METABOLISM DURING HIBERNATION

In cold-blooded animals in a state of hibernation metabolism is almost at a complete standstill. Indeed, in some of them, as well as in frozen caterpillars, it must be at a complete standstill. Not so the metabolism of warm-blooded animals. These must maintain a minimum of physiological activity and keep up a certain amount of body heat, or freeze to death. At the same time, they must maintain metabolism at as low a level as is compatible with continued existence, else their food reserves may be exhausted before the end of winter, at which time they will also die of freezing.

The low rate of metabolism in the hibernating bat, manifest by slow respiration, slow heart action and sluggish circulation, means a very slow use of nutritive reserves. The same slow circulation, slow heart action and lessened rate of breathing seen in the hibernating bear also mean the same slow consumption of reserves. Exhaustion of reserves before the return of warm weather would result in death from starvation.

Griffin says that "in spite of the low level to which the metabolic processes have fallen, a hibernating bat will awaken in a few minutes if handled or even disturbed by lights and talking. Once awake, the bat is as lively and active as ever. His temperature, circulation, and respiration have returned to normal." Were this activity continued, exhaustion of food stores would rapidly result. He tells us that "after flying around for a few minutes they hang up again and relapse into the torpor of hibernation."

Mr. Griffin tells us that the metabolic rate of an animal in hibernation depends on the temperature of his surroundings: "he will burn more fat at a higher temperature, just as any chemical reaction is speeded up by a rise in temperature." This is not good physiology and I doubt the correctness of his statement. He, himself, shows that the hibernating bat may be awakened and become active, his temperature, circulation and respiration becoming normal in spite of the low temperature of his surroundings. I think we must regard hibernation as a function of life that is vitally controlled and not absolutely determined by the temperature of the surrounding air. The control of metabolism is from within and not from without. There is a purposive conserving of food stores, not a mere passive non-use of these.

We witness, not a mere slowing down of "chemical reactions" by a lowering of temperature, but a reduction of physiological activities by a process somewhat analogous to sleep. By his own showing, these physiological activities are not helpless in the grip of temperature. They are speeded up or slowed down by the bat in the same temperature. Mr. Griffin may be a biologist, but he talks like a chemist. He thinks of the bat in terms of test-tubes, reagent bottles, retorts, etc., and not as a living organism that takes an active part in the control of its behavior.

The bat is not a cold-blooded animal and, even in hibernation, with metabolism reduced to the lowest point compatible with continued life, is able to maintain a body temperature slightly higher than that of its surroundings. It is able to increase or decrease its metabolism in the same temperature. It can be active or dormant in the same temperature. Hibernation seems to be an adjustment to certain environmental conditions--the absence of food supply seems to be more important in inducing this state than the reduction of temperature--rather than a passive yielding to outside influences. The reduction of metabolism is not the result of cold, but the result of the need to conserve food reserves. Oxydation in the animal body, while a chemical process, is a rigidly controlled process. The body does not start to burn and just continue to burn until it is consumed. The body's fat stores do not catch fire on hot days and go up in flames. Even in the hottest weather the fasting animal reduces its metabolic rate and conserves its food reserves. As a matter of fact, non-hibernating animals conserve their food reserves better in hot than in cold weather. This is due to the fact that more heat must be produced in cold weather to maintain normal body temperature. This "chemical reaction" is not speeded up by a rise in temperature; for, internally, there is no rise in temperature, though the surface of the body may feel chilly and the faster may complain of being cold even in hot weather.

It would be interesting to know what is the internal temperature of the bat in hibernation. It is, no doubt, much lower than in the active state. But the question remains to be answered: Is lowered, temperature due to reduced metabolism, or is lowered metabolism due to lowered temperature?

If the lowering of temperature comes from without and is responsible for the reduction of metabolism, it would seem to be impossible for the bat to arouse itself or be aroused from its state of "torpor" by anything short of an increase of temperature. So long as the temperature of the cave is thirty-three degrees, Fahrenheit, that of the bat should remain nearly as low and "torpor" should persist. It could not fly out of its cave to see if Spring has arrived, or more accurately, perhaps, to see if there is a food supply in evidence. If control is from without, the bat should be helpless until the control--temperature--is changed. Only the coming of warm weather should awaken him. Bats leaving a cave and flying to another when its temperature starts to drop to too low levels shows that the reduction of their metabolism is not a result of lowered temperature. For, if it were, a further lowering of temperature would further decrease metabolism and make it impossible for the bat to awaken and fly in search of a more sheltered abode.

The fact that some species commence their period of hibernation while the temperature is still relatively high and food is still to be had, indicates that the control of metabolism is from within, not from without. The hibernating animal is not helpless in the grip of external conditions.

Date: 2015-01-11; view: 783