Flatworm Love and Hates Its Mate

The marine flatworm is a hermaphrodite – each has both male and female sex organs. When it has the urge to reproduce, an exceptional sense of smell is used to track a mate from long distances. When they finally join, they are extremely hostile. The two would-be lovers rear up immediately, like two swordsmen at the ready, revealing sharp, double-headed organs. This tool, also used for hunting food, serves the purpose of stabbing the other violently with genetic material. The scientific community refers to this activity as "penis fencing", and it can last up to 20 hours.

When we meet people who are so much like ourselves, we can confuse commonalities with compatibility. The desire to be with someone can be a reason to stay. It is easy to overlook tiny disagreements – about decorating the flat, for example, or where to eat.

For marine flatworms, an extended stay with an unfriendly mate results in their bodies being ravaged with holes. Afterwards both worms go off alone. The loser lost a game of "traumatic insemination", and if you hadn't guessed, her female self will later endure the costly undertaking of bearing children. Alone, torn and pregnant, she must wonder if it was worthwhile.decides fighting with someone is better than fighting with no one.