Today I Found Out's video: Deciding Who is Gets to Be Male Via a Swordfight

Like many aquatic invertebrates, flatworms are hermaphrodites, which means they have both male and female reproductive organs. For reproductive purposes, they determine who’s going to be the mother or father with a good old fashioned fight- a penis fight, to be exact, in an act scientists call "penis fencing". Yes, in many species of flatworm the creatures literally duke it out with their junk- sometimes for as much as an hour- trying to stab each other until one (or in some species both) succeed and ejaculate sperm through the skin of the other flatworm. The ‘loser’ flatworm takes on the female role and later gives birth. The winner male in many species will sometimes then go on to fence with other flatworms until it eventually loses and becomes impregnated.