Birds Do It, Bonobos Do It

PERTH, Australia — Anyone who doubts the relevance of homosexuality in animals to the current debate about gay marriage should consider Roy and Silo. The two male chinstrap penguins live together at the Central Park Zoo in Manhattan, eat raw fish and are rearing a chick of their own -- kind of like those sushi-loving San Franciscans who've been flocking to the courthouse in droves lately. Roy and Silo, the New York Times reported recently, have been inseparable for six years, having sex with each other and showing no interest in female penguins (who also showed little in them). After the pair tried to incubate a rock, a sympathetic keeper gave the penguins an orphan egg to hatch, which they sat on for more than a month. Then they fed the chick until it was able to fend for itself.

Animal homosexuality comes up in part because arguments against gay marriage often invoke phrases like "natural order," "natural law" or "crime against nature," which make it, well, natural to wonder about whether birds and bees do that, too. It turns out that sexual behavior directed at members of the same sex, up to and including copulation, is widespread among animals. According to Bruce Bagemihl, author of "Biological Exuberance: Animal Homosexuality and Natural Diversity," some form of it occurs in more than 450 species, from plovers to bighorn sheep to, yes, penguins. We don't know how common it is in the wild, because if the sexes look alike, as they do in penguins, it can be hard to tell whether the members of a pair are male or female.

Scientists, who are as subject to social biases as anyone else, have often ignored or explained away homosexual behavior in animals, although the tide now seems to be turning. In primates like the sexually adventurous bonobos, smaller cousins of chimpanzees and close relatives of humans, same-sex behavior is now understood to be part of a complex social life in which sexual gestures are often used to defuse tense situations. So the idea that homosexuality is unnatural or deviant simply doesn't hold up when we compare ourselves to other animals.

That said, analogues to gay marriage in the animal kingdom can go only so far. If homosexuality evolved in animals, it has to have a genetic component. Obviously, Roy and Silo cannot impart the biological tendency to pair with another male to their offspring -- they required the zoo equivalent of an adoption agency to obtain their chick. And exclusive same-sex pairing cannot persist in large numbers in a wild population since the genes associated with doing so by definition are not perpetuated.