Why bats are more efficient fliers than birds

By Ker Than

updated 4:28 p.m. ET Jan. 22, 2007

Their motions might seem erratic and graceless, but bats are more efficient fliers than birds, thanks to an airlift mechanism that is unique among aerial creatures, new wind-tunnel tests show.

Previous studies that compared oxygen consumption among birds, insects and bats of similar sizes — a hummingbird, a small bat and a large moth, for example — found that bats use less energy to fly, but “no one’s really had an explanation for this phenomenon,” said study team member Sharon Swartz, an associate professor in ecology and evolutionary biology at Brown University.

The wind tunnel tests suggest the secret to efficient bat flight lies in the furry creature’s flexible skin membrane and its many-jointed wings, which together creates a shape-shifting structure that provides more lift, less drag and greater maneuverability.

Unlike insects and birds, which have relatively rigid wings that can move in only a few directions, a bat’s wing contains more than two dozen joints that are overlaid by a thin elastic membrane that can stretch to catch air and generate lift in many different ways.

This gives bats an extraordinary amount of control over the three-dimensional shape their wings take during flight, Swartz explained.

“Insects can move the joint at the insect equivalent of a shoulder, but that’s the only place where they can exert force and control movement,” she said. Birds have many more joints in their wings, but it’s nothing compared to bats.

“Bats are operating with the same skeleton that we have. Every joint in the human hand is there in the bat’s wing and actually a couple more,” Swartz told LiveScience. “Think about the degree of control that we have over the shape of our hands — bats are able to extend that to make fine-scale adjustments during flight.”

It was once thought that despite having so many wing joints, it was more efficient for bats to stabilize their wings and wave them up and down like relatively rigid paddles the way birds do.

“What we see when we look more closely is that in fact, it’s not what they’re doing,” Swartz said in a telephone interview. “It suggests that they’re able to take advantage of this highly jointed system to make subtle adjustments to the wing shape during flight.”

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