Ali and I were talking over lunch and got on the subject of homosexuality in animals, which is a subject neither of us know anything about. We were wondering the extent of it in the animal kingdom, as well as how and why it happens, so I decided to do some research and write a post about it.
First a definition—homosexual behavior in animals isn't exactly the same as it is in humans. It's a bit more widely encompassing, including sex, bonding, courtship and parenting by same-sex couples. Most homosexual behavior is seen in addition to normal heterosexual activity; few species have members who engage exclusively in homosexual behavior, but a few (domesticated sheep for example) do seem to have a small percentage of every population that are exclusively homosexual.
A fundamental idea behind homosexual behavior in animals is that "not all sexual acts lead to reproduction"—which is to say, homosexual behavior probably exists not because animals think it'll help them reproduce, but because it helps lower stress levels or strengthens social bonds. Some male homosexual pairings occur after heterosexual activity has produced an egg or offspring, when male pairs take over care of the newborn. Young cared for this way often have a higher survival rate as male pairs can more effectively guard territory from potential predators. Bonobos frequently mount one another to diffuse tension arising in response to novel stimuli or conflict, while Amazon dolphins often engage in casual gender-blind sex in groups 3-5 members large.
Basis of homosexuality varies from species to species, but a few trends emerge: abnormal levels of sex hormones (either too high or too low) are correlated to homosexual behavior, as are differing gonad sizes and alterations of neurotransmitter levels. One of the dominant hypotheses about the cause of homosexuality in humans, exposure to differing levels of sex hormones in the womb during gestation, seems not to have been explored with other animals.
Interesting stuff—makes you wonder what leads to exclusively homosexuality in humans rather than the more occasional or functional same-sex encounters that seem more common in the animal kingdom.