3.2 Physiological changes during entry
Under normal euthermic circumstances, animals kept in an ambient temperature of 0° C would be expected to show a marked increase in metabolic rate and adaptive thermogenesis. However, the response in hibernators is the opposite. Figure 11 shows data from a woodchuck (Marmota monax; Figure 8) about to enter torpor. Following a period of 2 hours or so when T b is held more or less constant, but oxygen consumption and heart rate are highly irregular, the woodchuck lowers its oxygen consumption (a measure of its metabolic rate). Within 8 hours of the start of entry into torpor, metabolism appears to be at a minimal base-line, withT b subsiding smoothly to about 12° C within 14 hours (Lyman and O'Brien, 1960).
Entry into hibernation can take a great deal longer than the few hours it takes in the woodchuck. In the 1960s at Harvard University, Felix Strumwasser recorded the brain temperature (T brain) of the Californian ground squirrel (Citellus beecheyi) entering hibernation (Figure 12). T brain dropped during each dark period but rose again before the period of light. On the first, third and fifth ‘nights’, the drop was to less than 34° C, but on the remaining ‘nights’ the drops were successively greater. Strumwasser (1960) argued that these test drops indicated metabolic and neuronal preparation for deep hibernation. Such test drops are quite common in mammals entering hibernation, but are by no means universal.
The blood pressure of ground squirrels entering hibernation has been measured by inserting a catheter into the aorta. Once the catheter is in place and the animal has recovered from the operation, blood pressure can be measured with no stress to the animal. At first, the mean blood pressure remains within the range of the active animal, but as hibernation deepens the blood pressure decreases. A mild peripheral vasoconstriction takes place and it persists throughout the period of hibernation. This vascular response may be important in maintaining adequate blood pressure to the brain, given the huge reduction in heart rate.
However, during entry into hibernation, there are large fluctations in vasomotor tone, and periods of superficial vasodilation occur, which may have the effect of accelerating heat loss. Indeed, it is quite likely that vasomotor control largely determines the changes in T b seen in test drops. These fluctuations alternate with short periods of shivering, suggesting that the rate at which the T b is allowed to drop is being carefully controlled.