Does a Bear Sleep in the Woods?
Photo by Susan Elizabeth Jones on Unsplash
The answer to this age old question is complicated, but the short answer is yes. Most people think bears hibernate during the winter months, but it’s not that simple. Hibernation in its true form means inactivity where metabolism slows and energy is conserved. It can be an adaptation to a seasonal shortage of food, low environmental temperatures, or snow cover. People have called bear hibernation torpor, winter sleep, dormancy, and carnivorean lethargy. The leading physiologists now simply call it hibernation. But, bears are not considered true hibernators – what does that mean?
With the help of the National Park Service website, let’s learn more. Mammals who are considered true, or deep hibernators, such as the groundhog, experience a drastic decrease in body temperature during hibernation. Body temperature for hibernating bears remains above 88°F, which is within 12°F of their normal body temperature of 100°–101°F. Many scientists now consider bears to be super hibernators, a highly efficient hibernator. Due to the highly insulating pelts of bears and their lower surface area to mass ratio than smaller hibernators, body heat is lost slowly which enables bears to cut their metabolic rate by 50-60%. Respirations in bears decrease from 6-10 breaths per minute normally to 1 breath every 45 seconds during hibernation. They experience a drop in heart rate from 40-50 beats per minute during the summer to 8-19 beats per minute during hibernation. Mammals that experience lower body temperatures during hibernation, like the groundhog, must awaken every few days to raise their body temperature, move around, urinate, and eat. Then they lower body temperature and repeat the cycle. Black bears generally do not eat, drink, defecate, or urinate during hibernation. Bears live off a layer of fat built up during the summer and fall months prior to hibernation. Waste products are produced, however, instead of disposing of their waste, bears recycle it. The urea produced from fat metabolism is broken down and the resulting nitrogen is used by the bear to build protein, which allows them to maintain muscle mass and organ tissues. Bears lose fat and may actually increase lean-body mass while hibernating due to this nitrogen recycling.
Where is all this amazing science happening? In a den! Black bears den under windfalls, in hollow trees or caves, in previously occupied dens, or excavate their own. The chamber floor is covered with bedding material ranging from tree boughs to organic matter which have air pockets trapping body heat helping keep the bear warm. Movement to dens is correlated to weather with most movement usually occurring from late October to mid-November. However, scientists have found hibernation onset varies by as much as one month depending on weather conditions. Location also influences denning onset, with bears in northern latitudes denning earlier and longer than bears in southern latitudes. Bears will remain in the area of their den for a few weeks and enter a state of lethargy during which they eat nothing and sleep frequently.
Black bears breed from May through July but implantation does not occur until around December, about one month after solitary females den. The cubs are born in late January or early February naked, blind, and helpless. They measure about 8 inches long and weigh from 8 - 12 ounces. The newborn cubs do not hibernate, but sleep next to their mother, nurse, and grow rapidly. When black bear cubs emerge from the den at about three months of age, they weigh about 4 - 8 pounds and are able to follow their mother around in search of food. When temperatures warm up and food is available, bears emerge from their dens. Male bears emerge first, usually from early to mid-March (average days denned = 131 days), followed by solitary females and females with yearlings or two-years olds in late March through mid-April (average days denned = 151 days). The last to emerge are females with new-born cubs from mid-April through early May (average days denned = 171). Females with new-born cubs remain in the general vicinity of the den for several weeks while all others leave the vicinity of their den within a week of emergence.
As usual, what’s happening in nature can be of benefit to humans. Several physiological processes bears undergo during hibernation are of interest to medical researchers. When bears are hibernating and metabolizing body fat, their cholesterol levels are twice as high as during the summer and twice as high as the cholesterol levels of most humans. Bears, however, do not suffer from hardening of the arteries or gallstones, conditions resulting from high levels of cholesterol in humans. The bear's liver secretes a substance that dissolves gallstones in humans without surgery.
So as the days shorten and the temps drop, think of our neighbor, the black bear, and his peaceful sleep.
