Fast and loose. By being promiscuous, inbred red flour beetles can choose better sperm (inset).

Łukasz Michalczyk

The Benefits of Being Promiscuous

Biologists have long wondered why some female animals bother to play the field. With males, spreading one's sperm as far and wide as possible makes sense, as they sire more offspring that way. For the female, however, extra sex takes a toll on her body, potentially reducing her life span. Yet it doesn't expand her chances of spreading her genes around. Researchers have now shown that promiscuity in the fairer sex has its benefits. By mating multiple times, females can compensate for the downsides of inbreeding. And that's why promiscuity may evolve in small populations.

When populations of many organisms are small, their members often wind up mating with kin. Such inbreeding can lead to a double dose of bad genes that result in sickly offspring and reduced fertility. Matthew Gage, an evolutionary ecologist at the University of East Anglia in the United Kingdom, wanted to know if promiscuous females could offset problems from inbreeding by providing more possibilities for their eggs to be fertilized by genetically suitable sperm. He and his colleagues tested this idea in the red flour beetle, a 3-millimeter-long insect pest found in flour and grains and sometimes used in laboratory studies.

Gage and colleagues found that multiple mating, also called polyandry, did benefit inbred females. When allowed to mate with just one male, inbred females have 50% fewer surviving offspring than outbred counterparts that mated with nonkin. But the number of offspring is equal if the inbred females breed with five males, the team reports today in Science. The inbred females are apparently able to weed out sperm from kin that would lead to less fit offspring. If they mate with just one male, however, they don't have that option.

The researchers also examined whether promiscuity could evolve in inbred beetles. They first created inbred beetles by mating sisters with brothers for several generations. Then they allowed multiple populations of the inbred strains to pass through 15 generations with no interference from the researchers. The researchers also raised outbred populations for comparison. Gage and his colleagues assessed the females' promiscuity by watching them interact with a series of 10 males presented to them one at a time.

The inbred females became fast and loose compared with their outbred counterparts. They were quicker to start mating, and they hooked up with males more frequently and for a longer time, the researchers report. They also produced more surviving offspring.

"No other work has shown that inbreeding over evolutionary time frames also changes female behavior," says Rhonda Snook, an evolutionary biologist at the University of Sheffield in the United Kingdom, who was not involved in the study.

"The issue of multiple mating by females is a long-standing conundrum in biology," adds Göran Arnqvist, an evolutionary biologist at Uppsala University in Sweden, who was not associated with the study. "This [work] suggests one scenario in which multiple mating can be beneficial." In theory, females in small populations of other species should follow suit.