Ducks have taken the battle of the sexes to a whole new level.

To impregnate female ducks as quickly as possible, a certain species of male ducks has evolved corkscrew-shaped penises that shoot out of the ducks’ bodies in less than half a second, Yale researchers have found. To thwart the males, female duck vaginas have evolved to spiral the other way — an adaptation the researchers say is one of the most dramatic examples of the conflict between the sexes to control fertilization.

The finding was officially announced today by Yale researcher Patricia Brennan, who worked with fellow Yale researchers Christopher Clark and Richard Prum on the study. They published their results in the Dec. 23 issue of the journal Proceedings of the Royal Society, Series B.

Male ducks evolved their quick-ejecting penises — some of which are up to eight inches long — to mate forcibly with females, Brennan said in a press release. But female ducks’ convoluted vaginas, Brennan said, should make it difficult for a male duck to force its way in if the female duck isn’t interested.

Using a set of glass tubes with different shapes to test her hypothesis, Brennan found that male duck penises, which spiral in a counter-clockwise direction, could not extend into a glass tube that spun clockwise like a female duck vagina.

This finding, Brennan said, will give scientists new insights into how sexual conflict could lead to evolution.