The wings of bats provide them with support and lift as they fly. But they are also giant sensors that tell bats about the flow of air around their bodies, helping them to execute sharp manoeuvres without crashing.
The wings’ ability to monitor airflow depends on tiny hairs that cover their surfaces. The hairs were discovered almost a century ago. Scientists suggested that they are sense organs that allow bats to fly in complete darkness. That idea fell out of favour in the 1940s when Donald Griffin and Robert Galambos showed that bats navigate by listening for the echoes of their own calls. The discovery of bat sonar solved the mystery of their night-time aerobatics, and the wing hairs fell into obscurity.
But John Zook at Ohio University had not forgotten about them. He has shown that the pre-sonar theories were partly correct. The hairs complement a bat’s echolocation and turn it into a better flier, allowing the animal to “feel” its way through the sky.
By studying the wings of two species – the big brown bat and the short-tailed fruit bat – Zook showed that their wings have two types of hair. The first looks much like the fur on the rest of their bodies, but the second is very different. These hairs are invisible to the naked eye, and they cover the entire wing. They’re barely half a millimetre long, and less than a micrometre wide at their tip – you could fit a bundle of 10 to 100 of these inside a single human hair. Each one sticks out of a domed structure and connects to a touch-sensitive cell.
Zook teamed up with Susanne Sterbing-D’Angelo from the University of Maryland to find out how the wing hairs work. They blew gentle puffs of air over different parts of the wing, while recording signals in a part of the bats’ brains that controls their sense of touch – the somatosensory cortex. They found that each bit of the wing is sensitive to air flowing in from a specific direction. For example, the hairs on the leading edge of the wing responded strongly to air flowing in from ahead, while those on the back edge responded to airflow from behind.
Based on this pattern, Zook suggested that the hairs allow the bat to detect turbulence. To fly properly, bats need a smooth flow of air across their wings. If the wing takes the wrong shape, that flow becomes warped into spirals and vortices. If the bat can’t correct itself, it could stall and crash. The hairs could help them avoid this by giving them constant information about the air around their wings.
“Bats can stabilize their flight successfully when confronted with wind gusts, very much in contrast to current small air vehicles,” says Sterbing-D’Angelo. “During flight manoeuvres involving large angles of attack, like hovering and steep banking, the risk of vortex formation and stalling is high. If the bat gets aerodynamic feedback from the wing hair sensors, it can take these manoeuvres to the limit.”
To test that idea, the team gave the wings a chemical shave, using a liquid that dissolves the keratin in their hairs. The liquid didn’t affect the wing’s general sense of touch, but it did remove all the tiny hairs. And without them, the bats lost their ability to respond to the puffs of air across their wings. They also became poorer fliers.
Sterbing-D’Angelo let them loose in an “artificial forest” – a maze of nets and posts in a darkened room. With their hairs intact, they managed to fly within inches of the obstacles, and avoided them by executing very sharp turns. Without the hairs, the bats still avoided the obstacles but they did so with wider turns and greater distances. On the sharpest of turns, their movements were jerkier than normal and they would erratically waver in altitude. They also flew slightly faster, perhaps to help them recover from stalls (human pilots use a similar trick).
Our planes have instruments called Pitot tubes that perform the same role, but in the natural world, the bats’ hairy sensors might be a unique innovation. Certainly, birds and insects control their flight using very different structures – insects, for example, use deformable bumps on the wings, special gyroscopes that evolved from wings themselves, and more. But pterosaurs – giant reptiles that flew over the dinosaurs – did have small fine hairs on their wings. Perhaps these played a similar role to the wing hairs of bats?
Reference: Sterbing-D’Angelo, Chadha, Chiu, Falk, Xian, Barcelo, Zook and Moss. 2011. Bat wing sensors support flight control. PNAS. http://dx.doi.org/10.1073/pnas.1018740108
Image by Furryscaly