Energetic consequences of metabolic depression in tropical and temperate-zone lizards

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.