Editing Animal sexual behaviour (section)

==Notes on specific species ==
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===Bonobos===
The [[Bonobo]], which has a peaceful, egalitarian and [[matriarchal]] society, is a fully bisexual species — both males and females engage in sexual behaviour with the same and the opposite sex, with females being particularly noted for engaging in sexual behaviour with each other and at up to 75% of sexual activity being bisexual. Bonobos often use sexual activity to prevent violence and conflict.<ref>National Geographic: Homosexual Activity Among Animals Stirs Debate [http://news.nationalgeographic.com/news/2004/07/0722_040722_gayanimal_2.html]; Retrieved Nov 6 2007</ref>

===Birds===

Some black swans of Australia form sexually active male-male mated pairs and steal nests, or form temporary [[threesomes]] with females to obtain eggs, driving away the female after she lays the eggs. More of their cygnets survive to adulthood than those of different-sex pairs possibly due to their superior ability to defend large portions of land.

In early February [[2004]] the ''[[New York Times]]'' [http://query.nytimes.com/gst/abstract.html?res=FA0C1EF83A5F0C748CDDAB0894DC404482 reported] that a male pair of [[chinstrap penguin]]s named [[Roy and Silo]] in the [[Central Park Zoo]] in [[New York City]] were partnered and had successfully hatched a female chick from an egg. Other penguins in New York have also been [http://www.jrn.columbia.edu/studentwork/cns/2002-06-10/591.asp reported] to be forming same-sex pairs.

Zoos in [[Japan]] and [[Germany]] have also [http://www.365gay.com/newscon05/02/021105penguins.htm documented] male penguin couples.  The couples have been shown to build nests together and use a stone to replace an egg in the nest.  Researchers at [[Rikkyo University]] in [[Tokyo]], found twenty such pairs at sixteen major aquariums and zoos in Japan.  Bremerhaven Zoo in Germany attempted to break up the male couples by importing female penguins from [[Sweden]] and separating the male couples; they were unsuccessful.  The zoo director stated the relationships were too strong between the couples.  

Recently, a mated pair of [[swan]]s in [[Boston, Massachusetts|Boston]] were found to both be female. They too had attempted to raise eggs together. [http://www.365gay.com/newscon05/08/081505ramSwan.htm]

[http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2004/02/07/MNG3N4RAV41.DTL Studies have shown] that ten to fifteen percent of female western gulls in some populations in the wild prefer other females.

As many as 19% of Mallard pairs in a given population have been observed to consist of male-male homosexuals.<ref>Bagemihl, Bruce (1999): ''Biological Exuberance: Animal Homosexuality and Natural Diversity'': 479-481. St. Martin's Press. <small>ISBN 0312192398</small></ref>

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

Lizards that perform the courtship ritual have greater fecundity than those kept in isolation due to an increase in hormones triggered by the sexual behaviours. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success. 

From an [[evolutionary]] standpoint these females are passing their full genetic code to all of their offspring rather than the 50% of genes that would be passed in sexual reproduction.  Certain species of [[gecko]] also reproduce by parthenogenesis.

===Flatworm===
[[Penis fencing]] is a mating behaviour engaged in by certain species of [[flatworm]], such as Pseudobiceros hancockanus. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes.<ref>{{cite web |url=http://www.pbs.org/kcet/shapeoflife/episodes/hunt_explo2.html |title=Fighting to mate: flatworm penis fencing |author=Leslie Newman |publisher=[[Public Broadcasting Service|PBS]]}}</ref>

Child-bearing, while necessary for the continuation of a species, requires considerable resources from the mother. Thus, from a biological point of view, it is preferable to be the father rather than the mother.

The species "fence" using two-headed dagger-like penises which are pointed, and white in color. The "winner" is the organism that inseminates the other. The sperm is absorbed through pores in the skin, causing fertilization in the "loser".

