Porter, who first hit it big in the 1920s, wouldn't risk parading his homosexuality in public. In his day "the birds and the bees" generally meant only one thing—sex between a male and female.

But, actually, some same-sex birds do do it. So do beetles, sheep, fruit bats, dolphins, and orangutans. Zoologists are discovering that homosexual and bisexual activity is not unknown within the animal kingdom.

Roy and Silo, two male chinstrap penguins at New York's Central Park Zoo have been inseparable for six years now. They display classic pair-bonding behavior—entwining of necks, mutual preening, flipper flapping, and the rest. They also have sex, while ignoring potential female mates.

Wild birds exhibit similar behavior. There are male ostriches that only court their own gender, and pairs of male flamingos that mate, build nests, and even raise foster chicks.

Filmmakers recently went in search of homosexual wild animals as part of a National Geographic Ultimate Explorer documentary about the female's role in the mating game. (The film, Girl Power, will be screened in the U.S this Saturday at 8 p.m. ET, 5 p.m PT on MSNBC TV.)

The team caught female Japanese macaques engaged in intimate acts which, if observed in humans, would be in the X-rated category.

"The homosexual behavior that goes on is completely baffling and intriguing," says National Geographic Ultimate Explorer correspondent, Mireya Mayor. "You would have thought females that want to be mated, especially over their fertile period, would be seeking out males."

Well, perhaps, in a roundabout way, they are seeking males, suggests primatologist Amy Parish.

She argues that female macaques may enhance their social position through homosexual intimacy which in turn influences breeding success. Parish says, "Taking something that's nonreproductive, like mounting another female—if it leads to control of a resource or acquisition of a resource or a good alliance partner, that could directly impact your reproductive success."

Sexual Gratification

On the other hand, they could just be enjoying themselves, suggests Paul Vasey, animal behavior professor at the University of Lethbridge, Alberta, Canada. "They're engaging in the behavior because it's gratifying sexually or it's sexually pleasurable," he says. "They just like it. It doesn't have any sort of adaptive payoff."

Matthew Grober, biology professor at Georgia State University, agrees, saying, "If [sex] wasn't fun, we wouldn't have any kids around. So I think that maybe Japanese macaques have taken the fun aspect of sex and really run with it."

The bonobo, an African ape closely related to humans, has an even bigger sexual appetite. Studies suggest 75 percent of bonobo sex is nonreproductive and that nearly all bonobos are bisexual. Frans de Waal, author of Bonobo: The Forgotten Ape, calls the species a "make love, not war" primate. He believes bonobos use sex to resolve conflicts between individuals.

Other animals appear to go through a homosexual phase before they become fully mature. For instance, male dolphin calves often form temporary sexual partnerships, which scientists believe help to establish lifelong bonds. Such sexual behavior has been documented only relatively recently. Zoologists have been accused of skirting round the subject for fear of stepping into a political minefield.

"There was a lot of hiding of what was going on, I think, because people were maybe afraid that they would get into trouble by talking about it," notes de Waal. Whether it's a good idea or not, it's hard not make comparisons between humans and other animals, especially primates. The fact that homosexuality does, after all, exist in the natural world is bound to be used against people who insist such behavior is unnatural.

In the U.S., in particular, the moral debate over this issue rages on. Many on the religious right regard homosexuality as a sin. And only this month, President Bush vowed to continue his bid to ban gay marriages after the Senate blocked the proposal.

Already, cases of animal homosexuality have been cited in successful court cases brought against states like Texas, where gay sex was, until recently, illegal.

Yet scientists say we should be wary of referring to animals when considering what's acceptable in human society. For instance, infanticide, as practiced by lions and many other animals, isn't something people, gay or straight, generally approve of in humans.

Human Homosexuality

So how far can we go in using animals to help us understand human homosexuality? Robin Dunbar is a professor of evolutionary psychology at the University of Liverpool, England. "The bottom line is that anything that happens in other primates, and particularly other apes, is likely to have strong evolutionary continuity with what happens in humans," he said.

Dunbar says the bonobo's use of homosexual activity for social bonding is a possible example, adding, "One of the main arguments for human homosexual behavior is that it helps bond male groups together, particularly where a group of individuals are dependent on each other, as they might be in hunting or warfare."

For instance, the Spartans, in ancient Greece, encouraged homosexuality among their elite troops. "They had the not unreasonable belief that individuals would stick by and make all efforts to rescue other individuals if they had a lover relationship," Dunbar added.

Another suggestion is that homosexuality is a developmental phase people go through. He said, "This is similar to the argument of play in young animals to get their brain and muscles to work effectively and together. Off the back of this, there's the possibility you can get individuals locked into this phase for the rest of their lives as a result of the social environment they grow up in."

But he adds that homosexuality doesn't necessarily have to have a function. It could be a spin-off or by-product of something else and in itself carries no evolutionary weight."

He cites sexual gratification, which encourages procreation, as an example. "An organism is designed to maximize its motivational systems," he adds.

In other words, if the urge to have sex is strong enough it may spill over into nonreproductive sex, as suggested by the actions of the bonobos and macaques. However, as Dunbar admits, there's a long way to go before the causes of homosexuality in humans are fully understood.