Hydrodynamics of the feet of fish‐catching bats: Influence of the water surface on drag and morphological design

Abstract

Abstract The bats, Noctilio leporinus and Pizonyx vivesi , display similar hind foot morphologies specialized for their fish‐catching habits. The hydrodynamic drag force of isolated hind feet from fishing and closely related non‐fishing bats was measured in a water channel at velocities up to 0.74 ms −1 . Drag force increased curvilinearly with increasing water velocity. The highest values of drag force for a given velocity were measured for the fishing bats, which have hind feet 1.8–3.9 times larger than related non‐fishing bats. However at high Froude Numbers, the dimensionless drag coefficient (C D ) was 23–39% lower for fishing bats compared to non‐fishing bats. Cross‐sections of the laterally compressed toes of N. leporinus and P. vivesi conformed to hydrofoil designs with minimum drag capabilities, indicated by Fineness Ratio and shoulder position. Moveing at high Froude Numbers at the water surface results in the addition of spray drag to the other components of the total drag force (i.e., frictional, pressure, and wave). Spray drag is suppressed by a foil shape with a long pointed leading edge with a short rounded trailing edge. The configuration of the fishing bat toe with a sharp leading edge and relatively long forebody region compared to the maximum thickness provided a measurable drag reduction at the air‐water interface. It was concluded that hydrodynamic drag considerations at the water surface were important in the design of hind feed for fish‐catching bats.