Apparently, there are these microscopic worms that are hermaphrodites by nature, and during the courtship process they are able to assume the female or male role. The most desireable role is that of the male, because it allows the worm to continue to distribute its genetic material while the "female" has to waste time in gestation before it can have another go, so to say. So before they have sex, they fight, and the winner gets to be male. So these worms have developed some rather aggressive sexual adaptations, including acidic sperm that eats away at the partner during one of these duels, comically called penis fencing.