Why do animals hibernate in winter? They hibernate to conserve energy and resources during a time when both are difficult to find. It would be a better question of how do animals hibernate, why do not more animals hibernate, or how did hibernation evolve! Those questions are much more difficult to answer.
Winter, particularly the farther north you go gets extremely cold and most plants go dormant. The out door temperatures start dropping free water becomes scarce as ponds and even lakes start to freeze. This means that food is only available in limited amounts, and the same applies to water. Animals also have more difficulty keeping their body warm. . Small animals such as hedgehogs and bats hibernate because their food supply is unavailable during the winter.
To hibernate is defined by the Free Dictionary as "(of some animals) to pass the winter in a resting state in which heartbeat, temperature, and breathing rate are very low [Latin hibernare to spend the winter". To most people it means a very long, very deep sleep, to the animal it means survival.
To hibernate the animal stuffs itself with food throughout the spring, summer and fall, building up numerous layers of fat and stores of nutrients in its body. Then it finds a small enclosed area where it is safe from predators, not exposed to the elements, and can curl up and slowly fall to sleep. The small enclosure blocks wind, but also conserves heat. The body of the animal undergoes physical changes and virtually shuts down. These are metabolic changes as a result of chemicals produced by the brain, the Science Clarified has a website that explains it in non-technical terms.
Why do not all animals hibernate? Because there are numerous ways to adapt and each animal is different. Some semi-hibernate, such as bears. Many animals migrate to follow their food supply. Nature is diverse and there are often more than one way to overcome problems, and nature tries them all.
How did hibernation first begin? That is also a question for debate and speculation. No one really knows!
Hibernation by animals in winter is a bizarre function, but animals do it to get through the cold times when food is limited. To find a listing of the different types of hibernators, check out the Enchanted Learning Website, it is good for both adults and children, as well as an excellent teacher's resource!