Male ducks are notorious for attempting "forced copulation" with females.

So females seem to have evolved vaginas that make it hard for a male duck to actually inseminate them, if they don't want it to, by forcing it towards the dead ends.

Of course, the male ducks haven't taken this lying down. The more forced copulation a duck engages in, the longer the males' penises tend to be, according to a 2010 study in Proceedings of the Royal Society B.

In fact, male ducks grow a new penis every year (yes, you read that right). Which means they can vary the length depending on that year's competition.