When it comes to sex, some female ducks have taken "no" to a new level. They have evolved vaginas with clockwise spirals that keep out the oppositely spiraled penises of undesirable males, scientists have discovered.

When the female wishes to say "yes," she has some tricks that make it easier for her preferred mate to slip his corkscrew-shaped penis easily inside and achieve fertilization.

"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 lead researcher Patricia Brennan of Yale University. "This coevolution results from conflict between the sexes over who is going to control fertilization."