Do parasites explain female promiscuity?

An experiment with bumblebees provides the most direct evidence yet for a theory explaining why females of so many species go to the trouble of mating with more than one male.

Such behavior has puzzled biologists because of "the obvious costs of time, energy, and exposure to predation," as Boris Baer and Paul Schmid-Hempel put it in the Jan. 14 NATURE. The researchers, from ETH Zurich in Switzerland, point out that some social insects "carry this behavior to extremes." Virgin honeybee queens mate with 10 to 20 males during a once-in-a-lifetime round of midair sex.

Female insects can give birth to broods with multiple fathers, and theorists have proposed that boosting the genetic diversity of a brood should make the colony better able to withstand parasites.

Baer and Schmid-Hempel artificially inseminated bumblebee queens with either low- or high-diversity sperm. The colonies that the queens founded foraged outdoors, where workers encounter all sorts of menaces. The seven high-diversity colonies ended up with fewer parasites and greater reproductive success, on average, than the low-diversity colonies.

William D. Hamilton of the University of Oxford in England, one of the theorists who proposed the parasite idea, greeted the work warmly. Besides helping explain the forces behind insect orgies, he says, the paper may also help resolve another mystery, "perhaps the very greatest of the subject--that of why sexual reproduction so often prevails over its obviously far more efficient alternative, female-female parthenogenesis."

Would female animals be more likely just to give birth without male input if it weren't for the risks of parasites? The new study, Hamilton muses, "reflects on a lot that we all care about--on love, for example, and all its troubles, and on all the rest of the wonderful, yet confusing, patterns that sex creates."