A Bat's Secret to Flying Like a Boss? Tiny, Tiny Hairs

Scientists show that bats use microscopic hairs on their wings to feel their way through the air.
Well hello to you too.
Well hello to you too.CYNTHIA MOSS LAB

It's hard to overstate just how amazing bats are. After all, they’re the only mammals with the true power of flight (the flying squirrel can glide its heart out, but that doesn’t really count). Bats can perform incredible banks and turns and dives a bird would envy: An eagle may be able to swoop in and snag a fish from a river, but a bat can pinpoint a tiny insect and run it down mid-air.

Bats have the power of echolocation to guide their fighter pilot-status flight, yes, but a team of scientists just uncovered an extra way the proto-vampires boost their hunts: They use microscopic hairs on their wings to feel their way through the air. The hairs act like levers of sorts, tripping nerves that send signals to the brain, allowing the bat to precisely correct its flight to avoid obstacles and pick off insects---no small feat in the pitch blackness.

Neuroscientist Cynthia F. Moss of Johns Hopkins University and her colleagues published the findings today in the journal Cell Reports. But their research began with an earlier experiment, in which they applied depilatory cream to bat wings---that is, they Naired them. Very carefully, of course. “The membrane is very, very thin, it’s about 40 or 50 microns, very delicate,” Moss says. “And so we diluted the depilatory cream and put it on briefly and cleaned it off, and that removed the hairs." The bats could still fly in a dark room without the hairs, but not quite as well as before.

While they didn’t exactly slam into things in the darkness, the de-haired bats didn’t slow as quickly as they approached obstacles. And their turns were wider, which is telling considering bats can make extremely tight banks. It seemed that they could still form a picture of their surroundings with echolocation, but without the hairs, the subtle precision of their flight was gone.

Moss' new study, focusing on North America's big brown bats, details what’s going on here. The researchers applied puffs of air to the wings of shorn and non-shorn bats while measuring activity in a part of their brains known as the somatosensory cortex, where things like touch and pain are handled. Sure enough, the brains of non-shorn bats lit up in response, while the brains of the bats with the missing hair did not.

One of the many sensory hairs on a bat's wing.

Cynthia Moss Lab

That isn’t surprising when you consider that there’s a disproportionately large number of sensory cells, called Merkel cells, that are associated with these hairs. "In addition there were these other receptors called lanceolate endings, which are also associated with hairs,” says Moss. She thinks those receptors transmit information about hair movements that occur both during flight and as bats use their wings in other surprisingly dextrous ways, like when they envelop prey or cradle their young.

A bat’s wing, you see, is highly manipulable compared to a bird’s. Much of the muscle in a bat’s wing isn’t actually attached to bone, so by contracting or relaxing its muscles, a bat can change the stretchiness of its wings so it can make turns or hit the brakes. And that level of control is only possible because of the combination of echolocation data and this newly discovered air flow info.

Survival of the Illest

The discovery of the function of these hairs has interesting evolutionary implications as well. Mammals, quite frankly, have no business being in the air. They’ve got heavy bones and a rather glaring lack of wings. But over evolutionary time, bats rigged the system. Like flying squirrels, they probably started off gliding, and got better and better at powered flight. “Not all bats are insectivorous, but many are,” Moss says, “and so with the ability to fly, they could chase after insect prey in the dark, using their echolocation to track the prey in the absence of light.”

But that wasn’t enough. “Over millions of years the ability to execute fine maneuvers in true powered flight evolved, and that really required fine sensory motor control,” Moss adds. “So it's not just, OK, I set myself to glide. I've got to feel the air, feel the stretch in my wing, and know how to make adjustments.” That’s where the hairs came in. At what point they came in during the evolution of bat flight isn’t clear, but what’s abundantly clear is that bats have arrived at a remarkably effective system to not only hear their way through the dark, but feel their way too.

As it happens, just yesterday another team of scientists reported that they’d found a bizarre fossil dinosaur that had both long, thin, bat-like fingers, which could have supported a membrane, plus the feathers of a bird. Whether it was capable of powered flight or just gliding isn’t clear, but it’s yet another twist in the evolution of flight on Earth. Pterosaurs (which weren’t technically dinosaurs) had membranes, bats have membranes, birds have feathers (and kind of a membrane, but it’s obscured by feathers), and now Yi qi, as the new dino is known, had both. There’s been a whole lot of experimentation among critters who’ve taken to the air, and science is still trying to piece together how flight has evolved. Hell, science is clearly still figuring out how bats do it so well in our modern world.

Really, it’d be hard to argue that anything flies as nimbly as the bats, thanks to that partnership of sensory hairs and echolocation, and Moss thinks that her research could have big implications for human technology. “All of these sensors collectively guide not only the flapping and the exact wing position, but also the wing shape,” she says. “So in theory, if there's interest in developing highly maneuverable unmanned vehicles, having additional sensors, having the ability to control the shape of the wing could bestow greater agility and maneuverability.”

Whether or not these flexible, bat-wing drones will hunt insects is to be seen. But it never hurts to dream.