Need more bats? Learn about all the amazing things bats can do in our radio interview with Sharon Swartz and Cynthia Moss, and check out a spotlight on batty science.
You’d think that bats and birds fly in similar ways—in fact, many scientists used to consider bat flight a minor variation of bird flight. But, with the aid of high-speed video, researchers have discovered that bat flight is much more complex than initially thought.
Sharon Swartz, a professor of biology at Brown University who studies bat flight and the structure of their wings, and Kenny Breuer, a professor of engineering at Brown University who studies animal flight and fluid mechanics, teamed up to study how these tiny mammals actually fly. Their research lab includes an eclectic mix of high speed cameras, motion tracking, flight corridors, wind tunnels (or a “treadmill for flying animals,” as Swartz puts it), and more to solve this batty mystery.
“When we first started, really very little was known about the precise nature of bat flight,” Breuer says. “We’re really interested in how [bats] have evolved to generate these kinds of forces of motions.”
So, how exactly does bat flight differ from bird flight? For one, bats have way more flexibility and control over their wings—bats have a whole hand in theirs, and are made of muscle and skin. Bird wings, by comparison, are a “relatively stiff airfoil,” says Swartz.
“That allows [bats] to change the conformation and shape of the wing with incredible dexterity and precision,” says Breuer.
“We don’t usually think of skin as being a muscular organ,” explains Swartz. “But the skin of the wing membranes of bats is invested with a whole series of muscles.” Bats can use these muscles to change how stiff the skin in their wing membrane is, allowing them to have minute control over their flight path.
Unlike bird wings, “[bat wings] bend, flex, and puff out,” says Breuer. This manifests in flight behaviors that are unique to bats, like being able to land upside down. “They have to slow down. They have to flip themselves upside down and land hanging onto the ceiling or hanging onto a tree roost. It’s like doing a high dive backwards,” says Breuer.
Due to the control on their wings, bats are able to recover quickly from the challenges of flight, like gusts of wind. The researchers tested this by constructing an apparatus of lasers and air jets. When the bat flew through the laser beams, it triggered a puff of air, aimed directly at the bat.
“We find that even [with] really strong gusts of wind, they recover stability in less than a single wing-beat. So what we’re trying to understand now is, what are the mechanisms that they used to recover so quickly?” says Swartz.
To find the answers, Swartz and Breuer use the motion-capture data to reconstruct the bats’ wing movement inside the computer—and even make mechanical, robotic wings that mimic the movements of bats. The mechanical wings allow them to run tests “that we can’t ask the animals to do,” Breuer says. “And it does provide a lot of inspiration and ideas of things that we might try for building robotic flying vehicles.”
The researchers still have several questions they hope to answer: How did flying animals evolve different methods of flight? Do the bats actively control the wing when reacting to a gust, or are the movements passive?
Swartz finds it all compelling. “I love those moments where you’ve recorded something that no one has seen before, a moment of insight into the natural world. It doesn’t matter how tiny it is. There’s nothing like that.”
SHARON SWARTZ: Before we had high speed video, people had this view that bat flight was just kind of a minor variant of bird flight. But what we found over and over again is how unbelievably maneuverable these animals are.
KENNY BREUER: Being able to manipulate their wings and their bodies in such a way that they can adjust and maneuver really boggles the mind
SHARON SWARTZ: A bat wings framed by the skeleton allows bats to have a kind of control over a three dimensional shape that would be impossible for any other kind of flying animal.
SHARON SCHWARTZ: I’m Sharon Schwartz. I study bats, how they fly, and the structure of their wings.
KENNY BRYER: And I’m Kenny Bryer. I’m a professor of engineering and I study animal fight and fluid mechanics.
KENNY BREUER: So the collaboration between Sharon’s lab and my lab allows us to approach the same problem from two different perspectives.
SHARON SWARTZ: We really find that we can do much more interesting things together than either of us can do by ourselves because we’re able to combine aerodynamics with the study of the morphology of the wings.
SHARON SWARTZ: There’s a lot of really fundamental differences between the flight of birds and bats. So a bird wing is a relatively stiff airfoil.
KENNY BREUER: Bats have a whole hand in their wing and that allows them to change the conformation and shape of the wing with incredible dexterity and precision.
SHARON SWARTZ: So the bones of the part of the wing that’s closest to the shoulder, the humerus and the radius, have the kind of geometry that we see in birds. But once you cross the wrist joint, we see bones that are less mineralized. And that makes that bone itself less stiff. It actually promotes spinning.
