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Bats and birds use their wing-flapping powers differently, a video study of the night flyers reveals. Bats leave a more complicated pattern of swirling vortices in their wake, and work harder on their upstroke than birds. This, the researchers say, may be the secret to their super-maneuverability. And it means that they fly a bit like a bee.

Anders Hedenström, from Lund University, Sweden, filmed small nectar-feeding bats in a stream of fog, so that the fog particles could be used to visualize the shape of the air currents after each stroke.

Researchers know from previous work that birds create a vortex in the air behind each wing, then both of these combine into a single loop of air. This creates a minimal amount of turbulence and drag behind the bird. But the vortexes behind bat wings stay separate from each other, and each wing is, in essence, operating in isolation to the other. Although this is less aerodynamically efficient, the bat uses it to its advantage. Having independent drag on each wing could be used to help it turn more quickly.

"Bats are obviously extremely good flyers," says Graham Taylor, an animal behaviourist at Oxford University, UK, who agrees that independent wing flapping might be in part responsible for the bat's flying prowess.

Hedenström's research also shows that at low speeds the bat creates a large force as it flaps its wings upwards. Birds deliberately avoid this by spreading their feathery wing tips apart. But again, this doesn't have to be a bad thing for the bat.

"Bats can use the backstroke for a useful purpose," says Hedenström. "It generates lift." Its membranous wing twists on each stroke and uses the wind against it in the same way as a sailor uses wind to create force in the desired direction — in the bat's case, up.

Hedenström suggests that in this way the bat is like a vertebrate version of the bumble bee, which also generates a huge upward lift on the backstroke.

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• References

  1. Hedenström A., et al. Science, 316 . 894 - 897 (2007). | Article | PubMed |