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.
Thats 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.
This forceful mating is something the females venture to avoid.
Luckily, evolution is on their side.
The clockwise vaginas managed to stop the intruding organ, protecting the female duck form unwanted advances.
This helps stop undesired advances, and lets females retain control over who will reproduce successfully with them.
It may not be avagina dentata, but its a close approximation.
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