Why Some People Are Gay: Notes (and Clues) from the Animal Kingdom

We have known for at least a decade that hundreds of animal species — including birds, reptiles, mollusks and, of course, humans — engage in same-gender sexual acts. But no one is quite sure why. After all, same-sex couplings don’t usually result in offspring. (I say usually because when male marine snails pair with other males, one partner conveniently changes sex, allowing for reproduction.) Evolutionarily speaking, homosexuality should have disappeared long ago.

A yearlong study just completed at the University of California at Riverside offers several fascinating competing theories about why same-gender sexual behavior has endured. And although it’s gay-pride month — and the 40th anniversary of the Stonewall riots that sparked the gay-rights movement — not all the theories will give same-gender-loving humans a reason to celebrate. (See the top 10 animal stories of 2007.)

One particularly charged finding is that in most species besides humans, same-gender pairings rarely lead to lifelong relationships. In other words, when one attractive bonobo male eyes another in a lovely patch of Congo swamp forest, they occasionally kiss and then move on to other oral pleasures, but they don’t bother anyone afterward about trying to legalize their right to an open-banana-bar ceremony. In fact, they are likely to move on to girl bonobos: most animals that engage in same-gender sex acts do so only when an opposite-sex partner is unavailable.

And yet the study’s authors, Nathan Bailey and Marlene Zuk of UC Riverside’s biology department, report some exceptions, like the laysan albatross. Last year, researchers studying a Hawaiian colony of albatrosses found that nearly a third of all the couples involved two females who courted and then shared parenting responsibilities. (Albatrosses don’t have U-Hauls, so no lesbian jokes, please.) Male chinstrap penguins also form long-term relationships, at least in captivity. And some male bighorn sheep will mount females only after the females adopt male-like behaviors.