Dynamic Complexity of Wing Form in Bats: Implications for Flight Performance

Abstract For many decades, students of animal flight have taken advantage of the body of engineering theory used to design fixed-wing aircraft. Here, we review important ways in which bat wings and bat flight violate the assumptions of this body of theory, and we identify features of bat wing morphology and kinematics that may be of particular importance for flight performance. In particular, the complexity and heterogeneity of the mechanical properties of wing membrane skin and the low material and structural stiffness of wing bones result in wings that deform extensively under aerodynamic loading in ways not seen in aircraft engineered by humans. INTRODUCTION Biologists have long used aircraft aerodynamics to estimate lift and drag forces encountered by wings, aspects of flight energetics, and limits of flight performance, particularly regarding speed and maneuverability. Here, we examine how well the wings of bats meet the assumptions of conventional aerodynamic theory, and try to identify those areas where such theory can be effectively employed to understand bat flight, as well as those for which conventional aerodynamic theory poorly models bat flight. Aerodynamic principles suited to large fixed-wing aircraft have been applied to bat flight largely because of the lack of more appropriate theory; the mechanics and aerodynamics of small, flexible, flapping wings has traditionally been beyond the scope of fluid mechanics. Recent technological, computational, and theoretical advances, however, have dramatically changed the field of fluid dynamics. As a consequence, biologists can gain considerable new insight into bat flight and its morphological basis. We may be able to better identify the most functionally significant features of wing morphology and flight kinematics, and more accurately estimate the mechanics and energetics of flight. This will enable us to draw more realistic and informative comparisons among taxa that differ in wing shape, body size, etc. One important first step