These ideas generally aim to explain only particular behaviors in a particular species. So far, the only real conclusion this relatively small body of literature seems to point to, collectively, is a kind of deflating, meta-conclusion: a single explanation of homosexual behavior in animals may not be possible, because thinking of “homosexual behavior in animals” as a single scientific subject might not make much sense. “Biologists want to build these unified theories to explain everything they see,” Vasey told me. So do journalists, he added — all people, really. “But none of this lends itself to a linear story. My take on it is that homosexual behavior is not a uniform phenomenon. Having one unifying body of theory that explains why it’s happening in all these different species might be a chimera.”My feelings about this reflect what I have come to believe about homosexuality in human history: lots of people in lots of cultures have had plenty of gay sex, for various reasons, but being "gay" in our sense seems to me to be a modern social creation. We tend to think that most people are born gay or straight and can't do much about it. In ancient Greece they tended to see sexuality as situational. Every Greek man was expected to marry a woman and have children, but in some cities they were also expected to participate in social rituals that required sex with men. The Greeks seem to have recognized that some people had strong preferences (Sappho, for example), but not so strong that they wouldn't carry out their social duties in the way of both reproduction and same-sex bonding. According to biologist Joan Roughgarden (what a name!), this seems to be the case with many species of animals:
Male big horn sheep live in what are often called "homosexual societies." They bond through genital licking and anal intercourse, which often ends in ejaculation. If a male sheep chooses to not have gay sex, it becomes a social outcast. Ironically, scientists call such straight-laced males "effeminate." Giraffes have all-male orgies. So do bottlenose dolphins, killer whales, gray whales, and West Indian manatees. Japanese macaques, on the other hand, are ardent lesbians; the females enthusiastically mount each other.None of these animals is necessarily gay in the sense of preferring same-sex partners to opposite-sex partners. They simply spend all or part of their lives in single-sex societies, within which they relate to each other partly through sex. (Like, say, human sailors who spent years at sea on small wooden ships.) All would probably breed if given the chance.
It is important to note that none of this means people and animals don't have preferences. They may. But "gay" preferences do not keep either people or animals from breeding, which, it seems to me, renders moot the question of what the "evolutionary purpose" of homosexuality might be. "Straight" preferences do not keep people or animals from engaging in homosexual sex when it seems like a good idea.