Survival strategies to endure harsh environmental conditions involve entering a state of dormancy, a broad term for reduced physical activity and metabolism. While many people use the word “hibernation” loosely to describe any long winter sleep, the term “true hibernation” refers to a highly specific, regulated physiological state. Understanding the biological markers of this deep suppression is necessary to distinguish the few animals that meet the criteria from those that merely take a winter rest.
The Physiological Markers of True Hibernation
True hibernation represents a profound, controlled suppression of the mammal’s entire system. The most dramatic and defining change is the regulated drop in core body temperature, which can fall close to the ambient temperature of the den, often reaching just a few degrees above freezing. For instance, the body temperature of a hibernating ground squirrel may drop from a normal 37°C to as low as 2 to 5°C.
This decline is coupled with an extreme decline in metabolic activity to conserve energy reserves. The metabolic rate of a true hibernator plummets to as low as 2 to 5% of its active state. Correspondingly, the heart rate slows from hundreds of beats per minute to just a handful, such as the alpine marmot’s heart rate falling from 120 beats to a mere three or four beats per minute. This state is not simply a passive freezing but an active, regulated process that prevents tissue damage.
Separating Hibernation from Other Dormancy States
Many animals enter other forms of seasonal dormancy that lack the extreme physiological suppression. One common form is torpor, a short-term, shallow metabolic drop lasting from a few hours to a few days, typically used by small animals like hummingbirds or bats to survive a cold night. These animals are easily roused and cycle in and out of the state rapidly.
Brumation describes the winter dormancy of cold-blooded animals like reptiles and amphibians, such as turtles and snakes. Since these creatures are ectotherms, their body temperature and metabolism are entirely dependent on the surrounding environmental temperature, and their dormancy is not an internally regulated process. The confusion often centers on bears, which enter a state sometimes called winter sleep or shallow torpor. A bear’s body temperature drops only slightly, typically by 5 to 10°C, remaining well above 30°C, which allows them to wake up quickly and defend themselves or give birth, unlike the deep unresponsiveness of a true hibernator.
The Exclusive Club: Animals That Achieve True Hibernation
The most rigorous criteria for true hibernation are met predominantly by smaller mammals, particularly rodents, which have evolved the capacity for the dramatic, sustained physiological shutdown. Groundhogs, also known as woodchucks, are classic North American examples, entering a deep sleep that can last up to five months, surviving entirely on the fat reserves accumulated during the summer. Specific species of ground squirrels, such as the Arctic ground squirrel and the thirteen-lined ground squirrel, are among the most studied true hibernators.
These small mammals push their physiology to the extreme, with their body temperature sometimes dropping below 0°C without the formation of damaging ice crystals. Other members of this exclusive club include the European hedgehog, the fat-tailed dwarf lemur, and many species of bats, with some bats hibernating for up to six months. Before entering this state, these animals engage in hyperphagia, eating massive amounts of food to build up the fat layer to fuel their minimal metabolic needs.
The Complex Mechanics of Arousal
Arousal, the process of reawakening from true hibernation, is arguably more complex and energetically demanding than the entry into the dormant state. True hibernators do not remain continuously cold for the entire season but undergo periodic, spontaneous bouts of warming, called interbout arousals, typically lasting 12 to 24 hours every one to three weeks. These cyclical arousals are metabolically costly, consuming a significant portion of the animal’s fat reserves.
The rapid rewarming is accomplished through non-shivering thermogenesis, a process that relies heavily on specialized brown adipose tissue (BAT). BAT is densely packed with mitochondria, and its activation allows the animal to generate substantial heat without relying on muscle movement or shivering. During an arousal event, the heat produced by BAT can cause the animal’s body temperature to rise by 20°C in less than an hour, with the heart rate surging from a few beats to hundreds per minute. This mechanism is essential for the hibernator to return to a fully active, warm-blooded state.