Abstract
Although we typically think of mammals as homeotherms that maintain a constant body temperature of about 37℃, the reality is that various species, scattered among the orders of the class Mammalia, are considered to be "hibernators". Hibernation is an unusual physiological phenomenon which occurs under stress conditions such as hypothermia, hypoxia or ischaemia. The exact definition of "hibernation" is open to considerable argument since it has a physiological spectrum from the black bear (Ursus americanus) which remains inactive most of the winter but lowers its body temperature only 3 or 4℃ to the smaller rodents which remain virtually motionless for days with a body temperature near 0℃ (deep hibernation). In deep hibernation, animals undergo a dramatic drop in body temperature and a fall in heart and respiratory rates. Despite this decline, the peripheral vascular resistance increases with deepening hibernation to keep the blood pressure within a reasonable range, and it has been thought that an increase in viscosity of blood and responsiveness of vasculature to noradrenaline are the major contributing factors for this regulation. However, recent studies have shown that other factors such as enhancement of sympathetic neurotransmission and modulation of endothelial function may also be involved in the regulation.
