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air–water interface, unsteady flow, biomechanics, running on water. (983). However, without a fundamental physical understanding f the dependence of water-running ability on body size, any laim of causal relationships among water-running ability, abitat preference and body size is only conjecture. Previously, to develop such a physical understanding, we nvestigated the forces that are produced when a flat disk enters he water, taking care to approximate the size and speed of the isks to those of the lizards ’ feet (Glasheen and McMahon, 996a). Three fundamental phenomena were found to be of articular importance in low-speed water entry: (1) impact; (2) to avoid cavity collapse. In this investigation, we incorporate morphological, hydrodynamic and kinematic data in an allometric model which probes the effect of body size on water-running ability