10 MAY 2007
How do bats do the things they do? Some catch insects in midair. Others take a page from the hummingbird's playbook and hover while they drink nectar from flowers. A study published this week gives a first answer to the aerodynamics of bat flight, and it turns out to be surprisingly complicated.
All images courtesy Anders Hedenström, © Science
Although bats and birds are both vertebrates, they evolved flight separately. Bats move the air with thin skin rather than feathers. And the bones in bat wings reach the wingtip, while birds have short wing bones and long flight feathers.
These anatomical differences create a distinct mode of flight.
Bats, birds and airplanes all create lift due to the shape of the top and bottom of the wings. When the wing moves through the air, it creates a low-pressure zone above it that counteracts gravity. Moving objects in general also create vortices—tiny horizontal tornadoes that sap the object's kinetic energy, but also supply clues about the interaction of flying object and air.
The complicated aerodynamics of bat flight may help explain the animal's extreme maneuverability, says Anders Hedenström, professor of theoretical ecology at Lund University (Sweden), who was first author of the paper in Science. Hedenström studied Glossophaga soricina, a nectar-feeding bat native to Mexico and South America, partly because it's easy to study when it hovers to slurp honey-laced water from a tube.
"We work in a wind tunnel, and we have quite a bit of experience working with birds in this setup," he says. "The main problem in these experiments is that you want an animal that repeatedly flies to a predictable point, so you can direct the cameras and the laser light sheet to the animal's position."
"Laser light sheet" refers to the illumination that Hedenström used to track air movement. In this so-called digital particle image velocimetry, the laser illuminates one plane near the bat, and the camera detects droplets of water vapor in the air. The swirls and vortices in the wake behind the animal give a good picture of how it affects the air, and while there have been several studies of bird flight using this technique, Hedenström says, "This is the first comprehensive view of the bat wake, with modern technique."
Birds create a single, continuous vortex loop as a result of the downstroke during slow flight, while bats create separate vortices from each wing, says Hedenström. "Vortices form and reform within each beat. Mainly it's one loop from the downstroke and a new one from the upstroke. When speed increases, there is almost a continuous vortex," he says. "We don't know if insects do this ... but among vertebrates, this is a new aerodynamic effect. It was totally unexpected; it was never predicted from any flight model."
The absence of feathers may help explain the complicated and distinct nature of bat flight. At slow speed, bird wings allow air to filter through the feathers on the upstroke. This cannot occur in a membrane-covered bat's wing; on its upstroke, the outer wing produces a downward force, while the inner wing produces lift.
Courtesy Anders Hedenström, © Science
Bird wing bones are also shorter and stiffer than bat wing bones; birds rely on long, flexible flight feathers to efficiently push air on the downstroke. The wing bones of bats (which are mammals, after all) feature distinct "finger bones" that reach the wingtip. Other researchers have found that bat wing bones flex, much like those flight feathers, Hedenström says. "That is what was so fascinating," he says. "Evolution has made two independent solutions to the same problem. They are completely different, but they show in many respects very similar performance measures."
The multiple lifts and vortices of the bat wing may help it maneuver, Hedenström says. "At the slowest speed, while hovering, we found higher momentum in the wake than we would expect from ... conventional aerodynamic models. The wings generate higher lift forces than we first thought, and higher force gives more capacity for acceleration."
Finally, because each wing creates a separate vortex, "You could also say that the wings are more aerodynamically independent of each other, which might also help in tight maneuvering flights."
— David J. Tenenbaum
• Bat Flight Generates Complex Aerodynamic Tracks, by Anders Hedenström et al, Science, May 11, 2007.