The concept of animals sleeping through the winter is common knowledge, but the term “hibernation” is frequently misused. Many animals enter a state of winter dormancy to survive cold temperatures and food scarcity, yet not all of these states meet the biological definition of true hibernation. The distinction lies in the depth of physiological suppression and the nature of the metabolic shutdown. Understanding this standard allows for a more accurate classification of the survival strategies animals employ during the harshest months. True hibernation involves a profound, regulated hypothermia, separating it from other forms of deep, prolonged winter sleep.
The Physiological Markers of True Hibernation
True hibernation is a complex, regulated process defined by an extreme reduction in an endotherm’s metabolic rate and core body temperature. The primary feature is the dramatic drop in metabolism, which can fall to as low as 2 to 5% of the animal’s normal active rate. This profound suppression allows the animal to conserve energy by slowing down virtually all biological functions. The heart rate of a true hibernator, such as the 13-lined ground squirrel, can plummet from over 300 beats per minute to fewer than five beats per minute.
The second marker is a massive drop in core body temperature, a state known as regulated hypothermia. The hibernator’s temperature often approaches the ambient temperature of its burrow, sometimes falling to near freezing (34 to 39 degrees Fahrenheit or 1 to 4 degrees Celsius). The Arctic ground squirrel is an exception, capable of maintaining its body temperature below the freezing point of water, as low as 27 degrees Fahrenheit (-2.9 degrees Celsius), without cellular damage.
These periods of deep torpor are not continuous, as true hibernators periodically experience arousals. During these brief awakenings, which occur every few days or weeks, the animal rapidly rewarms its body to a near-normal temperature for about 12 to 20 hours. Rewarming is accomplished through non-shivering thermogenesis, a process relying heavily on specialized brown adipose tissue (BAT) to generate heat efficiently before re-entering the hypothermic state.
Mammals Classified as True Hibernators
The creatures that meet the criteria of regulated, deep hypothermia are generally small-to-medium-sized mammals, primarily rodents and insectivores. Ground squirrels are considered champion hibernators, with species like the Arctic ground squirrel spending up to seven or eight months in this profound state. Marmots, including the woodchuck or groundhog, are also classic examples, capable of long, deep hibernation periods lasting several months.
Many species of bats are true hibernators, hanging upside down in caves and abandoned mines where the temperature is stable and cool. They sustain themselves through the winter when their insect prey is unavailable by dropping their heart rate from hundreds of beats per minute to just a few. The European hedgehog is another true hibernator, rolling into a tight ball and drastically reducing its metabolism. Small rodents such as chipmunks and hamsters also exhibit true hibernation, though the duration and depth can vary depending on environmental conditions.
Dormancy States That Are Not True Hibernation
Many large mammals and cold-blooded animals are commonly mistaken for true hibernators, but their winter dormancy states are fundamentally different. The most famous example is the bear, which enters a state often called “winter sleep” or deep torpor, not true hibernation. A black bear’s body temperature drops only slightly, usually settling between 88 and 93 degrees Fahrenheit (30 to 34 degrees Celsius), which is only a 10 to 12-degree drop from its normal active temperature.
This relatively high body temperature allows the bear to be easily and quickly aroused, which is essential for defense and for female bears to give birth and nurse cubs while dormant. Unlike true hibernators, bears do not urinate or defecate for the entire period, recycling metabolic waste products. This physiological stability and ability to wake rapidly exclude them from the classification of true hibernators.
Torpor
Another distinct state is torpor, which is essentially a short-term, daily version of metabolic suppression. Animals like hummingbirds and certain mice use torpor nightly to survive when food is scarce, dropping their body temperature and metabolism for a few hours before rewarming quickly to resume activity. Torpor is a temporary survival strategy, lasting from a few hours to a few days, and is often governed by the animal’s circadian rhythm. This contrasts sharply with hibernation, which is a seasonal, multi-week or multi-month commitment.
Brumation
Finally, cold-blooded animals, such as reptiles and amphibians, enter a state called brumation. Because these animals are ectotherms, their body temperature is directly dependent on the environment, not regulated internally. During brumation, their metabolism slows significantly as the surrounding temperature drops, but they may still wake up on warmer days to drink water. This process is a response to external cold, not a complex, internally controlled physiological shutdown like true hibernation.