How DUCK got the twist

SYDNEY: Female ducks have evolved "maze-like" genitals with many twists, pouches and dead ends, in a bid to prevent rape and retain control of who fathers their offspring – while male ducks have evolved equally convoluted penises to keep up.
Patricia Brennan, an American reproductive biologist at Yale University in Connecticut, found that some ducks and geese have co-evolved elaborate genitals in an "evolutionary arms race" between the sexes.
Ducks, especially mallards, are one of the few species of birds in which males will often rape females, in a violent act which can result in injuries or death by drowning. The females fight back by preventing successful fertilisation, said Brennan. In fact, of the 40 per cent of matings which are a result of forced copulations, only around four per cent are successful.

Storyteller: Patricia Brennan, Reproductive biologist, Yale University, Connecticut.

http://www.cosmosmagazine.com/node/1277

Coevolution of Male and Female Genital Morphology in Waterfowl