Correction Appended

Roy and Silo, two chinstrap penguins at the Central Park Zoo in Manhattan, are completely devoted to each other. For nearly six years now, they have been inseparable. They exhibit what in penguin parlance is called ''ecstatic behavior'': that is, they entwine their necks, they vocalize to each other, they have sex. Silo and Roy are, to anthropomorphize a bit, gay penguins. When offered female companionship, they have adamantly refused it. And the females aren't interested in them, either.

At one time, the two seemed so desperate to incubate an egg together that they put a rock in their nest and sat on it, keeping it warm in the folds of their abdomens, said their chief keeper, Rob Gramzay. Finally, he gave them a fertile egg that needed care to hatch. Things went perfectly. Roy and Silo sat on it for the typical 34 days until a chick, Tango, was born. For the next two and a half months they raised Tango, keeping her warm and feeding her food from their beaks until she could go out into the world on her own. Mr. Gramzay is full of praise for them.

''They did a great job,'' he said. He was standing inside the glassed-in penguin exhibit, where Roy and Silo had just finished lunch. Penguins usually like a swim after they eat, and Silo was in the water. Roy had finished his dip and was up on the beach.

Roy and Silo are hardly unusual. Milou and Squawk, two young males, are also beginning to exhibit courtship behavior, hanging out with each other, billing and bowing. Before them, the Central Park Zoo had Georgey and Mickey, two female Gentoo penguins who tried to incubate eggs together. And Wendell and Cass, a devoted male African penguin pair, live at the New York Aquarium in Coney Island. Indeed, scientists have found homosexual behavior throughout the animal world.

This growing body of science has been increasingly drawn into charged debates about homosexuality in American society, on subjects from gay marriage to sodomy laws, despite reluctance from experts in the field to extrapolate from animals to humans. Gay groups argue that if homosexual behavior occurs in animals, it is natural, and therefore the rights of homosexuals should be protected. On the other hand, some conservative religious groups have condemned the same practices in the past, calling them ''animalistic.''

But if homosexuality occurs among animals, does that necessarily mean that it is natural for humans, too? And that raises a familiar question: if homosexuality is not a choice, but a result of natural forces that cannot be controlled, can it be immoral?

The open discussion of homosexual behavior in animals is relatively new. ''There has been a certain cultural shyness about admitting it,'' said Frans de Waal, whose 1997 book, ''Bonobo: The Forgotten Ape'' (University of California Press), unleashed a torrent of discussion about animal sexuality. Bonobos, apes closely related to humans, are wildly energetic sexually. Studies show that whether observed in the wild or in captivity, nearly all are bisexual, and nearly half their sexual interactions are with the same sex. Female bonobos have been observed to engage in homosexual activity almost hourly.

Before his own book, ''American scientists who investigated bonobos never discussed sex at all,'' said Mr. de Waal, director of the Living Links Center of the Yerkes Primate Center at Emory University in Atlanta. ''Or they sometimes would show two females having sex together, and would say, 'The females are very affectionate.' ''

Then in 1999, Bruce Bagemihl published ''Biological Exuberance: Animal Homosexuality and Natural Diversity'' (St. Martin's Press), one of the first books of its kind to provide an overview of scholarly studies of same-sex behavior in animals. Mr. Bagemihl said homosexual behavior had been documented in some 450 species. (Homosexuality, he says, refers to any of these behaviors between members of the same sex: long-term bonding, sexual contact, courtship displays or the rearing of young.) Last summer the book was cited by the American Psychiatric Association and other groups in a ''friend of the court'' brief submitted to the Supreme Court in Lawrence v. Texas, a case challenging a Texas anti-sodomy law. The court struck down the law.

''Sexual Exuberance'' was also cited in 2000 by gay rights groups opposed to Ballot Measure 9, a proposed Oregon statute prohibiting teaching about homosexuality or bisexuality in public schools. The measure lost.

In his book Mr. Bagemihl describes homosexual activity in a broad spectrum of animals. He asserts that while same-sex behavior is sometimes found in captivity, it is actually seen more frequently in studies of animals in the wild.

Among birds, for instance, studies show that 10 to 15 percent of female western gulls in some populations in the wild are homosexual. Females perform courtship rituals, like tossing their heads at each other or offering small gifts of food to each other, and they establish nests together. Occasionally they mate with males and produce fertile eggs but then return to their original same-sex partners. Their bonds, too, may persist for years.