SPRAWLING LOCOMOTION IN THE LIZARD SCELOPORUS-CLARKII - THE EFFECTS OF SPEED ON GAIT, HINDLIMB KINEMATICS, AND AXIAL BENDING DURING WALKING

Although the hindlimb is widely considered to provide the propulsive f orce in lizard locomotion, no study to date has analysed kinematic pat terns of hindlimb movements for more than one stride for a single indi vidual and no study has considered limb and axial kinematics together. In this study, kinematic data from several individuals of the Scelopo rus clarkii are used to describe the movement patterns of the axial sk eleton and hindlimb at different speeds, to analyse how kinematics cha nge with speed, and to compare and contrast these findings with the in ferred effects of speed cited in the literature. Angular limb movement s and axial bending patterns (standing wave with nodes on the girdles) did not change with speed. Only the relative speed of retracting the femur and flexing the knee during limb retraction changes with speed. Based on these data and similar results from a recent study of salaman ders, it appears that, over a range of speeds involving a walking trot , sprawling vertebrates increase speed by simply retracting the femur relatively faster, thus this simple functional adjustment may be a gen eral mechanism to increase speed in tetrapods. The demonstration that femoral retraction alone is the major speed effector in Sceloporus cla rkii lends strong functional support to ecomorphological implications of limb length (and especially femur length and caudifemoralis size) i n locomotory ecology and performance in phrynosomatid lizards. It also lends support to inferences about the caudifemoralis muscle as a prea daptation to terrestrial locomotion and as a key innovation in the evo lution of bipedalism.