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I heard that some monkeys can be homosexual? a friend of mine was saying that he saw a male monkey having sex with another male monkey in the zoo, I honestly didn't believe him, I think he just got confused or he made it up.

Could this be true? can monkeys become homosexuals? does homosexuality (other than human) exist in the animal kingdom?

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    $\begingroup$ its likely they saw right. many animals have been shown to exhibit homosexual behavior. en.wikipedia.org/wiki/Homosexual_behavior_in_animals $\endgroup$
    – shigeta
    Apr 25, 2013 at 4:01
  • $\begingroup$ I have seen dogs doing so.I do believe that sexuality [ homo or bi or hetero ] is one's right. $\endgroup$
    – S.Sunil
    Aug 26, 2013 at 6:20
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    $\begingroup$ I would just like to point out that there is a difference, most obvious in humans, between homosexual behavior and a homosexual orientation. The former is often very dependent on cultural norms and context: for example, holding hands or kissing on the cheek may or may not be considered homosexual behavior or indicative of sexual orientation depending on what country you're in. $\endgroup$
    – Amory
    Aug 26, 2013 at 14:14

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Cases of homosexuality have been described in numerous species, including primates, and the wikipedia link posted by @shigeta lists and describes some of them.

It could be difficult to untangle the difference between an animal not being able to identify sexes very well, or, deliberately mating to anyone and those actively making a choice to engage in intercourse with a same sex individual.

I there are some species where sexes are very difficult to tell apart so they may just go around having sex with anything that looks like the right species because the mating is relatively low cost so doesn't matter if the partner was male or female. This is one such example in sea snails:

"In a laboratory mate-choice experiment, male N. radiata preferred to mate with females, indicating precopulatory sex identification. They copulated with males, however, at the frequency of 37%, perhaps because of sex misidentification."

Other likely candidates are those with little sexual dimorphism and those with sneaker males that mimic females.

I have also heard theoretical discussions (oh the joys of lunchtime in an evolutionary biology department!) which suggest it may be a way of individuals (particularly males) practising or keeping up fitness relating to sex or stimulating better sperm production etc.

Once you decide what constitutes "a homosexual animal" then it is perhaps easier to examine. I would say a homosexual animal is one that mates exclusively (or highly preferentially) with same sex individuals when given the choice.

Models of genetic and epigenetic inheritance have been used to suggest just how homosexuality could evolve and you can read more about it in this Biology.Stackexchange post.

So it seems that monkeys, and other animals, have been documented to exhibit homosexuality, reasons for them to do so could exist, and it could be a heritable trait which persists despite seemingly strong negative selection.

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    $\begingroup$ Just as a comment, I'd be surprised if animals got confused and couldn't tell the gender of the individuals they are trying to mate with. If a vet can tell the difference, which is the case in most animals, the animals themselves shouldn't have any problem. $\endgroup$
    – shigeta
    Apr 25, 2013 at 19:14
  • $\begingroup$ "Everything in the world is about sex except sex. Sex is about power." ~Oscar Wilde $\endgroup$
    – Kevin
    Apr 25, 2013 at 19:49
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    $\begingroup$ @shigeta here is one such example - the males mated with the wrong sex 37% of the time link.springer.com/article/10.1007%2Fs10164-005-0159-1 "In a laboratory mate-choice experiment, male N. radiata preferred to mate with females, indicating precopulatory sex identification. They copulated with males, however, at the frequency of 37%, perhaps because of sex misidentification." .. other likely candidates are those with little sexual dimorphism and those with sneaker males that mimic females. $\endgroup$
    – rg255
    Apr 26, 2013 at 10:17
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    $\begingroup$ @rg255 I wasn't disputing you could find such examples. I suppose there may be animals that can't tell the difference between genders or individuals, but on the average I would doubt it. Even microoganisms can be specific; this would be an evolutionary 'feature' not a 'bug'. So its possible if there were something it does for the organism because there is a cost - disease spreads easily and there is some opportunity cost to mating. Competition would be on sperm competition rather than mate choice. They ought to be induced to mate with inanimate objects with specific visual cues. $\endgroup$
    – shigeta
    Apr 26, 2013 at 13:32
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    $\begingroup$ bisexuality for instance is very much a feature for bonobos - it creates their social structure... $\endgroup$
    – shigeta
    Apr 26, 2013 at 13:33
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Here's a Ted talk about a case of homosexual necrophilia in mallard duck. The speaker lists a few other examples of weird animal mating (frog with a fish). Apparently animals may have non-traditional preferences as well as humans.

The talk does not deal with whether this is a pathology or not, just lists a few weird examples.

Additionally, there's this section on sexual dimorphism of hypothalamus, which does mention pathology of hypothalamus. The statement below lacks a proper citation at the time of this writing, so it may be incorrect:

If a female rat is injected once with testosterone in the first few days of postnatal life (during the "critical period" of sex-steroid influence), the hypothalamus is irreversibly masculinized; the adult rat will be incapable of generating an LH surge in response to estrogen (a characteristic of females), but will be capable of exhibiting male sexual behaviors (mounting a sexually receptive female

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