The North American black bear (Ursus americanus) is the sole bear species inhabiting New Hampshire’s forests. These animals engage in a specialized winter sleep behavior known as denning, which differs significantly from the true hibernation observed in smaller mammals. Understanding the timing of this denning period is important as it dictates when bears are most active and visible. This annual cycle is driven by the scarcity of food during the cold months.
Seasonal Denning Timing in New Hampshire
Black bears typically enter their winter dens in New Hampshire between mid-October and late November. This timeframe is an average, and the precise date is subject to year-to-year variation. Dens are constructed in various locations, including burrows, caves, hollow logs, or within brush piles. Emergence from the dens typically occurs between late March and early April as the weather moderates.
There are distinctions based on a bear’s sex and reproductive status. Male bears have greater flexibility and may occasionally leave the den during mid-winter thaws before returning to sleep. Pregnant females and those with dependent offspring adhere to a stricter schedule. Pregnant females enter dens earlier than males to build fat reserves and prepare for giving birth in January. Sows with new cubs remain denned until April, often emerging later than solitary males.
Factors Influencing Denning Decisions
The timing of den entry and exit is highly sensitive to several environmental and biological cues. The most significant factor influencing a bear’s decision to den is the availability of food sources, particularly natural mast crops like acorns and beechnuts. Bears engage in hyperphagia, an intense period of feeding during the fall, to accumulate the body fat necessary to sustain them through the winter.
In years when natural foods are scarce, bears may enter their dens as early as September, as the rewards of remaining active no longer outweigh the energy expenditure. Conversely, an abundant supply of nuts and other forage can keep bears active well into December. The amount of stored body fat is particularly important for females, who require a minimum percentage of fat mass to support a pregnancy and nurse cubs. Weather conditions also play a part; early or deep snow and prolonged cold snaps hasten the denning process by reducing foraging efficiency. Reproductive status dictates the timeline; pregnant females and those with cubs must den earlier and stay longer.
The Physiology of Bear Winter Sleep
The winter sleep of the black bear is properly termed denning or torpor, as it involves physiological changes distinct from the deep hibernation of smaller mammals. A black bear remains easily arousable during this period, unlike true hibernators, which can be handled without waking. This moderate state of dormancy allows the bear to react quickly to threats or to tend to newborn cubs.
The bear’s body temperature drops moderately, typically falling by only 10 to 15 degrees Fahrenheit, from about 100°F to around 90°F. The metabolic rate is significantly reduced, decreasing by approximately 50% to conserve energy. This slowdown is also reflected in the heart rate, which drops substantially, from 40–50 beats per minute down to as low as 8–19 beats per minute. During the entire denning period, bears rely exclusively on accumulated fat reserves. They do not eat, drink, defecate, or urinate. The bear’s unique physiological adaptation allows it to recycle metabolic waste, preventing muscle atrophy and maintaining bone density.