Changes in heart rate are important for thermoregulation in the varanid lizard Varanus varius

Laboratory studies and a single field study have shown that heart rate in s ome reptiles is faster during heating than during cooling at any given body temperature. This phenomenon, which has been shown to reflect changes in p eripheral blood flow, is shown here to occur in the lizard Varanus varius ( lace monitor) in the wild. On a typical clear day, lizards emerged from the ir shelters in the morning to warm in the sun. Following this, animals were active, moving until they again entered a shelter in the evening. During t heir period of activity, body temperature was 34-36 degreesC in all six stu dy animals (4.0-5.6 kg), but the animals rarely shuttled between sun and sh ade exposure. Heart rate during the morning heating period was significantl y faster than during the evening cooling period. However, the ratio of heat ing to cooling heart rate decreased with increasing body temperature, being close to 2 at body temperatures of 22-24 degreesC and decreasing to 1.2-1. 3 at body temperatures of 34-36 degreesC. There was a significant decrease in thermal time constants with increasing heart rate during heating and coo ling confirming that changes in heart rate are linked to rates of heat exch ange.