PHYSIOLOGICAL ECOLOGY OF FRILLNECK LIZARDS IN A SEASONAL TROPICAL ENVIRONMENT

The frillneck lizard, Chlamydosaurus kingii, is a conspicuous componen t of the fauna of the wet-dry 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 facto r involved in this seasonal drop is a change in the behavioural thermo regulation of the species such that lower body temperatures (T(b)s) ar e selected during dry-season days. Here we examine other factors that could be responsible for the seasonal change in FMR: standard metaboli c rates (SMR) and activity. Samples from stomach flushing revealed tha t the lizards in the dry season continued to feed, but the volume of f ood was half as much as in the wet season. SMR in the laboratory was 3 0% less in the dry season. During the dry season, the energy expended by the lizards is 60.4 kJ kg(-1) day(-1) less than during the wet seas on. Combining laboratory and field data, we determined the relative co ntribution of the factors involved in this energy savings: 10% can be attributed to lower nighttime T-b, 12% is attributable to lower daytim e T-b, 12% is attributable to decreased metabolism, and the remaining 66% is attributable to other activities (including e.g. locomotion, re productive costs, digestion). Calculations indicate that if FMR did no t 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 s eason suggesting modest levels of water stress.