Meeting Abstract
Loons (Gaviiformes) are remarkable swimmers, capable of diving underwater for minutes at a time and out-maneuvering fish prey. But, unlike for other foot-propelled swimming birds, the swimming behavior and hydrodynamics of loons has never been quantitatively studied. Here, we employ a novel robotic method to accurately actuate cadaveric loon feet in 3D motions tracked from freely swimming common loons (Gavia immer). A load cell attached between an industrial robot and the loon foot measures lift and drag forces throughout the swimming motion. This method poses significant benefits compared to alternative current methodologies that often require small study animals and repeatable motions (i.e. DPIV) or involve using simplifying assumptions to predict fluid behavior (i.e. Computational Fluid Dynamics modeling). Using seven real feet, our robotic loon replicated swimming foot motions at 3 to 14 times slower than the real loon. Extrapolating from these trials, we find that real loons produce a maximum instantaneous force of at least 5N drag and 2.5N lift with each foot. Force generation throughout the power stroke is dominated by drag, with lift contributing up to 40% of the total hydrodynamic force at any point in time. We also find that lift develops during the end of the recovery stroke, potentially contributing to propulsion despite a collapsed position of the toes. These findings represent the most accurate experimental measurement of swimming forces produced by a freely swimming bird. By understanding how loons produce forces underwater, we can directly compare swimming strategies across the avian phylogeny and assess convergent swimming strategies within foot-based propulsion. Furthermore, we hope that future studies can use this robotic method to investigate how other animals swim.
