ONE of the great mysteries of vertebrate evolution – how bats acquired the ability to fly – may be a little closer to being solved, thanks to a new aerodynamic analysis of the shift from gliding to flapping flight.
Bats, together with birds and the now-extinct pterosaurs, are the only vertebrates capable of true powered flight. Most experts agree that bats evolved from ancestors that glided, but no intermediate fossils exist to suggest how they made this transition. “It’s been a topic that people have shied away from because of the lack of fossil evidence,” says Kristin Bishop of the University of California at Davis.
To learn more, Bishop surveyed what is known about the aerodynamics of gliding and flapping. Her results suggest that the gap between the two is indeed large. Gliders such as flying squirrels have short, stubby (low aspect ratio) “wings” (see Diagram). They tend to “surf” in a nose-up position, gaining lift – but also suffering drag – from air hitting the undersurface of their wings and from vortices swirling around their relatively broad wing tips. By contrast, the high-aspect-ratio (long, pointed) wings of flappers such as bats generate plenty of lift with minimal drag. Because of these basic differences in wing performance, gliders never try to flap and few bats ever try to glide, suggesting that a wing shape suited to one form of flight does poorly at the other (Quarterly Review of Biology, vol 83, p 153).
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To understand how ancestral bats made the transition from gliding to flapping, we need to know more about the aerodynamics of wings of intermediate aspect ratio, Bishop says. She suggests that gliders faced declining lift-to-drag ratios as their wings evolved towards a shape suited to flight, and that they coped by adopting a rudimentary form of flapping. Tests on intermediate wing shapes should reveal if flapping is able to maintain the degree of lift required. “All the experiments have been done on stationary wings. We don’t know what flapping might have contributed,” she says.
Bishop’s ideas help define what to investigate next, says Brock Fenton at the University of Western Ontario in London, Canada. His favoured hypothesis concerning flapping is that it helped early bats gain better control over gliding so that they could use echolocation to catch airborne insects at night, tapping into an ecological niche no other flying animals were exploiting. “You get more to eat, because there’s nobody else playing that game,” he says.
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