View source for Homosexual behavior in animals

{{Short description|Sexual behavior among non-human species that is interpreted as homosexual}}
{{For|homosexuality in humans|Homosexuality}}
{{pp-semi-indef}}
{{Use dmy dates|date=June 2023}}

[[File:Males Anas platyrhynchos 2.jpg|300px|thumb|Two male [[mallard]]s (''Anas platyrhynchos'')]]

Various non-human [[animal]] species exhibit behavior that can be interpreted<!-- NOTE: For why "interpreted" is used, see the second paragraph (Bruce Bagemihl) and points made lower in the article.--> as [[homosexual]] or [[Bisexuality|bisexual]], often referred to as ''same-sex sexual behavior'' (SSSB) by scientists. This may include same-sex [[Animal sexual behaviour|sexual activity]], [[Courtship display|courtship]], [[affection]], [[pair bond]]ing, and [[Homosexual parenting in animals|parenting among same-sex animal pairs]].<ref name="ES">{{cite journal|last=Braithwaite|first=L. W.|title=Ecological studies of the Black Swan III – Behaviour and social organization |journal=Wildlife Research |date=1981 |volume=8 |issue=1 |pages=134–146 |doi=10.1071/WR9810135 |publisher=[[CSIRO]] |location=Canberra, Australia|bibcode=1981WildR...8..135B }}</ref><ref name="Bailey_2009">{{cite journal | last1 = Bailey|first1=N. W.|last2=Zuk|first2=M. | title = Same-sex sexual behavior and evolution | journal = [[Trends in Ecology and Evolution]] | volume = 24 | issue = 8 | pages = 439–46 | date = August 2009 | pmid = 19539396 | doi = 10.1016/j.tree.2009.03.014|bibcode=2009TEcoE..24..439B }}</ref><ref>{{cite press release |date=June 17, 2009 |title=Same-sex Behavior Seen In Nearly All Animals, Review Finds |website=[[ScienceDaily]] |url=https://www.sciencedaily.com/releases/2009/06/090616122106.htm}}</ref> Various forms of this are found among a variety of [[vertebrate]] and [[arthropod]] taxonomic [[class (biology)|classes]]. The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from [[Sociality|social species]].

Scientists observe same-sex sexual behavior in animals in different degrees and forms among different species and [[clade]]s. A 2019 paper states that it has been observed in over 1,500 species.<ref>{{Cite journal |last1=Monk |first1=Julia D. |last2=Giglio |first2=Erin |last3=Kamath |first3=Ambika |last4=Lambert |first4=Max R. |last5=McDonough |first5=Caitlin E. |date=December 2019 |title=An alternative hypothesis for the evolution of same-sex sexual behaviour in animals |journal=[[Nature Ecology and Evolution]] |volume=3 |issue=12 |pages=1622–1631 |doi=10.1038/s41559-019-1019-7 |pmid=31740842 |s2cid=256708244 |issn=2397-334X|doi-access=free |bibcode=2019NatEE...3.1622M }}</ref> Although same-sex interactions involving genital contact have been reported in many animal species, they are routinely manifested in only a few, including humans.<ref>{{cite journal | vauthors = Bailey JM, Vasey PL, Diamond LM, Breedlove SM, Vilain E, Epprecht M | title = Sexual Orientation, Controversy, and Science | journal = [[Psychological Science in the Public Interest]] | volume = 17 | issue = 2 | pages = 45–101 | date = September 2016 | pmid = 27113562 | doi = 10.1177/1529100616637616 | s2cid = 42281410 | doi-access = free }}</ref> Other than humans, the only known species to exhibit exclusive homosexual orientation is the domesticated sheep (''[[Ovis aries]]''), involving about 10% of males.<ref name="Poiani2010">{{cite book| first1 = Aldo | last1 = Poiani | first2 = A. F. | last2 = Dixson | name-list-style = vanc |title=Animal Homosexuality: A Biosocial Perspective|url=https://books.google.com/books?id=EftT_1bsPOAC&pg=PA179|date=2010|publisher=[[Cambridge University Press]]|page=179|quote=This makes O. aries (ram) only the second mammal known, apart from humans, capable of displaying exclusive homosexuality.|isbn=9781139490382}}</ref><ref name="levay">{{cite book |title=Gay, Straight, and The Reason Why: The Science of Sexual Orientation|last=Levay| first=Simon | name-list-style = vanc |year=2017| url=https://books.google.com/books?id=-J6kDAAAQBAJ |via=[[Google Books]]|edition=Second |url-access=limited |publisher=[[Oxford University Press]] |location=Cambridge, Massachusetts |pages=38, 119|isbn=978-0-19-029737-4 }}</ref><ref>{{cite journal |last1=Raymond |first1=Michel |last2=Crochet |first2=Pierre-André |title=Carving Non-Proximal Explanations for Same-Sex Sexual Orientation |journal=Archives of Sexual Behavior |date=October 2023 |volume=52 |issue=7 |pages=3007–3012 |doi=10.1007/s10508-022-02497-z |pmid=36469147 |url=https://hal.science/hal-04284860/file/Carving%20non-proximal%20explanations%20for%20same-sex%20sexual%20orientation_pour_HAL.pdf |quote=There are numerous reports of homosexual behavior in many animal species in the wild, but there are no reports of exclusive same-sex sexual orientation (Bagemihl, 2000). It is, however, well-established that some domestic rams exhibit exclusive same-sex sexual orientation.}}</ref> The motivations for and implications of these behaviors are often lensed through anthropocentric thinking; [[Bruce Bagemihl]] states that any hypothesis is "necessarily an account of human interpretations of these phenomena".<ref name=Bagemihl>{{cite book| last1=Bagemihl| first1=Bruce|url=https://archive.org/details/biologicalexuber00bage/page/164/mode/1up|title=Biological Exuberance: Animal Homosexuality and Natural Diversity|date=1999|publisher=[[St. Martin's Press]]|isbn=9780312253776|edition=Stone Wall Inn|location=New York City|name-list-style=vanc|via=[[Internet Archive]]|url-access=registration|author-link=Bruce Bagemihl}}</ref>{{rp|2}}

Proposed causes for same-sex sexual behavior vary across species. Theories include mistaken identity (especially for arthropods<!--arachnids verified below-->), [[sexual conflict|sexually antagonistic selection]], [[balancing selection]], practice of behaviors needed for reproduction, expression of social dominance or submission, and social bonding.<ref>{{cite journal |last1=Balfour |first1=Vicki L. |last2=Shuker |first2=David M. |title=Same-sex sexual behaviour |journal=Current Biology |date=November 2020 |volume=30 |issue=22 |pages=R1345–R1346 |doi=10.1016/j.cub.2020.09.005 |pmid=33202225 |bibcode=2020CBio...30R1345B |url=https://www.cell.com/current-biology/fulltext/S0960-9822(20)31336-1 |access-date=25 June 2024}}</ref> Genetic, hormonal, and neurological variations as a basis for individual behavioral differences within species have been proposed, and same-sex sexual behavior has been induced in laboratory animals by these means.<!--citations in Basis section-->

==In relation to humans==
[[File:Canis anthus - Cécile Bloch 12.png|300px|thumb|Two male [[African golden wolf|African golden wolves]] (''Canis anthus'')]]
[[File:Cant wait for the bull.jpg|300px|thumb|A cow "bulling" during oestrus.]]
[[File:Reduction-of-Dopamine-Level-Enhances-the-Attractiveness-of-Male-Drosophila-to-Other-Males-pone.0004574.s001.ogv |300px|thumb|Courtship behavior in male [[Drosophila|fruit flies]]]]

===Applying the term ''homosexual'' to animals===
The term ''homosexual'' was coined by the Hungarian writer and campaigner [[Karl Maria Kertbeny]] in 1868 to describe same-sex sexual attraction and sexual behavior in humans.<ref>The first known use of the word ''Homoseksuäl'' is found in Benkert Kertbeny, K. M. (1869): Paragraph 143 des Preussischen 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</ref> 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.<ref>{{cite web | last=Dorit | first=Robert | name-list-style=vanc | title=Rethinking Sex | work=[[American Scientist]] | date=September–October 2004 | url=http://www.americanscientist.org/bookshelf/pub/rethinking-sex | access-date=2007-09-11 | archive-date=25 June 2010 | archive-url=https://web.archive.org/web/20100625055424/http://www.americanscientist.org/bookshelf/pub/rethinking-sex | url-status=dead}}</ref> Thus homosexual behavior has been given a number of terms over the years. According to Bagemihl, 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.<ref name=Bagemihl/>{{rp|122–166}}

Bailey et al. says:<ref name="Bailey_2009" /><blockquote>Homosexual: in animals, this has been used to refer to same-sex behavior that is not sexual in character (e.g. 'homosexual tandem running' in termites), same-sex courtship or copulatory behavior occurring over a short period of time (e.g. 'homosexual mounting' in cockroaches and rams) or long-term pair bonds between same-sex     partners that might involve any combination of courting, copulating, parenting and affectional behaviors (e.g. 'homosexual pair bonds' in gulls). In humans, the term is used to describe individual sexual behaviors as well as long-term relationships, but in some usages connotes a gay or lesbian social identity. Scientific writing would benefit from reserving this anthropomorphic term for humans and not using it to describe behavior in other animals, because of its deeply rooted context in human society.</blockquote>Animal preference and motivation is always inferred from behavior. In wild animals, researchers will 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,{{refn|<ref name=Bagemihl/>{{rp|122–166}}<ref name=Rainbow>{{cite book|first=Joan |last=Roughgarden |author-link=Joan Roughgarden| title=Evolutions rainbow: Diversity, gender and sexuality in nature and people| publisher=[[University of California Press]]| location=Berkeley, California | date=2004| pages=13–183|isbn=9780520957978 | url=https://books.google.com/books?id=7UqqAAAAQBAJ |via=[[Google Books]]| url-access=limited}}</ref><ref>{{cite journal | last1 = Vasey | first1 = Paul L | name-list-style = vanc | year = 1995 | title = Homosexual behaviour in primates: A review of evidence and theory | journal = [[International Journal of Primatology]] | volume = 16 | issue = 2| pages = 173–204 | doi=10.1007/bf02735477| s2cid = 26021360 }}</ref><ref name="Sommer & Vasey">{{cite book | last1 = Sommer | first1 = Volker | first2 = Paul L. | last2 = Vasey | name-list-style = vanc | date = 2006 | title = Homosexual Behaviour in Animals, An Evolutionary Perspective | publisher = [[Cambridge University Press]] | location = Cambridge | isbn = 978-0-521-86446-6 }}</ref>{{page needed|date=May 2016}}<ref name="Homosexual selection: The power of same-sex liaisons">{{cite web | last =Douglas | first =Kate | name-list-style = vanc | title= Homosexual selection: The power of same-sex liaisons | work=[[New Scientist]] | date=December 7, 2009 | url=https://www.newscientist.com/article/mg20427370.800-homosexual-selection-the-power-of-samesex-liaisons.html?page=1 | access-date = 2009-12-21}}</ref>}} applying the term ''homosexuality'' to all sexual behavior ([[Copulation (zoology)|copulation]], [[Animal sexual behaviour#Pleasure|genital stimulation]], mating games and sexual [[Display (zoology)|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.<ref name="Sommer & Vasey" />{{page needed|date=May 2016}}

===Nature===
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"). For instance, homosexuality in animals was cited by the [[American Psychological Association]] and other groups in their ''[[amici curiae]]'' brief to the [[United States Supreme Court]] in ''[[Lawrence v. Texas]]'', which ultimately struck down the [[sodomy laws]] of 14 states.<ref name="Love That Dare Not Squeak" /><ref>{{cite web| url=http://www.apa.org/about/offices/ogc/amicus/lawrence.pdf |title=Brief for Amici Curiae in Support of Petitioners, ''Lawrence v. Texas''|publisher=[[American Psychological Association]]|date=January 2003}}</ref>

