Flatworms Engage In Violent Penis Fencing

Flatworms are hermaphrodites that literally duel with their reproductive organs to determine who becomes the parent. The loser of this bizarre fencing match ends up burdened with the high energy cost of bearing offspring.

These flatworms possess both male and female reproductive systems, but they do not want to be the one who carries the eggs. By using their penises like swords, they try to pierce their partner to inject sperm while avoiding being hit themselves. It is a strange evolutionary trade-off where the loser effectively takes on the role of the mother. This high-stakes joust ensures that reproduction is a dangerous and competitive game rather than a simple pairing.