How Do Animals Survive the Winter?

With the winter months upon us, Julia from State College, PA, writes in to ask how animals survive during the colder months of winter. She asks does the cold weather affect them? She is also curious about the food they eat during the winter months.
Some animals hibernate all winter, which is actually just a very deep sleep. This allows the animal to avoid the cold weather without having to move to a warmer climate. Hibernation 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. These animals get ready for winter by eating extra food and storing it as body fat, which is used for energy while hibernating. Some also store food to eat later in the winter. Some animals that hibernate include bears, skunks and chipmunks. Cold-blooded animals like fish, turtles, frogs, and snakes, find shelter in holes or burrows. There they spend the winter inactive, or dormant, which is similar to hibernation.
Other animals stay pretty active during the winter, but they must adapt to the changing weather. Many make changes in their behavior or bodies. For example, animals may grow new, thicker fur in the fall to keep warm. Food is often hard to find in the winter, so the animals must adapt to this as well. Squirrels, mice and beavers, gather extra food in the fall and store it to eat later. Other animals like rabbits and deer, spend the winter months looking for moss, twigs, bark and leaves to eat. Some animals even eat different kinds of foods as the seasons change. For example, the red fox eats fruit and insects in the spring, summer and fall. However, in the winter, the red fox cannot find these foods so it eats small rodents instead. These active winter animals must also stay warm through the cold months. They sometimes find shelter in holes in trees or logs, under rocks or leaves, or even under the ground. Some mice and squirrels even huddle close together to stay warm.
Some birds migrate or travel to other places where the weather is warmer or they can find food. Many birds migrate in the fall. The trip can be dangerous, so the birds often travel in large flocks. Birds can fly very long distance but most birds will migrate shorter distances. Other animals migrate, too, including bats, caribou, elk, and whales.

Little Lion Experiment:
This experiment will allow you to determine how much energy animals are saving while hibernating! You will need ice cubes, a small pot and a thermometer that goes down to 40°F (5°C) or lower. If you don't have a thermometer like that, then put a cup of cold water in the fridge, which is almost exactly the same temperature (41°F) as a deep hibernating animal. Put a cup of warm water on the kitchen table. Let both cups sit for 20 minutes. You will need a pencil and some paper to record your observations.
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 41°F. We'll call this the "deep hibernator time." Also note how much longer it takes to heat up to 60°F (the body temperature of an animal that is dormant). We'll call this the "dormant 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 98.6°F, 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 when the animal is hibernating) to the animal's body temperature. Similarly, the "dormant time" shows how much energy is needed to go from freezing to that animal's body temperature. The "human time" shows how much energy is needed to go from freezing to our body temperature. 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 41°F or 60°F instead of normal body temperature.

How Do Animals Cope With The Winter?

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.