==Research==
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, [[Bruce Bagemihl]], author of the book ''Biological Exuberence: Animal Homosexuality and Natural Diversity'', emphasizes that there are no anatomical or endocrinological differences between exclusively homosexual and exclusively heterosexual animal pairs.<ref name=Bagemihl/>{{rp|164}} 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.<ref name=Bagemihl/>{{rp|164}}

Many of the animals used in laboratory-based 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.<ref name="Somer_2010">{{cite book| veditors = Somer V, Vasey PL |title=Homosexual Behavior In Animals: An Evolutionary Perspective |year=2010 |publisher=Cambridge Press|location=Cambridge | isbn = 978-0-521-18230-0 |id= {{ASIN|0521182301|country=uk}}}}</ref>{{page needed|date=May 2016}} Information gathered from these studies is limited when applied to spontaneously occurring same-sex behavior in animals outside of the laboratory.<ref name="Somer_2010"/>

Homosexual behaviour in animals has been discussed since [[classical antiquity]]. The earliest written mention of animal homosexuality appears to date back to 2,300 years ago, when [[Aristotle]] (384–322 BC) described copulation between pigeons, partridges and quails of the same sex.<ref name="Riccucci2011">{{cite journal| vauthors = Riccucci M |title=Same-sex sexual behaviour in bats|journal=Hystrix It. J. Mammal.|date=2011|volume=22|issue=1|pages=139–47|doi=10.4404/hystrix-22.1-4478}}</ref><ref name="OxfordAcademicZoo">{{cite journal |url=https://academic.oup.com/zoolinnean/article/195/1/1/6568055 |author = Aristotle |author2 = Pliny (transl. Rackham, 1947, Vol. 3: 399; Aristotle, transl. Peck, 1970, Vol. 2: 233; see also: Aelian, transl. Wilson, 1997: 37) |title=Oxford Academic Zoologic Journal|journal = Zoological Journal of the Linnean Society |date = May 2022 |volume = 195 |issue = 1 |pages = 1–32 |doi = 10.1093/zoolinnean/zlab127 |doi-access = free }}</ref> The Hieroglyphics of Horapollo, written in the 4th century AD by the Egyptian writer [[Horapollo]], mentions "hermaphroditism" in hyenas and homosexuality in partridges.<ref name="Riccucci2011"/> The first review of animal homosexuality was written by the zoologist [[Ferdinand Karsch-Haack]] in 1900.<ref name="Riccucci2011"/><ref>{{cite journal |last1=Karsch |first1=F. |title=Päderastie und Tribadie bei den Tieren auf Grund Literatur |journal=Jahrbuch für sexuelle Zwischenstufen |date=1900 |volume=2 |hdl-access=free |hdl=2027/mdp.39015016472477?urlappend=%3Bseq=442}}</ref>

Academic research into the ubiquity of same-sex sexual behavior was not carried out on a large scale, possibly due to [[observer bias]] caused by social attitudes to same-sex sexual behavior,<ref name=Rainbow/> innocent confusion, lack of interest, distaste, scientists fearing loss of their [[Grant (money)|grants]] or even from a fear of "being ridiculed by their colleagues".<ref name="1,500 Animal Species Practice">{{cite web|url=http://www.news-medical.net/?id=20718 |title=1,500 Animal Species Practice Homosexuality |date=2006-10-23 |publisher=News-medical.net |access-date=2007-09-10 |url-status=dead |archive-url=https://web.archive.org/web/20070528115402/http://www.news-medical.net/?id=20718 |archive-date=May 28, 2007 }}</ref><ref name="HITAK">{{cite web|title=Homosexuality in the Animal kingdom |date=February 2009|access-date=October 6, 2017 |url=http://www.nhm.uio.no/besok-oss/utstillinger/skiftende/againstnature/gayanimals.html|website=nhm.uio.no |publisher=Natural History Museum, University of Oslo}}</ref> 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."<ref name="Homosexuality Commo">{{cite news | last = Moskowitz | first =Clara | name-list-style = vanc | title=Homosexuality Common in the Wild, Scientists Say| publisher=Fox News | date=19 May 2008 | url=http://www.foxnews.com/story/0,2933,356639,00.html| archive-url=https://web.archive.org/web/20080528041830/http://www.foxnews.com/story/0,2933,356639,00.html| url-status=dead| archive-date=28 May 2008| access-date = 2008-07-02}}</ref> They also noted "Not every sexual act has a reproductive function ... that's true of humans and non-humans."<ref name="Homosexuality Commo"/> Studies have demonstrated homosexual behavior in a number of species,<ref>{{cite journal | url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2907.1984.tb00344.x | doi=10.1111/j.1365-2907.1984.tb00344.x | title=Homosexual behaviour and female-male mounting in mammals—a first survey | year=1984 | last1=Dagg | first1=Anne Innis | author-link1=Anne Innis Dagg | journal=Mammal Review | volume=14 | issue=4 | pages=155–185 | bibcode=1984MamRv..14..155D | url-access=subscription }}</ref> but the true extent of homosexuality in animals is not known.

[[File:Two Giraffes.PNG|thumb|Two male [[giraffe]]s in [[Kenya]]]]
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 Bagemihl, [[Joan Roughgarden]], [[Thierry Lodé]]<ref>[[Thierry Lodé]] ''La guerre des sexes chez les animaux'' Eds O Jacb, Paris, 2006, {{ISBN|2-7381-1901-8}}</ref> 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. 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,<ref name="Homosexual selection: The power of same-sex liaisons"/> but also in non-monogamous species like sheep. One report on sheep found that 8% of rams exhibited homosexual preferences—that is, even when given a choice, they chose male over female partners.<ref name="The Volume of a Sexually Dimorphic">{{cite journal | vauthors = Roselli CE, Larkin K, Resko JA, Stellflug JN, Stormshak F | title = The volume of a sexually dimorphic nucleus in the ovine medial preoptic area/anterior hypothalamus varies with sexual partner preference | journal = Endocrinology | volume = 145 | issue = 2 | pages = 478–83 | date = February 2004 | pmid = 14525915 | doi = 10.1210/en.2003-1098 | doi-access = free }}</ref> In fact, apparent homosexual individuals are known from all of the traditional domestic species, from sheep, cattle and horses to cats, dogs and [[budgerigars]].<ref name=Bagemihl/>{{Rp|pages=|page=81}} In October 2023, biologists reported  studies of animals (over 1,500 different species) that found [[same-sex behavior]] (not necessarily related to human orientation) may help improve social stability by reducing conflict within the groups studied.<ref name="NYT-20231003cz">{{cite news |last=Zimmer |first=Carl |authorlink=Carl Zimmer |title=Same-Sex Behavior Evolved in Many Mammals to Reduce Conflict, Study Suggests|url=https://www.nytimes.com/2023/10/03/science/same-sex-behavior-evolution-mammals.html |date=3 October 2023 |work=[[The New York Times]] |url-status=live |archive-url=https://web.archive.org/web/20231020175203/https://www.nytimes.com/2023/10/03/science/same-sex-behavior-evolution-mammals.html |archive-date=20 October 2023 |url-access=limited |via=[[Internet Archive]]}}</ref><ref name="NAT-20231003">{{cite journal |author=Gómez, José M. |display-authors=et al. |title=The evolution of same-sex sexual behaviour in mammals |date=3 October 2023 |journal=[[Nature (journal)|Nature]] |volume=14 |number=5719 |page=5719 |doi=10.1038/s41467-023-41290-x |doi-access=free |pmid=37788987 |pmc=10547684 |bibcode=2023NatCo..14.5719G }}</ref>

==Basis==
Sexual behaviors often require a significant energy investment. When sexual behaviors produce offspring, there is an obvious benefit for the animal. However, the benefit from performing homosexual behaviors (which cannot result in the production of offspring) is less obvious, and some scientists have called it a "Darwinian paradox" because it is non-reproductive. A number of non-exclusive different explanations for the emergence of such traits have been put forward.<ref>{{cite encyclopedia|author1-link=Vincent Savolainen |last1=Savolainen |first1=Vincent |title=Encyclopedia of Evolutionary Psychological Science |entry=Evolution of Homosexuality |publisher=[[Springer Publishing]] |year=2016 |pages=1–8|doi=10.1007/978-3-319-16999-6_3403-1 |isbn=978-3-319-16999-6|s2cid=220448278 }}</ref>

===Physiological basis===
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,<ref name="Adler 1997 8–9">{{cite journal |last=Adler |first=Tina| name-list-style = vanc |title=Animal's Fancies|journal=Society for Science and the Public|date=4 Jan 1997|volume=151|issue=1|pages=8–9|doi=10.2307/3980720|jstor=3980720}}</ref> in addition to the size of the animal's gonads,<ref name=Bagemihl/>{{rp|164}} 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 do not. Ultimately, empirical support from comprehensive [[endocrinological]] studies exist for both interpretations.<ref name="Adler 1997 8–9"/><ref name="Somer_2010" /> 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]].<ref name=Bagemihl/>{{rp|164}}

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.<ref name="Adler 1997 8–9"/>

===Genetic basis===
Researchers found that disabling the fucose mutarotase<ref>{{cite web | url=http://www.wikigenes.org/e/gene/e/945842.html | archive-url=https://web.archive.org/web/20160610084742/http://www.wikigenes.org/e/gene/e/945842.html | archive-date=10 June 2016 | title=WikiGenes - Collaborative Publishing }}</ref> (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 [[Knockout mouse|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 findings were published in the [[BioMed Central|''BMC Genetics'' journal]] on July 7, 2010.<ref>{{cite news|url=https://www.telegraph.co.uk/science/science-news/7877774/Female-mice-can-be-turned-lesbian-by-deleting-gene.html |archive-url=https://web.archive.org/web/20100710155012/http://www.telegraph.co.uk/science/science-news/7877774/Female-mice-can-be-turned-lesbian-by-deleting-gene.html |url-status=dead |archive-date=2010-07-10 |title=Female mice 'can be turned lesbian by deleting gene' |newspaper=The Telegraph |date=2010-07-08 |access-date=2010-11-17 |location=London |first=Matthew |last=Moore| name-list-style = vanc }}</ref><ref>{{cite journal | vauthors = Park D, Choi D, Lee J, Lim DS, Park C | title = Male-like sexual behavior of female mouse lacking fucose mutarotase | journal = BMC Genetics | volume = 11 | pages = 62 | date = July 2010 | pmid = 20609214 | pmc = 2912782 | doi = 10.1186/1471-2156-11-62 | publisher = BioMed Central | doi-access = free }}</ref> 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.<ref>{{cite journal| vauthors = Terry J |title='Unnatural Acts' in Nature: The Scientific Fascination with Queer Animals |journal=GLQ: A Journal of Lesbian and Gay Studies|year= 2000|volume=6|issue=2|pages=151–193|doi=10.1215/10642684-6-2-151|s2cid=145504677 }}</ref>

