What if an animal's main food source is very scarce in the winter? The1may be hibernation (冬眠), that is, a deep sleep which allows them to2the winter with little or no food.
Ways of hibernation 3widely. Some animals store food in their burrows (洞穴) to eat when they awake for short periods. Many4extra food in the fall while it is plentiful, which is5as body fat to be used later for energy.
Hibernators have two kinds of 6: regular white fat and special brown fat. The brown fat7near the animal's brain, heart and lungs. Its main8is to provide extra body heat. It also sends a quick burst of 9 to warm these organs first when it's time to wake up.
True hibernators go into such a deep sleep that they are difficult to10and may appear dead. Their11goes down. Their breathing and heart beat slow significantly. 12a hibernating woodchuck 's heart rate13 from 80 to 4 beats per minute，and its temperature 14from 37C to as low as 4C. Other hibernating animals don' t experience major 15in temperature, heart rate or breathing. They are usually the 16sleepers, easily awakened. They may sleep during the 17weather and wake to wander and eat during milder weather.
If an animal lives in an area where the winter is mild, it may hibernate only18or not at all. However, even when the19 is severe, the animals may wake up for short periods every few weeks to use their toilet rooms and eat if food is20.
