SEASONAL-CHANGES IN THERMOREGULATION BY THE FRILLNECK LIZARD, CHLAMYDOSAURUS-KINGII, IN TROPICAL AUSTRALIA

The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet-dry tropics of northern Australia during the wet season. During the dry season, h owever, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rate s only 28% of the wet season levels. Body temperatures (T-b's) of the lizards were measured by radio telemetry throughout the day during the wet and dry seasons. The midday T-b's during the wet season were high (grand mean = 36.7 degrees C) and typical for heliothermic lizards, b ut the dry season midday T-b's were significantly lower (grand mean = 32.8 degrees C). Microclimatic data and physical characteristics of th e lizards were used in a biophysical model to calculate the operative temperatures (T-e) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seas ons. The T-e's revealed the physical possibility for the lizards to ac hieve much higher T-b's during the dry season than were measured. Thus , the lower T-b's in the dry season represent a shift in preference ra ther than an inability to attain a high T-b during the cooler dry seas on. Inspection of the T-b's and T-e's revealed that although the lizar ds remained cooler in the dry season, they did not thermoregulate at t he lowest possible T-b's. During both seasons the lizards basked in th e sun early and late in the day, but during the dry season the lizards stopped intensive basking at a T-b approximate to 4 degrees C lower t han in the wet season. An index of the extent to which the lizards exp loit the available thermal environment indicates that they thermoregul ate carefully in both seasons. T-b's were also measured in a laborator y thermal gradient during both seasons, and the T-b's selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference i s an acclimatization response or an endogenous seasonal cycle rather t han a response to a simple thermal cue. The lower T-b's in the dry sea son result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the liza rds do not thermoregulate at the lowest possible T-b's suggests that t he dry season T-b's represent a compromise between conservation of res ources and the ability to perform other functions such as escape preda tors and/or digest food.