Why bats are more efficient fliers than birds

Stretchy wings
The other key to a bat’s efficient flight lies in its highly elastic wing. Videos from the wind tunnel tests show that a bat’s wing is mostly extended for the down stroke during straightforward flight. But because the membrane can curve and stretch much more than a bird’s wing can, bats can generate greater lift for less energy.

By blowing non-toxic smoke over the bats as they were flying, the researchers were also able to create a video that revealed how air flows around the creatures as they flap their wings.

The data showed that during the down stroke, the air vortex — which generates much of the lift in flapping-wing flight — closely tracks the animals’ wingtips. But in the upstroke, the vortex appears to come from another location entirely, perhaps the wrist joint.

The researchers think this unusual pattern helps to make bat flight more efficient and credit it to the tremendous flexibility and articulation of the wing.

The findings, detailed in the Dec. 2006 issue of the journal Bioinspiration and Biomimetics, suggest the furry fliers might make good templates for flying machines.

“Bats have unique capabilities, but the goal is not to build something that looks like a bat,” said study team member Kenny Breuer, also of Brown University. “We want to understand bat flight and be able to incorporate some of the features of bat flight into an engineered vehicle.”

The complexity of bat’s wings also challenges some current theories that say bats evolved from some kind of flying squirrel-type creature.

“That might still be true, but what we know today is that although gliding appears to have evolved seven times in mammals,” Swartz said, “not a single one of those groups is closely related to bats.”

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