Editing Animal sexual behaviour (section)

===Homosexual behaviour===
{{Main|Homosexuality in animals}}
[[Image:Males Anas platyrhynchos 2 .jpg|thumb|Male [[Mallard]]s, ''Anas platyrhynchos''.]]
The presence of [[same-sex sexual behaviour]] was not scientifically observed on a large scale until recent times. Homosexual behaviour does occur in the [[animal]] kingdom outside humans, especially in social species, particularly in marine birds and mammals, monkeys, and the [[Hominidae|great apes]]. Homosexual behaviour has been observed among 1,500 species, and in 500 of those it is well documented.<ref>{{cite web |url=http://news.bbc.co.uk/2/hi/europe/6066606.stm |title=Oslo gay animal show draws crowds |accessdate=2006-10-19 |date=2006-10-19|work=BBC News}}</ref> 

{{quote|To turn the approach on its head: No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue.|<ref name="news-medical" />}}

[[Georgetown University]] professor [[Janet Mann]] has specifically theorised that homosexual behaviour, at least in [[dolphin]]s, is an evolutionary advantage that minimizes intraspecies aggression, especially among males.
*Male [[penguin]] couples have been documented to mate for life, build nests together, and to use a stone as a [[surrogate]] egg in nesting and brooding. In 2004, the [[Central Park Zoo]] in the [[United States]] replaced one male couple's stone with a fertilized egg, which the couple then raised as their own offspring.<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"] by Dinitia Smith, ''San Francisco Chronicle'', February 7, 2004</ref> [[Germany|German]] and [[Japan]]ese [[zoo]]s have also reported homosexual behaviour among their penguins. This phenomenon has also been reported at Kelly Tarlton's Aquarium in [[Auckland]], [[New Zealand]].  "Humans have created the myth that sexuality can be justified only by reproduction, which by definition limits it to hetero sex," says Michael Bronski, author of The Pleasure Principle: Culture, Backlash, and the Struggle for Gay Freedom. "But here is an animal society that uses homosexuality to improve its social life."
*Mounting of one female by another is common among [[cattle]]. (''See also, [[Freemartin]]. Freemartins occur because of clearly causal hormonal factors at work during gestation''.)
*[[Bonobo]]s in zoos. After studying the primates for his book ''Bonobo: The Forgotten Ape'', primatologist [[Frans de Waal]], a professor of psychology at Emory University in Atlanta, says that such expressions of [[intimacy]] are consistent with the homosexual behaviour of what he terms "the erotic champions of the world." "Same-sex, opposite-sex — bonobos just [[love]] sex play," de Waal said in an interview. "They have so much sex, it gets boring."
*Homosexual behaviour in male sheep (found in 6-10% of rams) is associated with variations in cerebral mass distribution and chemical activity. A study reported in ''[[Endocrinology (journal)|Endocrinology]]'' concluded that biological and physiological factors are in effect.<ref>[http://endo.endojournals.org/cgi/content/abstract/145/2/478 "The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference"] by Charles E. Roselli, et al., The Endocrine Society, October 2, 2003</ref> These findings are similar to human findings reported by [[Simon LeVay]].

:<blockquote>Approximately eight percent of [male] rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes...</blockquote>
*Male bighorn sheep are divisible into two kinds: the typical males among whom homosexual behaviour, including intercourse, is common and "effeminate sheep", or "behavioural transvestites", which are not known to engage in homosexual behaviour.<ref>[http://www.washingtonpost.com/ac2/wp-dyn/A22107-2004Jul1?language=printer In BriefRams Will Be Rams]</ref><ref>[http://www.stanfordalumni.org/news/magazine/2004/mayjun/features/roughgarden.html On the Originality of Species]</ref>