When the Animals Sleep

Most people tend to associate hibernation with bears. However, only small animals can enter the torpor that defines true hibernation. Hibernation is a process that involves a drastic lowering of body temperature, and bears simply have too large a mass to dissipate enough heat for them to enter a true state of hibernation. A wintering bear's temperature rarely falls below 86 degrees Fahrenheit, so they are rather easily awakened. It takes a true hibernator several hours to arouse from a near-freezing body temperature.

In the January/February issue of Science 84, Carrol Fleming reports on some recent findings made by various researchers in the field. When a ground squirrel, for instance, curls up for the winter, its body temperature drops from a normal level of about 98 degrees to as low as 34 or 35 degrees. Its heart, over a period of three to four hours slows from 350 beats a minute to as few as two to four beats.

The obvious purpose of hibernation is to conserve energy during the lean months of winter, and for years scientists thought that the cooler weather of autumn triggered its onset. However, it was found that even if squirrels were kept in a constant temperature of 95 degrees year-round, they still showed signs of hibernation every 300 to 360 days or so. In such temperatures, the squirrels were unable to lower their body heat, but they lost weight as if in hibernation nonetheless.

At periodic intervals--about two weeks--the hibernating animal's body temperature returns to normal and it rouses itself to visit the bathroom and maybe grab a bite to eat. It will remain alert for maybe a day of activity before returning to hibernate. During these active times, the squirrel uses up 80 to 90 percent of the total calories consumed during the entire period of hibernation.

There remain many unanswered questions about how animals live through such a seemingly punishing ordeal. In the case of the ground squirrel, one puzzle is why it does not suffocate itself. Even though it may breathe only once every several minutes, it hibernates in a pocket only barely larger than itself and sleeps with its head tucked underneath its body. Under such circumstances, it might be thought that the level of carbon dioxide buildup would become intolerable, yet the squirrel's blood remains well oxygenated.

The main question, though, is how tissue can remain vibrant through such temperatures. Human hearts would simply stop beating at about 59 to 68 degrees, yet the hibernator's heart continues to function at 34 degrees, nerves continue to conduct impulses, and life goes on.