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Mystery & Miracle of Hibernation
Despite a huge amount of research, we do not fully understand how grizzly bears survive the long, cold winter months as they snooze in dens. We do know that they do not eat, drink, urinate, or defecate. Amazingly, they also do not lose bone or muscle mass, or kidney function. Somehow, in January, the females bear young, usually one or two. At less than a pound in weight, at birth a bear cub is the smallest of any placental mammal compared to its size as an adult (400-700 pounds or so). In a groggy state, the mother nurses her young until they emerge together in the springtime. Her milk has one of the highest fat contents of any land mammal -- liquid lard – so cubs grow amazingly fast.
Mom and cubs move considerable distances from high, snow-covered slopes to lower meadows in April to graze on emerging vegetation, or to feed on winter-killed or weakened big game. Then they follow the herbs, roots, berries and meat as they become available. Mom always remembers where diverse foods such as horsetails or biscuitroot were in the past, and her cubs learn from her what tends to be where and how to stay out of trouble.
Image © Roger Hayden - all rights reserved
Mom and cubs move considerable distances from high, snow-covered slopes to lower meadows in April to graze on emerging vegetation, or to feed on winter-killed or weakened big game. Then they follow the herbs, roots, berries and meat as they become available. Mom always remembers where diverse foods such as horsetails or biscuitroot were in the past, and her cubs learn from her what tends to be where and how to stay out of trouble.
In preparation for hibernation in late November, grizzly bears pork up, starting in July. Grizzly bears must have access to foods rich in fats, carbohydrates, and proteins in order to build up sufficient fat reserves to survive denning and the thin early spring season. Bears can eat 50,000 calories or more a day during hyperphagia. (That is 10,000 times what a big athletic human would eat).
Grizzly bears have been the subject of intense interest among medical researchers, because of their ability to survive such long periods without eating or eliminating waste, getting the equivalent of bedsores, losing muscle mass, developing osteoporosis, or experiencing failure of their kidneys. Maybe someday hibernation will no longer be a miracle, but that would not be nearly as much fun.
For more detail on the miracle of hibernation, see: Grizzly Bears, Dreaming and the Frontier of Wonder