Hibernation is a biological strategy used by certain animals to survive extended periods of cold temperatures and scarce food resources. It is a profound physiological adjustment, fundamentally different from normal sleep. The term is often used incorrectly, leading to a misunderstanding of which animals truly engage in this deep dormancy. True hibernation is distinguished by the degree of metabolic suppression and the corresponding drop in body temperature an animal sustains for months.
Defining True Hibernation
True hibernation is controlled, long-term hypothermia that allows an animal to conserve energy during winter. It is characterized by a massive reduction in metabolic rate, often dropping to as little as 1–2% of the active state. This profound slowdown is accompanied by a dramatic decrease in core body temperature, which in some small mammals can fall to just a few degrees above freezing (37–41°F or 3–5°C).
Heart rate and respiration slow significantly; a woodchuck’s heartbeat, for instance, may decrease from 80 to just four beats per minute. This state of regulated hypothermia is not continuous, as true hibernators must periodically undergo arousals, returning their body temperature to a normal, warm-blooded state for several hours. This process uses a specialized energy source, brown adipose tissue (BAT), which generates heat through non-shivering thermogenesis to quickly rewarm the animal. The purpose of these expensive periodic awakenings is unclear but may involve biological processes like immune system maintenance or managing a sleep deficit.
The Specialized Group: True Hibernators
The animals that fit the strict physiological definition of true hibernation are typically small-to-medium-sized mammals. Their high surface area-to-volume ratio makes them vulnerable to rapid heat loss in cold environments. These obligate hibernators enter their dormant state annually. The list includes many rodents, such as groundhogs (also known as woodchucks), marmots, and ground squirrels.
Woodchucks, for instance, can sustain this deep rest for up to 150 days, with their body temperature dropping to near 38°F (3.3°C). Certain species of bats, like the little brown bat, are true hibernators, slowing their heart rate to as low as 10 beats per minute. Hedgehogs are another classic example, relying on stored fat reserves and exhibiting a barely detectable rate of breathing throughout the winter.
Confusion and Misconceptions: Torpor and Winter Sleep
Many animals commonly associated with winter dormancy, most notably bears, do not meet the physiological criteria for true hibernation. They are better described as engaging in “winter sleep” or “denning.” The difference lies primarily in the degree of metabolic slowdown and the ability to be roused quickly. Bears, including American black bears and brown bears, only experience a slight drop in body temperature, usually from around 100°F to 88°F (37.8°C to 31.1°C).
This limited temperature reduction means their metabolic rate is not suppressed to the extreme levels seen in true hibernators. This allows a bear to wake up and become active relatively quickly, which is necessary for defense. Female bears also use this state to give birth and nurse cubs during the winter months. Other mammals like skunks and raccoons enter a similar state of winter lethargy; they will sleep for prolonged periods but can stir and become active during warmer spells to forage.
The term torpor describes a short-term, involuntary state of decreased physiological activity, often lasting only a few hours or days. Animals like chipmunks and mice use torpor daily to survive cold nights or temporary food scarcity. Torpor involves a less dramatic reduction in body temperature and metabolic rate compared to true hibernation. The Common Poorwill is the only known bird species to enter a sustained state of torpor to survive the winter.
Beyond Winter: Aestivation
Dormancy is not exclusively a winter survival strategy, as some animals must cope with periods of extreme heat and drought. Aestivation is the equivalent of hibernation but is triggered by hot, arid conditions rather than cold. This state of summer dormancy is a hypometabolic condition where the animal’s activity and metabolism slow down to prevent water loss and conserve energy.
The animals that aestivate include:
- Certain land snails, which seal their shells with a protective mucous membrane.
- The African lungfish, which burrows into the mud and secretes a cocoon.
- Desert tortoises, which retreat underground to cooler environments.
- Some amphibians, such as the water-holding frog.
Aestivation demonstrates that metabolic suppression is an adaptation to cope with any major environmental stressor, not only the winter cold.