In order to understand why bears hibernate, it is first important to understand what hibernation is. In addition, it does need to be pointed out that bears don’t truly hibernate. Rather they fall into a deep sleep that can last days, weeks, or even months.
When an animal hibernates, its body functions slow greatly. Heart rate and breathing rate drop to almost nothing. This conserves energy, which is useful in the winter when food supplies fall, both snow, and cold would make it difficult for the animal to eat enough food to stay alive.
In true hibernation, such as that experienced by rodents such as squirrels, the animal becomes nearly comatose, and it is exceptionally difficult to rouse them. Since heart and respiration rates drop so much, they often appear to be dead.
In bears, it is a little different, though it serves the same purpose. In a bear, when they fall into a deep sleep, the heartbeat and respiration also drop. However, they don’t drop as much, which means that the bear is still using vital energy supplies, usually supplied from a buildup of fat formed during the summer and fall.
They also aren’t as difficult to rouse, and will take advantage of times between storms to wake up and look for food. This trait helps greatly in padding the energy reserves during cold and snowy weather. Crater Lake National Park has a large resident population of black bears, and it isn’t unusual to see them up and walking around, looking for food, in the middle of winter. This usually doesn’t last longer than a day or two, and then they go back to their slumber.
It isn’t difficult to understand why bears hibernate, once hibernation is understood, even if it isn’t hibernation but rather a deep sleep. Bears are huge omnivorous carnivores. They eat meat, carrion, and plants. During the worst ravages of winter, most of these are in short supply, though since a bear has an acute sense of smell, they will often come awake even in a snowstorm, if they smell carrion.
An animal with the bulk of a bear requires a great deal of food to sustain its life. The amount of food needed increases when the temperatures and conditions worsen, since it must also then keep warm. It amounts to a drop in food supply and an increase in the need for more food to survive. This is where the deep sleep is the lifesaver, literally.
In deep sleep, a bear’s metabolism slows to a trickle, so the animal can survive on stores of built up fat. Since movements are limited, little energy is burned. The bear doesn’t need to hunt for prey or to rummage.
Whether it is real hibernation or not, the question is, why do bears hibernate. The answer is that if they didn’t, they’d starve to death or freeze. Nature has a way of allowing for the continuance of the species.