When facing cold temperatures and scarce food, animals have evolved various strategies to conserve energy and survive the winter months. The umbrella term for this survival mechanism is “dormancy,” but this obscures a vast range of physiological responses. While the popular understanding of “hibernation” suggests a simple long sleep, the biological reality is a continuum of specific adaptations. The mechanism an animal employs determines whether it stays active, enters a deep metabolic state, or adopts a lighter, more flexible form of winter rest.
Defining True Hibernators
True hibernation is a state of deep, sustained metabolic depression that allows certain mammals to survive extended periods of low temperature and resource scarcity. This is not mere sleep; it is an actively controlled physiological reset triggered by internal cues like changing day length and external factors like temperature and food availability. The defining characteristic is a dramatic drop in body temperature, which may fall close to ambient temperature, sometimes to just a few degrees above freezing.
An animal’s heart rate slows profoundly, dropping from hundreds of beats per minute to as low as five to ten beats per minute in animals like the Arctic ground squirrel. Respiration rate also plummets, with some true hibernators taking only one breath every few minutes. This physiological slowdown dramatically reduces energy demand, allowing the animal to survive on stored body fat for months without eating or drinking.
This deep state of torpor is not continuous throughout the winter, as true hibernators must undergo periodic arousals. These brief periods involve rapidly raising their body temperature back to normal, a process that is extremely costly in terms of energy expenditure. Scientists believe these arousals are necessary for vital physiological maintenance, such as immune function, waste removal, and repairing cellular damage. Examples of true hibernators include ground squirrels, hedgehogs, dormice, and many species of bats.
Strategies of Alternative Dormancy
Many animals popularly called hibernators employ alternative forms of dormancy that are physiologically distinct from true hibernation. These strategies, which include winter sleep, brumation, and daily torpor, involve less severe physiological changes, allowing for greater flexibility.
Winter Sleep and Shallow Torpor
Large mammals famously associated with winter rest, such as bears, do not enter true hibernation but rather a state described as winter sleep or shallow torpor. Unlike true hibernators, a bear’s body temperature only drops minimally, typically between 3 to 5 degrees Celsius. This modest temperature reduction allows the bear to remain responsive and capable of quick arousal if disturbed or threatened.
Bears retain physiological control during this time, allowing them to perform processes impossible for a true hibernator. For instance, female black bears often give birth to and nurse their cubs while in the den. Bears also do not urinate or defecate for months, achieving this through metabolic adaptations, such as recycling urea to maintain muscle mass and bone density.
Brumation
A different strategy is employed by cold-blooded animals (ectotherms), such as reptiles and amphibians, in a state known as brumation. Because these animals cannot internally regulate their body temperature, their body functions slow down as a consequence of external cooling. Brumation is temperature-dependent, triggered by decreasing temperatures and shorter daylight hours.
During brumation, metabolism, heart rate, and activity levels decrease significantly, but the animal is not in the deep, comatose state of true mammalian hibernation. Brumating reptiles, such as snakes and turtles, may occasionally wake to bask or drink water. They refrain from eating because their digestive systems are too slow to process food. This process allows them to conserve energy until external conditions become warm enough to support normal activity.
Daily Torpor
Daily torpor is a short-term, opportunistic form of energy saving common in small mammals and birds that possess high metabolic rates. This state is a shallow, temporary reduction in body temperature and metabolic rate that typically lasts only a few hours, often overnight. Animals use this mechanism to survive short-term cold snaps or periods of food scarcity.
Hummingbirds are a classic example, entering torpor nightly when their primary food source of nectar is unavailable. Their high metabolism would otherwise quickly deplete their fat reserves. Unlike hibernation, daily torpor is quickly reversible, allowing the animal to warm up and resume normal activity to forage.
Animals That Remain Active
Many species survive winter without entering any form of dormancy, relying instead on physical and behavioral modifications to manage cold and scarcity. These animals must continuously find food to fuel their high metabolic rate and generate internal heat.
Physical adaptations center on insulation and heat regulation in the extremities. Mammals like deer and elk grow specialized winter coats featuring hollow guard hairs that trap a thick layer of insulating air, providing superior protection against the cold. The Arctic fox has a dense undercoat and utilizes a counter-current heat exchange system in its legs, allowing the paws to remain cold while preventing core body heat loss.
Behavioral strategies focus on resource management and shelter. Many bird species avoid winter entirely through long-distance migration to warmer regions where food remains abundant. Other animals, such as squirrels, red foxes, and beavers, employ food caching, spending autumn storing large quantities of nuts, carcasses, or plant material to be retrieved later.
Active animals become specialized in finding or creating winter shelter. White-tailed deer gather in sheltered areas called “deer yards,” usually thick conifer stands that offer protection from wind and deep snow. Small mammals and Arctic foxes utilize the subnivean zone, a protected layer of space that forms between the ground and the insulating blanket of snow. This provides a microclimate warmer than the harsh air temperature above.