Seebacher F, James RS. Plasticity of muscle function in a thermoregulating ectotherm (Crocodylus porosus): biomechanics and metabolism. Am J Physiol Regul Integr Comp Physiol 294: R1024-R1032, 2008. First published January 16, 2007 doi:10.1152/ajpregu.00755.2007.-Thermoregulation and thermal sensitivity of performance are thought to have coevolved so that performance is optimized within the selected body temperature range. However, locomotor performance in thermoregulating crocodiles (Crocodylus porosus) is plastic and maxima shift to different selected body temperatures in different thermal environments. Here we test the hypothesis that muscle metabolic and biomechanical parameters are optimized at the body temperatures selected in different thermal environments. Hence, we related indices of anaerobic (lactate dehydrogenase) and aerobic (cytochrome c oxidase) metabolic capacities and myofibrillar ATPase activity to the biomechanics of isometric and work loop caudofemoralis muscle function. Maximal isometric stress (force per muscle cross-sectional area) did not change with thermal acclimation, but muscle work loop power output increased with cold acclimation as a result of shorter activation and relaxation times. The thermal sensitivity of myofibrillar ATPase activity decreased with cold acclimation in caudofemoralis muscle. Neither aerobic nor anaerobic metabolic capacities were directly linked to changes in muscle performance during thermal acclimation, although there was a negative relationship between anaerobic capacity and isometric twitch stress in cold-acclimated animals. We conclude that by combining thermoregulation with plasticity in biomechanical function, crocodiles maximize performance in environments with highly variable thermal properties. body temperature; thermal acclimation; locomotion; work loop; enzymes PHYSICAL PERFORMANCE OF VERTEBRATES may be constrained by limited capacity to respond to environmental change, and individuals that are most effective in compensating for environmental variability will gain a selective advantage (75). Selection can be partitioned into a performance gradient and a fitness gradient (4). The former relates variation in biochemical, physiological, or morphological traits to a measure of ecologically relevant performance, and the latter quantifies the effect of variance in performance on fitness (4, 56). Locomotion is closely linked to Darwinian fitness because of its influence on foraging success and escape from predators (36,40,45,63,83,84) as well as on social status (23). For example, maximal sprint speed in male lizards (Crotaphytus collaris) is a reliable predictor of territory size and defense (39, 68), and faster lizards have significantly greater reproductive output (38).Although locomotor performance is heritable (21,46,58,88), there is considerable within-generation variability as a result of environmental effects and gene-environment interactions (14, 58, 73). Locomotor performance is determined by underlying intrinsic muscle properties, such as muscl...