Metabolism in lizards: low lactate turnover and advantages of heterothermy

The bobtail lizard (Tiliqua rugosa) encounters environmental temperatures that range from 7 to 40 degrees C. We have studied the effects of temperature on the whole-body turnover of glucose, lactate, and fatty acid. At 35 degrees C turnover rates of glucose and fatty acid account for the majority of O2 uptake and reflect the low metabolic rate of the bobtail compared with that of a mammal of equivalent mass. Lactate turnover at rest is very low. The low rates of lactate production correlate with a lack, or relative lack, of those tissues that normally produce most of the lactate in a resting mammal, such as nonnucleated red blood cells, renal medulla, and smooth muscle associated with blood vessels. Q10 (change in rate with a 10 degrees C change in temperature) values for turnover rates of glucose and fatty acid increase as temperature decreases. It is concluded that there is an inverse temperature effect that minimizes fuel usage during torpor.