Many mammals live through the winter by hibernating (冬眠). There is a nice problem of definition here. Most experts now agree that a mammal can be said to hibernate only if body temperature drops greatly and its whole metabolism —including respiration and heart rate—is much reduced. This is the case with animals like the woodchucks, hamsters, and hedgehogs, which hibernate in their burrows , and bats, which gather in caves. Bears, however, are not classed as hibernators. They pass most of the winter sleeping, but their body temperature drops only a few degrees, and they can become active without going through a slow process of warming up. Bears, relying on fat reserves to keep them going, even produce their young during this period. Mammals are warm-blooded, that is, they have a means of keeping the temperature of their bodies quite constant despite the normal temperature changes of the outside world. Cold-blooded animals, fish, reptiles, insects, amphibians, also have a certain amount of control over their body temperature. They can warm up, if they are too cold, by sunning themselves, or cool off by getting into the shade. But it is hard for cold-blooded animals to keep warm in the winter. For any animal to be active, its body temperature must be above freezing. This is because the chemistry of life depends on water in a liquid state. If the body actually freezes, the whole system is disrupted and the animal dies. So cold-blooded animals in the north must either find some place to spend the winter where temperatures do not reach the freezing points, or develop a special resting stage in which the water content of the protoplasm (原生质) is much reduced. This will make its freezing point much lower than usual. Thus animals about to hibernate often dig down into the soil. Cold-blooded animals are so called because they________.