Black bears in Colorado undergo a period of dormancy each year, a natural adaptation that allows them to survive through periods of scarce food and harsh weather conditions. While often referred to as hibernation, this state for bears is distinct from the deep sleep experienced by smaller hibernators. The patterns of this annual cycle are closely tied to environmental cues and physiological changes.
Seasonal Timing of Hibernation
Black bears in Colorado typically begin to enter their dens for hibernation in late fall or early winter, often around October or November. Females with cubs are usually among the first to den, followed by other females, and then adult males. This period of inactivity generally lasts until late winter or early spring, with bears emerging from their dens around March or April.
Male bears often emerge earlier than females, sometimes by a couple of weeks. Females that have given birth to cubs during the winter tend to be the last to leave their dens, typically in late April or early May.
What Prompts Bears to Hibernate
The decision for black bears to enter hibernation is primarily driven by environmental and physiological signals. A major factor is the decreasing availability of natural food sources, such as berries, nuts, and other vegetation, which become scarce as colder weather sets in. Bears do not hibernate because they are cold, but rather because their food supply diminishes significantly.
Another important cue is the change in photoperiod, or the shortening of daylight hours in the fall. This reduction in daylight triggers metabolic changes in bears, signaling their bodies to prepare for dormancy. Leading up to hibernation, bears enter a phase known as hyperphagia, a period of intense eating and drinking. During hyperphagia, bears can consume vast amounts of calories, up to 20,000 per day, to build up fat reserves necessary for sustaining them through the winter without food. This excessive eating can involve foraging for up to 20 hours a day.
Life in the Den and Emergence
Once in their dens, black bears enter a state of reduced activity often referred to as torpor, denning, or winter lethargy. Though a deep rest, bears can be roused if disturbed. During this time, their physiological processes undergo significant changes to conserve energy.
A bear’s heart rate can drop substantially, from 40-50 beats per minute to as low as 8-21 beats per minute. Their metabolism also slows, reducing to about 25-50% of their normal summer rates, and their breathing becomes much slower, sometimes as infrequent as one breath every 15 to 45 seconds. Unlike many other hibernators, a black bear’s body temperature does not drop drastically, typically decreasing by only about 12 degrees Fahrenheit, remaining between 88 and 98 degrees Fahrenheit. Bears do not eat, drink, urinate, or defecate for up to 200 days while in the den, relying entirely on their stored fat reserves. Pregnant females give birth to their cubs in the den during mid-winter, and the cubs will nurse and grow there until emergence. When bears emerge in the spring, they have lost a significant portion of their body weight, typically 15-30%. They emerge with a strong appetite and immediately begin seeking food, often starting with tender vegetation like grasses to help their digestive systems readjust.