Hydrodynamics of the Feet of Fish-Catching Bats: Influence of the Water Surface on Drag and Morphological Design

The bats, Noctilio leporinus and Pizonyx uiuesi, display similar hind foot mor- phologies 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-l. 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 dimen- sionless drag coefficient (CD) 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. Mov- ing 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 con- cluded that hydrodynamic drag considerations at the water surface were important in the design of hind feet for fish-catching bats. Noctilio leporinus of the family Noctilionidae and Pizonyx vivesi of the Vespertilionidae are unique among the various species of chiroptera, because they are highly specialized in their habits for catching and eating fish (Burt, '32; Bloedel, '55; Gudger, '45; Hill and Smith, '84; Patten and Findley, '70; Suthers, '65, '67). Both species dem- onstrate considerable parallelism in their behav- iors and morphology for piscivory (Altenbach, '89; Bloedel, '55; Blood, '87; Reeder and Norris, '54). The bats use their echolocation to detect fish by ripples or breaks in the water surface and then drag their feet 0.3 to 3.0 m through the water to gaff the fish (Bloedel, '55; Hood and Jones, '84; Suthers, '65, '67; Wenstrup and Suthers, '84). Structurally, fishing bats are large bodied with elongate digits on the hind feet, hooklike claws are directed forward due to a 180 degree rotation of the hindlimb from the typical mammalian pat- tern, the plantar surfaces are smooth with a re- duced number of wrinkles and hair, the claws and digits are laterally compressed, and plagio- patagium is truncated and attaches high on the tibia (Blood, '87; Gudger, '45; Reeder and Norris, '54; Vaughan, '86; Walker, '75). These morpholog- ical similarities in the hind limbs are believed to reduce the drag associated with raking the water for fish (Blood, '87). The present examination was initiated to deter- mine the extent of drag reduction due to mor- phological specializations by the fishing bats. Comparisons of fishing bats with related non- fishing species were made to determine which particular anatomical modifications were derived for fish-catching.