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Duck penises show battle of the sexes

mallard ducks

Female ducks, like the one in the foreground, have evolved clever ways of choosing which male to mate with. So, will the genitalia of this green-faced drake do the trick? (Image: iStockphoto)(Source: iStockphoto)

Ducks have evolved complicated genitals in what appears to be an 'arms race' between the sexes, researchers report, and females may be coming out ahead.

The findings not only open a window into a little-studied area of biology, but could help shed light on how evolution works to help both males and females control their own breeding, the US and UK researchers say.

Dr Patricia Brennan, of both Yale University and the University of Sheffield, and colleagues publish their study online today in the Public Library of Science journal PLoS One.


"Birds are the only group where it mostly has been lost; 97% of birds do not have phalluses at all," she says.




Ducks, especially mallards, are an exception. Although mallards pair off to mate, stray males often force themselves onto the females.

Yet studies show that these forced copulations do not pay off for the males.

"Even in a species where 40% of the copulations are forced copulations, the ducklings still are mostly sired by the mates," Brennan says.


Getting to the bottom of duck sex

So Brennan's team looked at a lot of duck bottoms.

What they found surprised them - corkscrew shaped oviducts, with plenty of potential dead-ends.

"Interestingly, the male phallus is also a spiral, but it twists in the opposite, counterclockwise, direction," says Yale ornithologist Professor Richard Prum.


Brennan believes females evolved convoluted oviducts to foil stray males' attempts at forced copulation.


Only if the female is relaxed and cooperative can the male's sperm get anywhere near the unfertilised eggs, the researchers suggest.


And it may mean something for humans.


Tags: animals, biology