===Neurobiological basis===
In March 2011, research showed that [[serotonin]] is involved in the mechanism of sexual orientation of mice.<ref>{{cite news|url=https://www.bbc.co.uk/news/health-12825688 |title=Sexual preference chemical found in mice |work=BBC News |date=2011-03-23 |access-date=2011-03-24}}</ref><ref>{{cite journal | vauthors = Liu Y, Jiang Y, Si Y, Kim JY, Chen ZF, Rao Y | title = Molecular regulation of sexual preference revealed by genetic studies of 5-HT in the brains of male mice | journal = Nature | volume = 472 | issue = 7341 | pages = 95–9 | date = April 2011 | pmid = 21441904 | pmc = 4094133 | doi = 10.1038/nature09822 | bibcode = 2011Natur.472...95L }}</ref> A study conducted on fruit flies found that inhibiting the dopamine neurotransmitter inhibited lab-induced homosexual behavior.<ref>{{cite journal|last=Zuk|first=Marlene|name-list-style=vanc|title=Same-sex insects: what do bees-or at least flies-have to tell us about homosexuality?|journal=Natural History|date=22 November 2011|volume=119|issue=10|pages=22 |url=https://experts.umn.edu/en/publications/same-sex-insects-what-do-bees-or-at-least-flies-have-to-tell-us-a}}</ref>
<!-- Homosexual behaviour in insects and arachnids have been difficult to interpret. Most species has a very limited registry of social interactions, making homosexuality as a functional adoptions as seen in social vertebrates less likely. In a review of reports of homosexual behaviour 102 arthropod species, as much as 80% of the observations was suggested as being due to mistaken identity. The authors speculate that the fitness costs to males in same-sex copulation would be less than the cost of ignoring a female, and that the energetic cost of a sensory apparatus with better accuracy outweigh the fitness cost of occasional homosexual behaviour.<ref>{{cite journal| vauthors = Scharf I, Martin OY |title=Same-sex sexual behavior in insects and arachnids: prevalence, causes, and consequences|journal=Behavioral Ecology and Sociobiology|date=2 August 2013|volume=67|issue=11|pages=1719–1730|doi=10.1007/s00265-013-1610-x}}</ref> In some species exhibiting [[traumatic insemination]] (e.g. [[Bed bug|bedbugs]]), male homosexual behaviour will usually cause the death of the inseminated male, which may make such act a form of male combat against competitors.<ref>{{cite web| vauthors = Hansen J |title=A sex life with rape and pepper spray|url=http://dca.au.dk/en/tempside/show/artikel/et-sexliv-med-voldtaegt-og-peberspray/|publisher=DCA - DanishCentre For Food And Agriculture|access-date=8 March 2018}}</ref>> -->

=== Other hypotheses ===

One proposal for the adaptive function of homosexual behavior is the formation of alliances and mutual social benefit to the animals. Studies support this in specific species, such as [[black swan]]s, where a quarter of mate pairs consist of two males, who mate with a female and chase her away once she lays the egg, then raise it themselves. These M-M pairs have great success in defending their territory and resources, and keep their young alive until fledgling 80% of the time, compared to 30% for M-F pairs.<ref>{{cite web |last1=Braithwaite |first1=L.Wayne |title=Ecological studies of the black swan |url=http://hdl.handle.net/102.100.100/292286?index=1 |publisher=Australian Wildlife Research |hdl=102.100.100/292286?index=1 |access-date=3 December 2021}}</ref>

Studies done on [[homosexual behavior in birds]] showed a negative correlation between a species' relative parental investment and F-F homosexual behaviors, i.e. in species whose females invested more time and care into their young relative to males, F-F sexual encounters happened less often. Similarly, there was a negative correlation between relative parental investment and M-M homosexual behaviors. This meant that species exhibiting a high degree of [[Animal sexual behaviour#Polygamy|polygamy]] (where females often are the exclusive caretakers of the young) F-F sexual behaviors were very rare, whereas in a socially [[Monogamy in animals|monogamous]] species (in which a M-F pair works together to care for young) they were much more common. The trend was opposite for males, in polygamous species M-M sexual behaviors were quite common and in socially monogamous species they were rare. The study argues that release from parental care, a very energy intensive investment, allows the opportunities for homosexual behaviors to be exhibited, and higher parental care prevents homosexual behaviors from occurring because of the energy cost of the behaviors.<ref>{{cite journal |last1=MacFarlane |first1=Geoff |title=Homosexual behaviour in birds: frequency of expression is related to parental care disparity between the sexes |url=https://www.sciencedirect.com/science/article/pii/S0003347210001855 |journal=Animal Behaviour |year=2010 |volume=80 |issue=3 |pages=375–390 |doi=10.1016/j.anbehav.2010.05.009 |s2cid=53148085 |access-date=3 December 2021|url-access=subscription }}</ref>

A 2019 paper hypothesized that when sex first began to evolve, there was no distinction between homosexuality and [[heterosexuality]], and animals mated with other members of their species indiscriminately. This is a contrast to most perspectives, which try to find explanations for the evolution of homosexual behaviors and separate it completely from the evolution of heterosexual behaviors. The study states that it is unlikely that sexual behaviors evolved simultaneously to the evolution of traits necessary to recognize a compatible sexual mate, such as size, shape, odor, and color. As those secondary sex characteristics evolved, sexuality would have become more discriminatory, leading to less homosexuality, but homosexual behaviors would rarely have had enough cost to be selected against and removed entirely from a population. Additionally, the cost of homosexual behavior would be offset by the cost of mate recognition, which requires psychological adaptations, and excessive discrimination in mate choice can lead to missing out of mating opportunities. With indiscriminate mating, these factors are irrelevant. The paper notes that in some species, especially where survival is very difficult and each energy-related decision could mean the animal's death, homosexual behavior would be strongly selected against, leading to strictly heterosexual species.<ref>{{cite journal |last1=Monk |first1=Julia |title=An alternative hypothesis for the evolution of same-sex sexual behaviour in animals |journal=Nature Ecology & Evolution |year=2019 |volume=3 |issue=12 |pages=1622–1631 |doi=10.1038/s41559-019-1019-7 |pmid=31740842 |s2cid=208144026 |doi-access=free |bibcode=2019NatEE...3.1622M }}</ref>

== {{anchor|Some selected species and groups}} Some select species and groups==
{{See also|List of animals displaying homosexual behavior}}

===Birds===
{{See also|List of birds displaying homosexual behavior|Seabird breeding behavior#Same-sex pairing}}

====Black swans====
[[File:Black Swans.jpg|right|thumb|Male [[Black swan|black swans]] (''Cygnus atratus'')]]

An estimated one-quarter of all [[black swan]] pairings are of males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. The males spent time in each other's society, guarded the common territory, performed greeting ceremonies before each other, and (in the reproductive period) pre-marital rituals, and if one of the birds tried to sit on the other, an intense fight began.<ref name="ES" /><ref>{{cite journal | last = Braithwaite |first=L. W. | year = 1970 | title = The Black Swan | journal = Australian Natural History | volume = 16 | pages = 375–379 }}</ref> More of their [[swan|cygnet]]s 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.<ref name=Bagemihl/>{{rp|487–491}}<ref name="Oslo gay animal show draws crowds">{{cite news| title=Oslo gay animal show draws crowds| work=[[BBC News]]| date=19 October 2006| url=http://news.bbc.co.uk/2/hi/europe/6066606.stm| access-date = 2009-06-15}}</ref>

====Albatrosses====
Female [[Laysan albatross]]es, on the north-western tip of the island of Oahu, Hawaii, form pairs for co-growing offspring. On the observed island, the number of females considerably exceeds the number of males (59% N=102/172), so 31% of females, after mating with males, create partnerships for hatching and feeding chicks. Compared to male-female couples, female-female partnerships have a lower hatching rate (87% vs 41%) and lower overall [[reproductive success]] (67% vs 31%).<ref>{{cite journal | vauthors = Young LC, Zaun BJ, Vanderwerf EA | title = Successful same-sex pairing in Laysan albatross | journal = Biology Letters | volume = 4 | issue = 4 | pages = 323–5 | date = August 2008 | pmid = 18505710 | pmc = 2610150 | doi = 10.1098/rsbl.2008.0191 }}</ref>

Warming waters from [[climate change]] have led to increased foraging times and thus increased mortality among female [[black-browed albatross]]es on the [[Antipodes Islands]] in [[New Zealand]]. The skewed gender imbalance has led to many male albatrosses forming homosexual relationships. Male-male pairs now comprise between 2–5% of the albatross population on the islands.<ref>{{Cite news|last=Frost|first=Natasha|date=2021-11-29|title=Climate Change Is Driving Some Albatrosses to 'Divorce,' Study Finds|language=en-US|work=The New York Times|url=https://www.nytimes.com/2021/11/29/world/asia/albatross-climate-change.html|access-date=2021-11-29|issn=0362-4331}}</ref><ref>{{Cite web|date=2021-11-24|title=Climate crisis pushes albatross 'divorce' rates higher – study|url=https://www.theguardian.com/environment/2021/nov/24/climate-crisis-pushes-albatross-divorce-rates-higher-study|access-date=2021-11-29|website=The Guardian|language=en}}</ref>

==== Blue ducks ====
In 2009, a UK-based [[captive breeding]] program for [[blue duck]]s (involving two males and one female) was derailed when the two males paired with each other instead of with the female that they were assigned to mate with.<ref>{{Cite web|last=Lite|first=Jordan|title=Gay ducks derail repopulation plan|url=https://blogs.scientificamerican.com/extinction-countdown/gay-ducks-derail-repopulation-plan/|access-date=2021-12-04|website=Scientific American Blog Network|language=en}}</ref>

====Ibises====
{{main|American white ibis#Breeding and lifespan}}
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.<ref>{{cite journal|journal=Nature|title=Mercury causes homosexuality in male ibises|last1=Milton|first1=Joseph| name-list-style = vanc |doi=10.1038/news.2010.641|date=1 December 2010}}</ref><ref>{{cite journal | vauthors = Frederick P, Jayasena N | title = Altered pairing behaviour and reproductive success in white ibises exposed to environmentally relevant concentrations of methylmercury | journal = Proceedings. Biological Sciences | volume = 278 | issue = 1713 | pages = 1851–7 | date = June 2011 | pmid = 21123262 | pmc = 3097836 | doi = 10.1098/rspb.2010.2189 }}</ref>

====Mallards====
{{main|Mallard#Breeding}}
[[File:Couple of two male mallard ducks - homosexual Anas platyrhynchos - Moenchbruch - Mönchbruch - May 3rd 2013 - 01.jpg|thumb|Two male [[mallard]]s (''Anas platyrhynchos'')]]

[[Mallard]]s 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.<ref name=Bagemihl/>{{Rp|page=493}} [[Kees Moeliker]] of the [[Natural History Museum Rotterdam]] has observed one male mallard engage in homosexual [[necrophilia]].<ref>{{cite journal| vauthors = Moeliker CW |title=The First Case of Homosexual Necrophilia in the Mallard ''Anas platyrhynchos'' (Aves: Anatidae)|journal=Deinsea|volume=8|issue=2001|pages=243–247|date=9 November 2001|url=http://www.hetnatuurhistorisch.nl/fileadmin/user_upload/documents-nmr/Publicaties/Deinsea/Deinsea_08/Deinsea_8_15_Moeliker_.pdf |access-date=6 October 2014}}</ref>

====Penguins====
{{further|Penguin#Behaviour}}
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 penguin]]s 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 had the English text partly written in Greek letters, to prevent this knowledge becoming more widely known. The report was unearthed only a century later, and published in ''[[Polar Record]]'' in June 2012.<ref>{{cite web|url=http://www.abc.net.au/news/2012-06-10/explorer27s-study-of-sexually-depraved-penguins-unearthed/4062488|title=Unearthed study on 'sexual depravity' in penguins | date=10 June 2012|website=abc.net.au|access-date=25 March 2018}}</ref><ref>{{cite journal |last1=Russell |first1=Douglas G. D. |last2=Sladen |first2=William J. L. |last3=Ainley |first3=David G. |title=Dr. George Murray Levick (1876–1956): unpublished notes on the sexual habits of the Adélie penguin |journal=Polar Record |date=2012 |volume=48 |issue=4 |pages=387–393 |doi=10.1017/S0032247412000216|bibcode=2012PoRec..48..387R |s2cid=146584734 }}</ref>

