Hibernation

In marked contrast to all large mammals, including man, a number of small northern and alpine mammals are capable of reducing voluntarily, and often conspicuously, their metabolic rates and their body temperatures for extended periods of time. This process, because it occurs in the winter, is known as hibernation and provides a mechanism by which the animal can escape the rigors of a long and often hard winter, when food is largely very limited. Hibernation is largely confined to small herbivores such as marmots, ground squirrels, chipmunks, dormice, and northern bats (the black bear is an acknowledged hibernator although it does not conform to the accepted pattern).

Entry into the hibernating state is characterized by a large reduction of the metabolic rate (sometimes down to as much as 1/200th of the normal rate) and body temperature (in some cases down to 32°F). Some animals arouse periodically and quite regularly, especially the ground squirrels, and experience wide variations of body temperatures. These are conditions which cannot be tolerated by animals such as man whose mechanism for regulating his metabolism is extremely sensitive to temperature. In fact, it is the latter observation that gives research on this problem some force, as hypothermia (becoming too cold) in man is often injurious, if not fatal. It is therefore interesting to understand how a hibernator can survive hypothermia, thus possibly identifying some mechanisms that can be used as therapeutic measures in treating it in man.

For some years, the Institute of Arctic Biology of the University of Alaska at Fairbanks has had a large program concerned with the metabolic adjustments that the hibernator must make before its entry into, or exit from, the hibernating state. Within this institute I currently perform research on this topic under a grant from the National Institutes of Health. Marmots and Arctic ground squirrels are used almost exclusively in this research.

When an animal enters the hibernation chamber it ceases feeding for the entire period, thus facing prolonged starvation and widely varying body temperatures. It is obvious that the animal has some elaborate mechanisms that permit it to regulate its metabolism over a wide temperature range, and to husband efficiently its finite energy reserves for long periods. These mechanisms are manifested at the physiological and at the biochemical levels. It appears that, between regular bouts of arousal from deep hibernation, the animal allows its blood glucose to fall slowly to extremely low levels. Upon arousal, the glucose is restored and the animal once again descends into deep hibernation.

The source of almost all of the energy used for this is fat, which in some animals may be well over 50% of the total body weight. That there are fundamental differences between hibernating and non-hibernating ground squirrels was made clear by some interesting experiments conducted by (among others) Dr. L. Keith Miller of the Institute of Arctic Biology. He demonstrated that the nerves of a hibernating animal continue to conduct impulses down to temperatures far below those at which similar impulses from summer (non-hibernating) animals cease to conduct. This is further supported by the finding that most, if not all, of the animal appears to "restructure" much of its intracellular machinery into a new form that can function and be regulated efficiently over a wide temperature range.

These alterations are at the molecular level, and available evidence indicates that they are "turned on" by a hormonal stimulus or "trigger". One of the most provocative results of the current research is that all of the enzymes so far studied undergo this transition 3 to 4 weeks before the onset of hibernation. The opposite change also occurs before the end of the hibernation season when the animal finally emerges. The significance of this is that the new characteristics would not do the hibernating animal much good if it did not possess them upon entry into hibernation--there is little value in adapting to something that has already occurred. Further, this means that the ground squirrel anticipates, apparently without any recognizable external signal, a very dramatic change in its metabolic status. It is this last aspect that draws together the separate but converging threads of the annual cycles of the hormonal "trigger" and the resultant biochemical and metabolic changes that turn a summer ground squirrel into a hibernating one.