Penis Fencing
Hermaphroditic flatworms fence with their two-headed penises — the loser gets pregnant and must bear the energy cost of producing eggs.
Many flatworm species are hermaphrodites, possessing both male and female reproductive organs. When two flatworms meet to mate, neither wants to be the "female" — producing eggs is energetically expensive. The solution? Penis fencing.
Two flatworms will literally fight using their sharp, two-headed penises as weapons, each trying to stab the other and inject sperm while avoiding being stabbed themselves. These duels can last over an hour.
The "loser" — the one who gets stabbed first — absorbs the winner's sperm through the wound and becomes pregnant. The winner can continue on, potentially fencing with more partners, while the loser must invest significant energy into developing and laying eggs.
The fencing behavior involves sophisticated movements: rearing up, lunging, parrying, and dodging. Some species have evolved penis stylets (hardened points) that can pierce right through the other flatworm's body.
This violent mating ritual demonstrates how sexual conflict can drive the evolution of bizarre behaviors, even in simple organisms.