Journal
ANNUAL REVIEW OF PHYSIOLOGY
Volume 66, Issue -, Pages 239-274Publisher
ANNUAL REVIEWS
DOI: 10.1146/annurev.physiol.66.032102.115105
Keywords
body mass; body temperature; metabolic inhibition; temperature effects; thermal conductance; thermoregulation
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Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (T-b) for MR reduction, perhaps with the exception of very small species and at high torpor T-b, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to T-b effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of T-b are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and T-b reduction at torpor onset. Over the ambient temperature (T-a) range where torpid heterotherms are thermo-conforming, the T-b-T-a differential is more or less constant despite a decline of MR with T-a; however, in thermo-regulating torpid individuals, the T-b-T-a differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower T-b. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.
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