Hibernation in animals

   In some parts of the world, when it gets cold for long periods, certain animals just seem to disappear. This could be, of course, because they have died from the cold. Or it might be that they suffer from cold so much, or find it so hard to obtain food, that they have moved away somewhere warmer. In the winter, many birds do migrate to warmer countries. It is easy for them, because they can fly. But many other animals do not migrate. They hide themselves away and have a long winter sleep until the warm weather returns. This is hibernation.

   During hibernation, all the workings of the body slow down, very nearly to a stop, and the body temperature drops until it is nearly that of the surroundings. The heart slows, and so does the rate of breathing. And because the animal is in such a deep sleep, you might almost think that it had died.

Only one species of bird is known to hiber­nate. This is the poor-will of western North America. It has been found to hide away during the winter in a very sleepy state, with a body temperature only half its normal one.

   Mammals that hide away during the winter and hibernate include small rodents, such as dormice, and some of the small insect-eating animals. But very often they wake up once or twice and have a meal of food that they have stored away with them in their hiding places.

The kinds of bat that live in countries with cold winters hibernate hanging upside down in caves and sheltered places. When the warm weather returns and brings with it the insects that they need for food, they wake up again.

   As a preparation for the long winter sleep many animals begin to put on weight. They store up fat in their bodies. During hibernation this fat is gradually used up to provide the energy that is needed to keep the body working at a very low rate. Because during the sleep the animals are not as active - they do not move around looking for food - they do not need much energy. However, at the end of the winter, nearly all the stored fat has been used up, and the animal wakes up much thinner, and very hungry.