LOW-COST OF LOCOMOTION INCREASES PERFORMANCE AT LOW-TEMPERATURE IN A NOCTURNAL LIZARD

Thermal optima for physiological processes are generally high (30 degrees-40 degrees C) in lizards. Performance decreases substantially at low temperatures, yet some lizards are nocturnal and are active with body temperatures below 15 degrees C. We corroborated three hypotheses about the ecophysiological consequences of the evolution of nocturnality in lizards: (1) nocturnality requires activity at low temperature; (2) activity at low temperature imposes a thermal handicap that constrains performance capacity; (3) nocturnal species have higher performance capacity at low temperature than do comparable diurnal species. Field body temperatures during activity averaged 15.3 degrees C in Teratoscincus przewalskii, a nocturnal, terrestrial gecko from northwestern China. Individuals of T. przewalskii sustained exercise at 15 degrees C on a treadmill for more than 60 min at 0.18 km . h(-1). However, 15 degrees C was suboptimal for sustained locomotion. Resting and maximum oxygen consumption at 15 degrees and 25 degrees C were similar to predicted values for diurnal lizards, supporting the hypothesis that much of thermal physiology in lizards is evolutionarily conservative. The minimum cost of transport (C-min, 0.73 mL O(2)g(-1) km(-1)) for T. przewalskii was only 34% of the predicted value for a diurnal lizard of the same mass. This low cost yielded a maximum aerobic speed (MAS) of 0.27 km . h(-1) at 15 degrees C, which is 2.5 times the predicted MAS for a diurnal lizard of the same mass. In comparison with predicted values for diurnal lizards, T. przewalskii showed increased but thermally submaximal locomotor performance capacity at nighttime temperatures.