Sleeping It Off

Winter in the Rockies is far more enjoyable for the areas human residents than it is for most of the areas animals. We nestle within heated dens and only venture forth after covering ourselves with successive layers of clothing. However, some of the valleys residents manage to make it through the winter without suffering the rigors of -50°C weather--they simply sleep through it all.

Spending the winter in a state of dormancy is much more complicated than just heading to bed for an extended nap. Normally referred to as hibernation, it is a marvel of nature and allows animals to survive in an environment in which they would otherwise have little food to eat.

When entering a state of hibernation, the metabolism slows to a rate far lower than that found in even the deepest sleep. Slowly, the heart rate and metabolism drop, followed by the body temperature. Our high metabolic rate is responsible for helping us to maintain our normally high body temperature.

There are a number of triggers that bring on hibernation. A sudden drop in temperature is one trigger, but the onset may be more gradual for other animals. Many ground squirrels enter a state of incomplete dormancy. With each successive day, it's body temperature drops a few degrees, stays low for a while, and then rises back to a normal temperature. Each day it drops a little lower. This day by day temperature yo yo continues until the body temperature reaches a level similar to that of its environment, and slowly the animal drifts into full hibernation. These successive false starts are thought to adapt the animals body and brain to the lower temperatures found during the long winter dormancy. In a full state of hibernation, only the animals most vital functions take place. It's breathing is very shallow, it's circulation slow, and it slowly absorbs it's body fats for energy. The breathing rate may slow to as little as one breath per minute while the body temperature may drop from about 18.5°C to around 4.2°C. The heart beats only two or three times per minute.

The body temperature will vary with the surrounding temperature until it rises above freezing and slowly begins to rouse the ground squirrel from its winter nap.

Bears and skunks do not truly hibernate. Their body temperature and pulse drop only slightly lower than normal and they breath much more frequently than true hibernators. They also wake up during warm spells and may even leave their den for short periods. However, during the long period of winter dormancy, they may lose as much as 40% of their body weight. As a result, during the summer, when food is plentiful, bears will spend most of their waking hours eating.