In early February 2004, ''[[The New York Times]]'' reported that [[Roy and Silo]], a male pair of [[chinstrap penguin]]s 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.<ref name="Love That Dare Not Squeak">{{cite news| last =Smith| first =Dinitia | name-list-style = vanc | title=Love That Dare Not Squeak Its Name| newspaper=The New York Times| date=February 7, 2004| url=https://query.nytimes.com/gst/abstract.html?res=FA0C1EF83A5F0C748CDDAB0894DC404482| access-date = 2007-09-10}}</ref> Other [[penguin]]s in New York zoos have also been reported to have formed same-sex pairs.<ref name="They're in love. They're gay">{{cite web|title= They're in love. They're gay. They're penguins... And they're not alone.|publisher= [[Columbia University]]|agency= Columbia News Service|date= June 10, 2002|url= http://www.timelessspirit.com/SEPT04/cristina.shtml|url-status=dead|archive-url= https://web.archive.org/web/20090123213518/http://www.timelessspirit.com/SEPT04/cristina.shtml|archive-date = January 23, 2009}}</ref><ref>[http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2004/02/07/MNG3N4RAV41.DTL "Central Park Zoo's gay penguins ignite debate"], 2002-02-07, ''San Francisco Chronicle''</ref>

In Odense Zoo in Denmark, a pair of male king penguins adopted an egg that had been abandoned by a female, proceeding to incubate it and raise the chick.<ref>{{cite news|title=Denmark's Gay Penguins Become Fathers|url=http://www.huffingtonpost.com/2012/11/09/denmark-gay-penguins-fathers_n_2104190.html |newspaper=Huffington Post|date=November 11, 2012}}</ref><ref>{{cite news|title=Gay Penguins Become Dads|url=https://www.buzzfeed.com/stacylambe/gay-penguins-become-dads|newspaper=BuzzFeed|date=November 9, 2012}}</ref>
Zoos in [[Japan]] and [[Germany]] have also documented homosexual male penguin couples.<ref name="DW">{{cite news |url=http://www.dw.com/en/cold-shoulder-for-swedish-seductresses/a-1484083 |title=Cold Shoulder for Swedish Seductresses &#124; Germany|publisher=Deutsche Welle |date=2005-02-10 |access-date=2010-11-17}}</ref><ref name="CBC">{{cite news| url=https://www.cbc.ca/news/world/gay-penguin-couple-adopts-abandoned-egg-in-german-zoo-1.794702 | publisher=CBC News | title=Gay penguin couple adopts abandoned egg in German zoo | date=2009-06-05}}</ref> 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 [[Humboldt penguin]]s 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.<ref>{{cite web|url=http://www.ananova.com/news/story/sm_1275591.html |title=Ananova Article on "Tempting Gay Penguins Straight" |publisher=Ananova.com |access-date=2010-11-17}}</ref> German gay groups protested at this attempt to break up the male-male pairs<ref>{{cite web |url=http://www.ananova.com/news/story/sm_1284769.html |title=Followup Ananova Article on German authorities stopping trying to change the penguins' sexual orientation, after GLBTQI organizations protest |publisher=Ananova.com |access-date=2010-11-17 |archive-url=https://web.archive.org/web/20090208215419/http://www.ananova.com/news/story/sm_1284769.html |archive-date=2009-02-08 |url-status=dead }}</ref> 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."<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/world/europe/4264913.stm | work=BBC News | title=Gay outrage over penguin sex test | date=2005-02-14 | access-date=2010-05-04}}</ref>

A pair of male [[Magellanic penguin]]s at the [[San Francisco Zoo]] shared a burrow for six years and raised a surrogate chick; the pair split when the male of a pair in the next burrow died and the female sought a new mate.<ref>{{cite news|url=http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2009/07/14/BAUS18NTE7.DTL |title=Widow a wedge between zoo's male penguin pair |newspaper=San Francisco Chronicle |date=2009-07-14 |access-date=2010-11-17 |first=Meredith |last=May| name-list-style = vanc }}</ref>

Buddy and Pedro, a pair of male [[African penguin]]s, were separated by the [[Toronto Zoo]] to mate with female penguins in 2011.<ref>{{Citation |last= McCormack|first= Simon| name-list-style = vanc |date=14 November 2011 |title=Gay Penguins Reunion: Buddy And Pedro Will Be Back Together By Spring |work=Gay Voices |publisher=[[The Huffington Post]] |url=http://www.huffingtonpost.com/2011/11/14/gay-penguin-reunion_n_1093298.html |access-date=16 November 2011}}</ref><ref name="GPBT">{{cite news| url=https://www.cbc.ca/news/canada/toronto/same-sex-penguin-pair-pursue-female-partners-1.1004938 | publisher=CBC News | title=Same-sex penguin pair pursue female partners | date=2011-12-12}}</ref> Buddy has since paired off with a female.<ref name="GPBT"/>

Suki and Chupchikoni are two female African penguins that pair bonded at the [[Ramat Gan Safari]] in Israel in 2013. Chupchikoni was assumed to be male until her blood was tested.<ref>{{cite news|last=Schuster|first=Ruth| name-list-style = vanc |title=Lesbian Penguins Shack Up at Israeli Zoo|url=http://forward.com/articles/189552/lesbian-penguins-shack-up-at-israeli-zoo/|newspaper=The Jewish Daily Forward|date=December 19, 2013}}</ref>

Jumbs and Kermit, two male Humboldt penguins at [[Wingham Wildlife Park]], were given an egg from heterosexual couple Hurricane and Isobel. The egg failed to hatch. However, they were given another abandoned egg from the same couple which successfully hatched on 12 April 2014.<ref name="penguin">[[Wingham Wildlife Park]]</ref>

As of 2018, two female King penguins at [[Kelly Tarlton's Sea Life Aquarium|Kelly Tarltons]] in [[Auckland]], New Zealand, called Thelma and Louise (named after the [[Thelma & Louise|1991 film]]) have been in a relationship for eight years, when most of the other eligible penguins switch partners each mating season, regardless of their orientation. The two penguins were both taking care of an egg that Thelma hatched, but is unknown whether it was fertilized.<ref>{{Cite news|url=https://www.newshub.co.nz/home/new-zealand/2018/12/auckland-s-gay-penguin-couple-prepare-for-special-christmas-bundle-of-joy.html|title=Auckland's gay penguin couple prepare for special Christmas bundle of joy|work=Newshub|access-date=2018-12-24|language=en|archive-url=https://web.archive.org/web/20181224143719/https://www.newshub.co.nz/home/new-zealand/2018/12/auckland-s-gay-penguin-couple-prepare-for-special-christmas-bundle-of-joy.html|archive-date=2018-12-24|url-status=dead}}</ref>

[[Sphen and Magic]], two male [[gentoo penguin|gentoo penguins]] in [[Sea Life Sydney Aquarium]], paired in 2018 and went on to raise two chicks, Lara (formerly Sphengic) and Clancy.<ref>{{Cite news |last=Bowles |first=Nellie |date=2019-01-15 |title=The Gay Penguins of Australia |url=https://www.nytimes.com/2019/01/15/style/gay-penguins-australia.html |access-date=2024-08-22 |work=[[The New York Times]] |language=en-US |issn=0362-4331}}</ref><ref>{{Cite news |last=Press |first= |date=2024-08-21 |title=When globally famous gay penguin Sphen died in Sydney, his partner began to sing |url=https://www.theguardian.com/australia-news/article/2024/aug/22/when-globally-famous-gay-penguin-sphen-died-in-sydney-his-partner-began-to-sing |access-date=2024-08-22 |work=[[The Guardian]] |language=en-GB |issn=0261-3077}}</ref> Sphen died in 2024.<ref>{{Cite web |date=2024-08-22 |title=Gay penguin Sphen dies in Australian aquarium |url=https://www.dw.com/en/gay-penguin-sphen-dies-in-australian-aquarium/a-70011352 |access-date=2024-08-22 |website=[[Deutsche Welle]] |language=en}}</ref>

In September 2018 at [[Odense Zoo]] in Denmark, a male penguin couple took a chick from a heterosexual couple while they were swimming. Zoo staff speculated that the couple saw the biological parents as neglectful of the chick. The biological parents later confronted the couple and zoo staff returned the chick to them.<ref>{{Cite web |last=Kacala |first=Alexander |date=2018-09-27 |title=Gay penguins 'kidnap' chick from straight parents at Denmark zoo |url=https://www.nbcnews.com/feature/nbc-out/gay-penguins-kidnap-chick-straight-parents-denmark-zoo-n913611 |access-date=2024-08-22 |website=[[NBC News]] |language=en}}</ref>

Marama and Rocky, two female gentoo penguins at [[Sea Life London Aquarium]], had an "incredibly strong bond" for five years prior to adopting a chick who was born in June 2019.<ref>{{Cite web |last=Konstantinides |first=Anneta |title=A same-sex penguin couple at a London aquarium become mothers for the first time after adopting a chick |url=https://www.businessinsider.com/same-sex-penguin-couple-adopt-first-chick-london-aquarium-2019-7 |access-date=2024-08-22 |website=[[Business Insider]] |language=en-US}}</ref> The chick was intentionally not gendered and was given a [[Gender neutrality|gender-neutral]] purple band for identification purposes.<ref>{{Cite news |last=Gawronski |first=Quinn |date=2019-09-10 |title=Gay penguins at London aquarium are raising 'genderless' chick |url=https://www.nbcnews.com/feature/nbc-out/gay-penguins-london-aquarium-are-raising-genderless-chick-n1052096 |access-date=2024-08-22 |work=[[NBC News]]}}</ref> They were still a couple in 2021.<ref name="SS">{{Cite news |title=Penguins: Same-sex couples formed at London aquarium |url=https://www.bbc.co.uk/news/uk-england-london-57076071 |access-date=2024-08-22 |work=[[BBC News]]}}</ref>

Skipper and Ping, two male penguins at [[Berlin Zoo]], adopted an abandoned egg in July 2019.<ref>{{Cite news |date=2019-08-12 |title=Berlin gay penguins adopt abandoned egg |url=https://www.bbc.co.uk/news/blogs-news-from-elsewhere-49318080 |access-date=2024-08-22 |work=[[BBC News]] |language=en-GB}}</ref>

A female penguin couple named Electra and Viola adopted, incubated and raised an egg from another couple at [[L'Oceanogràfic]] in [[Valencia]] in 2020.<ref>{{Cite web |last=Silva |first=Cynthia |date=2020-08-26 |title=Lesbian penguin 'super moms' welcome chick at Spanish aquarium |url=https://www.nbcnews.com/feature/nbc-out/lesbian-penguin-super-moms-welcome-chick-spanish-aquarium-n1238072 |access-date=2024-08-22 |website=[[NBC News]] |language=en}}</ref>

A second same-sex female couple, Marmalade and Chickpea, formed at Sea Life London during the 2021 mating season.<ref name="SS" />

====Vultures====
In 1998, two male [[griffon vulture]]s 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.<ref>{{cite news |url=https://www.independent.co.uk/news/gay-vulture-couple-raise-surrogate-chicks-1110120.html |title=Gay vulture couple raise surrogate chicks | first = Eric | last = Silver | name-list-style = vanc |newspaper=The Independent |date=2 August 1999 |access-date=2009-09-21 | location=London }}</ref> 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.<ref>{{cite news|url=http://www.haaretz.com/hasen/spages/1115739.html|title='Gay' vulture couple split up at Jerusalem zoo, then become fathers|first=Jonathan|last=Lis|name-list-style=vanc|newspaper=[[Haaretz]]|date=21 September 2009|access-date=2009-09-21|archive-date=16 June 2013|archive-url=https://web.archive.org/web/20130616033627/http://www.haaretz.com/print-edition/news/gay-vulture-couple-split-up-at-jerusalem-zoo-then-become-fathers-1.7562|url-status=dead}}</ref>

