From male killer whales that ride the dorsal fin of another male to female bonobos that rub their genitals together, the animal kingdom tolerates all kinds of lifestyles.
A first-ever museum display, "Against Nature?," which opened last month at the University of Oslo's Natural History Museum in Norway, presents 51 species of animals exhibiting homosexuality.
"Homosexuality has been observed in more than 1,500 species, and the phenomenon has been well described for 500 of them," said Petter Bockman, project coordinator of the exhibition.
The idea, however, is rarely discussed in the scientific community and is often dismissed as unnatural because it doesn't appear to benefit the larger cause of species continuation.
"I think to some extent people don't think it's important because we went through all this time period in sociobiology where everything had to be tied to reproduction and reproductive success," said Linda Wolfe, who heads the Department of Anthropology at East Carolina University. "If it doesn't have [something to do] with reproduction it's not important."
However, species continuation may not always be the ultimate goal, as many animals, including humans, engage in sexual activities more than is necessary for reproduction.
"You can make up all kinds of stories: Oh it's for dominance, it's for this, it's for that, but when it comes down to the bottom I think it's just for sexual pleasure," Wolfe told LiveScience.
Conversely, some argue that homosexual sex could have a bigger natural cause than just pure pleasure: namely evolutionary benefits.
Copulation could be used for alliance and protection among animals of the same sex. In situations when a species is mostly bisexual, homosexual relationships allow an animal to join a pack.
"In bonobos for instance, strict heterosexual individuals would not be able to make friends in the flock and thus never be able to breed," Bockman told LiveScience. "In some bird species that bond for life, homosexual pairs raise young. If they are females, a male may fertilize their eggs. If they are males, a solitary female may mate with them and deposit her eggs in their nest."
Mom and Dad and Dad
Almost a quarter of black swan families are parented by homosexual couples. Male couples sometimes mate with a female just to have a baby. Once she lays the egg, they chase her away, hatch the egg, and raise a family on their own.
"Homosexuality" and "heterosexuality" are terms defined by societal boundaries, invisible in the animal kingdom.
"Many species are hermaphrodites," Bockman said. Hermaphrodites have both male and female sex organs. A lot of marine species have no sex life at all, but just squirt their eggs or semen into sea.
Some creatures even reproduce asexually, by dividing themselves into two organisms. In one species of gecko, females clone themselves.
Like most complex issues, animal homosexuality is challenging and poorly understood. Therefore, educators tend to shy away from covering it in their teaching. Many scientists don't even want to be associated with this type of research.
"I've had primatologists offer to give me their data on homosexual behavior because they didn't want to publish it," Wolfe said.
"Against Nature?" was set up partly to demystify the concept.
The argument that a homosexual way of living cannot be accepted because it is against the "laws of nature" can now be rejected scientifically, said Geir Soli, project leader for the exhibition. "A main target for this project was to get museums involved in current debate; to show that museums are more than just a gallery for the past."
To learn more, see LiveScience's Top 10 presentation, Gay Animals: Alternate Lifestyles in the Wild.