Life

If ever there was a story that illustrated the lengths a male will go to have sex, it is this. Male Muscovy ducks have a penis up to 40 centimetres long – almost half the length of their body – but that's just one of the twists and turns in the story of how female and male ducks try to outsmart each other.

A female Muscovy duck chooses a mate based on her assessment of his courtship and plumage. But rejected males don't give up easily, and can force copulation on unwilling females. The long, flexible penis helps them do so.

So females have evolved to wrest back control of copulation, says Patricia Brennan at Yale University. "The males and females become locked in this arms race, each trying to dominate the outcome. It's fascinating to find such a clear and obvious example of sexual conflict."

Do the twist

The male duck's penis is spiral-shaped: like a corkscrew, it twists in a counter-clockwise direction so that sperm will target the oviduct on the female's left-hand side. In almost all birds only the left ovary is functional, but in a 2007 study, Brennan and colleagues noticed that in ducks the female's vagina twists in the opposite direction.

To see whether these clockwise twists and turns would make copulation more difficult for the male, the group managed to get male ducks to unfurl their penises into glass tubes shaped like a female's vagina. The males found it difficult to evert their penises down the length of the tube – which is what the team reckons happens in the vagina.

Brennan thinks that, while the males are evolving long and flexible penises to help them force copulations, the females are using their complex vaginal anatomy to take back control over which sperm fertilises their eggs. When a female wants to mate with her chosen partner, she can make the process easier by relaxing the muscles around the vagina entrance.

Brennan's team also timed the male's penis eversion, which took a mere one-third of a second – around 60 times faster than was previously thought (see video above). "This definitely gives the males a mechanism by which they can copulate," says Brennan, who was taken aback by the speed. "To be totally honest, I'm still in shock," she says.

Locke Rowe, an evolutionary ecologist at the University of Toronto, Canada, says the findings show a good example of antagonistic genital anatomy in animals. "This is a good piece of evidence that conflict, rather than confluence, is shaping evolution," he says.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2009.2139

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