Two male vultures at the [[Allwetter Zoo]] in [[Münster|Muenster]] built a nest together, although they were picked on and their nest materials were often 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.<ref>{{cite web |url=http://entertainment.stv.tv/showbiz/209259-gay-vultures-split-up-to-go-straight/ |title=Gay vultures split up to go straight &#124; Showbiz: Latest News &#124; STV Entertainment |publisher=Entertainment.stv.tv |access-date=2010-11-17 |archive-date=21 November 2010 |archive-url=https://web.archive.org/web/20101121075904/http://entertainment.stv.tv/showbiz/209259-gay-vultures-split-up-to-go-straight/ |url-status=dead }}</ref>

====Pigeons====
{{further|Pigeon#Courtship and reproduction}}
Both male and female [[pigeon]]s 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.<ref>{{Cite journal|last1=Jankowiak|first1=Łukasz|last2=Tryjanowski|first2=Piotr|last3=Hetmański|first3=Tomasz|last4=Skórka|first4=Piotr|date=2018-01-26|title=Experimentally evoked same-sex sexual behaviour in pigeons: better to be in a female-female pair than alone|journal=Scientific Reports|language=en|volume=8|issue=1|page=1654|doi=10.1038/s41598-018-20128-3|pmid=29374281|issn=2045-2322|pmc=5785962|bibcode=2018NatSR...8.1654J}}</ref>

===Mammals===
{{see also|List of mammals displaying homosexual behavior|Animal sexual behaviour#Mammals}}

===={{anchor|Amazon_Dolphin}}Amazon dolphins====
The [[Amazon river dolphin]] or [[boto]] has been reported to form up in bands of 3–5 individuals engaging in sexual activity. 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.<ref name=Bagemihl/>{{rp|339–348}} In captivity, they have been observed to sometimes perform homosexual and heterosexual penetration of the [[Blowhole (biology)|blowhole]], a hole homologous with the [[nostril]] of other mammals, making this the only known example of nasal sex in the animal kingdom.<ref name=Bagemihl/>{{rp|339–348}}<ref>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 [http://www.aquaticmammalsjournal.org/share/AquaticMammalsIssueArchives/1985/Aquatic_Mammals_11_2/Sylvestre.pdf article] {{Webarchive|url=https://web.archive.org/web/20161022010800/http://www.aquaticmammalsjournal.org/share/aquaticmammalsissuearchives/1985/aquatic_mammals_11_2/sylvestre.pdf |date=22 October 2016 }}</ref> The males will sometimes also perform sex with males from the [[tucuxi]] species, a type of small [[porpoise]].<ref name=Bagemihl/>{{rp|339–348}}

====American bisons====
[[File:American bison k5680-1.jpg|thumb|The American bison is a [[bovine]] [[mammal]] which displays homosexual behavior.]]
{{further|American bison#Behavior and ecology}}
Courtship, mounting, and full [[anal sex|anal penetration]] between bulls has been noted to occur among [[American bison]]. The [[Mandan]] nation {{lang|mhq|[[Okipa]]}} festival concludes with a ceremonial enactment of this behavior, to "ensure the return of the buffalo in the coming season".<ref name="Left-Handed Bears & Androgynous">{{cite news|last=Bagemihl |first=Bruce | name-list-style = vanc |title=Left-Handed Bears & Androgynous Cassowaries: Homosexual/transgendered animals and indigenous knowledge |work=[[Whole Earth Review|Whole Earth Magazine]] |date=May 2000 |url=http://www.wholeearthmag.com/ArticleBin/338.html |access-date=13 December 2013 |url-status=dead |archive-url=https://web.archive.org/web/20000815195419/http://www.wholeearthmag.com/ArticleBin/338.html |archive-date=August 15, 2000 |via=[[Internet Archive]]}}</ref> Also, mounting of one female by another (known as "[[Bulling (cattle)|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.

====Bats====
[[File:Homosexual-Fellatio-Erect-Penis-Licking-between-Male-Bonin-Flying-Foxes-Pteropus-pselaphon-pone.0166024.s002.ogv|thumb|Two male [[Bonin flying fox]]es (''Pteropus pselaphon'') performing [[fellatio]] on each other<ref name="Sugita2016"/>]]

More than 20 species of bat have been documented to engage in homosexual behavior.<ref name="Riccucci2011"/><ref name="Sugita2016">{{cite journal | vauthors = Sugita N | title = Homosexual Fellatio: Erect Penis Licking between Male Bonin Flying Foxes Pteropus pselaphon | journal = PLOS ONE | volume = 11 | issue = 11 | pages = e0166024 | date = Nov 2016 | pmid = 27824953 | pmc = 5100941 | doi = 10.1371/journal.pone.0166024 | bibcode = 2016PLoSO..1166024S | doi-access = free }}</ref> Bat species that have been observed engaging in homosexual behavior in the wild include:<ref name="Riccucci2011"/>
{{columns-list|colwidth=30em|
* the [[grey-headed flying fox]] (''Pteropus poliocephalus'')
* the [[Bonin flying fox]] (''Pteropus pselaphon'')<ref name="Sugita2016"/>
* the [[Indian flying fox]] (''Pteropus giganteus'')
* [[Rafinesque's big-eared bat]] (''Corynorhinus rafinesquii'')
* the [[common bent-wing bat]] (''Miniopterus schreibersii'')
* the [[serotine bat]] (''Eptesicus serotinus'')
* [[Bechstein's bat]] (''Myotis bechsteinii'')
* the [[long-fingered bat]] (''Myotis capaccinii'')
* [[Daubenton's bat]] (''Myotis daubentonii'')
* the  [[little brown bat]] (''Myotis lucifugus'')
* the [[greater mouse-eared bat]] (''Myotis myotis'')
* the [[whiskered bat]] (''Myotis mystacinus'')
* [[Natterer's bat]] (''Myotis nattereri'')
* the [[common noctule]] (''Nyctalus noctula'')
* [[Leisler's bat]] (''Nyctalus leisleri'')
* the [[common pipistrelle]] (''Pipistrellus pipistrellus'')
* the [[brown long-eared bat]] (''Plecotus auritus'')
* the [[barbastelle]] (''Barbastella barbastellus'')
* the [[greater horseshoe bat]] (''Rhinolophus ferrumequinum'')
* the [[lesser horseshoe bat]] (''Rhinolophus hipposideros'')}}

Bat species that have been observed engaging in homosexual behavior in captivity include the [[Livingstone's fruit bat|Comoro flying fox]] (''Pteropus livingstonii''), the [[Rodrigues flying fox]] (''Pteropus rodricensis'') and the [[common vampire bat]] (''Desmodus rotundus'').<ref name="Riccucci2011"/>

Homosexual behavior in bats has been categorized into 6 groups: mutual homosexual grooming and licking, homosexual masturbation, homosexual play, homosexual mounting, coercive sex, and cross-species homosexual sex.<ref name="Riccucci2011"/><ref name="Sugita2016"/>

In the wild, the [[grey-headed flying fox]] (''Pteropus poliocephalus'') engages in [[Social grooming|allogrooming]] wherein one partner licks and gently bites the chest and wing membrane of the other partner. Both sexes display this form of mutual homosexual grooming and it is more common in males. Males often have erect penises while they are mutually grooming each other. Like opposite-sex grooming partners, same-sex grooming partners continuously utter a "pre-copulation call", which is described as a "pulsed grating call", while engaged in this activity.<ref name="Riccucci2011"/><ref name="Sugita2016"/>

In wild [[Bonin flying fox]]es (''Pteropus pselaphon''), males perform fellatio or 'male-male genital licking' on other males. Male–male genital licking events occur repeatedly several times in the same pair, and reciprocal genital licking also occurs. The male-male genital licking in these bats is considered a sexual behavior. Allogrooming in Bonin flying foxes has never been observed, hence the male-male genital licking in this species does not seem to be a byproduct of allogrooming, but rather a behavior of directly licking the male genital area, independent of allogrooming.<ref name="Sugita2016"/> In captivity, same-sex genital licking has been observed among males of the [[Livingstone's fruit bat|Comoro flying fox]] (''Pteropus livingstonii'') as well as among males of the [[common vampire bat]] (''Desmodus rotundus'').<ref name="Riccucci2011"/><ref name="Sugita2016"/>

In wild [[Indian flying fox]]es (''Pteropus giganteus''), males often mount one another, with erections and thrusting, while play-wrestling.<ref name="Riccucci2011"/> Males of the [[long-fingered bat]] (''Myotis capaccinii'') have been observed in the same position of male-female mounting, with one gripping the back of the other's fur. A similar behavior was also observed in the [[common bent-wing bat]] (''Miniopterus schreibersii'').<ref name="Riccucci2011"/>

In wild [[little brown bat]]s (''Myotis lucifugus''), males often mount other males (and females) during late autumn and winter, when many of the mounted individuals are [[torpor|torpid]].<ref name="Riccucci2011"/> 35% of matings during this period are homosexual.<ref name="Myotis lucifugusADW">{{cite journal| vauthors = Havens A |editor1-last=Dewey|editor1-first=Tanya|editor2-last=Shefferly|editor2-first=Nancy|title=Myotis lucifugus|journal=Animal Diversity Web|date=2006|url=http://animaldiversity.org/accounts/Myotis_lucifugus/}}</ref> These coercive copulations usually include ejaculation and the mounted bat often makes a typical copulation call consisting of a long squawk.<ref name="Riccucci2011"/> Similarly, in [[Hibernaculum (zoology)|hibernacula]] of the [[common noctule]] (''Nyctalus noctula''), active males were observed to wake up from lethargy on a warm day and engage in mating with lethargic males and (active or lethargic) females. The lethargic males, like females, called out loudly and presented their buccal glands with opened mouth during copulation.<ref name="Riccucci2011"/>

Vesey-Fitzgerald (1949) observed homosexual behaviours in all 12 British bat species known at the time: "Homosexuality is common in the spring in all species, and, since the males are in full possession of their powers, I suspect throughout the summer...I have even seen homosexuality between [[Natterer's bat|Natterer's]] and [[Daubenton's bat]]s (''Myotis nattereri'' and ''M. daubentonii'')."<ref name="Riccucci2011"/>

====Bottlenose dolphins====
{{see also|Animal sexual behavior#Bottlenose dolphins|Bottlenose dolphin#Life history}}
Dolphins of several species engage in homosexual acts, though it is best studied in the [[bottlenose dolphin]]s.<ref name=Bagemihl/>{{page needed|date=May 2016}} 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.<ref>{{cite journal| vauthors = Norris KS, Dohl TP |title=Behaviour of the Hawaiian spinner dolphin, ''Stenella longirostris''|journal=Fishery Bulletin|year=1980|volume=77|pages=821–849}}</ref> Between males, homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting [[dolphin penis|the penis]] in the other's [[genital slit]] and sometimes anus.<ref>{{cite web | vauthors = Wells RS |title=Community structure of Bottlenose Dolphins near Sarasota, Florida|year=1995|series=Paper presented at the 24th International Ethological Conference, Honoluly, Hawaii|url= https://www.researchgate.net/publication/285070874}}</ref>

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.<ref>{{cite web|url=http://www.livescience.com/2534-sex-couples-common-wild.html|title=Same Sex Couples Common in the Wild |website=livescience.com|date=16 May 2008 |access-date=25 March 2018}}</ref> 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.<ref>{{cite journal| vauthors = Herzing DL, Johnson CM |title=Interspecific Interaction between Spotted Dolphins (''Stenella frontalis'') and Bottlenose Dolphins (''Tursiops truncatus'') in the Bahamas, 1985–1995|journal=Aquatic Mammals|year=1997|volume=23|pages=85–99}}</ref>

