Very tiny muscles let bats control wing shape and stiffness as they fly.
The nearly hair-thin muscles are embedded in the membrane of their inherently floppy wings. Birds and insects have stiff wings, but the new evidence suggests bats have evolved this muscular means of preserving or adjusting wing shape.
“Aerodynamic performance depends upon wing shape,” says Brown University biology graduate student Jorn Cheney, lead author of the newly published paper in Bioinspiration and Biomimetics.
“The shape of a membrane wing might initially begin flat but as soon as it starts producing lift it’s not going to remain flat because it has to deform in response to that aerodynamic load.
“The shape it adopts could be a terrible one—it could make the animal crash—or it could be beneficial,” Cheney says. “But they are not locked into that shape.
“Because bats have these muscles in their wings, and also bones that can control the general shape as well, they can adopt any number of profiles.”
Wind tunnel tests
Cheney wasn’t sure what to make of the tiny muscles, called plagiopatagiales, heading into the experiments reported in the paper. They have been known for more than a century but their function has never been demonstrated.
When Cheney considered the muscle function, he estimated that each individual muscle would be too weak to reshape the wing. That led him to form two competing hypotheses: either that the muscles would activate together to enhance force or that these oddly shaped, weak muscles might exist solely as sensors of stretch.
Only experiments could settle the question, so Cheney attached electrode sensors to a few muscles on the wings of a few Jamaican fruit bats and filmed them as they flew in the lab’s wind tunnel.
Turn wings on and off
Three key findings emerged from the data. They all point to the plagiopatagiales modulating skin stiffness.
One result was that the muscle activation and relaxation follows a distinct pattern during flight: they tense on the downstroke and relax on the upstroke.
“This is the first study showing that bats turn these muscles on and off during a typical wingbeat cycle,” says co-author Sharon Swartz, a biology professor at Brown.
Another finding was that the muscles don’t act individually. Instead they exert their force in synchrony, providing enough collective strength to stiffen the wing.
Finally, Cheney found, the muscles appeared to activate with different timing at different flight speeds. As the bats flew faster, they tensed the muscles sooner in the upstroke-downstroke cycle.
In other words, the data suggested that the muscles do not behave passively but actively and collectively in keeping with conditions of flight.
None of the data, however, preclude the muscles from serving a sensory function as well.
In addition to Cheney, Breuer, and Swartz, the paper’s other authors are Erika Giblin, Nicolai Konow, and Thomas Roberts at Brown and Kevin Middleton at the University of Missouri.
The Air Force Ofﬁce of Scientiﬁc Research and the National Science Foundation funded the research.
Source: Brown University