DEBBIE ELLIOTT, host:

From the politics of gender in academia to an academic study of sex and survival in the insect world. It's the topic for this week's Science Out of the Box.

Our report is about sex-crazed female insects. Yes, you heard me right. Not too long ago such bugs were widely thought to be both rare and unimportant. Now there's growing evidence that many species owe their very survival to these fluttering females.

NPR's John Nielsen explains.

JOHN NIELSEN: Male insects get no respect. Their mates bite their heads off and their baby sisters eat them alive. Even worse, some male insects have to deal with a bacteria that kills them off by the trillions, without giving females as much as a sniffle.

Dr. OLIVIA JUDSON (Imperial College): I mean that's very, very sinister. And fortunately, humans do not have anything like it. Nor, as far as I know, do any mammals.

NIELSEN: Biologist Olivia Judson of London's Imperial College. She's the author of "Dr. Tatiana's Sex Advice to All Creation," a guide to the evolutionary history of sex. And she says it's possible that one-fifth of the insect species in the world have now been infected by the dreaded Wolbachia germ. Until now, experts have wondered why this hasn't triggered a mass bug extinction.

Dr. JUDSON: Because if you arrive at a situation, as in these - some of these butterflies where you have extremely female biased sex ratios, what happens if you lose the last male? You're done.

NIELSEN: Judson suspects that many germ-ridden species avoid this fate because the females produce bumper crops of eggs, raising the odds that enough males will survive. But this raises another question. How do all these eggs get fertilized when you've got 50 willing females in line for every sperm-filled male? Sylvain Charlat of London's University College says the answer turns out to be a simple one. Those 50 sex-crazed females mustn't take no for an answer.

Charlat studies isolated groups of blue moon butterflies in the South Pacific. Some groups have been ravaged by the male-killing germs, while others have never been infected. In the uninfected groups, Charlat says he found both life and sex to be fairly unhurried things. After all, there were plenty of males to go around.

Professor SYLVAIN CHARLAT (University College, London): Females which fly slowly at the ground level, and they would be generally pretty calm.

NIELSEN: But Charlat says the scene was very different in groups that had been hammered by the male-killing bacteria. In those places, females that had never been satisfied flew around in hordes, chasing the few remaining males.

Prof. CHARLAT: At two, three meters high, and they would fly really fast. And that's most likely a behavior induced by the lack of sperm in these females.

NIELSEN: Right. They're not looking for leaves.

Prof. CHARLAT: No. They're looking for something else. Yeah, they're looking for sperm.

NIELSEN: When these females found their male, they kept him very busy, to the point where some males mated with several dozen partners. Charlat says the sex-crazed females didn't rest until they were sure that all 500 of their eggs had been fertilized. How they knew this is a mystery, but Charlat says he's got some theories.

Prof. CHARLAT: One is that males transfer some chemical cues. They could also be able to feel in some way the science of the sperm package they receive.

NIELSEN: Yeah, it's hard to figure out a way to say that on the radio. The size of the package makes a big difference.

Prof. CHARLAT: Yeah, the size of the package plus its contents may be.

NIELSEN: Charlat says he's certain that this frenzied mating process is the thing that saved the infected groups of butterflies. He suspects that other insects use the same technique.

But Olivia Judson says there's another lesson to be learned here. In the world of insects, female promiscuity is turning out to be much more common and much more important than had been previously supposed.

Dr. JUDSON: The females respond to the situation they're in. And when males are in short supply - as anyone who's been to a girls school can tell you - things get pretty nasty.

NIELSEN: Charlat's findings appear in the journal Current Biology.

John Nielsen, NPR News, Washington.

ELLIOTT: To see pictures of Charlat's butterflies and read about some of Olivia Judson's favorite promiscuous animals, visit npr.org.

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