When sex is a dangerous game

New Scientist, 06 September 1997

THE flatworm Pseudoceros bifurcus is no romantic. When it wants sex, it simply rears up and stabs its mate with its penis. Because it is a hermaphroditic species, its mate has a penis too, so it may get a jab in return. This leads to an unusual foreplay ritual that its discoverers in Germany have dubbed "penis fencing".

Nicolaas Michiels and Leslie Newman of the Max Planck Institute for Behavioural Physiology in Seewiesen stumbled upon the 6-centimetre worm in the sea off Heron Island at the southern end of Australia's Great Barrier Reef. For 20 hours they observed 16 pairs of worms, which they had housed in ice-cream tubs. At first, the animals did nothing. "They can't smell, so they couldn't find each other," says Michiels.

But when the worms happened to meet, they started sexual fencing bouts that lasted up to an hour. The two animals would sit up, evert their penises and try to inject sperm into each other. Only about one out of six strikes leads to successful insemination, says Michiels, who calls the behaviour "brutally inefficient". The animals are left severely wounded, with prominent punctures.

Michiels says each worm may be trying to inseminate others and avoid being stabbed itself so that others have to bear the energetic costs of reproducing its genes. "It's hard to know whether the individuals are really trying to avoid being hit," comments Janet Leonard of Oregon State University in Newport. "It's an interesting hypothesis."