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, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tb's) of the lizards were measured by ratio telemetry throughout the day during the wet and dry seasons. The midday Tb's during the wet season high (grand mean = 36.7°C) and typical for heliothermic lizards, but the dry season midday Tb's were significantly lower (grand mean = 32.8°C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) 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 seasons. The Te's revealed the physical possibility for the lizards to achieve much higher Tb's during the dry season than were measured. Thus, the lower Tb's in the dry season represent a shift in preference rather than an inability to attain a high Tb's during the cooler dry season. Inspection of the Tb's and Te's revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tb's. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb °4°C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregulate carefully in both seasons. Tb's were also measured in a laboratory thermal gradient during both seasons, and the Tb'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 is acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tb's in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tb's suggests that the dry season Tb's represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.

One response of ectothermic animals to periods of inactivity is inverse acclimation, or metabolic depression, which results in the conservation of energy. Most studies of metabolic depression and acclimation have involved temperate-zone species, and the information from tropical species has been largely restricted to laboratory studies that failed to demonstrate thermal acclimation of metabolism. Recently, metabolic depression has been shown in several species of reptiles from the wet-dry tropics of northern Australia during the dry season. We review existing data on the energy budgets of temperate and tropical species during periods of inactivity and make calculations of energy saved due to metabolic depression across a range of temperatures. Because tropical species experience relatively high temperatures during periods of inactivity, they have a greater potential for energy savings, any enhancement of their metabolic depression is disproportionately advantageous with respect to energy savings, and in some species metabolic depression is probably essential for survival. Thus, we would expect metabolic depression to be well developed in some tropical reptiles. The lack of thermal acclimation in laboratory studies indicates that environmental parameters other than temperature (such as food or water) may initiate metabolic depression in tropical species. Higher temperatures, however, magnify the energy savings accomplished by metabolic depression.

The field metabolic rates (FMR) and water fluxes of Varanus scalaris were measured during the wet and dry seasons by the doubly‐labelled water technique. Seasonal measurements of standard (night‐time) metabolism (SMR) and resting (daytime) metabolism (RMR) were made in the laboratory at 18, 24, 30 and 36°C, and maximal oxygen consumption was measured at 36°C on a motorized treadmill. This population was active throughout the year. In the wet season, the mean FMR was 7.8 kJ day−1 (128.0 kJkg−1 day−1; mean mass = 66.4 g, n= 13), and during the dry season the mean was 5.0 kJ day−1 (67.6 kJ kg−1 day−1; mean mass = 77.4 g, n= 17). The mean water flux rates for these animals were 3.6 and 1.2 ml day−1, respectively (60.4 and 16.6 ml kg−1 day−1). The seasonal means of FMR and water flux were significantly different by ANCOVA (P < 0.0001). Measurements of SMR and RMR were significantly higher in the wet season (ANCOVA: P < 0.0001), but we found no difference in the maximal oxygen consumption between seasons (ANCOVA: P= 0.6). The maximal oxygen consumption of the lizards on the treadmill (2.9 ml min−1= 1.8 ml g−1 h−1), mean mass = 97.4 g, n= 16) was 20 times that of the SMR at the same temperature during the dry season, and 11 times that of the SMR during the wet season. The seasonal differences in FMR were attributable to: changes in SMR (12.2%) and RMR (16.4%); differences in night‐time body temperatures (11.3) and daytime body temperatures (16.4%); and activity (broadly defined to include locomotion, digestion, and reproductive costs (43.7%).

The frillneck lizard, Chlamydosaurus kingii, is a conspicuous component of the fauna of the wetdry tropics of northern Australia during the wet season, but it is rarely seen in the dry season. Previous studies have demonstrated that during the dry season the field metabolic rate (FMR) is only about one-quarter of the wet-season rate, and one factor involved in this seasonal drop is a change in the behavioural thermoregulation of the species such that lower body temperatures (T s) are selected during dry-season days. Here we examine other factors that could be responsible for the seasonal change in FMR: standard metabolic rates (SMR) and activity. Samples from stomach flushing revealed that the lizards in the dry season continued to feed, but the volume of food was half as much as in the wet season. SMR in the laboratory was 30% less in the dry season. During the dry season, the energy expended by the lizards is 60.4 kJ kg day less than during the wet season. Combining laboratory and field data, we determined the relative contribution of the factors involved in this energy savings: 10% can be attributed to lower nighttime T , 12% is attributable to lower daytime T, 12% is attributable to decreased metabolism, and the remaining 66% is attributable to other activities (including e.g. locomotion, reproductive costs, digestion). Calculations indicate that if FMR did not drop in the dry season the lizards would not survive on the observed food intake during this season. Seasonal analysis of blood plasma and urine indicated an accumulation of some electrolytes during the dry season suggesting modest levels of water stress.

Lophognathus temporalis is an arboreal lizard from the wet-dry tropics of Australia. During the wet season the field metabolic rate (FMR) of the lizards was 209 kJ kg Ϫ1 d Ϫ1 , but during the dry season FMR was only 62 kJ kg Ϫ1 d Ϫ1 . Similarly, water flux decreased from 73.6 mL kg Ϫ1 d Ϫ1 in the wet season to 18.5 mL kg Ϫ1 d Ϫ1 in the dry season. Body temperatures (Tb) were significantly lower in the dry season, and operative temperatures, calculated by incorporating microclimatic data with characteristics of the lizards, indicated that the seasonal shift was due to changes in thermoregulatory behaviour rather than limitations of the thermal environment. By combining field measurements of Tb and FMR with laboratory measurements of standard metabolic rate over a range of Tb, we were able to subdivide the FMR into its components and to determine which factors contributed to the seasonal reduction in energy expenditure. During the dry season, lizards used 147 kJ kg Ϫ1 d Ϫ1 less energy than during the wet season, and 24% of this decrease was estimated to be due to the passive effects of lower nighttime Tb, 14% was due to the active selection of lower daytime Tb, 27% was due to the physiological shift to lower standard metabolic rates, and 35% was due to reduced activity in the dry season. Although the population size remained relatively constant (107 lizards ha Ϫ1 during the wet season and 125 lizards ha Ϫ1 during the dry season), the population structure changed, reflecting the seasonal patterns of recruitment and mortality. The number of lizards active at any one time was much lower in the dry season, reflecting the lower levels of activity in this season. The energy expenditure of the population of L. temporalis was 612 kJ ha Ϫ1 d Ϫ1 during the wet season and 113 kJ ha Ϫ1 d Ϫ1 during the dry season.