====Elephants====
{{further|Elephant#Mating}}
[[African elephant|African]] and [[Asian elephant|Asian]] male elephants 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 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.<ref name=Bagemihl/>{{rp|427–430}}

====Giraffes====
{{further|Giraffe#Necking}}
Male [[giraffe]]s 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.<ref>{{cite journal | vauthors = Coe MJ | year = 1967 | title = "Necking" behavior in the giraffe | journal = Journal of Zoology | volume = 151 | issue =3 | pages = 313–321 | doi=10.1111/j.1469-7998.1967.tb02117.x }}</ref> 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.<ref name=Bagemihl/>{{rp|391–393}}

==== Humpback whales ====
In 2022, photographers captured the first-ever images of [[Humpback whale|humpback whales]] mating, both of which were males, off the coast of [[Maui]]. The whales were observed for 30 minutes, with one whale using its penis to penetrate the genital slit of the other.<ref>{{Cite web |last=Kuta |first=Sarah |title=These Photos Are the First to Show Humpback Whales Mating—and Both Are Males |url=https://www.smithsonianmag.com/smart-news/these-photos-are-the-first-to-show-humpback-whales-mating-and-both-are-males-180983890/ |access-date=2024-03-11 |website=Smithsonian Magazine |language=en}}</ref><ref>{{Cite journal |last1=Stack |first1=Stephanie H. |last2=Krannichfeld |first2=Lyle |last3=Romano |first3=Brandi |date=2024-02-27 |title=An observation of sexual behavior between two male humpback whales |journal=Marine Mammal Science |volume=40 |issue=3 |language=en |doi=10.1111/mms.13119 |issn=0824-0469|doi-access=free |bibcode=2024MMamS..40E3119S |hdl=10072/430033 |hdl-access=free }}</ref>

====Marmots====
{{Multiple image
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| footer            = [[Olympic marmot]] (left) and [[hoary marmot]] (right)
| direction         = 
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Homosexual behavior is quite common in wild marmots.<ref name=Bagemihl/>{{rp|525–527}} In [[Olympic marmot]]s (''Marmota olympus'') and [[hoary marmot]]s (''Marmota caligata''), females often mount other females as well as engage in other affectionate and sexual behaviors with females of the same species.<ref name=Bagemihl/>{{rp|525–527}} They display a high frequency of these behaviors especially when they are [[estrus|in heat]].<ref name=Bagemihl/>{{rp|525–527}}<ref name="Barash1973">{{cite journal| vauthors = Barash DP|title=The Social Biology of the Olympic Marmot|journal=Animal Behaviour Monographs |date=1973|volume=6|issue=3 |pages=171–245 |doi=10.1016/0003-3472(73)90002-X}}</ref> A homosexual encounter often begins with a greeting interaction in which one female nuzzles her nose on the other female's cheek or mouth, or both females touch noses or mouths. Additionally, a female may gently chew on the ear or neck of her partner, who responds by raising her tail. The first female may sniff the other's genital region or nuzzle that region with her mouth. She may then proceed to mount the other female, during which the mounting female gently grasps the mounted female's dorsal neck fur in her jaws while thrusting. The mounted female arches her back and holds her tail to one side to facilitate their sexual interaction.<ref name=Bagemihl/>{{rp|525–527}}<ref name="Barash1974">{{cite journal| vauthors = Barash DP |title=The social behaviour of the hoary marmot (Marmota caligata)| journal=Anim. Behav.|date=Feb 1974|volume=22 |issue=1| pages=256–61| doi=10.1016/S0003-3472(74)80077-1}}</ref>

====Lions====
{{further|Sexual behavior of lions}}
[[File:Wuppertal - Zoo - Panthera leo 01 (1) ies.webm|thumb|right|Male lions mating]]

Both male and female [[lion]]s have been seen to interact [[animal sexuality|homosexually]].{{refn|<ref name=Bagemihl/>{{rp|302–305}}<ref>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</ref><ref>{{cite web | last =Srivastav | first =Suvira | name-list-style = vanc | title =Lion, Without Lioness | work =TerraGreen: News to Save the Earth | publisher =Terragreen | date =15–31 December 2001 | url =http://www.teri.res.in/teriin/terragreen/issue3/feature.htm | access-date =2007-09-02 | url-status=dead | archive-url =https://web.archive.org/web/20070914010938/http://www.teri.res.in/teriin/terragreen/issue3/feature.htm | archive-date =14 September 2007 }}</ref>}} Male lions pair-bond for a number of days and initiate homosexual activity with affectionate nuzzling and caressing, leading to mounting and [[pelvic thrusting|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.

====Polecats====
[[European polecat]]s (''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.<ref>[[Thierry Lodé]] ''La guerre des sexes chez les animaux'' Eds O Jacob, Paris, 2006.{{ISBN|2-7381-1901-8}}</ref>{{page needed|date=May 2016}}

====Primates====
Same-sex sexual behaviors have been observed in New World monkeys, Old World monkeys, and apes. These behaviors are much more common in Old World monkeys than New World monkeys, and all ape genera have been observed engaging in same-sex sexual behavior. Prosimians have not been observed engaging in same-sex sexual behavior. Same-sex mounting, for instance, has been observed in 16% of New World primate genera. In contrast, 76% of Old World monkey genera contain at least one species that has been observed in male-male mounts, and 70% of genera for female-female mounts. Same-sex mounting has been observed in all ape genera. Same-sex sexual behaviors likely evolved following the divergence of New World monkeys;<ref>{{Cite journal |last1=Pfau |first1=Daniel |last2=Jordan |first2=Cynthia L. |last3=Breedlove |first3=S. Marc |date=2021-08-01 |title=The De-Scent of Sexuality: Did Loss of a Pheromone Signaling Protein Permit the Evolution of Same-Sex Sexual Behavior in Primates? |url=https://doi.org/10.1007/s10508-018-1377-2 |journal=Archives of Sexual Behavior |language=en |volume=50 |issue=6 |pages=2267–2276 |doi=10.1007/s10508-018-1377-2 |pmid=31016493 |s2cid=254251448 |issn=1573-2800|url-access=subscription }}</ref> these behaviors are a conserved trait among Old World monkeys and apes.<ref>{{Citation |last=Dixson |first=Alan |title=Homosexual behaviour in primates |date=2010-08-19 |url=http://dx.doi.org/10.1017/cbo9780511762192.010 |work=Animal Homosexuality |pages=381–400 |access-date=2023-10-22 |publisher=Cambridge University Press|doi=10.1017/cbo9780511762192.010 |isbn=978-0-521-19675-8 |url-access=subscription }}</ref> Among Old World monkeys, same-sex genital interactions are much more common in multi-male systems as opposed to uni-male systems.<ref>{{Cite journal |last1=MacFarlane |first1=Geoff R. |last2=Vasey |first2=Paul L. |date=2016-05-01 |title=Promiscuous primates engage in same-sex genital interactions |url=https://www.sciencedirect.com/science/article/pii/S0376635716300377 |journal=Behavioural Processes |volume=126 |pages=21–26 |doi=10.1016/j.beproc.2016.02.016 |pmid=26930251 |s2cid=39603917 |issn=0376-6357|url-access=subscription }}</ref>

=====Bonobos=====
{{See also|Bonobo#Sociosexual behaviour}}[[File:Bonobo 011.jpg|thumb|right|Two male bonobos]]
[[Bonobo]]s form a [[Matriarchy|matriarchal]] society, unusual among [[ape]]s. They are fully [[Bisexuality|bisexual]]: both males and females engage in hetero- and homosexual behavior, being noted for female–female sex in particular, including between juveniles and adults.<ref name="De_Waal_1995">{{cite journal |last=de Waal |first=Frans B. M. |author-link=Frans de Waal |name-list-style=vanc |date=March 1995 |title=Bonobo Sex and Society: The behavior of a close relative challenges assumptions about male supremacy in human evolution |url=http://www.biolinguagem.com/ling_cog_cult/dewaal_1995_bonobosexsociety.pdf |url-status=dead |journal=[[Scientific American]] |volume=272 |issue=3 |pages=82–88 |bibcode=1995SciAm.272c..82W |doi=10.1038/scientificamerican0395-82 |pmid=7871411 |archive-url=https://web.archive.org/web/20160801020429/http://www.biolinguagem.com/ling_cog_cult/dewaal_1995_bonobosexsociety.pdf |archive-date=1 August 2016 |access-date=10 March 2019}}</ref> 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 primate species, homosexuality has been reported for all [[Hominidae|great apes]], as well as a number of other [[primate]] species.{{refn|<ref>{{Cite web |title=Bonobo |url=http://www.colszoo.org/animalareas/aforest/bonobo.html |url-status=dead |archive-url=https://web.archive.org/web/20070817012421/http://www.colszoo.org/animalareas/aforest/bonobo.html |archive-date=17 August 2007 |publisher=[[Columbus Zoo and Aquarium]]}}</ref><ref>{{cite book |last=Dawkins |first=Richard |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Life |title-link=The Ancestor's Tale |publisher=[[Houghton Mifflin]] (US) |year=2004 |isbn=978-1-155-16265-2 |pages=92–93 |chapter=Rendezvous I: Chimpanzees |author-link=Richard Dawkins |name-list-style=vanc}}</ref><ref>{{cite web |last=Laird |first=Courtney |name-list-style=vanc |title=Social Organization |url=http://www.bio.davidson.edu/people/vecase/Behavior/Spring2004/laird/Social%20Organization.htm |url-status=dead |archive-url=https://web.archive.org/web/20110519005633/http://www.bio.davidson.edu/people/vecase/Behavior/Spring2004/laird/Social%20Organization.htm |archive-date=19 May 2011 |website=bio.davidson.edu}}</ref><ref>{{cite journal |vauthors=Stanford CB |year=1998 |title=The social behavior of chimpanzees and bonobos |journal=[[Current Anthropology]] |volume=39 |issue=4 |pages=399–407 |doi=10.1086/204757 |s2cid=8452514}}</ref><ref>{{Cite book |last=Kano |first=Takayoshi |title=The Last Ape: Pygmy Chimpanzee Behavior and Ecology |publisher=[[Stanford University Press]] |year=1992 |isbn=9780804716123 |name-list-style=vanc}}</ref>}}

Dutch [[Primatology|primatologist]] [[Frans de Waal]], who extensively observed and filmed bonobos, believed that sexual activity is the bonobo's way of 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.<ref name="De_Waal_1995" /><ref>{{cite book |last=de Waal |first=Frans B. M. |title=The Ape and the Sushi Master: Cultural Reflections by a Primatologist |title-link=The Ape and the Sushi Master |publisher=[[Basic Books]] |year=2001 |isbn=978-84-493-1325-7 |chapter=Bonobos and Fig Leaves |author-link=Frans de Waal |name-list-style=vanc}}</ref>

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 [[scrotum|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.<ref name="De_Waal_1995" />

===== Chimpanzees =====
Compared to bonobos, less research has been conducted on same-sex sexual behaviors in chimpanzees (''Pan troglodytes''). In contrast to bonobos, same-sex sexual behaviors are less common in Kibale chimpanzees (''Pan troglodytes schweinfurthii'') and primarily occur between adult males.<ref>{{Cite journal |last1=Sandel |first1=Aaron A. |last2=Reddy |first2=Rachna B. |date=2021-02-02 |title=Sociosexual behaviour in wild chimpanzees occurs in variable contexts and is frequent between same-sex partners |url=https://brill.com/view/journals/beh/158/3-4/article-p249_3.xml |journal=Behaviour |volume=158 |issue=3–4 |pages=249–276 |doi=10.1163/1568539X-bja10062 |s2cid=234056789 |issn=0005-7959|url-access=subscription }}</ref> While less frequent than in female bonobos, same-sex sexual behaviors between male chimpanzees occur regularly and in a variety of contexts. Female chimpanzees are much less likely to engage in same-sex sexual behaviors than males.

