The evolutionary interests of the sexes are not congruent and therefore the sexes are often in conflict over reproductive decisions.  In some extreme cases, conflict can lead to an evolutionary arms race where evolution of a trait that allows one sex to get the upper hand in reproduction, results in the evolution of a trait to mitigate the fitness loss in the other sex, with further escalation.  This is the case in waterfowl genitalia, where males use their large corkscrew penises to force females to copulate, resulting in females evolving a complex genital morphology to counteract the male advantage.

Eversion of the penis is explosive, and occurs at 1.6 m/s! This gives the males a mechanism to force the females to copulate despite their resistance.

We tested the ability of the penis to evert in glass tubes that mimic the geometry of the female vagina and found that female like shapes prevent the penis from everting.  Read all about it in our Proc. R. Soc 2010 paper.