For ectotherms, Tb is essentially equal to Ta during hibernation/estivation. This means that any decrease in Ta during hibernation or estivation is accompanied by a decrease in Tb, which in turn is accompanied by an exponential decline in metabolic rate (MR) as described by the relationship, that is,
MRTb2 = mrtMq 10
where MRTb2 is the metabolic rate at Tb2 and MRTb1 is MR at Tb1. For most physiological variables, . 10 is generally about 2.5. This decrease in MR results in substantial energy savings and thus a prolonged survival period in the cold.
For some ectotherms there is an unequivocal intrinsic metabolic depression during estivation that occurs without any change in Tb (e.g., snails, fishes, and amphibians). Some plant seeds during dormancy are also hypometa-bolic or even ametabolic. This intrinsic metabolic depression, which is often a decrease in MR to about 20% of normal, occurs in the absence of any Tb, ionic, osmotic, or any other discernable physiological perturbation. The cue for intrinsic metabolic depression would appear to be a change in environmental conditions that indicates impending potential for desiccation. Intrinsic metabolic depression is not a short-term (e.g., daily) event; it often takes about 2-4 weeks for metabolic depression to become fully developed. It is probably more important for estivation, which has a lesser hypo-metabolism by lowered Tb than hibernation. The molecular or biochemical mechanisms for this intrinsic metabolic depression are not well understood; however its physiological significance is clearly extension of the hibernation/estivation period that can be survived by conserving energy.
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