Black bears (Ursus americanus) and grizzly bears (Ursus arctos horribilis) share similar winter strategies. Bears are widely known as hibernators, although their strategy departs from the classic physiological definition of hibernation.
Bears enter dens in early winter when food becomes scarce and temperatures plummet. Prior to entering the den, bears have gone through a stage of hyperphagia, consuming up to 28,000 calories per day in order to accumulate fat reserves to survive the winter. The reserves need to be significant to last the bear until March.
Sows, or female bears, need the reserves not only for themselves but also for their unborn cubs. Bears mate in May through early June and females give birth in the den between mid-January to early February. Although the bears mate in early spring, implantation of the egg in the uterus does not occur immediately. This is called delayed implantation, and it serves two purposes: first, if the sow is not healthy when she is preparing for hibernation, the egg is aborted and the female will not have cubs; second, delayed implantation lengthens the gestation time to 200 to 220 days, which allows birth to take place in late winter.
When the cubs are born, they suckle milk and sleep. The sow is semiconscious during the birthing process, then falls back into hibernation after the cubs are born. The number of cubs a female gives birth to is directly related to the amount of fat reserves of the mother upon entering hibernation. With more fat reserves, the female has more resources to devote to her developing cubs.
It is debated whether or not bears are true hibernators because their body temperature only drops two degrees centigrade, which is a very small amount. Even though the temperature drop is small, it still functions by lowering the metabolic rate of the bear.
While hibernating, the bear will not eat, drink, urinate or defecate. Each of these activities, or rather absence of activities, would normally cause major health problems: not moving combined with not eating or drinking should cause muscle atrophy; not urinating or defecating should cause a life-threatening build-up of toxins; remaining in the same position should cause bed sores; however, in March, bears leave the den without toxin build-up, with the same muscle mass as when they began hibernating, and without bed sores, bone loss or artery damage. How bears manage this incredible physiological feat is still being researched (the medical implications of avoiding bed sores would be incredible), and the full story is still incomplete; however, we do know that bears are able to convert urea to creatine, which is a protein used in building muscles. This adaptation, along with many other mechanisms yet to be explained, allows these creatures to survive winter through hibernation.
References: Lecture by Kevin Taylor