Black bear hibernating with 3 month old cub -news.nationalgeoraphic photo

An unusually warm autumn and a good hard mast crop, especially red oak acorns, may keep some Appalachian black bears on the prowl a bit longer than normal, but area biologists don’t see anything out of the ordinary.

Dr. Frank van Manen of the University of Tennessee expects pregnant females to start denning in the next couple of weeks. According to van Manen, pregnant females are the first to den and adult males are the last.

It’s all an energy game. Bears understand that winter is approaching. Many different environmental triggers, like dropping temperatures, are involved. The number of daylight hours, photo-periodism, is believed to be one of the primary triggers. As the days begin to shorten, bears begin to feed voraciously trying to put on enough fat to see them through the winter.

The limiting physical environmental factor is food availability. Bears are highly adapted to summer time food supplies. Their digestive tract, stomach structure and dentition are designed for the consumption of easily digestible foods like nuts, berries, succulent vegetation, colonial insects and newborn animals. They are not well adapted to digest tough winter vegetation or designed to chase down adult prey in the snow.

When it gets to the point bears are spending more energy obtaining food than they are storing, it’s time to hit the hay. Pregnant females know instinctively they will need an abundant energy reserve to give birth and begin nursing in the den, therefore they den early before they get to that diminishing food-to-energy ratio.

There has been a long-standing controversy regarding the actual state of the black bear’s winter respite. The bear’s high winter temperature, only about 12 degrees below norm, and the fact that bears can react instantaneously to disturbances during their winter sleep didn’t correspond with traditional definitions of hibernation.

The body temperature of other animals, like woodchucks and chipmunks, often referred to as true hibernators, may fall as low as 40 degrees Fahrenheit during their winter sleep. Yet it is these animals that must rouse every few days, warm up and urinate. Some must even eat and defecate during these periodic arousals to make it through the winter. Bears, on the other hand, neither defecate nor urinate while denned.

Innovative biologists have come up with a way to address these differences in winter sleep patterns to determine who the real hibernators are. They changed the definition of mammalian hibernation to: “the specialized seasonal reduction of metabolism in response to the concurrent pressures of food unavailability and low environmental temperatures.” Now chipmunks and bears can both be true hibernators.

Bears have developed truly remarkable adaptations that allow them to hibernate for months (up to seven in northern climates) without eating, defecating or urinating and still maintain a hibernating temperature of more than 85 degrees Fahrenheit.

Their hibernating cholesterol level is double that of their summertime level, yet they experience no hardening of the arteries. And even though they don’t urinate during hibernation they don’t suffer from gallstones. In fact hibernating bears produce a bile known as ursodeoxycholic acid that has been known to dissolve gallstones in humans. Bears are able to maintain muscle and organ tissue during hibernation by breaking down urea and using the nitrogen to build protein. Researchers believe hormone-like substances may be responsible for physiological changes in black bears during hibernation.

But hibernation is in response to food availability, and in southern climes like Florida and Louisiana hibernation is greatly shortened and for some males may not occur at all. Pregnant females in the south still den to give birth.

Another hint that photo-periodism plays an important role in black bear hibernation is the fact that even scarcity of food cannot promote hibernation in the summer. Bears are susceptible to starvation if there is no food in the summer months.

Van Manen notes that the average home range for female black bears in the Smokies is about five square miles. Males have larger home ranges, perhaps 11 square miles. While these ranges may increase in years of poor food production and bears may roam widely in search of food during summer and autumn months, they invariably return to their home ranges to hibernate. Van Manen said that records show about 5 percent of the bears in the Smokies return to the same den site.