Penguins regularly form same-sex pairs. Photo: Per Aas/NHM-Oslo

The Oslo Natural History Museum opened the show last week and says it has been well attended, not least by families.

Organisers reported early criticism of the project, and being told by one opponent they would "burn in hell".

But there has been strong interest in an aspect of animal behaviour the museum says is quite common.

It says homosexuality has been observed among 1,500 species, and that in 500 of those it is well documented.

The exhibition - entitled Against Nature? - includes photographs of one male giraffe mounting another, of apes stimulating others of the same sex, and two aroused male right whales rubbing against each other.

We hope to reject the all too well known argument that homosexual behaviour is a crime against nature

Oslo Natural History Museum

"Homosexuality is a common and widespread phenomenon in the animal world," says an exhibition statement.

"Not only short-lived sexual relationships, but even long-lasting partnerships; partnerships that may last a lifetime."

The museum says it is the first exhibition in the world to touch on a subject that has been taboo in the past.

It says sex between animals - as between humans - is often a matter of enjoyment, rather than procreation, and that this applies to animals of the same sex as well as opposite sexes.

'Bisexual species'

While homosexuality would appear to contradict evolutionary imperatives, scientists involved in the exhibition say it appears to do no harm and may actually help in some circumstances.

Sometimes a pair of male birds may rear eggs "donated" by a female.

In the case of flamingos, for instance, "two males can hold a much larger territory than a regular flamingo pair, thus more chicks can grow up", the exhibition states.

Pairs of male flamingos have been known to raise young

In some colonies, as many as one in 10 pairs of penguins may be same-sex, while "in some animals the whole species is bisexual", the exhibition says, giving bonobo chimpanzees as an example.

There has been some hostility to the exhibition. An American commentator said it was an example of "propaganda invading the scientific world".

Petter Bockman, a zoologist who helped put the show together, admitted that "there is a political motive".

In Norway there was a desire among publicly funded museums to be "deliverers of truth" and to "put on display controversial subjects, things that are not said and are swept under the carpet".

The museum says one of its aims is to "help to de-mystify homosexuality among people... we hope to reject the all too well known argument that homosexual behaviour is a crime against nature".