Animals have different action plans to survive winter. It is common to find birds migrate during winter months. Some mammals like bats, reindeers and even whales migrate to find a warm winter place. Some tend to adapt. Ex. Whitetail deer grow a grayish thicker coat during winter. They get changed back to reddish-brown during summer and spring.
Many of us think bears hibernate during winter, but they actually participate in a similar but not quite the same stage called Torpor.
Hibernation is a voluntary state an animal enters to save energy, when food is scarce. During hibernation an animal lowers its body temperature, slows its breathing rate, heart rate, and metabolic rate. It allows animals to survive only on their energy stores (fat and muscle) until the hunting and eating begins in the spring.
True or deep hibernators like ground squirrels, hedgehogs, bats, mouse lemurs can sleep without any disturbance for months.
Bears can sleep more than 100 days without eating, drinking, or passing waste! Bears can actually turn their pee into protein.
The main difference is during torpor, the animal is able to wake up quickly. Waking involves violent shaking and muscle contractions, much like shivering. Also to avoid any danger, or if the opportunity exists it can go out for food.
Bears are under evolutionary pressure. In the colder, northern parts of Alaska, bears hibernate about 7 months of the year. Bears in the warmer, coastal regions of the state hibernate for 2-5 months.
If food is present a bear does not have to hibernate, unless it is pregnant or winter conditions are severe enough (such as found in Alaska). Cubs are born about two months into hibernation. The den is a safe environment for the cubs to grow for at least 5 months.
Bears in zoos will not hibernate if food is available. They are fed year round. Since they do not undergo a winter weight loss like wild bears, they can get overweight, which is very unhealthy. Fortunately more zoos are allowing their bears hibernate during winter. Though it makes the animals unavailable for viewing, it helps the bears stay leaner and healthier.
Bears do not always stays in a den. It is possible to find a bear hibernating in road culverts, digging and crawling into the root structure of the oversized trees, and even underneath home porches.