19 JULY 2007
Moving past monogamy: Honeybees beg to differ!
Humans mainly mate on the principle of one man, one mate, although some people have a thing for cutting corners....
Honeybees have a different take on the matter. One queen, many mates. Call it polyandry.
Photo: State of Missouri
In the average colony, one queen mates with 10 to 20 males, says Heather Mattila, a post-doctoral fellow in the department of neurobiology and behavior at Cornell University. We immediately intuited the origin of the phrase, "queen bee," then turned to an evolutionary conundrum: How come?
If a queen has one mate, all the other members of her hive are her full-sisters. And according to evolutionary theory, full-sisters have a strong incentive to act altruistically. And what could be more selfless than bees, which will sting intruders to protect the hive -- and then die.
Biologists have explained this altruism as follows: If you promote the survival of close relatives, they will pass your genes along. You have become redundant, so in terms of protecting the hive, it's okay to die trying...
But if a hive has a dozen fathers, your return on your "ultimate investment" is diluted: Dying to protect your half-sisters is dumber, in evolutionary terms, than dying to protect your full-sisters, because it is less effective at promoting the appearance of your genes in later generations.
Courtesy Heather Mattila
Failure is an orphan, but success has 15 fathers
Could bees know something we don't? Perhaps, and here we get to the results of an intriguing, long-term comparison of honeybee colonies descended from one male (the "full-sister" colonies), to colonies with 15 fathers (the "diverse" colonies).
Mattila and Cornell professor Thomas Seeley put 9 full-sister colonies and 12 diverse colonies through an experiment designed to simulate the normal struggle for survival in bee-dom. Bee colonies face a big test each summer, when one queen and a few thousand bees "swarm" from the hive and try to establish a new colony.
Swarming bees must immediately build enough comb to house the new colony, and amass enough food to satisfy the munchies. Swarming often fails.
A lotta dad will do ya!
So how did the full-sister colonies compare to the diverse colonies?
When the foraging was good, the diverse colonies increased their weight by 305 percent, compared to 163 percent for the full-sister colonies.
The workers in the diverse colonies were more active, and by the end of August, those colonies averaged 26,700 workers, versus 5,300 for the full-sister colonies.
Every diverse colony survived a cold stretch in late August that snuffed five of the nine full-sister colonies.
The diverse colonies produced about eight times as many drones as the full-sister colonies. Because drones are gents that can distribute a colony's genes, they are a key measure of "genetic fitness." (In genetics, this aerobic-sounding term is defined as the ability to move genes to successive generations and new populations.)
Five of the diverse colonies survived the winter, but all of the full-sister colonies perished.
On every measure, the results provided a stunning contrast between the two strategies. "I was pretty surprised, I thought maybe there would be one [full-sister] colony that would have the right genes and do as well," says Mattila. Within a couple of days of entering the new hive, the diverse bees were already building more comb than the full sisters, she adds. "It wasn't that the bees necessarily looked more active to the eye, but it was obvious by how much comb they made."
Photo by Dr. S.L. Buchmann, Courtesy USDA
A honey of a study
So why is genetic diversity so helpful? The benefit may lie in bees' genetic predisposition to adopt certain tasks: Some lineages may be better at building comb, say, while others are more likely to collect pollen or nectar. If you need to fill a lot of jobs, it obviously helps to have a range of talents in the hive.
But whatever the specific advantage of genetic diversity, it clearly overpowers the benefits from selfless behavior that results from genetic uniformity, Mattila says. "Mathematically, it is better for a worker to rear her own offspring than her less-related sisters," because that does a better job of promoting the reproduction of her genes. "Why don't these workers start rearing only their own offspring? With honeybees, the benefits from extreme polyandry must outweigh any fitness they would gain by a different strategy."
Just wondering: Does this make you bee any more receptive to the idea of polyandry?
-- David Tenenbaum
• Genetic Diversity in Honey Bee Colonies Enhances Productivity and Fitness, Heather R. Mattila and Thomas D. Seeley, Science, 20 July, 2007.