Negotiating obstacles:: running kinematics of the lizard Sceloporus malachiticus

The kinematics of lizard locomotion has been extensively studied, focusing mainly on steady running over level ground or over uniformly inclined surfaces. However, lizards inhabit structurally complex environments and frequently encounter small physical barriers located along their traveling routes. Consequently, the ability to climb obstacles is likely ecologically advantageous for small vertebrates because it allows expansion of their spatial niche across distinct microhabitats. Our study tests which aspects of lizard kinematics change to improve the negotiation of obstacles that might be encountered along traveling routes. We examined the kinematics of Sceloporus malachiticus using high-speed (500 fps) digital video as individuals moved over rectangular obstacles with three different heights (0.8, 1.8 and 3.8 cm; 22, 49 and 103% of the lizards' mean forelimb length: wrist to shoulder), as well as during steady level running. Our results show that S. malachiticus lizards increase vertical relative to horizontal limb motion and elevate their head and trunk in anticipation of moving over an obstacle. The lizards generally appeared to use their forelimbs as levers to pull their body over higher obstacles, whereas the hindlimbs pushed against the ground to propel the pelvis forward and upward. Elevation of the center of the trunk in preparatory strides (23.83 and 37.85 mm, low vs. high obstacle) and forelimb support duty factor (0.33 vs. 0.45, low vs. high obstacle) increased with obstacle height, whereas locomotor speed and stepping frequency decreased from steady level running (0.89 m s(-1) and 6.87 Hz) when moving over the highest obstacle (0.62 m s(-1) and 5.74 Hz). Lizards paused more frequently on high obstacles, and made more adjustments of hand and foot contact positions with increased barrier height. They also jumped and ran bipedally more frequently to negotiate higher obstacles. This study shows that both kinematic and behavioral strategies are used by small vertebrates with a sprawled posture to help negotiate obstacles.