Hang around with other guys if you want your penis to get bigger. At least that's how it works in ducks.
A new study shows that male ducks that live with other guys grow a bigger penis than male ducks that only live with females. Apparently, the competition of having other guys around stimulates ducks to produce a bigger penis, in hopes of winning the mating game.
This is just one of the crazy aspects of duck mating. Duck phalluses are not only big, they have a corkscrew shape. And female ducks have strange, multi-pocketed vaginas that allow female ducks to control which aggressive male gets to fertilize her eggs. Evolving a labyrinthine vagina seems like a useful strategy for a species where forced copulation is common. Since female ducks don't get to control who mates with them, they have evolved anatomy that allows them to control who fathers their ducklings.
All of which makes great fodder for feminists and masculinists to argue forever about what all of this mating strangeness means.