Transitions in body wave dynamics in lizards with varying body and limb proportions

One of the best-known transitions in vertebrate evolution is from a short-bodied, limbed form to a limbless, snake-like form. Such transitions occur concurrently with a transition in locomotion patterns from almost standing-wave body bending to traveling-wave body undulation. How does such a transition in locomotion pattern occur in the context of the body form continuum? Here we studied the locomotion patterns of three species of Brachymeles skinks (B. kadwa with effective hind leg lengths (HLL)=0.17 relative to snout-vent length (SVL), B. taylori with HLL= 0.15 SVL , and B. mungtingkamay with the HLL=0.09 SVL) and compared them with stereotypical lizards, Uma scoparia (HLL=0.33 SVL) and Callisaurus draconoides (HLL=0.44 SVL), and an almost legless lizard, Lerista praepedita. We use new neural net tracking and theoretical geometric modeling tools to analyze animals’ locomotion on a fine granular[PB1] surface. All animals used body-leg coordination that geometric theory predicted to maximize forward propulsion. Surprisingly, the use of a traveling wave occurred after even a modest reduction in limb length, suggesting a sharp transition in locomotor mode along a gradual body form continuum.