africa.

The family structure of the 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 rather than a vagina, early naturalists thought hyenas were hermaphroditic males who commonly practiced homosexuality.[84] 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,"[85] and they have "been masculinized without being defeminized.”[84]

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.[84][86] Research has shown that "elevated levels of testosterone in utero"[87] contribute to extra aggressiveness; both males and females mount members of both the same and opposite sex,[87][88] who in turn are possibly acting more submissive because of lower levels of testosterone in utero.[85]

Others[edit]

Lizards[edit]

Template:Further2 Whiptail lizard (Teiidae genus) females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard.[89] 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.[90]

Insects[edit]

Dragonflies[edit]
File:Head of dragonfly 2.jpg
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[91][92] 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[edit]

Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males.[18] The genetic basis of animal homosexuality has been studied in the fly Drosophila melanogaster.[93] Here, multiple genes have been identified that can cause homosexual courtship and mating.[94] These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains.[95][96] These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.[97][98]

Bed bugs[edit]

Template:Main Male bed bugs (Cimex lectularius) are sexually attracted to any newly fed individual and this results in homosexual mounting. This occurs in heterosexual mounting by the traumatic insemination in which the male pierces the female abdomen with his needle-like penis. In homosexual mating this risks abdominal injuries as males lack the female counteradaptive spermalege structure. Males produce alarm pheromones to reduce such homosexual matings.[99]

See also[edit]

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References[edit]

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External links[edit]

  • 1.0 1.1 1.2 1.3 1.4 Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, St. Martin's Press, 1999; ISBN 978-0312253776
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  • Calvin Reid Gay Lib for the Animals: A New Look At Homosexuality in Nature. Volume 245 Issue 5 02/01/1999, Feb 01, 1999
  • "Same-sex Behavior Seen In Nearly All Animals, Review Finds", Science Daily
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  • 8.0 8.1 Animal Homosexuality: A Biosocial Perspective By Aldo Poiani, A. F. Dixson, Aldo Poiani, A. F. Dixson, p. 179, 2010, Cambridge University Press
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  • The first known use of the word Homoseksuäl is found in Benkert Kertbeny, K.M. (1869): Paragraph 143 des Preussichen Strafgesetzebuches vom 14/4-1851 und seine Aufrechterhaltung als Paragraph 152 im Entwurf eines Strafgesetzbuches fur den Norddeutschen Bundes, Leipzig, 1869. Reprinted in Jahrbuch fur sexuelle Zwischenstufen 7 (1905), pp. 1-66
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  • 13.0 13.1 Bagemihl 1999, pp. 122-166.
  • Joan Roughgarden, Evolutions rainbow: Diversity, gender and sexuality in nature and people, University of California Press, Berkeley, 2004; pp.13-183
  • Vasey, Paul L. (1995), Homosexual behaviour in primates: A review of evidence and theory, International Journal of Primatology 16: p 173-204
  • 16.0 16.1 Sommer, Volker & Paul L. Vasey (2006), Homosexual Behaviour in Animals, An Evolutionary Perspective. Cambridge University Press, Cambridge. ISBN 0-521-86446-1
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  • Joan Roughgarden, Evolutions rainbow: Diversity, gender and sexuality in nature and people, University of California Press, Berkeley, 2004
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  • Bruce Bagemihl, citing a study by Leuthold, W. (1977): African Ungulates: A Comparative Review of Their Ethology and Behavioural Ecology. Springer Verlag, Berlin, cited in Biological Exuberance: Animal Homosexuality and Natural Diversity, 1999;
  • Thierry Lodé "La guerre des sexes chez les animaux" Eds O Jacb, Paris, 2006, ISBN 2-7381-1901-8
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  • 29.0 29.1 29.2 Template:Cite journal Cite error: Invalid <ref> tag; name "The_Volume_of_a_Sexually_Dimorphic" defined multiple times with different content
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  • Braithwaite, L. W., 'Ecological studies of the Black Swan III – Behaviour and social organization', Australian Wildlife Research 8, 1981: 134-146
  • Braithwaite, L. W., 'The Black Swan', Australian Natural History 16, 1970: 375-379
  • Bagemihl 1999, pp. 487-491.
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  • ABC News, 10 June 2012
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  • "Central Park Zoo's gay penguins ignite debate", San Francisco Chronicle, hosted at SFGate.com
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  • 58.0 58.1 58.2 Bagemihl 1999, pp. 339-348.
  • Sylvestre, J.-P. (Some Observations on Behavior of Two Orinoco Dolphins (Inia geoffrensis humboldtiaba [Pilleri and Gihr 1977]), in Captivity, at Duisburg Zoo. Aquatic mammals no 11, pp 58-65 article
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  • Bonobos at the Columbus ZooTemplate:Dead link
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  • Frans de Waal, "Bonobo Sex and Society" in Scientific American (March 1995), p. 82ff
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  • Stanford, C. B. (1998). The social behavior of chimpanzees and bonobos. Current Anthropology 39: 399–407.
  • Kano, Takayoshi (1992). The Last Ape: Pygmy Chimpanzee Behavior and Ecology. Stanford, CA: Stanford University Press.
  • Frans B. M. de Waal, "Bonobo Sex and Society" Scientific American, Mar. 1995, pp. 82-88
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  • http://www.livescience.com/2534-sex-couples-common-wild.html
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  • Bagemihl 1999, pp. 427-430.
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  • Bagemihl 1999, pp. 391-393.
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  • Bagemihl 1999, pp. 302-305.
  • Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, St. Martin's Press, 1999; pp.302-305. In his discussion of lion same-sex relations, Bagemihl is making use of published work by: J.B. Cooper, "An Exploratory Study on African Lions" in Comparative Psychology Monographs 17:1-48; R.L. Eaton, "The Biology and Social Behavior of Reproduction in the Lion" in Eaton, ed. The World's Cats, vol. II; pp.3-58; Seattle, 1974; G.B. Schaller, The Serengeti Lion; University of Chicago Press, 1972
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  • Scrivener, C.J (2008): Behavioural problems, Merck Manual of Veterinary Medicine.
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