How the duck got his spiral

Female ducks have evolved an intriguing way to avoid becoming impregnated by undesirable but aggressive males endowed with large corkscrew-shaped penises: vaginas with clockwise spirals that thwart oppositely spiraled males.
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“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 penis,” 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.”

The research builds upon a 2007 Yale study that first described the strange morphology of a duck’s sexual organs. While most birds have no phalluses, ducks turn out to have relatively large, flexible penises—up to 20 centimeters—tucked inside their bodies. During sex, male ducks extend, or evert, their phalluses inside the female. Brennan and her Yale colleagues used high-speed video to document the erection of the duck penis for the first time and found the whole process takes less than half a second—an act the Yale team described as “explosive.”

More Information:
Explosive eversion and functional morphology of duck penis supports sexual conflict in waterfowl( paper )