Abstract
The thermal sensitivity of physiological rates is a key characteristic of organisms. For tuatara (Sphenodon punctatus), the last surviving member of the reptilian order Rhynchocephalia and an unusually cold-tolerant reptile, we aimed to clarify responses in indices of metabolic rate (oxygen consumption [VO2] and carbon dioxide production [VCO2]) as well as rates of total evaporative water loss (TEWL) to temperatures at the warmer end of the known tolerated range; currently, patterns for metabolic rate are unclear above 25 degrees C, and TEWL has not been measured above 25 degrees C. We first established that metabolic rate was lowest during the photophase and then measured VO2, VCO2, and TEWL at six temperatures (12 degrees, 20 degrees, 24 degrees, 27 degrees, 29 degrees, and 30 degrees C) during this phase. Consistent with our predictions, we found that mass-adjusted VO2, VCO2, and TEWL increased at least 3.5-fold between 12 degrees and 30 degrees C (at 30 degrees C, rates were 2.509 mL g(-1) h(-1), 2.001 mL g(-1) h(-1), and 1.829 mg(-1) g(-1) h(-1), respectively). Temperature coefficients (Q(10) values) for mass-adjusted VO2 and TEWL showed thermal dependence between 12 degrees and 29 degrees C but with a reduced increase or thermal independence between 29 degrees and 30 degrees C. There was no observed effect of egg incubation temperature (inferred sex) on the subsequent metabolic rates of juveniles. The respiratory exchange ratio implied a switch from carbohydrate metabolism at <22 degrees C to lipid metabolism at >27 degrees C. The rigorous measurement of VO2 and TEWL provides a basis for future studies to predict the thermal sensitivity of tuatara to human-mediated climate change.