Flatworms are hermaphrodites, meaning they have both male and female sex organs, which triggers some seriously odd reproductive behavior. When trying to impregnate another worm, for example, a flatworm tries to pierce the skin of the other using its penis. Experts term this heated competition “penis fencing”: The first to successfully impregnate the other while fending off advances becomes the de facto male, who wins because he won’t have to expend the energy required to carry eggs.
From “The 9 weirdest animal penises on Earth,” posted to The Week on June 22nd, 2012.
Whoa. This brings fighting as foreplay to a whole new level…
