HSIEH, S.T.*; LAUDER, G.V.: Running on water: quantitative flow visualization of basilisk lizard locomotion

Previous work on water surface locomotion in basilisk lizards has focused on limb kinematics and mechanical models of force generation. Nothing is known about the effect of limb motion on subsurface water flow and the orientation of forces resulting from limb impacts. We examined how basilisk lizards (a.k.a. ‘Jesus lizards’) run on water from the standpoint of fluid mechanics, measuring the wake produced by the foot with digital particle imaging velocimetry (DPIV). DPIV allowed visualization of vortices produced by the foot, measurement of foot force direction, and calculation of impact force magnitude. We induced ten green basilisk lizards (Basiliscus plumifrons; mass 10-20g) to run across a water track. Only data from runs in which the animal ran bipedally (i.e. the front feet did not touch the water) were used in final analyses. Each step was broken down into four phases corresponding with the primary direction of motion of the foot: slap, stroke, upward recovery, and downward recovery. By suspending reflective particles in the water, we visualized fluid flow using a 6W laser and filmed with two synchronized high-speed Redlake cameras at 250fps. To more thoroughly assess the shape and orientation of generated vortex rings, we collected data from three different light sheet orientations: x-y plane (parallel to the direction of motion), y-z plane (perpendicular to the direction of motion), and x-z plane (horizontal). DPIV data show that forces from the feet are oriented posterior and medial to the lizard, rather than posterior and lateral, as have been reported for level terrestrial locomotion among all other tetrapods. Analyses include measurements of vortex circulation and calculation of the magnitudes of thrust and drag generated by the feet through each of the phases of a step.