===== Gibbons =====
There is one published study of same-sex sexual behavior in white-handed gibbons (''Hylobates lar''). The authors report on observations of an adult male mounting an adolescent male. In gibbons, same-sex sexual behavior is likely rare.<ref>{{Cite journal |last1=Edwards |first1=Anna-Marie A. R. |last2=Todd |first2=John D. |date=1991-04-01 |title=Homosexual behaviour in wild white-handed gibbons (Hylobates lar) |url=https://doi.org/10.1007/BF02381180 |journal=Primates |language=en |volume=32 |issue=2 |pages=231–236 |doi=10.1007/BF02381180 |s2cid=26662075 |issn=1610-7365|url-access=subscription }}</ref>

=====Gorillas=====
{{See also|Gorilla#Social structure}}Both male and female mountain gorillas (''Gorilla gorilla beringei'') have been observed engaging in same-sex sexual behaviors.<ref>{{Cite web |title=Does homosexual behaviour exist among gorillas? |url=https://www.berggorilla.org/en/gorillas/general/social-life/does-homosexual-behaviour-exist-among-gorillas/ |access-date=2023-10-22 |website=Berggorilla & Regenwald Direkthilfe e.V. |language=en}}</ref> Homosexual behavior among male gorillas has been studied.<ref>{{Cite journal |last=Yamagiwa |first=Juichi |name-list-style=vanc |date=1987-01-01 |title=Intra- and inter-group interactions of an all-male group of virunga mountain gorillas (Gorilla gorilla beringei) |journal=Primates |language=en |volume=28 |issue=1 |pages=1–30 |doi=10.1007/BF02382180 |issn=1610-7365 |s2cid=24667667}}</ref> This behavior occurs more often in all-male bachelor packs in the wild and it is believed to play a role in social bonding. Homosexual behavior among female [[mountain gorilla]]s has also been documented.<ref>{{cite journal |vauthors=Grueter CC, Stoinski TS |date=2016-05-11 |title=Homosexual Behavior in Female Mountain Gorillas: Reflection of Dominance, Affiliation, Reconciliation or Arousal? |journal=PLOS ONE |volume=11 |issue=5 |pages=e0154185 |bibcode=2016PLoSO..1154185G |doi=10.1371/journal.pone.0154185 |pmc=4864209 |pmid=27167861 |doi-access=free}}</ref> Among all-male "bachelor" groups, same-sex sexual interactions are extremely common.<ref>{{Cite journal |last=Yamagiwa |first=Juichi |date=1987-01-01 |title=Intra- and inter-group interactions of an all-male group of virunga mountain gorillas (Gorilla gorilla beringei) |url=https://doi.org/10.1007/BF02382180 |journal=Primates |language=en |volume=28 |issue=1 |pages=1–30 |doi=10.1007/BF02382180 |s2cid=24667667 |issn=1610-7365|url-access=subscription }}</ref> Female mountain gorillas engage in same-sex sexual behaviors frequently as well, and coincided in prevalence in accordance with intersexual copulations as well. As opposed to conferring social status, same-sex sexual behaviors are likely reflective of elevated arousal in female mountain gorillas.<ref>{{Cite web |last=Australia |first=University of Western |title=Homosexual activity documented in female gorillas for the first time |url=https://phys.org/news/2016-05-homosexual-documented-female-gorillas.html |access-date=2023-10-22 |website=phys.org |language=en}}</ref> Same-sex sexual behaviors are natural and common among mountain gorillas.

=====Orangutans=====
Homosexual behavior forms part of the natural repertoire of sexual or sociosexual behavior of [[orangutan]]s. Male homosexual behavior occurs both in the wild and in captivity, and it occurs in both adolescent and mature individuals. Homosexual behavior in orangutans is not an artifact of captivity or contact with humans.<ref name="Fox-2001">{{cite journal |vauthors=Fox EA |date=November 2001 |title=Homosexual behavior in wild Sumatran orangutans (Pongo pygmaeus abelii) |journal=[[American Journal of Primatology]] |volume=55 |issue=3 |pages=177–81 |doi=10.1002/ajp.1051 |pmid=11746281 |s2cid=21561581}}</ref>

Sociosexual behaviors among orangutans (''Pongo spp.'') are extremely common and invoke a variety of evolutionary strategies.<ref>{{Cite journal |last1=Kunz |first1=Julia A. |last2=Duvot |first2=Guilhem J. |last3=Willems |first3=Erik P. |last4=Stickelberger |first4=Julia |last5=Spillmann |first5=Brigitte |last6=Utami Atmoko |first6=Sri Suci |last7=van Noordwijk |first7=Maria A. |last8=van Schaik |first8=Carel P. |date=December 2021 |title=The context of sexual coercion in orang-utans: when do male and female mating interests collide? |journal=Animal Behaviour |volume=182 |pages=67–90 |doi=10.1016/j.anbehav.2021.09.012 |issn=0003-3472|doi-access=free }}</ref><ref>{{Cite journal |last1=O'Connell |first1=Caitlin A. |last2=Susanto |first2=Tri Wahyu |last3=Knott |first3=Cheryl D. |date=November 2020 |title=Sociosexual behavioral patterns involving nulliparous female orangutans ( Pongo sp.) reflect unique challenges during the adolescent period |url=https://onlinelibrary.wiley.com/doi/10.1002/ajp.23058 |journal=American Journal of Primatology |language=en |volume=82 |issue=11 |pages=e23058 |doi=10.1002/ajp.23058 |pmid=31583721 |s2cid=203659435 |issn=0275-2565|url-access=subscription }}</ref> However, only one study of same-sex sexual behavior has been published.<ref>{{Cite journal |last=Fox |first=Elizabeth A. |date=2001 |title=Homosexual behavior in wild Sumatran orangutans (Pongo pygmaeus abelii) |url=http://dx.doi.org/10.1002/ajp.1051 |journal=American Journal of Primatology |volume=55 |issue=3 |pages=177–181 |doi=10.1002/ajp.1051 |pmid=11746281 |s2cid=21561581 |issn=0275-2565|url-access=subscription }}</ref> In Sumatran orangutans (''Pongo abelii''), same-sex sexual behavior has been documented among two dyads of unflanged adolescent males. Oral contact with genitals and behavior similar to male-female forced copulation between sexually mature sub-adult males were observed.

=====Monkeys=====
Among monkeys{{clarify|date=June 2012}}, [[Lionel Tiger]] and [[Robin Fox]] conducted a study on how [[Depo-Provera]] [[contraceptive]]s lead to decreased male attraction to females.<ref>{{cite book|url=https://books.google.com/books?id=LiyrEhQPmRsC&pg=PA66| first = Lionel | last = Tiger | name-list-style = vanc |pages=66ff|year=1992|title=The Pursuit of Pleasure|publisher=Transaction Publishers|isbn=978-0-7658-0696-3}}</ref>

======Japanese macaques======
{{further|Japanese macaque#Behavior}}
With the [[Japanese macaque]], also known as the "snow [[monkey]]", [[Homosexuality|same-sex relations]] are frequent, though rates vary between troops. Females will form "[[wikt:consort|consort]]ships" 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.<ref name=Bagemihl/>{{rp|302–305}}

====Sheep====
{{main|Homosexual behavior in sheep}}
''[[Ovis aries]],'' the common domesticated sheep, has attracted much attention due to the fact that around 8–10% of rams have an exclusive homosexual orientation.{{refn|<ref name="Poiani2010"/><ref name="The Volume of a Sexually Dimorphic"/><ref>{{cite journal | vauthors = Roselli CE, Stormshak F | title = The neurobiology of sexual partner preferences in rams | journal = Hormones and Behavior | volume = 55 | issue = 5 | pages = 611–20 | date = May 2009 | pmid = 19446078 | pmc = 2684522 | doi = 10.1016/j.yhbeh.2009.03.013 }}</ref><ref name="ncbi.nlm.nih.gov">{{cite journal | vauthors = Roselli CE, Stormshak F | title = Prenatal programming of sexual partner preference: the ram model | journal = Journal of Neuroendocrinology | volume = 21 | issue = 4 | pages = 359–64 | date = March 2009 | pmid = 19207819 | pmc = 2668810 | doi = 10.1111/j.1365-2826.2009.01828.x }}</ref><ref>{{cite journal | vauthors = Perkins A, Fitzgerald JA, Price EO | title = Luteinizing hormone and testosterone response of sexually active and inactive rams | journal = Journal of Animal Science | volume = 70 | issue = 7 | pages = 2086–93 | date = July 1992 | pmid = 1644682 | doi = 10.2527/1992.7072086x | url = https://www.animalsciencepublications.org/publications/jas/articles/70/7/2086 | access-date = 2015-07-07 | archive-url = https://web.archive.org/web/20170630082018/https://www.animalsciencepublications.org/publications/jas/articles/70/7/2086 | archive-date = 2017-06-30 | url-status=dead | url-access = subscription }}</ref><ref name="Rosellietal2004">{{cite journal | vauthors = Roselli CE, Larkin K, Schrunk JM, Stormshak F | title = Sexual partner preference, hypothalamic morphology and aromatase in rams | journal = Physiology & Behavior | volume = 83 | issue = 2 | pages = 233–45 | date = November 2004 | pmid = 15488542 | doi = 10.1016/j.physbeh.2004.08.017 | s2cid = 156571 }}</ref>}} Such rams prefer to court and mount other rams only, even in the presence of estrous ewes.<ref name="Poiani2010"/> Moreover, around 18–22% of rams are bisexual.<ref name="ncbi.nlm.nih.gov"/>

Several observations indicate that male–male sexual preference in rams is sexually motivated. Rams routinely perform the same courtship behaviors (including foreleg kicks, nudges, vocalizations, anogenital sniffs and [[flehmen response|flehmen]]) prior to mounting other males as observed when other rams court and mount estrous females. Furthermore, pelvic thrusting and ejaculation often accompany same-sex mounts by rams.<ref name="Rosellietal2004"/>

====Spotted hyenas====
{{see also|Animal sexual behaviour#Spotted hyena|Spotted hyena#Reproduction and development}}

[[File:Spotted hyena2.jpg|right|thumb|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 [[Clitoris#Spotted hyenas|the female spotted hyena's unique urogenital system]], which looks more like a penis rather than a vagina, early naturalists thought hyenas were [[hermaphrodite|hermaphroditic]] males who commonly practiced [[homosexuality]].<ref name="Like mother, like cubs">{{cite web | title= Like mother, like cubs: Hyena alpha moms jumpstart cubs with hormonal jolt | publisher=[[Michigan State University]] | year=2006 | url=http://hyenas.zoology.msu.edu/crocuta/hyena_research_overview.html | access-date = 2007-09-11 |archive-url = https://web.archive.org/web/20070629155747/http://hyenas.zoology.msu.edu/crocuta/hyena_research_overview.html <!-- Bot retrieved archive --> |archive-date = 2007-06-29}}</ref>{{failed verification|date=May 2015}} 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,"<ref name="Research: Spotted Hyena - Introduction">{{cite web | last =Holekamp | first =Kay E. | name-list-style = vanc | title=Research: Spotted Hyena - Introduction and Overview | publisher=[[Michigan State University]], Department of [[Zoology]] |year=2003 |url=http://hyenas.zoology.msu.edu/crocuta/hyena_research_overview.html |access-date = 2007-09-11 |archive-url = https://web.archive.org/web/20070629155747/http://hyenas.zoology.msu.edu/crocuta/hyena_research_overview.html <!-- Bot retrieved archive --> |archive-date = 2007-06-29}}</ref> and they have "been masculinized without being defeminized".<ref name="Like mother, like cubs"/>{{failed verification|date=May 2015}}

