Strategies for overcoming thermal constraints on skeletal muscle function in ectotherms
Daily and seasonal fluctuations in the environmental temperature pose a challenge to ectotherms as they move through their environment. Their skeletal muscles must generate enough power to allow them to accelerate and move quickly enough to catch prey or escape predation, but muscle is highly temperature sensitive. Ectotherms circumvent these problems through behavioral modifications at low temperatures such as hiding and entering brumation, a hibernation-like state which can involve several months of inactivity without eating. A potential drawback to brumation is that long periods of inactivity can lead to skeletal muscle atrophy, which would lower muscle power upon the resumption of activity. Ectotherms have also evolved mechanisms to maintain locomotor performance at moderate temperatures, however the mechanisms used to maintain running performance are heretofore unknown. In my dissertation I use the western fence lizard, Sceloporus occidentalis, as a model running ectotherm. In chapter 1, I use in vitro muscle preparations and histology to find that the lowered metabolic rate conferred by a low body temperature is sufficient to mitigate muscle atrophy after long periods of muscle disuse. I then use inverse dynamics in chapter 2 to calculate hind-limb joint powers and determine that fence lizards amplify muscle power using tendons to maintain acceleration performance at moderate temperatures. In chapter 3, I measure EMG activity in an ankle extensor muscle of running fence lizards. I combine that data with an in silico muscle model to determine that lizards alter the timing of activation of their muscles to cycle energy through tendons rather than muscles while running, and that this helps them maintain speed at moderate temperatures.