Thermal effects on motor control and in vitro muscle dynamics of the ballistic tongue apparatus in chameleons.

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.