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

One response of ectothermic animals to periods of inactivity is inverse acc limation, or metabolic depression, which results in the conservation of ene rgy. Most studies of metabolic depression and acclimation have involved tem perate-zone species, and the information from tropical species has been lar gely restricted to laboratory studies that failed to demonstrate thermal ac climation of metabolism. Recently, metabolic depression has been shown in s everal species of reptiles from the wet-dry tropics of northern Australia d uring the dry season. We review existing data on the energy budgets of temp erate and tropical species during periods of inactivity and make calculatio ns of energy saved due to metabolic depression across a range of temperatur es. Because tropical species experience relatively high temperatures during periods of inactivity, they have a greater potential for energy savings, a ny enhancement of their metabolic depression is disproportionately advantag eous with respect to energy savings, and in some species metabolic depressi on is probably essential for survival. Thus, we would expect metabolic depr ession to be well developed in some tropical reptiles. The lack of thermal acclimation in laboratory studies indicates that environmental parameters o ther than temperature (such as food or water) may initiate metabolic depres sion in tropical species. Higher temperatures, however, magnify the energy savings accomplished by metabolic depression.