BMR is regulated independently of T b at least in hibernating mammals. Entry into hibernation is characterized by a gradual fall in RQ, which indicates a switch from carbohydrate to lipid metabolism for energy provision (through the phosphorylation of pyruvate dehydrogenase, the inhibitor of mitochondrial fatty acid uptake). There is evidence that some other vertebrates, such as hibernating frogs, may continue to use carbohydrate catabolism or activate gluconeogenic pathways when arousal episodes completely deplete stored triacylglycerols.
The most important adaptive modifications of the mitochondrion are reduced oxidative metabolism, the reversal of the proton leak across the inner membrane as an energy-saving mechanism during hibernation, and an increase in production of uncoupling proteins (UCP) in preparation for metabolic thermogenesis during arousal episodes.
Lung ventilation is linked to oxygen supply in euthermic animals, but in hibernation where tissue blood flow is almost absent it becomes less important. The characteristic inspiratory patterns of hibernation, as well as the fall in heart rate, have been shown to follow passively from the depression ofT b.
The energy budget of mammals for euthermia, torpor and arousal can be estimated both in the laboratory and natural habitats. For small mammals that are obligate hibernators, energy saving is in excess of 80% for the winter months and about 60% averaged over the year. Lactation and gestation can stretch the energy budget of female mammals considerably, and they may display further adaptations to minimize the cost. Hibernation should not, however, be seen as the universal solution for energy saving in winter and considerations of size, behaviour pattern and habitat all have an effect upon the useful duration of torpor bouts or whether torpor is an appropriate strategy at all. Hibernation physiology may be a major factor in determining the biogeographical and ecological diversity of mammals and perhaps other vertebrates.