SWARTZ, S.M.; MIDDLETON, K.M.; IRIARTE-DIAZ, J.; LEE, M.; WOFFORD, J.M.; BREUER, K.S.; RITTER, D.A.; Brown University, Providence, RI; Brown University, Providence, RI; Brown University, Providence, RI; Brown University, Providence, RI; Brown University, Providence, RI; Brown University, Providence, RI; Brown University, Providence, RI; : Can bats actively control the mechanical properties of the wing membrane?

During flight, the bat wing membrane deforms in spatially and temporally complex ways that can affect aerodynamic force production. One mechanism by which it has been proposed that bats actively control air flow is modulating wing camber. Camber of bat wings can be varied by movements of the thumb, which control the position of the propatagium or ‘leading edge flap’, and movements of the fifth digit, which control the position of the wing’s trailing edge. Some authors have also suggested that intrinsic muscles of the wing membrane skin contribute to camber. These muscles, ten to thirty discrete chordwise bundles in the armwing, have no bone attachments; instead they attach to the skin’s collagen-elastin network. No specific mechanism has yet been proposed to detail how activity of these muscles influences wing form. We hypothesize that the mm plagiopatagiales influence wing aerodynamics by modulating the non-linear stress-strain behavior of wing skin; intensity of muscle activity may directly influence skin strain, and given that skin modulus depends strongly on strain, thereby modify stiffness. For a given pressure difference across the membrane, the amount of billowing and wing curvature will depend on skin stiffness, so as stiffness decreases, billowing and camber will increase for a given aerodynamic force. We begin to test this hypothesis by recording electromyographic activity in the mm plagiopatagiales of Cynopterus brachiotis in slow flight. We find that the muscles are activated during each wingbeat cycle; these data are consistent with the hypothesized role of the muscles in modulating aerodynamic performance.