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

Thermal optima for physiological processes are generally high (30 degr ees-40 degrees C) in lizards. Performance decreases substantially at l ow temperatures, yet some lizards are nocturnal and are active with bo dy temperatures below 15 degrees C. We corroborated three hypotheses a bout the ecophysiological consequences of the evolution of nocturnalit y in lizards: (1) nocturnality requires activity at low temperature; ( 2) activity at low temperature imposes a thermal handicap that constra ins performance capacity; (3) nocturnal species have higher performanc e capacity at low temperature than do comparable diurnal species. Fiel d body temperatures during activity averaged 15.3 degrees C in Teratos cincus przewalskii, a nocturnal, terrestrial gecko from northwestern C hina. 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 d egrees C was suboptimal for sustained locomotion. Resting and maximum oxygen consumption at 15 degrees and 25 degrees C were similar to pred icted values for diurnal lizards, supporting the hypothesis that much of thermal physiology in lizards is evolutionarily conservative. The m inimum cost of transport (C-min, 0.73 mL O(2)g(-1) km(-1)) for T. prze walskii was only 34% of the predicted value for a diurnal lizard of th e 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 fo r a diurnal lizard of the same mass. In comparison with predicted valu es for diurnal lizards, T. przewalskii showed increased but thermally submaximal locomotor performance capacity at nighttime temperatures.