Why the Lady Is a Tramp

Biology has an easy explanation for Don Juan's motives--having many lovers lets him produce the greatest number of offspring. But scientists have long wondered why the females of many species seek out multiple mates, because each coupling can cost them precious time and energy without necessarily increasing the number of offspring they can bear. New research shows how, at least in flies, such female promiscuity can reduce the chances that the offspring will inherit bad genes.

Females take multiple partners, a behavior called polyandry, in many species, including mice. Some researchers have suggested that polyandry evolved to help females boost the likelihood that their offspring will carry certain positive traits such as virility. They can collect sperm through multiple matings, and only the most competitive of this "sperm cocktail" will fertilize their eggs, says Nina Wedell, an evolutionary biologist at the University of Exeter in the United Kingdom.

An alternative explanation is that cheating evolved to spare the offspring from potentially negative traits. Wedell and her colleagues examined one cause of such problematic traits: "selfish" genes. These genes break the 50-50 inheritance rule by being passed onto offspring more often than not. Selfish genes "fight" other genes to get passed on to the next generation, often harming the carrier by causing problems such as reduced fertility. For example, male fruit flies can carry a selfish gene that destroys all of their sperm with a Y chromosome, so they produce fewer sperm and can father only daughters. Yet, male fruit flies with the selfish gene are physically identical to those without it.

To test whether females might evolve polyandry in response to the gene, the researchers ran an evolutionary experiment. They compared mating behavior between four fruit fly lines in which the selfish gene was present in about 30% of the males, and eight fruit fly lines lacking the gene entirely. Females in all the fruit fly lines showed nearly identical mating rates at the outset of the experiment. But after 10 generations, females from the selfish gene population remated, on average, almost a full day sooner than females from the populations without the selfish gene, the researchers report today in Science (p. 1241).

Remating increased the females' chances of snagging a male fly with normal fertility, the researchers note. The findings present a new explanation for polyandry: that it represents an attempt to prevent the spread of selfish genes, Wedell says. "I suspect this may be more general [than just flies] simply because selfish genes are ubiquitous."

Carol Boggs, an evolutionary ecologist at Stanford University in Palo Alto, California, says the work makes an important contribution to our understanding of how mating systems evolve. It's the first study to look at the potential impact of selfish genes on mating systems, she says. "This is something that could very well be a factor in the determining the evolution of polyandry."