Bumblebee energy: what's the buzz?

Bumblebee energy: What's the buzz?

It flaps its wings an astonishing 160 times a second and consumes the caloric equivalent of 180 candy bars an hour. But exactly how the short-winged, big-bodied bumblebee stays aloft remains a mystery. Now, researchers have succeeded in measuring bumblebees' oxygen consumption during forward flight, obtaining the first such data for any free-flying insect. The feat represents a major step toward comprehending the physiology and mechanics of insect flight, they say.

It's no breeze measuring the oxygen consumption of a bee, explains study director Charles P. Ellington of the University of Cambridge in England. Miniature gas masks won't do for an animal that has more than 24 breathing holes. And commercial instruments can't detect changes in atmospheric oxygen concentration down to bee-sized levels of 5 parts per million or less. So Ellington, with co-workers at Cambridge and Rutgers University in New Brunswick, N.J., designed a highly sensitive and rather unorthodox apparatus for measuring the effect of flight speed on oxygen uptake in a mix of queen and worker bumblebees.

They inserted more than 100 bumblebees, one at a time, into a sealed wind tunnel 17 centimeters long. Though most of the captives refused to flap their wings, six bees buzzed along with the tunnel's speed-controlled airstream. Minuscule differences in gas pressure between this chamber and an identical but empty one revealed the insects' rate of oxygen consumption. The scientists eliminated extraneous pressure fluctuations that would have given a false reading by directing incoming air through a deflated condom, which Changed its volume slightly to equalize air pressure.

Though Ellington says the results didn't surprise his team, others may find them downright bee-wildering.

Hovering bumblebees consume about the same amount of oxygen -- and thus use the same amount of energy -- as bees cruising at speeds of up to 4 meters per second, the researchers report in the Oct. 4 NATURE. This matches the oxygen-uptake pattern detected in birds and bats, Ellington notes. In the January JOURNAL OF EXPERIMENTAL BIOLOGY, he and graduate student Robert Dudley presented an aerodynamic analysis suggesting that forward flight at moderate speeds requires about the same mechanical power as hovering. That might help explain the comparable oxygen uptake of hovering and forward-flying bees, Ellington says.

He points out, however, that the oxygen findings fly in the face of standard theories of animal aerodynamics, which maintain that hovering requires more oxygen due to the apparently greater exertion needed to stay aloft without moving forward. Theoretically, only at higher speeds -- when atmospheric drag increases significantly -- should the oxygen demands of forward flight equal or exceed those of hovering.

"The theories that everybody uses are too simple," Ellington says. "The main message of our paper is to tell people to stop using them."

Study coauthor Timothy M. Casey of Rutgers adds that by measuring oxygen uptake in hovering bees, researchers can reasonably estimate in-flight oxygen consumption, thus avoiding the need for the custom-built apparatus.