During courtship, some would-be lovers shoot themselves in the foot. Some snails, however, shoot each other in the foot: As a bizarre sort of foreplay, they routinely insert long needles into one another. Until recently, the point of all this sadomasochism was a mystery. But now, two researchers claim to have the answer: Stick a dart in your partner and it may use more of your sperm.
Some species of snail, which have both male and female organs, stab each other with a calcium carbonate "love dart" that may be more than 1 centimeter long. It penetrates the sole of the foot, the head, or even the brain of the loved one. Ever since it was discovered in the 17th century, scientists had been at a loss to explain this bizarre practice. But 3 years ago, researchers discovered that the mucus covering the dart contains substances that cause contractions near the recipient's bursa copulatrix, an organ that normally digests the vast majority of received sperm. With this antisperm device incapacitated, they reasoned, more sperm could find their way to eggs.
Now, zoologists David Rogers and Ronald Chase of McGill University in Montreal, Canada, have proof that this is actually the case. They observed hundreds of mating Helix aspersa snails and selected 18 that had successfully stabbed their partner with a love dart, as well as 21 that had shot and missed. Then they counted the number of sperm in the snails' partners. Successful shooters were rewarded with almost 2000 stored spermatozoa, whereas snails that had not been hit by a dart stored less than 1000 spermatozoa from the hapless marksmen, the researchers report in the 19 June issue of Behavioral Ecology and Sociobiology.
"It is a most valuable study," says Bruno Baur of the University of Basel in Switzerland who studies sexual behavior of land snails. But he stresses that many mysteries remain, such as why some species shoot darts each time they mate, while others do so only rarely.
The abstract of the report
Ronald Chase's home page