When you come inside after a walk in the snow, aren't you glad that you have a heater in your house? What would life be like without a heater? This challenge is faced by wild animals every winter.
It is very difficult to survive in the winter for two main reasons: the cold temperatures make it harder to stay warm, and there isn't as much food available (since most plants don't grow in the winter, and many animals migrate to warmer areas). So, animals either need to avoid the cold weather, or find ways to survive in it.
The most common strategy to survive the winter months is called hibernation, in which the animal goes into a deep sleep-like state until the weather becomes warmer. This allows the animal to avoid the cold weather without having to move to a warmer climate (like birds do when they migrate south for the winter).
Hibernation is more than just sleep. It is a way to conserve energy by slowing down all of the body's processes. The animal's body produces less heat so its body temperature gets colder, and the animal also breathes much more slowly. These two bodily changes, along with the fact that the animal isn't moving, allow it to use up much less energy than it does when it is awake.
This is important since animals get their energy from food, and obviously the animal is not eating any food while it is asleep! But if the animal doesn't eat while it hibernates, then how does it get energy? Hibernating animals eat enormous amounts of food right before they hibernate. This extra food energy gets stored as fat (bears can gain 40 pounds per week when they are preparing to hibernate!). Then, once asleep, their bodies use the extra fat for energy. Since the animals' bodies are in "slow mode" during hibernation, they do not require very much energy, and so the fat contains enough energy to sustain the animal through the winter.
Humans cannot hibernate, and so we need to eat every day, but hibernating animals are able to survive weeks or even months just by getting energy from their stored fat. Humans aren't the only animals that don't hibernate though. Grey squirrels, red foxes, and wild turkeys are just a few examples of other non-hibernators.
After you categorize an animal as a hibernator or non-hibernator, it is important to ask what type of hibernation it uses. This is because there are two kinds of hibernation: deep hibernation, and a more mild version called torpor. Deep hibernation is also called true hibernation since it is what we normally think of (sleeping through the whole winter without waking up) when we hear the word "hibernation." Examples of deep hibernators are: box turtles, toads, woodchucks, and garter snakes.
In contrast, when an animal is in torpor, it can wake up occasionally to look for food, then go back to sleep. In fact, some animals perform their normal activities during the day and just use torpor at night. In addition to being able to wake up easily, an animal in torpor has a higher body temperature (about 60F) than it would if it were in deep hibernation (around 41F, which is very close to the temperature of your refrigerator!). Black bears, skunks, and raccoons are examples of animals that use torpor during the winter.
Little Lion Experiment:
Get out a small pot and a thermometer that goes down to 40F (5C) or lower. If you don't have a thermometer like that, then put some cold water in the fridge, which is almost exactly the same temperature (41F) as a deep hibernating animal. Put a cup of warm water on the kitchen table. Let both cups sit for 20 minutes. Make some ice cubes (ask an adult if you need help).
Next, put an ice cube into the pot. Put the pot on the stove over low heat (get an adult to help you with this step). The ice cube will begin to melt into water. Keep checking the temperature of the water with your thermometer (or compare it to the refrigerated water) to see how long it takes for the water to reach 41F. We'll call this the "deep hibernator time." Also note how much longer it takes to heat up to 60F (the body temperature of an animal in torpor). We'll call this the "torpor time." If you don't have a thermometer, then you can just wait until the water is almost as warm as the room temperature water. Also record how much more time it takes to warm up to 98.6F, our body temperature (any household thermometer should be able to detect that temperature). We'll call this the "human time."
The amount of time it takes to reach a given temperature is directly related to the amount of energy (heat) that is needed to warm up the water to that temperature. So, the "deep hibernator time" shows how much energy is needed to go from freezing (which is about how cold it is where the animal is hibernating) to the animal's body temperature. Similarly, the "torpor time" shows how much energy is needed to go from freezing to that animal's body temperature, and the "human time" shows how much energy is needed to go from freezing to our body temperature. More importantly, the difference between the "human time" and one of the other times shows how much energy those animals are saving by only warming their bodies up to 41F or 60F instead of normal body temperature.