Multi - functional foot use during running in the zebra - tailed 1 lizard ( Callisaurus draconoides )

A diversity of animals that run on solid, level, flat, non-slip surfaces appear to bounce on 8 their legs; elastic elements in the limbs can store and return energy during each step. The 9 mechanics and energetics of running in natural terrain, particularly on surfaces that can 10 yield and flow under stress, is less understood. The zebra-tailed lizard (Callisaurus 11 draconoides), a small desert generalist with a large, elongate, tendinous hind foot, runs 12 rapidly across a variety of natural substrates. We use high speed video to obtain detailed 13 three-dimensional running kinematics on solid and granular surfaces to reveal how leg, foot, 14 and substrate mechanics contribute to its high locomotor performance. Running at ~ 10 15 body length/s (~ 1 m/s), the center of mass oscillates like a spring-mass system on both 16 substrates, with only 15% reduction in stride length on the granular surface. On the solid 17 surface, a strut-spring model of the hind limb reveals that the hind foot saves about 40% of 18 the mechanical work needed per step, significant for the lizard’s small size. On the granular 19 surface, a penetration force model and hypothesized subsurface foot rotation indicates that 20 the hind foot paddles through fluidized granular medium, and that the energy lost during 21 irreversible deformation of the substrate does not differ from the reduction in the 22 mechanical energy of the center of mass. The upper hind leg muscles must perform three 23 times as much mechanical work on the granular surface as on the solid surface to 24 compensate for the greater energy lost within the foot and to the substrate. 25