Image of B2, a robotic bat designed by collaborators at the University of Illinois (Photo and Design Credit: Drs. Seth Hutchinson and Soon-Jo Chung, University of Illinois)
Around 7:30 P.M. near the east shore of the Panama Canal, five of us gathered around the bed of an old, white pick-up truck. Strung between the rims hung twelve small cloth bags, each wriggling from the bats we netted only minutes before. Among the people was Sharon Swartz, anxiously awaiting the bag housing a favorite bat of hers—Noctilio albiventris or the lesser bulldog bat—which, after removed and measured, was carefully handed to her. As she held the bat, Sharon gently stroked its head, studied its wings, and spoke softly to it in adoration, almost as a mother to her precious child. It was an intimate moment, one that testified to a love cultivated by many years of exploration and fascination.
For Sharon, though, such tender moments are all too rare these days. As Professor in both the Department of Ecology and Evolutionary Biology and School of Engineering at Brown University, her schedule is packed to the brim with teaching, mentoring, writing, administrative work, and community outreach, many of which she has received awards for. But during the rare gaps in the hustle and bustle, she takes a moment to admire bats, the animals that have motivated the last 26 years of her research.
From her beginnings as a budding female scientist at the University of Chicago, Sharon’s interests focused on how animal anatomy and biological materials both evolve with the environment and function in locomotion. Those foundational interests led to investigations on subjects such as the mechanics of swinging in gibbons and gliding in flying squirrels, but bats have provided an unending source of biological and mechanical interest for her.
For an engineer, bat wings and flight are particularly attention-grabbing. Of extant flying animals, only bats use a wing made of a thin, skin membrane, roughly one-third the diameter of a human hair. Embedded within it are arrays of tiny muscles with still unknown functions, as well as elastin fibers that make the wing compliant or “stretchy”. Consequently bat wings look, and sometimes act, a little more like a parachute than a wing, making them a far stretch (pun intended) from the design and even theory of rigid airplane wings. Somehow, though, bats take to the air every night, where they fly with agility and dexterity.
So how do they do it? What is it that Nature knows that we don’t?
To answer such questions, Sharon, her Brown School of Engineering collaborator Kenny Breuer, and their lab group employ an amalgam of tools to study bats, including high-speed cameras, computational models, wind tunnels, airflow visualization, museum specimens, living bats, and even a robotic bat wing. Together, these diverse methods can unravel the mysteries of flying with compliant wings, and the team’s pioneering work on relations between bat wing deformability and slow flight exemplifies this approach.
Having rigid wings, birds, insects, and airplanes have limited ability to fly at slow speeds without stalling and plummeting. But bats circumvent the problem to somehow flourish at these speeds. Using membrane models, flow visualization experiments, and high-speed imaging of bats in flight, it’s now known that soft, skin membrane wings, unlike others, “stretch” to keep air moving smoothly over them; this means they produce a lifting force at pitching angles ranging from zero to thirty degrees, nearly doubling that of rigid wings. Consequently bats can course the sky at both high and low speed, giving many a necessary edge when hunting erratic and maneuverable prey. This discovery put bats and their wings on the radar of engineers seeking to construct flying micro-robots, and it is now an inspiration that engineers imitate in their designs.
Still image from an X-ray video of Carollia perspicillata, Seba’s short-tailed fruit bat. Surgically implanted beads mark specific anatomical locations in muscles, tendons and bones that researchers can track during flight.
But Sharon is, first and foremost, a biologist, and thus is interested in biological aspects of bat flight, be they anatomical, physiological, evolutionary, or ecological. Sharon sees a bright future where the studies her research group performs in controlled laboratory settings will migrate outdoors and integrate with the less-predictable natural world. Even now, lab members are uncovering how bats land on different substrates, how they respond to perturbations and turbulence, how bones move and muscles activate during flapping, and how bats control wing shape during flight. These studies foretell a future where scientists understand how bats truly interact with their environment and how they are potentially affected by an increasingly human-built world.
“How can anybody study something other than bats?” Sharon once asked in our one-on-one weekly meeting. Indeed the dozens of publications and plethora of talks she has given on bats demonstrate her enthusiasm for these animals. From the woman gently caressing the head of a lesser bulldog bat, to the scientist demystifying the secrets of how bats fly, Sharon exemplifies a person captivated by her subject.
So although I expect she asked the question facetiously, it may have only been half-facetiously.
-by Jeremy Rehm, Department of Ecology and Evolutionary Biology, Brown University.
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