NEW HAVEN, Conn., Dec. 23 (UPI) -- Female ducks likely developed vaginas with clockwise spirals to prevent fertilization from aggressive males, scientists in Connecticut said.
Male ducks are endowed with corkscrew penises shaped opposite that of the females, researchers at Yale said this week in the journal Proceedings of the Royal Society B.
"In species where forced copulation is common, males have evolved longer penises, but females have co-evolved convoluted vaginas with dead-end cul-de-sacs and spirals," researcher Patricia Brennan said. "This co-evolution results from conflict between the sexes over who is going to control fertilization."
While most birds have no phalluses, male ducks have flexible penises -- up to 8 inches long -- that can be thrust from within their bodies in less than half a second, Brennan said, describing the process as "explosive."
"Ducks are providing us with an incredible opportunity to understand the evolutionary consequences of conflict," Brennan said.