Fake sperm fool female butterflies.

One of the great puzzles of butterfly sex--why males produce so much junk sperm--may be nearing a solution.

The dud sperm, skinny little strings that have lost their cell nuclei and can't fertilize anything, may be protecting a male's reproductive investment. A new study finds that the more this dummy sperm bulks up a female's sperm storage organ, the longer she waits before seeking a second mate and the less likely she is to bother at all.

The idea that infertile sperm discourage females from remating is old, but "this is the first evidence," says Penny A. Cook of Liverpool (England) John Moores University. In the Feb. 11 NATURE, she and Nina Wedell from the University of Stockholm describe tests with greenveined white butterflies, Pieris napi.

Many other species--other butterflies, stalk-eyed flies, fruit flies, and mollusks--produce infertile sperm along with the real stuff. Infertile molluscan sperm grow so huge that the fertile sperm hitch a ride on them.

British scientists Robin Baker and Mark Bellis even proposed a decade ago that human sperm take multiple forms, including kamikazes to destroy leftovers from other men. Cook, however, cautions that "we're not extending our findings to humans."

She and Wedell let butterflies mate, then provided females with a new male each day to allow second courtships. Pairs remain coupled for an hour, so during the 10-day experiment, the researchers dashed around the 36 cages every 15 minutes. Fortunately, the butterflies don't mate at night.

If a mating began, the scientists interrupted it and checked the sperm store from the earlier mating. They tested the other females at the end of the study.

From their first mating, all the females had received similar stores of around 1,000 fertile sperm. However, the storage organs held about 1,000 infertile sperm in females who accepted a second male versus 4,000 in females who didn't.

Cook speculates that infertile sperm amount to cheap knockoffs that ease the physical demands on a male. In adulthood, the butterflies exist on a sort of soda-and-junk-food diet of nectar. Drawing on protein stored from their larval days, the males produce sperm packets as big as their heads for each encounter.

Geoffrey A. Parker of the University of Liverpool, who developed much of sperm-competition theory, rates the new study as "quite good evidence." For butterflies, he says, "it's highly significant," though he warns against generalizing.

That same caution was voiced by Rhonda R. Snook of the University of Nevada, Las Vegas. She did a similar experiment to see if infertile sperm delay remating in fruit flies. As reported in the December 1998 ANIMAL BEHAWOUR, they don't.

Snook laments the giggle factor associated with research in this area. Controlling sperm fertility might make a great way to control pest insects or aid inbred species, and insights into fertility might come from the mysterious process that generates two sperm types. "It's not just a gee-whiz thing," Snook says.