The Corkscrew Arms Race

Duck reproduction is the result of an evolutionary arms race between the sexes that has produced some of the most unusual reproductive anatomy in the bird world.

Male ducks have long, corkscrew-shaped penises that spiral counter-clockwise and can be explosively everted in under half a second. Some species, like the Argentine Lake Duck, have penises longer than their entire body.

But here's where it gets interesting: female ducks have evolved counter-measures. Their vaginas spiral in the opposite direction (clockwise) and feature dead-end pouches and blind alleys. This anatomy makes forced mating much less likely to result in fertilization.

This is a clear example of sexual conflict driving evolution. Because forced copulation is common in ducks, females evolved complex anatomy to retain control over which males successfully fertilize their eggs. Males, in turn, evolved longer and more elaborate penises. The result is an anatomical standoff.

Only about 3% of bird species have penises at all — most birds mate by briefly touching their cloacas together (called a "cloacal kiss"). Ducks are a notable exception.