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