Hedgehog

The more we study sleep and the special winter sleep known as hibernation the more we realize how it differs from one animal to another-and the more we realize how much more there is to be known about it! Probably the most profound sleeper of all is the European hedgehog, because it does not take any food during hibernation. The dormouse, by contrast, does occasionally awaken during the winter to feed, and the ground squirrel, and others like it, awaken often to feed in the course of the winter. The hedgehog is, therefore, a suitable example to take for a brief study of what is involved in true hibernation.

During the summer a hedgehog sleeps by day and comes out to feed at dusk. It may sometimes be seen abroad in daylight, even in the heat of the midday sun, but such individuals are usually ailing, or have been disturbed in their nests. Hedgehogs have hearty appetites for insects, worms and slugs, mainly, and as autumn approaches they feed even more heavily. As a result they lay in food-stores in the form of fat. This is, however, not the only change taking place in their bodies. There are numerous other changes in many of the organs. Some of the ductless glands, known as endocrine glands, such as the thyroid, parts of the adrenal glands, and the front part of the pituitary gland lying under the brain, undergo a reduction in size and in function. The reproductive organs diminish greatly. And the chemical composition of the blood alters radically. All these are, however, only the preparations for hibernation, and nothing more will happen to alter a hedgehog's daily routine until the temperature of the air drops below a particular point. Even then hibernation is at first incomplete, the hedgehog occasionally waking for spells of activity. From December to April, however, sleep is usually continuous and profound.

During the summer a hedgehog is warm-blooded, which means it can keep the temperature of its body at a steady level no matter how much the temperature of the air around it may fluctuate. In autumn it gives up its ability to control its own temperature, so that it becomes, in effect, a cold-blooded animal. What this means is hard to convey in simple terms but we can illustrate it by a single experiment. If a hedgehog, even at the height of summer, is injected with insulin and then put into an ice-box it will go into hibernation. So when we say it can relinquish its temperature control (that is, can pass from a warm-blooded to a cold-blooded state) we are merely putting into simple words the fact that automatic changes in the chemistry of the body, of a somewhat subtle nature, alter basically the body's reactions to fluctuations in the temperature of the surrounding air. It is hard to draw a distinct line between ordinary sleep and the sleep of hibernation. In many of their symptoms they are very alike, including an automatic awakening at the appropriate time. In hibernation, however, the breathing is slower and the pulse-rate slower than in ordinary sleep, and the skin and extremities are cold to the touch. However, the inside of the body is not so cold, and it becomes warmer the nearer we go to the hibernating hedgehog's heart. There is also a heavy concentration of the white blood corpuscles in the lining of the stomach and around the main blood-vessels, presumably to combat the invasion of bacteria from the intestine and stomach.

Even while in the profound slumber of complete hibernation a hedgehog is not "dead to the world". It will respond to sharp clicks by raising its spines slightly at each click. Moreover, the animal is still in touch with its surroundings in another way. Should the temperature drop too much the heart automatically begins to beat faster, the animal resumes its temperature control, becomes once more warm-blooded and will resume normal activity for a while, afterwards falling asleep again to continue its hibernation.