Flap Your Hands

To fly like a bat, you need flexible hand bones and stretchable skin across your fingers.

Story by Adam Summers   ~  Illustrations by Shawn Gould

Both the Boeing Company and bats (the furry, flying mammals) are leaders in aeronautical performance and versatility, yet they have strikingly different approaches to getting (and staying) off the ground. The kind of flight most of us have experienced begins with a stiff, strong airfoil, one that undergoes few changes of shape in flight. Built out of aluminum alloys and carbon-fiber composites, rigid wings provide the steady airflow needed to generate lift that is orderly, predictable, and well understood.

Bat flight is an entirely different affair. Rigid, strong, and heavy are out. Thin, whippy bones, stretchy skin, and wings that billow and change their shape with every stroke are in, a central part of the picture. Sharon M. Swartz, a biologist at Brown University in Providence, Rhode Island, and her students Kristin L. Bishop and Maryem-Fama Ismael Aguirre are investigating the fluttering flight of bats with both hands-on tests and computer simulations. They are learning what works, and what doesn't, when fliers must contend with unsteady airflows and with airfoils that continuously deform.

Nearly a quarter of all mammal species are bats, and they are the only winged animals in the class Mammalia. All bats belong to the order Chiroptera, meaning "hand-wing." They range from the bumblebee-size Kitti's hog-nosed bat to that fluttering horror, the vampire bat, to the Malayan flying fox, the largest species.

A bat's wings are not only different from a 747's; they are also quite unlike the wings of a bird. They lack feathers, obviously. And although the humerus, radius, and ulna of birds are quite similar to the humerus and radius of bats (which have only a vestigial ulna), avian hand bones have largely fused [see illustration above]. But bats' carpal bones conjoin at a point about halfway along the leading edge of the wing; the bones of the short, clawed first finger (homologous to our thumb) jut forward. The long second finger forms most of the distal half of the wing's leading edge. The third finger runs closely behind the second, but all the way to the tip of the wing. The fourth and fifth fingers run from the leading edge to the trailing edge of the wing, and stretched across all the fingers is a thin, flexible skin [see illustration below].

Bones don't bend-at least that's the message we get after an orthopedist applies a cast to the results of a misjudgment. But the bones of a bat's fingers have adaptations that promote bending. The digits' cartilage lacks calcium toward the fingertips, making them less apt than ordinary bone is to splinter under stress. Also, the cross section of the finger bone is not circular, as is the bone in a human finger, but flattened. This shape further encourages flexion (think about how much easier it is to bend a soda straw if you first give it a squeeze to flatten the thing).

Imagine wanting, as Swartz did, to measure how much bat wing bones bend. It's not easy. When bats fly, their wings flail up and down in such a complex path that a three-dimensional reconstruction of the flight would be impossible, even from a movie. Swartz and her colleagues David Carrier of the University of Utah in Salt Lake City and Michael Bennett of the University of Queensland in Brisbane solved the problem about a decade ago by gluing minute metal-foil strain gauges directly to the bones of bats.

The bat they studied was the gray-headed flying fox (Pteropus poliocephalus), about the size of a small chihuahua and sporting a nearly four-foot wingspan. It's huge for a bat, but just barely large enough to support the scientists' gauges. In the initial study, Swartz and the others attached gauges to the humerus and radius of the flying foxes; in later work, Swartz attached them to the fingers, between both the first and second and the second and third knuckles (to the proximal and medial phalanges, as an anatomist would say). As the animals flew about inside a long, spacious cage, the bending of a bone would also flex the gauge, thereby changing the electrical resistance in the foil. The tests demonstrated that the wing bones, about the same length as a person's index finger, deformed three-quarters of an inch or more with every beat of the wing.

Swartz went on to develop a computer model of bone deformation during flapping flight. She found that not only are flexible bones vital for bat flight, but so too is the skin that covers the hand-wing. The skin of most mammals can stretch equally in every direction, but bat-wing skin has many times more give along the direction between its body and its wingtip than it does between the leading edge and the trailing one. And when the skin billows out as the bat flies, it is stiff enough to transmit substantial force along the length of the wing and generate lift. In fact, if the skin were any stiffer, the delicate finger bones, despite their flexibility, would probably break.

The computer models, taking into account bones, skin, and the usual motions of flight, suggest that there are some limits to being batty. For one thing, a fruit bat that flies home with a mango in its mouth is pushing the limits of its flight equipment. The model predicts that even though the stresses of unladen flight bend finger bones less than halfway to breaking, the addition of a heavy fruit brings the bones dangerously close to failure. Counterintuitively, the model also predicts that heavier bones would cripple a bat. Its thin wing bones make up just 5 percent of the animal's weight, but if the bones' weight were doubled, the stresses on them would increase to dangerous levels rather than diminish. The wings' very lightness contributes to the safety of flight.

The computer model also makes clear that a bat's aerodynamics are far removed from those of fixed-wing airplanes. Unsteady airflow and flexible airfoils are the province of bat flight, and given the skittish nature of the average air traveler, those features are not likely to cross over to commercial aircraft. But because the complex movements of a bat's bones and skin do not require intricate muscular control, engineers still might try their hand at mimicking the bat's complicated but passive wing-designing a structure whose variable flight surfaces wouldn't require a motor at every joint. Perhaps, just as the wings of houseflies have been co-opted for microflyers, disembodied bat wings will also become an attractive option for flyers of medium scale-if not for Bruce Wayne in Gotham City, then for the designers of small, unmanned reconnaissance vehicles.

Adam Summers is an assistant professor of ecology and evolutionary biology at the University of California, Irvine (asummers@uci.edu).