  Although typically possessing four limbs and short bodies, lizards have
evolved a diversity of body plans, from short-bodied and fully-limbed to
elongate and nearly limbless. Such diversity in body morphology is hypothesized
as adaptations to locomotion cluttered terrestrial environments, but the mode
of propulsion -- e.g., the use of body and/or limbs to interact with the
substrate -- and potential body/limb coordination remain unstudied. Here, we
use biological experiments, a geometric theory of locomotion, and robophysical
experiments to comparatively and systematically investigate such dynamics in a
diverse sample of lizard morphologies. Locomotor field studies in short-limb,
elongated lizards (Brachymeles) and laboratory studies of full-limbed lizards
(Uma scoparia and Sceloporus olivaceus) and a limbless laterally undulating
organism (Chionactis occipitalis) reveal that the body wave dynamics can be
described by a combination of traveling and standing waves; the ratio of the
amplitudes of these components is inversely related to limb length. We use
geometric theory to analyze and explain the wave dynamics and body-leg
coordination observations; the theory predicts that leg thrust modulates the
body weight distribution and self-propulsion generation mechanism, which in
turn facilitates the choice of body waves. We test our hypothesis in biological
experiments by inducing the use of traveling wave in stereotyped lizards by
modulating the ground penetration resistance, as well as in controlled
non-biological experiments involving an undulating limbed robophysical model.
Our models could be valuable in understanding functional constraints on the
evolutionary process of elongation and limb reduction in lizards, as well as
advancing robot designs.
