When sex is a dangerous game
New Scientist, 06 September 1997
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".
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."