Homosexuality is not something exclusively human.

A recent exhibition at the Oslo Natural History Museum, named "Against Nature", depicts gay or lesbian behavior among dozens of bird and mammal species.

Thus, human homosexuality is not so "unnatural."

"We may have opinions on a lot of things, but one thing is clear - homosexuality is found throughout the animal kingdom, it is not against nature," said Geir Soeli, the project leader of the exhibition.

"Homosexuality has been observed for more than 1,500 animal species, and is well documented for 500 of them." said Soeli.

This is the first world exhibition on this subject.

Previously, a Dutch zoo exposed animal homosexual couples.

"The sexual urge is strong in all animals. ... It's a part of life, it's fun to have sex," Soeli explained homosexuality or bisexuality among animals.

One exhibit shows two stuffed female swans on a nest.

These birds sometimes raise young in homosexual couples, either after a female has left a male mate or donated an egg to a pair of males.

One photograph depicts two giant erect penises flailing above the water as two male right whales rub together.

Another depicts a giraffe bull mounting another for sex, another presents homosexuality among beetles.

This subject has often been ignored by researchers, perhaps because of distaste, lack of interest or fear of ridicule.

Bonobos (photo), also called dwarf chimpanzees, the closest species to human, are among extremes in having sex with either males or females, apparently as part of social bonding.

"Bonobos are bisexuals, all of them," Soeli said.

Since homosexual individuals leave no offspring, it is unclear why homosexuality survives.

One theory is that males can sometimes gain support in a pack by having homosexual contact.

"That in turn can help their chances of later mating with females," he said.

In a study made on families with homosexual men in Italy it was proven that their mothers and sisters had more offsprings.

"The same genes that give homosexuality in men could give higher fertility among women," he said.

In fact, the genes that confer high masculinity can produce lesbian female offsprings, and vice versa, genes of high femininity can render homosexual male offsprings.