Wantonness is everywhere. From honeybees to humans, females often like to have their children fathered by more than one male. Biologists have always had little hard data on why this should be so. But a report in today's Science shows that in ants, at least, promiscuity pays biological dividends.
The benefits that a female gains from polyandry are not immediately obvious. A single suitor can fertilize all of a female's eggs with sperm to spare. So why does she bother to waste energy by copulating with other mates? Scientists have speculated that by striving for the maximum genetic variability in her offspring, a female might ensure that many of her offspring survive the trials of life.
That is exactly what Blaine Cole and Diane Wiernasz of the University of Houston discovered in the western harvester ant, Pogonomyrmex occidentalis. Each year in Colorado, queens and males of these ants make their way to hilltops where they engage in unbridled copulation. "They look like clusters of grapes with all of the males hanging on the females," says Frank Trampus, a graduate student in Cole's lab. The morning after, the queens depart to establish their own colonies.
Some queens, it appears, play blushing brides during the mating frenzy. When the researchers took genetic profiles from 1492 nests, they found that the workers in some nests had all been fathered by a single male, while the queens in other nests clearly had mated with scores of males. After censusing all nests over 4 years, the scientists found that the more wanton a queen, the better her nest fared. The populations in multipaternity nests soared, increasing up to 35 times faster than single-father ones.
"The data set is really impressive," says Boris Baer, an ecologist from the Federal Institute of Technology in Zürich, Switzerland. Baer, who studies polyandry in bumblebees, suspects that the nests of promiscuous ants benefit from increased disease resistance. Because the workers are unrelated, they will differ in their susceptibility to parasites, so a disease may not easily spread through a colony, he says.