Find it here: Bisexual Species: Unorthodox Sex in the Animal Kingdom
You might have heard about two male penguins or flamingos breeding in captivity and maybe even raising a young of another couple most successfully.
[The penguins] entwined necks, called to each other and mated. They then built a nest together to prepare for an egg. But no egg was forthcoming: Roy and Silo were both male.
[...]Gramzay found an egg from another pair of penguins that was having difficulty hatching it and slipped it into Roy and Silo's nest. Roy and Silo took turns warming the egg with their blubbery underbellies until, after 34 days, a female chick pecked her way into the world. Roy and Silo kept the gray, fuzzy chick warm and regurgitated food into her tiny black beak.
[...]Roy and Silo belong to one of as many as 1,500 species of wild and captive animals that have been observed engaging in homosexual activity. Researchers have seen such same-sex goings-on in both male and female, old and young, and social and solitary creatures and on branches of the evolutionary tree ranging from insects to mammals.
I'm not emphasizing "homosexual behavior" for nothing. Succinctly put:
"Animals don't do sexual identity. They just do sex.
(Eric Anderson, sociologist)
Some knwoledgable people already know this, but homosexual behavior in animals doesn't only appear in captivity where stress might be high or individuals of the opposite gender scarce (though there it is more common). Many-a-times it's social behavior, helping with social tension, protecting the young or keeping fertile when there's no matching partner - or, because sex just feels too nice (the article has some nice elaborations on these reasons).
"[In humans] the categories of gay and straight are socially constructed," Anderson says.
... and I would ass 'artificially'. (I generally dislike having to identify politically, religiously, sexually, etc... as belong to one of a few socially constructed group for which we happen to have made up a word yet - though in communication, this does have some merit.)
Ah yes, that 'unnatural' stuff again...
Back [in the first half of the 1900s], scientists generally considered homosexual acts among animals to be abnormal. In some cases, they "treated" the animals by, say, castrating them or giving them lobotomies.
... until they figured that Japanese macaques and baboons (monkeys), as wells as bonobos (apes) like homosexual acts, as well (bonobos engage do same-sex half the time they have sex at all, including hand- and blowjobs!) - and not because they're sexually deprived, but for social reasons:
[Same sex behavior] in 20 Japanese macaques and two baboons occurred largely as a way of making peace with would-be foes. [...] Hamilton observed [...]: "homosexual behavior is of relatively frequent occurrence in the female when she is threatened by another female, but it is rarely manifested in response to sexual hunger." And in males, he penned, "homosexual alliances between mature and immature males may possess a defensive value for immature males, since they insure the assistance of an adult defender in the event of an attack."
Moreover, they made an interesting observation
"The more homosexuality, the more peaceful the species," asserts Petter Böckman, an academic adviser at the University of Oslo's Museum of Natural History in Norway. "Bonobos are peaceful."
They also add that two male swans beat any het couple at making nests, some birds form polygameous trios because of a lack of males, occassionally while sharing a nest, which increases their offsprings' chances of survival, and switching of a certain fruit fly gene makes them gender-confused. That is, they show that homosexual behavior has benefits and that it's probably genetically established.
But of course there are other reasons, too:
Many animals do it simply "because they want to," Böckman says. "People view animals as robots who behave as their genes say, but animals have feelings, and they react to those feelings."
Here's one for the LOL
Stress and the greater availability of same-sex partners may similarly contribute to the practice of homosexual acts among self-described heterosexual humans in environments such as the military, jails and sports teams.
(and they offer proof of this, too)
As for humans...
[S]ome experts argue that humans, like some other animals, are naturally bisexual. "We should be calling humans bisexual because this idea of exclusive homosexuality is not accurate of people," Roughgarden says. "Homosexuality is mixed in with heterosexuality across cultures and history."
:) All clear?