===Sheep===
An October [[2003]], [http://endo.endojournals.org/cgi/content/abstract/145/2/478 study] by Dr. Charles E. Roselli et al. ([[Oregon Health & Science University]]) states that homosexuality in male sheep (found in eight percent of rams) is associated with a region in the rams' brains which the authors call the "[[ovine Sexually Dimorphic Nucleus]]" (oSDN) which is half the size of the corresponding region in other male sheep.

However, some view this study to be flawed in that the determination of homosexuality within the sheep, (sample population of twenty-seven for the study), was to have animals who were unable to mount female ewes placed in a cage with two stanchioned males and two unstanchioned females (that is, the males could not move or struggle while the females could). Given the aggressive nature of the sheep copulation, the uneven treatment of males and females, many see this as simply evidence that the sheep in question were unable to be aggressive enough to mount females. Some say that the results were situational sexuality, unlike the bonds seen in human homosexuality.

The scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more [[neurons]] than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of [[aromatase]], a substance that converts [[testosterone]] to [[estradiol]], an [[estrogen]] [[hormone]] believed to facilitate typical male sexual behaviours. [[Aromatase]] expression was no different between male-oriented rams and ewes."

"The dense cluster of neurons that comprise the oSDN express [[cytochrome P450]] aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "...naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis."<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 - Roselli et al. 145 (2): 478 - Endocr...<!-- Bot generated title -->]</ref> As noted previously, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. The results of this study have not been confirmed by others.

===Spotted Hyena===
The female [[Spotted Hyena]] has a unique urinary-genital system, closely resembling the penis of the male, called a [[pseudo-penis]]. The family structure is matriarchal and dominance relationships with strong sexual elements are routinely observed between related females. 

They are notable for using visible sexual arousal as a sign of ''submission'' and not ''dominance'', in males as well as females (females have a sizable erectile [[clitoris]]), to the extent that biologist [[Robert Sapolsky]] speculates that in order to facilitate this, their [[sympathetic]] and [[parasympathetic]] nervous systems may be partially reversed in respect to their reproductive organs.<ref>Sapolsky, Why Zebras don't get Ulcers, p.127-129.</ref>

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

Janet Mann, a professor of biology and psychology at Georgetown University, argues<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]</ref> that the common same-sex behaviour among male dolphin calves is about bond formation and benefits the species evolutionarily.  They cite studies that have shown the dolphins later in life are bisexual and the male bonds forged from homosexuality work for protection as well as locating females to reproduce with.

===Seahorses===   
[[Seahorse]]s, long upheld as monogamous and mating for life, are identified as "promiscuous, flighty, and more than a little bit gay" according to research published in [[2007]].<ref name="seahorses">[http://www.timesonline.co.uk/tol/news/uk/science/article1303148.ece Promiscuous and bisexual — the ‘faithful’ seahorse has a secret sex life]</ref>   
    
Scientists at 15 [[aquarium]]s studied 90 seahorses of 3 [[species]]. Of 3168 sexual encounters, 37% were same sex acts.<ref name="seahorses"/> Flirting was common (up to 25 potential partners a day of both genders); only one species (the British Spiny Seahorse) included faithful representatives, and for these 5 of 17 were faithful, 12 were not.<ref name="seahorses"/> Bisexuality was widespread and considered "both a great surprise and a shock", with big bellied seahorses of both genders not showing partner preference.<ref name="seahorses"/> 1986 contacts were male-female, 836 were female-female and 346 were male-male.<ref name="seahorses"/>

===Lions===
Male [[lion]]s often lead their social groups jointly with one or more of their brothers. To ensure loyalty, the male co-leaders will "strengthen the bonds by often having sex with each other.<ref name="news-medical" />

=== Horses ===
Anecdotal evidence {{Fact|date=August 2008}} suggest that some Horses have environment or appearance preferences when selecting [[mates]].

There is also anecdotal evidence <ref>http://www.mini-horse.org/health_care_homo.html</ref> of limited bisexual behaviour in some stallions, although there is (as of 2008) no conclusive scientific confirmation. The anecdotal evidence claims this is most likely to occur in a single isolated group, with no access to mares.