He and she cooperate on anti-aphrodisiacs.

A common male butterfly deploys chemical weapons to keep rivals away from females--a great example of cooperation between the sexes that goes sour, says a team of scientists in Sweden.

Among green-veined white butterflies, Pieris napi, a male laces his sperm packages with methyl salicylate, report Johan Andersson of Stockholm University and his colleagues. This is the first time researchers have identified a chemical that serves as a butterfly anti-aphrodisiac, Andersson notes.

Females reeking of the stuff repulse males within seconds, the researchers report in the July 7 PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON B.

At first, that foul odor benefits both him and her, explains coauthor Christer Wiklund, also of Stockholm. "He'd prefer it if the female never mated again," Wiklund says. And she wouldn't mate again immediately, anyway. Her just-completed encounter left her with a sperm package, including nutrients, that can add up to 40 percent of her abdomen's weight. Rather than wasting energy fending off new suitors, she needs to start laying eggs.

Soon, however, she's ready for another mating--and more nutrients. "She needs to make herself attractive again," Wiklund says. This is when the sexes' once harmonious interests clash, but after 1 to 5 days, she can again attract males.

The discovery highlights recent emphasis on the Cold War between the sexes. "Twenty years ago, reproduction was seen as a joint venture with cooperation," Wiklund says. Since then, "conflict theory has been more in fashion," he explains.

Researchers have documented a few insect anti-aphrodisiacs, such as one that male fruit flies transfer to females. In 1976, butterfly specialist Lawrence E. Gilbert at the University of Texas at Austin suggested that males of a Heliconius butterfly also create anti-aphrodisiacs. Gilbert could smell the difference between mated and unmated females. Females of this species mate only once, and entomologists mused over the forces driving the evolution of the male-repelling substance.

Female green-veined white butterflies typically mate three to nine times. Analyzing butterfly odors with mass spectrometry, the researchers isolated and identified methyl salicylate as the volatile, repellent chemical from the sperm package. Suitors darted away within seconds when researchers applied the chemical to an otherwise alluring virgin female.

Wiklund can't distinguish mated and unmated females by sniffing, but in large amounts, the chemical reminds him of "old-fashioned bubble gum," he says. By feeding both sexes carbon-labeled precursors, the researchers found that only males turn the amino acid phenylalanine into the anti-aphrodisiac.

John Alcock of Arizona State University in Tempe, who studies mating-system evolution, wonders whether a female butterfly controls emission of the anti-aphrodisiac she receives. "In any event, this is the kind of story that appeals to evolutionists interested in figuring out who benefits and why from a trait whose adaptive function or functions are not immediately obvious," he says.

The P. napi study "is not going to compete with the human-genome frenzy," Alcock admits. "But for those of us who enjoy a good evolutionary puzzle, this is a nice problem to have solved."