Hibernation

HIBERNATION, a special state into which certain mammals periodically fall upon the onset of autumn cold; these include certain bats, hedgehogs, ground squirrels, hamsters, marmots, and others. All these animals possess a less perfect thermoregulatory apparatus than other mammals. The state of hibernation is characterized by the following features: the animal lies or hangs (bats) motionlessly; its heart beats very slowly and weakly, heart sounds are not audible in some, the blood flow is greatly slowed, blood pressure is low, peripheral vessels are partly empty and partly weakly filled, respiratory movements are very superficial and rare; body temperature is low and only slightly exceeds the ambient temperature. The output of CO₂ and consumption of O₂ are greatly reduced, the respiratory quotient remains at very low figures—0.57–0.23; amino acids predominate over urea in the urine. During hibernation, sleeping mammals seem to cease being warm-blooded and turn into cold-blooded ones. A parallel between sleeping animals and cold-blooded ones can be drawn in many respects. Body temperature in both changes in parallel with changes in the ambient temperature. With a drop in temperature, gas exchange falls, and with a rise, it increases, whereas in a non-hibernating animal the reverse relations take place. Organs excised from a sleeping animal survive similarly to the organs of cold-blooded animals; the heart of a sleeping marmot beats for several hours after respiration stops. In a sleeping animal, there is an increased resistance to poisons administered subcutaneously and intramuscularly, similarly to cold-blooded animals. Upon the cessation of hibernation, body temperature and all physiological functions return to normal within a few hours. The animal wakes up when the temperature rises to 16°; in the state of hibernation, it also wakes up spontaneously several times during the hibernation period to pass feces and urine, after which it soon falls asleep again. The cause of hibernation cannot be considered to be solely the drop in temperature, since hibernation still occurs even when the animal is kept in a warm room all winter. Recently, histological studies have shown atrophic changes in the endocrine glands, mainly the thyroid gland, in the sleeping animal. Reduced activity of the thyroid gland serves as the probable cause of hibernation, since the animal can be awakened by injecting extracts of this gland, as well as the thymus and adrenals, subcutaneously. Conversely, extracts of the pancreas inhibit awakening. Therefore, the probable cause of hibernation can be considered to be periodic seasonal changes in the internal secretory apparatus. Regarding the so-called "hibernation gland," see Adipose tissue.