It is not what you would expect to see when you take your children on a Sunday outing to the natural history museum: a giant photograph of one male giraffe humping another, or two whales sparring with giant penises. This, however, is Norway, where — for better or worse — the normal rules do not apply. Three years ago the Government told the country’s museums and libraries that they should do more to contribute to social debates and dare to tackle taboo subjects.

The results of that order are now coming through. One museum is staging an exhibition that debunks the national myth that every Norwegian was an heroic Resistance fighter in the Second World War. A second is planning an exhibition on Vidkun Quisling, the ultimate Norwegian collaborator. A third has an exhibition showing how badly Norway has treated Gypsies.

But the Natural History Museum in Oslo has gone one better. As America’s religious right fulminates against homosexuality, Europe embraces gay marriage, and leading homosexuals such as Martina Navratilova denounce scientists in Oregon for attempting to make gay sheep straight, the Naturhistorisk Museum is stepping squarely into the heart of a controversy that dates back to at least AD1120 when the Church Council of Nablus described homosexuality as a “sin against nature” .

It is staging a government-financed exhibition in its august halls that shows that homosexuality — far from being unnatural — is actually rampant in the animal world. Against Nature? is the first exhibition in the world dedicated to gay animals, claims Petter Bockman, its bearded and ponytailed scientific adviser, who also happens to be the University of Oslo’s leading — and only — frog expert (there are not many amphibians, gay or straight, this far north).

The facts have been staring scientists in the face for years, Bockman says, as he stands in front of the gay giraffes. “It’s fairly easy to see because the giraffe’s sex organs are not what you’d call modest.” The problem, he contends, is that when researchers are confronted by such behaviour, they choose to ignore it. They claim it is irrelevant to their work, or fear ridicule or the loss of their grants if they draw attention to it. They prefer to describe two animals of the same sex frolicking with each other as “competition, a form of greeting, ritualised combat, things like that — even when we are talking full anal intercourse with ejaculation”.

The taboo was finally broken in 1999 when Bruce Bagemihl, a gay biologist at the University of Wisconsin, published a book entitled Biological Exuberance: Animal Homosexuality and Natural Diversity.

Bagemihl had scoured every scientific journal and paper he could lay his hands on for references to homosexuality in animals. Tucked away at the end of long and erudite texts, or consigned to footnotes and appendices, he found that homosexuality had been observed in no fewer than 1,500 species, and well documented in 500 of them. The earliest mention of animal homosexuality probably came 2,300 years ago when Aristotle described two female hyenas cavorting with each other.

Bagemihl’s book provided the inspiration for this exhibition, and any notion that homosexuality is a uniquely human trait is quickly disposed of. You are greeted by a pair of swans — the very symbols of romantic love — who turn out to be a female couple. “Up to a fifth of all pairs are all male or all female,” reads the accompanying text.

Then you come to the photograph of the whales “penis fencing” above which hang — for no apparent reason — two actual whale penises, both several feet long and looking like stretched and desiccated turnips. Some of the male whales meet year after year, says Bockman, while their relations with females are fleeting at best.

A model — the one that invariably draws most giggles from the exhibition’s younger visitors — shows a male Amazonian river dolphin penetrating another’s blowhole. “This is the only example of nasal sex we have in nature,” Brockman observes.

Up to a fifth of all king penguin couples kept in captivity are gay, we learn from a display of stuffed penguins wearing pink scarves. Hooded seagulls, sea otters, fish, kangaroos, fruit bats, blue jays, storks, pine martens and owls make guest appearances. So does the lowly hedgehog (ouch).

Male and female bighorn sheep apparently unite during the rutting season, but the rest of the year the males stick together and homosexuality flourishes. “The females are boring. Only the males do it,” says Brockman. Insects, spiders, molluscs, crustaceans — they’re all at it. There is an 1896 sketch of two male scarab beetles enjoying each other. There are even gay gutworms; we know that, Brockman says, because “ they have sex organs and since they are translucent, it’s easy to find out what sex they are”.

Round a corner and you are confronted by a photograph of two female bonobo chimpanzees lovingly rubbing their swollen genitalia against each other while their offspring look on. “Their whole life revolves around sex,” Brockman explains with his trademark enthusiasm. “They will throw themselves into group sex and gender doesn’t seem to be relevant. Even children will give a helping hand.”