The Voyeur Biologist

Penis fencing is a mating behavior seen in certain species of flatworm, all of which are hermaphroditic, having both ovaries (which produce ova) and testicles (which produce sperm).

The flatworms “fence” with their penises, which are two-headed, pointed, and white. The “fencing” is a violent battle during which each flatworm attempts to pierce the skin of the other, to inseminate it. The sperm is absorbed through the skin, where it fertilizes the ova in the “loser,” who becomes the mother.

More: Fighting to Mate: Flatworm Penis Fencing