Hibernation in Mammals

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Abstract
Hibernation and enforced hypothermia in mammals are widely different physiologic states. Prior to hibernation there are various preparations for the hibernating state, including polyglandular endocrine involution, fattening and/or food storage, and changes in the saturation of depot fat in some animals. The actual causes for the onset of hibernation are unknown, for most hibernators can remain active at low environmental temperatures for long periods. Entrance into hibernation is under precise physiologic control, with heart rate, respiratory rate, and oxygen consumption slowing before a decline in body temperature. A reasonably high blood pressure is maintained during this period and in deep hibernation by an increased peripheral resistance produced in part by vasoconstriction. Homeostasis is continued in hibernation, as evidenced by a normal blood pH, a sensitivity to inspired CO2, and a response to ambient temperature below 0 C. by increased metabolic rate. At any time during entrance into hibernation or during hibernation the animal may arouse from this condition. Arousal is a coordinated physiologic event in which the anterior of the body is warmed rapidly by shivering and other heat generating mechanisms, while warmed blood is shunted from the posterior by differential vasoconstriction until the anterior reaches nearly 37 C. The tissues and organs of mammals that hibernate are capable of useful function at lower temperatures than the tissues of mammals that do not hibernate, but a hypothermed mammal that can hibernate will die in hypothermia even though it lives longer and at a lower temperature than a mammal that can not hibernate. Hibernation must involve a resetting of the "physiologic thermostat," which thus permits a controlled cooling of the animal, but the nature of this "resetting" is not known.