SUMMARY Temperature strongly affects whole-organism performance through its effect on muscle contractile rate properties, but movements powered by elastic recoil are liberated from much of the performance decline experienced by muscle-powered movements at low temperature. We examined the motor control and muscle contractile physiology underlying an elastically powered movement – tongue projection in chameleons – and the associated muscle powered retraction to test the premise that the thermal dependence of muscle contractile dynamics is conserved. We further tested the associated hypothesis that motor control patterns and muscle contractile dynamics must change as body temperature varies, despite the thermal robustness of tongue-projection performance. We found that, over 14–26°C, the latency between the onset of the tongue projector muscle activity and tongue projection was significantly affected by temperature ( Q 10 of 2.56), as were dynamic contractile properties of the tongue projector and retractor muscles ( Q 10 of 1.48–5.72), supporting our hypothesis that contractile rates slow with decreasing temperature and, as a result, activity durations of the projector muscle increase at low temperatures. Over 24–36°C, thermal effects on motor control and muscle contractile properties declined, indicating that temperature effects are more extreme across lower temperature ranges. Over the entire 14–36°C range, intensity of muscle activity for the tongue muscles was not affected by temperature, indicating that recruitment of motor units in neither muscle increases with decreasing temperature to compensate for declining contractile rates. These results reveal that specializations in morphology and motor control, not muscle contractile physiology, are responsible for the thermal robustness of tongue projection in chameleons.

SUMMARY Arboreal animals, especially lizards, often traverse three-dimensional networks of narrow perches with variable and steep inclines, but the effects of both incline and narrow surfaces on the locomotor movement and function of limbs are poorly understood. Thus, we quantified the three-dimensional hindlimb kinematics of a specialized arboreal lizard, Chamaeleo calyptratus , moving horizontally, and up and down a 30° incline on a narrow (2.4 cm) perch and a flat surface. We compared the flat-surface data of C. calyptratus with those of an anatomically generalized terrestrial lizard, Dipsosaurus dorsalis . Inclines had significant main effects for relatively few kinematic variables of C. calyptratus (11%)compared to D. dorsalis (73%). For C. calyptratus , the main effects of locomotor surface were nearly three times more widespread than those of incline. The foot of C. calyptratus was markedly anterior to the hip at footfall, primarily as a result of an unusually extended knee for a lizard. A large amount of knee flexion during early stance may be used by C. calyptratus to actively pull the body forward in a manner not found in D. dorsalis . Unexpectedly, the pelvic rotation of C. calyptratus greatly exceeded that of D. dorsalis and, unlike D. dorsalis , was not affected by incline. The more medial location of the foot of C. calyptratus on the narrow perch during stance was primarily a result of knee flexion rather than femur depression. Unlike previous qualitative descriptions of chameleons, our data for the hindlimb posture of C. calyptratus during stance indicate that the limb was not particularly erect.