OSLO - The birds and the bees may be gay, according to the world's first museum exhibition about homosexuality among animals.

With documentation of gay or lesbian behavior among giraffes, penguins, parrots, beetles, whales and dozens of other creatures, the Oslo Natural History Museum concludes human homosexuality cannot be viewed as "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," an exhibit statement said.

Geir Soeli, the project leader of the exhibition entitled "Against Nature," told Reuters: "Homosexuality has been observed for more than 1,500 animal species, and is well documented for 500 of them."

The museum said the exhibition, opening on Thursday despite condemnation from some Christians, was the first in the world on the subject. Soeli said a Dutch zoo had once organised tours to view homosexual couples among the animals.

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

One exhibit shows two stuffed female swans on a nest -- birds sometimes raise young in homosexual couples, either after a female has forsaken a male mate or donated an egg to a pair of males.

One photograph shows two giant erect penises flailing above the water as two male right whales rub together. Another shows a male giraffe mounting another for sex, another describes homosexuality among beetles.

BURN IN HELL

One radical Christian said organizers of the exhibition -- partly funded by the Norwegian government -- should "burn in hell," Soeli said. Laws describing homosexuality as a "crime against nature" are still on the statutes in some countries.

Greek philosopher Aristotle noted apparent homosexual behavior among hyenas 2,300 years ago but evidence of animal homosexuality has often been ignored by researchers, perhaps because of distaste, lack of interest or fear or ridicule.

Bonobos, a type of chimpanzee, 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.

Still, it is unclear why homosexuality survives since it seems a genetic dead-end.

Among theories, males can sometimes win greater acceptance in a pack by having homosexual contact. That in turn can help their chances of later mating with females, he said.

And a study of homosexual men in Italy suggested that their mothers and sisters had more offspring. "The same genes that give homosexuality in men could give higher fertility among women," he said.