Low Cost of Locomotion Increases Performance at Low Temperature in a Nocturnal Lizard

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Abstract
Thermal optima for physiological processes are generally high (30°-40° C) in lizards. Performance decreases substantially at low temperatures, yet some lizards are nocturnal and are active with body temperatures below 15° 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°C in Teratoscincus przewalskii, a nocturnal, terrestrial gecko from northwestern China. Individuals of T. przewalskii sustained exercise at 15° C on a treadmill for more than 60 min at 0.18 km · h⁻¹. However, 15° C was suboptimal for sustained locomotion. Resting and maximum oxygen consumption at 15° and 25° 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 ( , 0.73 mL O₂ 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° 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.