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

TL;DR

This study investigates how zebra-tailed lizards use their multifunctional hind feet to efficiently run across both solid and granular surfaces, revealing adaptations in limb mechanics and energy expenditure.

Contribution

It provides detailed 3D kinematic analysis and models of foot mechanics, highlighting how the lizard's foot functions differently on various natural substrates.

Findings

Hind foot saves about 40% of mechanical work on solid surfaces.

Lizard's center of mass oscillates like a spring-mass system on both substrates.

More muscle work is needed on granular surfaces to compensate for energy loss.

Abstract

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.