A "sexual arms race" is the phrase most often used to describe duck genitalia, and it's not hard to see why.
Male ducks have corkscrew penises.
Female ducks have corkscrew vaginas.
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.
But the females seem to be winning. Most times male ducks force themselves on females, it doesn't result in fertilisation.
Only 3% of duck inseminations come from forced copulation.
When a female does want to mate with a male, she will contract and relax internal muscles that scientists think help make sex easier.