The role of mechanical properties of the basilisk lizard foot in rejecting perturbations while running at the air-water interface*

During escape, basilisk lizards, B. basiliscus, run rapidly (~7-21 body lengths/second) on the surface of water. Previous work elucidated how the trajectory of the lizard's foot through water generates propulsion [Hsieh 2004]. However, the role of the lizard's long, flexible toes has not been as closely studied. Body weight support primarily occurs from the stroke of the foot through the water with high Reynold's number (3.6 × 10⁷) [Gaudet 1998]. We hypothesized that the properties of the foot, particularly the shape and stiffness, can passively correct for kinematic errors which would otherwise result in atypical perturbations to the reaction force. To test this, we developed several robophysical models of the foot varying in fidelity from simple 2D shapes cut from acrylic to 3D-printed and silicone cast feet based on biological scanning and testing of cadaveric specimens. These foot models were mounted to a robotic arm coupled with a load cell to measure the reaction forces during the interaction with a fluid medium. We began with a foot trajectory based on existing biological data before introducing error/noise into the path. We performed a sensitivity analysis of the reaction forces as a function of perturbation and foot model type, coupled with cavity-drag analysis to better characterize the stroke effectiveness of the foot models. This study allows us to begin to understand the role of mechanical properties in the foot in rejecting perturbations due to kinematic error.