Promiscuity in guppies has its virtues.

Female birds do it, female bees do it, even female guppies in the (freshwater) seas do it.

Choices by females often play the deciding role in mating behavior: Bronze-winged jacanas keep male harems and trade dalliances for childcare (SN: 3/6/99, p. 149), and honeybee queens solicit up to 20 males for their midair orgies and have parasite-resistant offspring as a result (SN: 1/30/99, p. 78). Yet often the evolutionary advantage conferred on females that risk disease and predation to love perhaps wisely and also too well hasn't been clear.

In the Aug. 29 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, two researchers from the University of St. Andrews in Fife, Scotland, report that promiscuity benefits female Trinidadian guppies by reducing gestation time and yielding offspring with better survival skills.

Unlike most fish, guppies bear live young. Females produce up to 30 eggs per month and give birth to a single monthly brood, says Anne Magurran, one of the study's authors. Fertilizing sperm can come from a newly solicited male or from reserves stored in the folds of the female's ovarian tissue. Biologists want to understand how females choose the sperm that fertilizes each of their eggs.

In Magurran's study, each of 76 virgin guppies mated over the course of 4 days with a single male or with four different males. Magurran found that multiply mated females produced significantly larger broods and gestated for an average of 8.76 days less than singly mated females did. Offspring of multiply mated females also spent significantly more time in protective schooling behavior and were more skilled at evading capture.

Each brood sired by several fathers may benefit from the genetic diversity, Magurran says. Or females might regulate the quality of their eggs depending on the males around them. Magurran points out that female birds permitted access to preferred males contribute more testosterone to their eggs, resulting in faster-maturing offspring.

The new findings are consistent with other studies showing an active female role in offspring quality, says Anne Houde, a biologist at Lake Forest (Ill.) College. Female guppies breed eagerly with a brightly colored male if they've already mated with a dull-colored one, she says. The best explanation may be that by regulating egg fertilization, the female guppy essentially upgrades her earlier mate choices.

"Usually, [evolutionary biologists] imagine that the benefits due to the effects of inheriting good genes are pretty small, so it's exciting to see that there are such large effects in [Magurran's ] study," says Houde.