Duck sex is far more interesting than it has any right to be, due to the twisted nature of the birds' genitals. Male and female ducks have corkscrew-shaped sex organs which spiral in different directions. Now we know why.

Ducks are known for their bizarre penises, stretching up to 20cm in size in an anti-clockwise spiral. That's an impressive organ for a bird often only 60cm long. The females, on the other hand, have vaginas that spiral clockwise, opposite to their menfolk. Ducks also engage in what is politely termed "forced copulation", where the male ducks attempt non-consensual sex with the female, and explosively extend their penises with a technique that takes less than half a second.

This forceful mating is something the females try to avoid. Luckily, evolution is on their side. Female ducks have evolved vaginas that spiral clockwise, and contain sharp turns, which scientists believe were used to prevent insemination by unwanted suitors. This theory has finally been tested by enterprising researchers at Yale, with too much time and glassware on their hands. With sets of cylindrical glass tubes shaped into clockwise or anti-clockwise spirals, they tested how easily the penis advanced through various vaginal configurations. The clockwise vaginas managed to stop the intruding organ, protecting the female duck form unwanted advances.

In fact, the majority of forced copulations don't result in fertilization, and it appears the two sexes are involved in an arms race over their genitalia, with males evolving new attacks and the females defences. The twisted vaginas can completely stop the penis from its explosive exertion, preventing unwanted genes from being passed on. This helps stop undesired advances, and lets females retain control over who will reproduce successfully with them.

It may not be a vagina dentata, but it's a close approximation.

[via Yale]