The Bizarre World of Flatworm Penis Fencing

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If you think human dating is complicated, wait until you hear about flatworms. These soft-bodied, unassuming sea noodles may look harmless, but when it’s time to decide who’s playing dad and who’s playing mum, things get spicy. Forget Tinder swipes, romantic dinners, or slow dancing under fairy lights. Flatworms settle it all with one activity: penis fencing.

These marine invertebrates quite literally duel with their penises. Two flatworms square up, whip out their twin jousting sticks (because most of them come equipped with not one but two), and begin a slow-motion sword fight that would make a medieval knight blush. The goal? Stab the other with your reproductive weapon and inseminate them. The loser of this intimate fencing match gets “volunteered” for motherhood.

It sounds absurd, but from the flatworm’s perspective, it makes perfect evolutionary sense. Carrying eggs and investing energy in producing offspring is hard work. If you can win the duel, you skip that whole ordeal. If you lose, well, congratulations—you’re now in charge of the nursery.

Scientists have actually studied these matches in detail, observing bouts that last for hours. Each combatant tries to avoid being stabbed while simultaneously attempting to land their own reproductive strike. It’s basically the biological version of “tag, you’re it,” except instead of a friendly tap, it’s an act that determines the future parenthood of one unwilling participant.

And if you think this is all a bit much, imagine being a flatworm in the mind of a homophobe or transphobe. Their binary brains would combust. Here you’ve got creatures that are simultaneously male and female, duelling with multiple penises, and casually switching parental roles depending on who got poked first. It’s biology throwing a glitter bomb at rigid gender norms, and the worms don’t even need a Pride parade to do it.

And before you start feeling smug about how “civilised” humans are, let’s pause. Flatworms aren’t the only ones with unusual reproductive rituals. Anglerfish fuse together into a single organism, male seahorses carry babies in their bellies, and some spiders casually eat their partners after sex like it’s a post-coital snack. Nature has no shame, only creativity.

The brilliance of the flatworm duel is that it flips our whole perspective on parenthood. Here, reproduction isn’t a romantic partnership or shared responsibility—it’s a fight, a literal contest of wills and wits. The winner walks away, free to fence another day, while the loser drifts off into parental destiny. Brutal, efficient, and bizarrely fair.

It makes you wonder: if humans had to duel like this to decide who carries the baby, how would our dating culture look? Would men suddenly discover a passion for Pilates, cardio, and fencing lessons? Would “gender reveal” parties be replaced by “penis fencing tournaments”? It’s an unsettling thought, but also one that highlights just how strange and diverse life on Earth really is.

So next time you feel awkward on a date, remember this: at least you’re not locked in a penis-jousting death match with the stakes of eternal parenthood hanging in the balance. Somewhere out there, two flatworms are still duelling, proving that love—and reproduction—can be as weird as it is wonderful.