Advances in the study of bat flight: the wing and the wind

Bats are diverse, speciose, and inhabit most of earth’s habitats, aided by powered flapping flight. The many traits that enable flight in these mammals have long attracted popular and research interest, but recent technological and conceptual advances have provided investigators with new kinds of information concerning diverse aspects of flight biology. As a consequence of these new data, our understanding of how bats fly has begun to undergo fundamental changes. Physical and neural science approaches are now beginning to inform understanding of structural architecture of wings. High-speed videography is dramatically expanding documentation of how bats fly. Experimental fluid dynamics and innovative physiological techniques profoundly influence how we interpret the ways bats produce aerodynamic forces as they execute distinctive flight behaviors and the mechanisms that underlie flight energetics. Here, we review how recent bat flight research has provided significant new insights into several important aspects of bat flight structure and function. We suggest that information coming from novel approaches offer opportunities to interconnect studies of wing structure, aerodynamics, and physiology more effectively, and to connect flight biology to newly emerging studies of bat evolution and ecology.

Flight energetics of the Marbled Murrelet, Brachyramphus marmoratus

We measured flight speeds (n = 3000) of Marbled Murrelet, Brachyramphus marmoratus (J.F. Gmelin, 1789), to determine whether flight speeds of an exceptionably fast bird coincide with the maximum-range speeds (Vmr) predicted by aerodynamic theory. The mean (±SE) speed of 22.6 ± 0.21 m·s–1 was significantly higher than the Vmr predicted by four models, using conventional values for the parasite drag coefficient (CDpar). In order for the Penny cuick model to predict a Vmr of 22 m·s–1, a CDpar of 0.05, which is lower than any previously reported, is necessary; the other models would need to assume even lower values for CDpar. We concluded that the cruising speed of Marbled Murrelets exceeds Vmr. Marbled Murrelets may exceed Vmr as a result of behavioural decisions, and we examined two behavioural hypotheses: that flight speeds exceed Vmr to (1) minimize predation rate and (2) maximize chick growth rate. However, there was no significant difference between flight speeds during high (daylight) and low (darkness) predation periods or between chick-rearing and non-breeding periods. Marbled Murrelets may also appear to fly at a speed that exceeds Vmr because the underlying aerodynamic theory is inaccurate for this species. To examine the reliability of aerodynamic theory for Marble Murrelets, we compared measured wingbeat frequencies (f) to those predicted by Pennycuick's model. The mean f was significantly lower than the fref predicted by Pennycuick's model, and generally, f = 7.9m–0.22 is a better model for auks than Pennycuick's model. In addition, the Strouhal number was particularly low (0.12 ± 0.02). We conclude that the current aerodynamic models are insufficient for an exceptionally fast-flying bird.