Homosexual behavior seems pointedly un-Darwinian. An animal that doesn't pass along genes by mating with the opposite sex at every, well, conceivable opportunity, seems to be at an evolutionary disadvantage. So what’s in it for the 450-plus species that go for same-sex sex?

Two evolutionary biologists from University of California, Riverside, set out to answer that question in a paper published today in Trends in Ecology and Evolution.

"It's been observed a lot," says Nathan Bailey, a post-doctoral researcher at U.C. Riverside and lead study author, of same-sex sexual behavior in animals. "But it took people a long time to put it in an evolutionary context." 

After studying dozens of published articles on the topic, Bailey and his colleague Marlene Zuk concluded that, in addition to being an adaptational strategy, "these behaviors can be a force," Bailey said. "They create a context in which selection can occur [differently] within a population." 

In the Laysan albatross, for example, previous research has shown that a third of all bonded pairs in a Hawaii colony are two females. This behavior helps the birds, whose colony has far more females than males, by allowing them to share parenting responsibilities. It also gives more stability to the offspring of males, already bonded to a female, who mate opportunistically with females in a same-sex couple. Such a dynamic, then may force gradual changes in behavior and even physical appearance of the birds, the authors note.

Other researchers, however, aren't convinced that everything must fit into the evolutionary, adaptive rubric. "You have to think outside of that," says Paul Vasey, who studies Japanese macaque monkeys as an associate professor at the University of Lethbridge in Canada.

His work has shown that in female macaque monkeys at least, same-sex sexual behavior doesn't seem to have any adaptational advantage, which "doesn't jibe with how people want to think about it," he says. But, he concludes, "You can't impose your perspective on the species you're studying. Attempt to understand the world on its own terms." 

What does all this mean for discussions about human homosexuality? To be sure, says Bailey, "there can be crosstalk" between the disciplines of human and animal study, and both arenas promise to be fertile ground for further research.