Many species of flatworms live in marine environments. They are simultaneous hermaphrodites, meaning they are both male and female. It's advantageous to be a hermaphrodite since the odds of successful reproduction are doubled. What’s the strategy for reproducing if you’re both male and female? It’s complicated. When a pair of flatworms of the same species meets, each can deliver sperm or receive it for fertilizing eggs internally. There’s a natural conflict since it takes a lot of energy to grow and carry fertilized eggs, so each animal wants to be the male.
In some species of flatworms, an animal can inject sperm directly through its partner’s body surface. Scientists call this “hypodermic insemination”. The first to stab becomes the father; the other bears the responsibility of motherhood. Mating then becomes a battle, commonly known as penis fencing. In some instances the fencing battle can last up to an hour. Some species even have more than one penis, making a more intense battle. The winner is the flatworm that successfully injects its sperm. The loser has battle wounds and carries the fertilized eggs – until the next encounter with another of its kind.