That's Why the Lady is a Tramp

When a new queen begins her reign over a colony of honeybees, she gives up her virginity with gusto. Venturing outside the hive and flying to a height of 6 meters or more, the queen mates in midair with a dozen or so male bees, called drones, who all die after ejaculating. The reason for this acrobatic orgy--polyandry is the polite term--has long been a puzzle.

Here's the conundrum: When the queen buzzes back to the hive and lays eggs, she fertilizes them with sperm from the various drones she mated with. That means many of the female workers will be half-sisters, and these bees should be less likely than full sisters to work for each others?benefit, at least according to the theory of kin selection. So why doesn抰 the queen mate with a single male and keep the hive as one tight family?

According to a study in the 20 July issue of Science, a genetically diverse hive can be vastly more productive than a homogenous hive of sisters. Heather Mattila, a postdoctoral fellow in ecology at Cornell University in Ithaca, New York, was inspired to undertake the research after her adviser, Thomas Seeley, discovered that genetically diverse hives better resisted a common bacterial disease. He and a colleague concluded that disease resistance is an adaption that would favor the evolution of polyandry. Mattila thought there might be more to the story, and Seeley bet her an ice cream there wasn抰.

In June last year, Mattila set up a dozen hives in which the queen had been artificially inseminated with sperm from 15 drones. Near them she placed another nine hives with queens who had sperm from just a single drone. All hives were treated with drugs to prevent disease, thus controlling for the effect of diversity on disease resistance. Within the first 2 weeks, bees in the diverse hives had built 30% more honeycomb and were more active at foraging. All that work paid off: The diverse bees collected 39% more pollen and nectar, and by August, their populations were five times larger. "It's a real snowball effect," Mattila says. She won her bet, and Seeley treated her to two scoops.

"This is an important paper because it demonstrates genetic diversity can lead to a marked increase in performance," says entomologist and neuroscientist Gene Robinson of the University of Illinois, Urbana-Champaign. Previous studies of the impact of diversity on hive function had yielded inconsistent results, he notes, but the new one is the longest and most comprehensive. It's not clear why diverse hives would be more productive, but one theory is that the bees are more sensitive to work-related stimuli and thus more likely to take on various tasks. As for the reported disappearance of honey bees and collapses of hives (Science, 18 May, p. 970), Robinson says that lack of diversity isn't thought to be a cause. The queens in most commercial beekeeping operations are mated with multiple drones.

In other bee news, another paper in Science shows how the queen bee might help maintain order inside the hive. A pheromone emitted by the queen prevents young bees from learning when to sting. Once the bees mature and leave the hive to forage, the effect wears off and they learn how to use their stingers. Robinson says that study opens "a rich stew of questions."