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.<ref name="Like mother, like cubs"/><ref name="Top Dogs: Steroid hormones">{{cite web | last =Selim | first =Jocelyn | name-list-style = vanc | title=Top Dogs: Steroid hormones give hyenas a head start. | work=[[Discover Magazine]] | date=2006-04-28 | url=http://discovermagazine.com/2006/apr/hormone-hyenas | access-date = 2007-09-11}}</ref> Research has shown that "elevated levels of [[testosterone]] [[Uterus|in utero]]"<ref name="Sexual Dimorphism of Perineal"/> contribute to extra aggressiveness; both males and females mount members of both the same and opposite sex,<ref name="Sexual Dimorphism of Perineal">{{cite journal | vauthors = Forger NG, Frank LG, Breedlove SM, Glickman SE | title = Sexual dimorphism of perineal muscles and motoneurons in spotted hyenas | journal = The Journal of Comparative Neurology | volume = 375 | issue = 2 | pages = 333–43 | date = November 1996 | pmid = 8915834 | doi = 10.1002/(SICI)1096-9861(19961111)375:2<333::AID-CNE11>3.0.CO;2-W | doi-access = free }}</ref><ref name="Wilson, Sexing the Hyena">{{cite journal |author=Anna Wilson |title=Sexing the Hyena: Intraspecies Readings of the Female Phallus |journal=Signs |volume=28 |issue=3 |pages=755–790 |date=Spring 2003 | publisher=[[University of Chicago Press]] |doi=10.1086/345320|s2cid=146640802 }}</ref> who in turn are possibly acting more [[submissive]] because of lower levels of testosterone in utero.<ref name="Research: Spotted Hyena - Introduction" />

===Reptiles===
====Lizards====
{{further|Animal sexual behaviour#Lizards}}
Several species of [[Teiidae|whiptail lizard]] (especially in the genus ''[[Aspidoscelis]]'') consist only of females that have the ability to reproduce through [[parthenogenesis]].<ref>{{cite journal |last1=Cole |first1=Charles J.| last2 = Townsend | first2 = Carol R.| name-list-style = vanc |date=9 May 2005|title=Parthenogenetic lizards as vertebrate systems |journal=Journal of Experimental Zoology|volume=256 |issue=S4|pages=174–176 |doi=10.1002/jez.1402560436|pmid=1974787}}</ref> 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. Some parthenogenetic 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 reproduction|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.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2005/06/050608053415.htm|title=Geckos: It's Not Always About Sex|last=Anon|date=9 June 2005|work=Science Daily|publisher=Lewis & Clark College |access-date=2008-12-14}}</ref>

Some species of sexually reproducing [[gecko]]s have also been found to display homosexual behavior, e.g. the day geckos [[Gold dust day gecko|''Phelsuma laticauda'']] and [[Blue-tailed day gecko|''Phelsuma cepediana'']].<ref>{{Cite journal|vauthors=Budzinski RM|date=1997|title=Homosexuelles Verhalten bei Geckos der Gattung Phelsuma.|url=http://www.sauria.de|journal=Sauria|volume=19|issue=3|pages=33–34|access-date=2020-07-25|archive-url=https://web.archive.org/web/20200607100011/https://sauria.de/|archive-date=2020-06-07|url-status=dead}}</ref>

====Tortoises====
[[Jonathan (tortoise)|Jonathan]], the world's oldest [[tortoise]] (an [[Aldabra giant tortoise]]), had been mating with another tortoise named Frederica since 1991. In 2017, it was discovered that Frederica was actually probably male all along, and was renamed Frederic.<ref name="thetimessthelenatortoise">{{cite news|last1=Binyon|first1=Michael| name-list-style = vanc |title=St Helena tortoise has a gay old time|url=https://www.thetimes.com/article/st-helena-tortoise-has-a-gay-old-time-6lcvnff7p|access-date=October 20, 2017|work=[[The Times]]|date=October 19, 2017}}</ref>

===Insects and arachnids===
There is evidence of same-sex sexual behavior in at least 110 species of insects and arachnids.<ref name="scharf2013">{{cite journal | vauthors = Scharf I, Martin OY | year = 2013 | title = Same-sex sexual behavior in insects and arachnids: prevalence, causes, and consequences | journal = Behavioral Ecology and Sociobiology | volume = 67 | issue = 11 | pages = 1719–1730 | doi=10.1007/s00265-013-1610-x| bibcode = 2013BEcoS..67.1719S | s2cid = 10899013 }}</ref> Scharf et al. says: "Males are more frequently involved in same-sex sexual (SSS)  behavior in the laboratory than in the field, and isolation, high density, and exposure to female pheromones increase its prevalence. SSS behavior is often shorter than the equivalent heterosexual behavior. Most cases can be explained via mistaken identification by the active (courting/mounting) male. Passive males often resist courting/mating attempts".<ref name="scharf2013" />

Scharf et al. continues: "SSS behavior has been reported in most insect orders, and Bagemihl (1999) provides a list of ~100 species of insects demonstrating such behavior. Yet, this list lacks detailed descriptions, and a more comprehensive summary of its prevalence in invertebrates, as well as ethology, causes, implications, and evolution of this behavior, remains lacking".<ref name="scharf2013" />

====Dragonflies====
[[File:Head of dragonfly 2.jpg|thumb|The head of darner dragonfly (''Basiaeschna janata'')]]

Male homosexuality has been inferred in several species of [[dragonflies]] (the [[Order (biology)|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<ref>{{cite journal | vauthors = Dunkle SW | year = 1991 | title = Head damage from mating attempts in dragonflies (Odonata:Anisoptera) | url =https://www.biodiversitylibrary.org/page/16252041 | journal = Entomological News | volume = 102 | issue =1 | pages = 37–41 }}</ref><ref>{{cite journal | vauthors = Utzeri C, Belfiore C | year = 1990 | title = Anomalous tandems in Odonata | journal = Fragmenta Entomologica | volume = 22 | issue = 2| pages = 271–288 }}</ref> 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)|fruitless]] gene court and attempt to mate exclusively with other males.<ref name="Gailey1">{{cite journal | vauthors = Gailey DA, Hall JC | title = Behavior and cytogenetics of fruitless in Drosophila melanogaster: different courtship defects caused by separate, closely linked lesions | journal = Genetics | volume = 121 | issue = 4 | pages = 773–85 | date = April 1989 | doi = 10.1093/genetics/121.4.773 | pmid = 2542123 | pmc = 1203660 | url = http://www.genetics.org/cgi/content/abstract/121/4/773 }}</ref> The genetic basis of animal homosexuality has been studied in the fly ''D. melanogaster''.<ref name="yamamoto3">{{cite journal | vauthors = Yamamoto D, Nakano Y | title = Sexual behavior mutants revisited: molecular and cellular basis of Drosophila mating | journal = Cellular and Molecular Life Sciences | volume = 56 | issue = 7–8 | pages = 634–46 | date = November 1999 | pmid = 11212311 | doi = 10.1007/s000180050458 | s2cid = 22055533 | pmc = 11146922 }}</ref> Here, multiple genes have been identified that can cause homosexual courtship and mating.<ref name=Yamamoto>{{cite journal | vauthors = Yamamoto D, Ito H, Fujitani K | title = Genetic dissection of sexual orientation: behavioral, cellular, and molecular approaches in Drosophila melanogaster | journal = Neuroscience Research | volume = 26 | issue = 2 | pages = 95–107 | date = October 1996 | pmid = 8953572 | doi = 10.1016/S0168-0102(96)01087-5 | s2cid = 36582275 }}</ref> These genes are thought to control behavior through [[pheromone]]s as well as altering the structure of the animal's brains.<ref name="ferveur">{{cite journal | vauthors = Ferveur JF, Savarit F, O'Kane CJ, Sureau G, Greenspan RJ, Jallon JM | title = Genetic feminization of pheromones and its behavioral consequences in Drosophila males | journal = Science | volume = 276 | issue = 5318 | pages = 1555–8 | date = June 1997 | pmid = 9171057 | doi = 10.1126/science.276.5318.1555 }}</ref><ref name="yamamoto2">{{cite journal | vauthors = Yamamoto D, Fujitani K, Usui K, Ito H, Nakano Y | title = From behavior to development: genes for sexual behavior define the neuronal sexual switch in Drosophila | journal = Mechanisms of Development | volume = 73 | issue = 2 | pages = 135–46 | date = May 1998 | pmid = 9622612 | doi = 10.1016/S0925-4773(98)00042-2 | s2cid = 14531382 }}</ref> These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.<ref name="zhang">{{cite journal | vauthors = Zhang SD, Odenwald WF | title = Misexpression of the white (w) gene triggers male-male courtship in Drosophila | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 92 | issue = 12 | pages = 5525–9 | date = June 1995 | pmid = 7777542 | pmc = 41728 | doi = 10.1073/pnas.92.12.5525 | bibcode = 1995PNAS...92.5525Z | doi-access = free }}</ref><ref name="svetec">{{cite journal | vauthors = Svetec N, Ferveur JF | title = Social experience and pheromonal perception can change male-male interactions in Drosophila melanogaster | journal = The Journal of Experimental Biology | volume = 208 | issue = Pt 5 | pages = 891–8 | date = March 2005 | pmid = 15755887 | doi = 10.1242/jeb.01454 | doi-access = free | bibcode = 2005JExpB.208..891S }}</ref>

====Bed bugs====
{{Main|Bed bug#Reproduction}}
Male [[Cimex lectularius|bed bug]]s (''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 [[pheromone]]s to reduce such homosexual mating.

==See also==
{{Portal|Animals|LGBTQ}}
{{columns-list|colwidth=30em|
*{{annotated link|Against Nature?}}
*[[Biology and sexual orientation]]
*{{annotated link|Freemartin}}
*{{section link|Hermaphrodite|Animals}}
*[[Homosexuality and psychology]]
*[[List of animals displaying homosexual behavior]]
*[[Non-reproductive sexual behavior in animals]]
*{{annotated link|Norms of reaction}}
*{{annotated link|Queer Planet}}
*{{annotated link|Sequential hermaphroditism}}
*[[Timeline of sexual orientation and medicine]]
*{{annotated link|Xq28}}
}}

==References==
{{Reflist|32em}}

==Further reading==
* {{cite book |title=Evolution's Rainbow: Diversity, Gender, and Sexuality in Nature and People |year=2013 |first=Joan |last=Roughgarden |publisher=University of California Press |isbn=978-0520280458}}

==External links==
{{Commons}}
*[https://web.archive.org/web/20040726025454/http://news.nationalgeographic.com/news/2004/07/0722_040722_gayanimal.html National Geographic]
*[https://web.archive.org/web/20080228025016/http://af.czu.cz/~bartos/publications/pdf/Bartos_Holeckova_2006.pdf Homosexual Behaviour in Animals An Evolutionary Perspective]
*[https://www.scientificamerican.com/article/bisexual-species/ Driscoll, E. V. (2008), Bisexual Species, Scientific American Mind, 19(3), pp. 68–73.]
*[https://medicalxpress.com/news/2017-10-nature-nurture-innate-social-behaviors.html Nature or nurture? Innate social behaviors in the mouse brain]

{{Animal sexual behavior}}
{{LGBTQ}}

{{DEFAULTSORT:Homosexual Behavior In Animals}}
[[Category:Sexual orientation and science]]
[[Category:Ethology]]
[[Category:Animal sexuality]]
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