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Thursday, Sept. 13, 2001
Making war, not love
By ROWAN HOOPER
"Love looks not with the eyes, but with the mind; And therefore is wing'd Cupid painted blind." So laments lovesick Helena in "A Midsummer-Night's Dream." Sorry to add to your woes, Helena, but not only is Cupid blind, he is more likely to glide on a trail of slime than fly on cherub wings. Cupid, it turns out, is rather like a snail.
Between ordinary land snails everywhere, in the back garden of your house and in your local park, one of the most extraordinary sex acts outside of Bangkok is taking place. Before copulation, snails circle each other for about half an hour, touching, nudging and biting. There is lip-to-lip as well as lip-to-genital contact. One of the snails then produces a bow-and-arrow apparatus from near the penis, and shoots a hard, sharp "love dart" into the body of the other. The partner replies with a dart of its own.
Snails are hermaphrodites, meaning they have male and female sexual functions in the same body. Unlike some fish species, in which individuals are males at one stage in their life and females at another, snails are both male and female at the same time. After the exchange of Cupid's arrows, each animal inserts its penis into the female genital tract of the partner, and each animal receives sperm, which it will digest or use to fertilize its eggs.
Slimy intimacies? Hardly.
"This is no lovey-dovey courtship," said Mike Siva-Jothy, an evolutionary biologist at the University of Sheffield, England, in an e-mail interview. "This isn't two teenagers snogging on a park bench. This is a knife-fight."
Biologists have pondered the puzzle of the snail's darts since the 17th century. It is the realization that sexual behavior can be like a knife-fight as well as a candle-lit dinner that has allowed biologists to solve the problem of the snail's darts. We tend to think of reproduction as a harmonious affair, but as in any other transaction (and at its core, that's what sex is, a transaction involving sperm) there is plenty of room for conflict.
For mammals, conflict might mean the male deserting a pregnant female to look for more females, leaving the abandoned mate literally holding the babies. For hermaphrodites, it might mean one partner giving sperm to the other, but cheating on the bargain and not hanging around to take the partner's sperm in return.
Formerly, biologists saw no conflict. They saw sex and reproduction as purely cooperative. Even when they saw a weapon, a calcium carbonate arrow shot into the body, they saw harmony; they saw Cupid at work. Hence the arrow was called a "love dart," and the mutual shooting was seen as an exchange of gifts. It made a kind of sense; after all, snails' growth is limited by calcium deficiency, so a free package might well be useful.
But 85 percent of darts are expelled, says Ronald Chase, a neurophysiologist and evolutionary biologist at McGill University, in Montreal. And in any case, it's not much of a gift: Each dart only contains a tiny amount of calcium. Darting only makes sense in the context of sexual conflict.
So Chase and his colleague Dave Rogers looked for another explanation. Their work was published earlier this year in the journal Behavioral Ecology and Sociobiology.
Some darts, it seems, miss their targets. The researchers compared the amount of sperm stored by snails that had been hit by darts and those that hadn't. About twice as much sperm is stored if the dart has hit home.
"We've finally established that the dart has a function after copulation, in promoting storage of the shooter's sperm," said Chase. "We believe that the dart acts as a hypodermic needle to transfer a chemical in the snail's mucus."
The female reproductive tract in most animals is a hostile place and snails are no exception. Between 1 million and 10 million sperm are transferred during copulation, but only a tiny proportion survive -- most are digested and absorbed by the partner snail. Mucus on the dart causes a change in the shape of the reproductive tract, leading to the storage of more sperm.
What further complicates the story is the fact that there are no separate sexes. A male who shoots a dart will soon himself -- or rather, herself -- be hit by a dart.
"There is a big conflict in hermaphrodites," agreed Joris Koene, who is studying the evolution of snails' darts at the University of Muenster in Germany. "Like in species with separate sexes, the male function of hermaphrodites will try to inseminate as many partners as possible, while the female function will look at quality rather than quantity."
"It remains to be seen whether the animal hit by the dart, representing the female function, benefits from the dart," said Chase.
Koene likened the situation to an arms-race. "It might be that hermaphrodites take this arms-race further than separate-sex species," he said. "These so-called 'love darts' seem to be manipulative devices to ensure that the donated sperm are used in fertilization, whether the partner likes it or not."
Cupid's message? Sex is even more complicated than the confusion suggested by "A Midsummer-Night's Dream."
E-mail Rowan Hooper at firstname.lastname@example.org