There are eight species of bears worldwide, and three of these occur in North America: brown (or grizzly), polar and black bears.

The black bear is the widest ranging and historically occurred in forested areas throughout much of North America. Today, it is found in about 39 of the lower 48 states.

Black bears enter an amazing semi-state of inactivity to survive winter, which is better termed winter lethargy. "Sleeping" black bears depend exclusively on fat reserves as an energy source over winter, and they lose about 20 to 27 percent of their body weight. Imagine being a 160-pound person and losing 30 to 45 pounds each winter.

In addition, body temperatures of black bears drop from a normal 100 degrees Fahrenheit to 91 degrees, heart rate goes from 40 to 50 beats per minute to 8 to 10 beats per minute, and oxygen consumption is about 50 to 60 percent of normal.

This lethargic state is maintained throughout winter without eating, drinking, defecating or urinating. In adult females, these remarkable physiological adaptations are occurring while also giving birth and lactating the young over winter. Hence, two factors have been given to explain winter lethargy in black bears: a seasonal scarcity of food and the need to take care of newborn young.

Bears begin entering their dens for winter lethargy when adequate body fat reserves have accumulated and food supplies become scarce in autumn. Entrance into lethargy occurs around November or December and terminates around April in Pennsylvania.

Typically, the first bears to enter lethargy are adult females. The moves into and out of lethargy are not instantaneous (like Yogi Bear entering and leaving his cave); instead, bears must gradually adapt physiologically and behaviorally to this extended period of inactivity by fasting and reducing daily activity during a period of about one month.

Cubs born during winter lethargy are tiny, weighing less than 1 percent of the body weight of the female (compared to 5 to 10 percent in humans). Because of this very small body size in newborn cubs, they experience a considerable loss of body heat, so the body heat produced by the dormant, lactating mother is vital in ensuring survival of her young over winter.

The survival of "sleeping" black bears is very dependent on den-site selection because dens provide protection from predators and inclement weather. Black bears in more southerly latitudes tend to use aboveground rather than ground-level dens to be protected from humans, dogs or floods caused by heavy winter rains.

If the fur of a bear becomes wet, the bear can lose 5 to 20 percent more body heat than one with dry fur.

Black bears in more northerly latitudes, like Pennsylvania, almost always use winter dens at or just below ground level. These dens may be under upturned roots or trunks of fallen trees, at the base of hollow trees, and in excavations, brush piles or rock crevices. Snow cover in northern latitudes helps conceal and insulate bears using dens at or slightly below ground level.

People should make every effort to avoid disturbing a black bear in winter lethargy. If a bear is aroused and forced to abandon the den, it may lose more than 56 percent more of its body weight, compared to normal weight loss. If a female with cubs is disturbed, weight loss associated with the disturbance can profoundly lower her energy levels for lactation and thereby reduce cub survival.

Furthermore, black bears in winter lethargy shunt blood away from skeletal muscles and into vital organs to better ration stored fat reserves. Thus, if "awakened" by a predator during winter lethargy, a black bear is quite sluggish and very susceptible to a predator's attack.

In summary, the black bear is truly a biological wonder during winter. Physiological and behavioral changes, coupled with strategic den-site selection, have evolved in black bears to enhance their survival and care of young during winter lethargy.

As we walk through the woodlands in Pennsylvania over the next couple of months, take a moment to reflect on how our largest carnivore is coping with the rigors of winter.

We also can do our part in conserving this magnificent mammal by staying clear of their dens while they are "asleep" over winter.

By the way, why do bears have fur coats? Because they look goofy in Penn State sweatshirts.