Now I'm a telling you here folks, I didn't know this interesting little tidbit about Duck evolution until today. 
Duck Sex Can Be ScrewyBy Brett French, The Billings Gazette
December 24, 2009
A female mallard duck in Billings. (Paul Ruhter/Gazette Staff)Some male ducks can be aggressive breeders, so female ducks interested in being selective have evolved an unusual means of eluding unwanted sexual encounters.
According to a
study by Yale University researchers, aggressive male ducks endowed with large corkscrew-shaped penises can't successfully copulate with females whose vaginas spiral clockwise. The researchers described their findings in the Dec. 23 issue of the journal
Proceedings of the Royal Society B.
"In species where forced copulation is common, males have evolved longer penises, but females have coevolved convoluted vaginas with dead-end cul-de-sacs and spirals in the opposite direction of the male (censored word)," said Patricia L.R. Brennan, lead author of the paper and postdoctoral researcher in the Yale Department of Ecology and Evolutionary Biology. "This coevolution results from conflict between the sexes over who is going to control fertilization."
This isn't the first foray into duck sex for Yale scientists. In 2007, they identified the daffy construction of some ducks' sex organs. They found that while most birds have no phalluses, ducks turn out to have relatively large, flexible penises -- up to 8 inches -- tucked inside their bodies. During sex, male ducks extend their phalluses inside the female, a process that takes less than half a second. Then the male duck smokes a cigarette and flies south, just kidding.
http://www.billingsgazette.com/lifestyles/recreation/gazoutdoors/article_30fbaa44-f0c3-11de-a274-001cc4c002e0.html
