Bear hibernation represents a remarkable biological adaptation that allows these animals to survive periods of resource scarcity and harsh environmental conditions. This extended state of dormancy is not initiated on a fixed calendar date for all bears. Instead, the timing of when bears begin this process is influenced by a complex interplay of environmental signals and internal biological programming.
Seasonal Cues for Hibernation
Bears begin hibernation in response to environmental signals that indicate the onset of winter. A primary external cue is the decreasing duration of daylight hours, known as photoperiod, which signals the changing seasons. As temperatures consistently drop in autumn, this prompts bears to prepare for dormancy. The reduced availability of natural food sources, such as berries, nuts, and fish, reinforces the need for metabolic slowdown.
Bears also possess an innate biological rhythm that works in conjunction with these external environmental cues. This internal clock regulates their preparedness for hibernation. Before entering their dens, bears engage in a period of intense feeding, called hyperphagia, to accumulate significant fat reserves. This fat accumulation is a necessary precursor to hibernation, providing the energy to sustain them through the winter.
Physiological Changes During Hibernation
Once a bear enters its den and begins hibernation, its body undergoes physiological changes to conserve energy and survive without food or water. The metabolic rate slows, leading to a decrease in heart rate, which can drop from 40-50 beats per minute to as low as 8-19 beats per minute. Breathing also becomes slower and shallower, often occurring only once every 15-45 seconds. The bear’s body temperature typically drops by several degrees, though it remains relatively high compared to smaller hibernators, usually staying above 30 degrees Celsius (86 degrees Fahrenheit).
Bears recycle waste products during this period, meaning they do not urinate or defecate while hibernating. Stored fat serves as the primary energy source, being metabolized to produce water and energy. This efficient use of fat helps bears minimize muscle atrophy and bone density loss. This allows them to emerge in spring with muscle mass and bone structure preserved.
Differences Across Bear Species and Habitats
The timing and depth of hibernation vary among different bear species and across their habitats. For instance, polar bears, especially males and non-pregnant females, may not hibernate at all if food sources, like seal populations, remain accessible throughout the arctic winter. In contrast, grizzly bears and black bears in colder, northern regions typically undergo deeper and longer hibernation periods.
Bears residing in milder, southern climates may experience shorter or even intermittent hibernation, particularly if food remains available year-round. Individual factors also play a role in this variability. Pregnant females often enter their dens earlier and emerge later than other bears, as they give birth and nurse cubs during this dormant phase. Age, overall health, and success of autumn feeding to build fat reserves also influence the duration and start of their hibernation.