Nocturnal lizards from a cool-temperate environment have high metabolic rates at low temperatures

Ectotherms from low-temperature environments have higher metabolic rates at low temperatures than those from warm-temperature environments. We predicted that nocturnal lizards, which are active at much lower environmental temperatures than diurnal lizards, would also have higher metabolic rates at low temperatures, and by association a lower thermal sensitivity (Q ₁₀) than diurnal and crepuscular lizards. We measured the rate of oxygen consumption ( [(V)dot]textO ₂ dot{V}{text{O}}_{ 2} ) of eight cool-temperate species of lizard (four nocturnal, three diurnal, and one crepuscular) at 13 and 26°C and analyzed log transformations of these data using log mass as a covariate. As expected, [(V)dot]textO ₂ dot{V}{text{O}}_{ 2} was positively correlated with temperature in all eight species, with [(V)dot]textO ₂ dot{V}{text{O}}_{ 2} being two to four times higher at 26°C than at 13°C. As predicted, at 13°C (but not 26°C) the [(V)dot]textO ₂ dot{V}{text{O}}_{ 2} was significantly higher in nocturnal than diurnal lizards. Species-specific differences and mass scaling factors explain the patterns of thermal sensitivity seen among these eight lizard species. Thermal sensitivity is strongly influenced by mass, with smaller species generally having higher thermal sensitivity of their metabolic rate, and this result deserves further exploration among other ectotherms. We conclude that, along with the previously reported lower cost of locomotion found in nocturnal lizards, they also partially offset the thermal handicap of activity at low body temperatures by having an elevated [(V)dot]textO ₂ dot{V}{text{O}}_{ 2} at lower temperatures.