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Published June 12, 2023 | Published
Journal Article Open

A fundamental propulsive mechanism employed by swimmers and flyers throughout the animal kingdom


Even casual observations of a crow in flight or a shark swimming demonstrate that animal propulsive structures bend in patterned sequences during movement. Detailed engineering studies using controlled models in combination with analysis of flows left in the wakes of moving animals or objects have largely confirmed that flexibility can confer speed and efficiency advantages. These studies have generally focused on the material properties of propulsive structures (propulsors). However, recent developments provide a different perspective on the operation of nature's flexible propulsors, which we consider in this Commentary. First, we discuss how comparative animal mechanics have demonstrated that natural propulsors constructed with very different material properties bend with remarkably similar kinematic patterns. This suggests that ordering principles beyond basic material properties govern natural propulsor bending. Second, we consider advances in hydrodynamic measurements demonstrating suction forces that dramatically enhance overall thrust produced by natural bending patterns. This is a previously unrecognized source of thrust production at bending surfaces that may dominate total thrust production. Together, these advances provide a new mechanistic perspective on bending by animal propulsors operating in fluids – either water or air. This shift in perspective offers new opportunities for understanding animal motion as well as new avenues for investigation into engineered designs of vehicles operating in fluids.

Additional Information

© 2023. Published by The Company of Biologists Ltd. We thank three reviewers for insightful improvements to this manuscript and Nick Bezio for artwork. We gratefully acknowledge funding support for this work from the US National Science Foundation CBET program (2100156 to S.P.C., 2100705 to J.H.C., 2019712 to J.O.D., 2100703 to B.J.G., 2100209 to E.A.K.), National Science Foundation RAISE (2034043 to E.A.K.) and US Office of Naval Research (N00014-22-1-2277 to J.H.C. and N00014-19-1-2035, N00014-22-1-2655 to E.A.K.). The authors declare no competing or financial interests.

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August 22, 2023
October 20, 2023