#4 Flatworms “Swordfight”
Many species of flatworms are hermaphrodites which means they have both male and female reproductory organs. These flatworms fight before the mating ritual to decide who gets to be the mommy. Because making eggs is way more difficult than shooting their sperm.
So what do they do? They “swordfight” officially known as “penis fencing”. Both flatworms use extendable two-headed sharp stylets to fight. Whoever is able to inject their sperm into the other flatworm’s abdominal through a wound wins. The loser gets inseminated.