It is not really understood exactly what tells the body of an animal that is about to hibernate to change its working. The effect of the falling outside temperature as winter approaches is certainly one of the signals. And it may be that another unconscious signal is the increasing length of the nights. As winter approaches the increasing difficulty of finding food may be another factor that influences the animal.
When the animal begins to hibernate, if it is warm-blooded - that is, if its body temperature is regulated, and not just that of its surroundings - the working of the temperature regulator alters. The animal's temperature starts to fall. It is thought that the working of the temperature regulator may be altered by the effect of a hormone in the bloodstream. A hormone is a chemical 'messenger that is carried around the body, and affects bodily activity in many crucial ways.
It may be that the increasing cold as winter approaches causes the production of this 'hibernating hormone'. Experiments have shown that if there is only a short spell of cold, then the animal may not respond by going into hibernation. It requires a longer and continuous cold spell before the animal's temperature control mechanism is affected, and its temperature begins to fall, and bodily processes slow down. When an animal is in hibernation its body's working processes may slow down to as little as one-hundredth of the normal rate.
If there is an especially great drop in temperature outside, then some of the body rates speed up. Otherwise the animal would freeze to death. The heart rate, for example, will increase to make sure that the tissues of the body are getting enough blood, carrying food and oxygen, to keep them alive.
When warm weather returns, in the spring, the process is reversed. The increase in outside temperature acts as a trigger, and the animal's own temperature begins to climb to normal. All the body processes speed up until it awakens and is active again. Very often, animals waking up from their winter sleep start shivering violently. This produces more heat in the body, and speeds the return to normal. Small animals such as ground squirrels and hamsters wake up from hibernation over a period of an hour or so. With larger animals the time taken for the normal temperature to return may take longer.
Animals that are cold-blooded have no body temperature control, such as we and other warm-blooded animals have. By keeping the temperature of our bodies steady even though the outside temperature varies, we can keep our bodies working at an even rate. But fish, amphibians (like frogs and toads), reptiles (like snakes, lizards and tortoises), and of course insects, depend on the outside temperature.
Fish do not normally hibernate. Even when there is a thick layer of ice on the water in the winter, most of them are able to move around, although more slowly. Some kinds of fish do very nearly hibernate. They remain motion-less in the water, sometimes partly buried in the mud, for long periods with their body processes almost at a standstill.
Frogs, newts, salamanders and toads do hibernate. They bury themselves in the mud to keep away from the frost. Sometimes they huddle together in groups to keep in warmth, and maintain their body temperature a little above the surroundings.
Tortoises, lizards and snakes find holes under stones and dig themselves in to hide and sleep in the rotting vegetation until the warm weather wakes them up again.
