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Fruit fly picture

"Sexier" female fruit flies (pictured, a fruit fly) hatch fewer babies than apparently unattractive mothers, according to a new study.

Photograph by Stanley Flegler, Visuals Unlimited


Brian Handwerk

for National Geographic News

December 29, 2009

There is a price to beauty: "Sexier" female fruit flies suffer more than unattractive ones, according to a new study.

The most fetching female fruit flies—that is, the biggest females, which can produce the most eggs—were inundated with suitors and hatched fewer babies than less desirable flies, said study leader Tristan Long, an evolutionary biologist at the University of Toronto.

The reason is twofold: The repeated advances hindered the females' ability to find food, and too much sex left the females overloaded with toxic semen, Long said.

(Related: "Female Flies Put Up a Fight to Keep Sex Short.")

Fruit flies are commonly used in lab studies, because the animals' roughly monthlong life spans allow scientists to quickly identify trends across generations.

Selfish Sex

Long and colleagues first developed a series of graphic models that predicted that too much male fruit fly persistence would harm the females' reproduction. 

When the team put an individual male in an area that contained a large and a small female, the male would pester the larger female more than the smaller female. 

Then the researchers compared lifetime fertility rates between large and small females either minimally or continuously exposed to males. In the continuously exposed group, the results showed that larger females had lower fertility rates than smaller females.

The findings, published December 8 in the journal PLoS Biology, fuel an ongoing debate about whether sexual selection and natural selection are always complimentary, Long said.

Natural selection promotes the genes that increase the likelihood of survival, while sexual selection promotes those genes that boost the odds of reproductive success. 

"To a certain extent it's recognized that maybe the most attractive individuals may be attractive because they carry the best genes," Long said. 

"[Sexual] strategies succeed because they benefit the individual that carries them," he explained, "even if it comes at a cost to their mates or to the population of the species as a whole." 

But by hounding attractive females, mobs of fruit fly males may be taking an evolutionary toll on their species. 

That's because their attention may reduce females' birthrates, keeping the gene that makes a female a better breeder from moving quickly through the population.

Not Even If You Were the Last Guppy on Earth

Females in other species suffer similar indignities at the hands of male suitors. (See pictures: "Animal Attraction" from National Geographic magazine.)

Females of many insect and lizard species, Long said, store sperm in their bodies and need to mate only once to fertilize a lifetime's worth of eggs—but are still subject to male pestering. 

And in guppies, males court attractive females so aggressively that the males limit the females' ability to find food—and worse. 

"Male courtship will force female guppies to move to areas where there is a greater risk of predation just to avoid the harassment," Long said. 

But it's certainly not all bad news for the beautiful. 

In certain cricket species, for example, the male delivers his sperm packaged with a large, nutritious meal.

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