The wood chucks of North America sleep for two thirds of the year.

They are active only for four months in the summer.

As the days begin to shorten, the woodchucks eat

far more than their normal amount of food.

The excess food is converted in to “brown fat” and is stored in its body.

When the fat stored becomes about one seventh of the body weight

of the woodchuck, it retreats into its burrow and becomes torpid.

It allows its body temperature to drop along with that of the surroundings

until it may become very close to the freezing point.

The temperature sensors continue to work and the temperature of the brain is kept up.

If about to freeze, some of the brown fat is used up to raise the body temperature rapidly.

When the spring arrives, the remaining brown fat is used up to bring the body temperature back to normal.