You wouldn’t necessarily expect to find out that ducks have a corkscrew penis or that snakes are endowed with a forked double-headed member while you’re at the Natural History Museum. You’re in for a surprise though: the venerable museum in South Kensington in London has put on a graphic show about animals’ sexual habits, called Sexual Nature.

The exhibition is a celebration of sex in all its forms and doesn’t shy away from tackling forced sex and homosexuality. Yes, you’ve heard right – animals don’t just do it to reproduce – gay sex has been observed in 450 species. Orang utans engaging in gay sex are much more affectionate than during heterosexual intercourse, kissing, grooming and hugging each other constantly.

The dimly lit show feels a bit seedy at first with a lurid red neon SEX sign greeting you at the entrance but it is all quite factual. Some 100 specimens from the museum’s collection, live creatures like guppies and stick insects and films will educate you about the animal kingdom’s dirty little secrets.

Bonobos – known as the erotic ape – have sex whenever they can without any regard for age or gender and unusually, sometimes have face-to-face or oral sex. Their promiscuity “seems to have a calming effect on their society which exhibits very little aggression,” the exhibition informs us. The labels are an odd mix of snappy, slightly naughty colloquialisms and science speak.

The films and stuffed exhibits are certainly worth seeing – so foxes do it back to back while most other animals do it from behind with the male mounting the female. It’s a brutal world: Male garter snakes force females to mate with them by gang raping them – as many as 25 can surround a single female.

Some males such as cats have barbed penises that hurt or even damage the female to prevent them from mating with others (in the case of cats, the spikes on the penis are also designed to stimulate ovulation). The leopard slug is so desperate to be the one and only that it chews off its own penis and leaves it inside its partner.

But some females strike back: many, for example the praying mantis and spiders, are bigger than the male and, mistaking him for prey, will try to eat him. That won’t stop the male copulating, though, as demonstrated by Isabella Rossellini’s hilarious films, also on show at the museum.

The actress, the daughter of Hollywood legend Ingrid Bergman and Italian film director Roberto Rossellini, has created a series of short films entitled Green Porno in which she dons a body stocking and acts out a number of bizarre sex acts with paper animals.

Pretending to be a duck, she tries to fight off an aggressive mallard and has her revenge by sending his corkscrew penis down to a dead end, taking advantage of her labyrinth of reproductive tracts. “I have evolved a vaginal complex to keep control,” shouts Rossellini.

The whiptail lizard has ditched males altogether and only gives birth to identical cloned daughters (prompting the curators to warn that due to the lack of genetic mixing fatal diseases could drive the species to extinction).

Much of what animals get up to looks familiar from a human perspective: from courtship (was Michael Jackson’s moonwalk inspired by the manakin bird’s courtship dance?)  to orgies (spinner dolphins have sex orgies called wuzzles). Chastity belts are nothing compared to vaginal plugs that stop rival males entering. Interestingly, very different animals can mate in similar ways.

Transsexuals are no strangers to the animal kingdom. Oysters start out as males that release sperm and later turn female to release eggs, and clownfish switch sides rather opportunistically as situations change.

Few genitals are on show per se but there are a few surprises. London Zoo’s Guy the Gorilla only had a one inch member – while barnacles have extraordinarily long penises, probably the largest in relation to body size in the animal kingdom. A barnacle can have a schlong 30 times its own body length.

Sexual Nature at the Natural History Museum, South Kensington, London, runs until 2 October 2011.