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Too much love. A male fruit fly mates with a large female.

Tristan Long

I'm Too Sexy for My Species

It's hard out there for a sexy female fruit fly. All she wants is a nice meal and a little sperm to fertilize her eggs, but male fruit flies harass her so much, according to a new study, that she lays fewer eggs than normal. And that, researchers say, could be bad for the evolution of the entire species.

When it comes to choosing a mating partner, females are usually the more picky sex. Female peacocks like the boys with the most colorful feathers, for example, and female deer go for big antlers. In the fruit fly world, however, males are the choosy ones. They prefer fatter females--they dance around them and constantly try to mate with them--probably because these females lay more eggs.

Evolutionary biologist Tristan Long of the University of California, Santa Barbara, and colleagues wondered if all of this attention was bad for the females. The team sorted female fruit flies by knocking them out with carbon dioxide and sifting them through a series of sieves, each with holes a little smaller than the level above. In some experiments, males were given a choice between large-bodied females and randomly chosen females of all sizes; in others, they chose between small-bodied females and randomly chosen females. In still others, males were put in vials with all large females or all small females.

When placed in vials with females of various sizes, the guys swarmed the largest females. "The males will court, they'll put their wings out and dance around, and they'll interfere with female foraging" by chasing them around while they're trying to eat, says Long. Eventually, the females gave in and mated many more times with the aggressive males than they needed to. (One time provides plenty of sperm.) "She's not mating because she needs sperm; she's probably mating because she's being worn down by this ongoing courtship," says Long.

The team found that large females lay fewer eggs when they've been harassed by males than they do when not harassed. By causing large females to lay fewer eggs—and thus give rise to fewer large female offspring--the males are selecting against a trait that's potentially beneficial for the whole species, the team reports today in PLoS Biology. Even when harassed, large females still lay about four more eggs than small females, notes Long, so though a determined male may be acting against the best interests of his species in the long run, he passes on more of his own genes by mating with a big female.

This experiment adds to evidence that evolution doesn't always lead inexorably to progress for the species, says evolutionary ecologist Hanna Kokko of the University of Helsinki in Finland. "Once we take into account that individuals differ in their interests, ... the effects [of evolution] can be pretty detrimental" to the population as a whole, she says. But not all choosy males work against the best interest of their species, says evolutionary biologist Russell Bonduriansky of the University of New South Wales in Sydney, Australia. Cricket males, for example, deliver a tasty nuptial meal to the female who takes their sperm, so it may be good for a female cricket to attract lots of suitors.