SHARON SWARTZ: We don’t usually think of skin as being a muscular organ, but the skin of the wing membranes of bats is invested with a whole series of muscles. And what we observe is that the muscles turn on and off in every wing-beat cycle. And so these muscles can change the stiffness of the skin in the wing membrane. And so that means the muscles change the aerodynamic properties of the airfoil.
KENNY BREUER: And that’s completely different from a bird in the way in which it operates. [bat wings] It bends, it flexes, it puffs out.
SHARON SWARTZ: So they’re able to continue to generate lift even as they’re moving more slowly.
KENNY BREUER: So when we first started, really very little was known about the precise nature of bat flight. We are really interested now in how the animal has evolved to generate these kinds of forces of motions. What can we learn about thrust, about lift, about unsteady flight mechanisms, about muscle activity. And we design these experiments at each stage to move ourselves forward.
KENNY BREUER: So when we do our test, we use two facilities. One is a flight corridor which is just a room. We have our camera set up in there.
SHARON SWARTZ: Just being able to see, in detail, how bats move their wings has turned out to give us a lot of insight.
KENNY BREUER: The other one is this wind tunnel.
SHARON SWARTZ: The equivalent of a treadmill for a flying animal.
KENNY BREUER: We take high resolution, high frequency motion of the wings from multiple angles and we reconstruct the kinematics of the motion that way. And then we fill the wind tunnel with a cloud and we record the motions, the particles of that cloud, and from that we can reconstruct the wake.
SHARON SWARTZ: And that lets us learn a lot about how it uses the wings to produce aerodynamic forces.
KENNY BREUER: Once we take measurements with the with the animals, we recreate aspects of that using engineered robotic flapping wings that we test in the wind tunnel and there we can do things that we can’t ask the animals to do. And it does provide a lot of inspiration and ideas of things that we might try for building robotic flying vehicles.
SHARON SWARTZ: So one of the things that bats do extremely well is landing.
KENNY BREUER: They have to slow down. They have to flip themselves upside down and land hanging onto the ceiling or hanging onto a tree roost. It’s like doing a high dive backwards.
SHARON SWARTZ: What we’ve found is that during the last two wing beats of a bat preparing to land, there there’s almost no aerodynamic force produced.
KENNY BREUER: They also use the mass in their wings to manipulate their body and that controls their rotation in the same way that a high diver controls her rotation when they dive.
KENNY BREUER: The bats are incredibly agile and maneuverable and they are very resistant to motivation to the air, to gust.
SHARON SWARTZ: If we want to understand how bats are able to do this so well, we have to have some way of providing a gust to the animal in the lab and then seeing what it does in detail.
DAVID BOERMA: We have two sets of laser cross beams here, so that when the bat flies through, the bat breaks the laser beams. That sends a trigger signal. The air jet delivers a puff of air and we can capture all of that in high speed video from above and below using an array of high speed cameras.
SHARON SWARTZ: We find that even really strong gusts of wind, they recover stability in less than a single wing-beat. And so what we’re trying to understand now is, what are the mechanisms that they used to recover so quickly? What is it about the properties of the body and the wing that might be turned control passively and how much is active?
KENNY BREUER: The ability to do these experiments really gives us a unique insight as to how these animals move and maneuver. And also just how they evolve. What is the evolution of flights in mammals
SHARON SWARTZ: I think that we understand enough now about how flight works where we can look at the origin and diversification of that flight to beautiful evolutionary laboratory.
SHARON SWARTZ: I love those moments where you’ve recorded something that no one has seen before, a moment of insight into the natural world it doesn’t matter how tiny it is. There’s nothing like that.
Produced by Luke Groskin
Music by Audio Network
Footage ands Stills Provided by Kenny Breuer and Sharon Swartz
Joe Bahlman, Atilla Bergou, David Boerma, Rhea von Busse, Jorn Cheney, Nick Hristov, Tatjana Hubel, Nicolai Konow, Lauren Reimnitz, Andrea Rummel, Cosima Schunk, Dave Willis, Dan Riskin, Hamid Vejdani. Bat Research supported by NSF, AFOSR and Brown University
All procedures involving animals were performed in an AAALAC-accredited facility in accordance with the Guide for the Care and Use of Laboratory Animals and approved by the Brown University Institutional Animal Care and Use Committee.