Winter dormancy is a biological strategy that allows certain animals to survive long periods of environmental adversity, primarily cold temperatures and the resulting scarcity of food. This state involves a profound shift in an animal’s physiology to drastically reduce the energy required for survival. Hibernation is characterized by a deep metabolic depression that conserves the internal energy stores built up during warmer months. The central question for animals in this extreme state is how they sustain themselves for months without access to external nourishment.
The Physiological State of True Hibernation
For a true hibernator, such as a groundhog or an Arctic ground squirrel, the answer to whether they eat is a definitive no during the deep state of torpor. This deep state involves a dramatic resetting of the animal’s internal thermostat, allowing its body temperature to drop from a normal mammalian range of around 37°C to just a few degrees above the surrounding burrow temperature, sometimes near 0°C. This profound hypothermia is accompanied by an extreme slowdown of all bodily functions.
The metabolic rate can decrease to as low as 2 to 4% of the resting rate, which is the mechanism for energy conservation. The heart rate of a small hibernator may slow from hundreds of beats per minute to just 5 to 10 beats per minute. Breathing becomes very infrequent and shallow, sometimes occurring only once every few minutes. The digestive system effectively ceases to function during this physiological shutdown.
True hibernators do not remain continuously in this state; they undergo periodic arousals, returning their body temperature to a normal level for a period of 12 to 24 hours every few weeks. This rapid rewarming is extraordinarily expensive, consuming the majority of the animal’s winter energy budget. The purpose of these arousals is homeostatic, allowing for essential processes like DNA repair, immune system maintenance, and the voiding of metabolic waste products.
Fueling the Long Sleep: Pre-Hibernation Preparation
The success of a hibernating animal is determined well before the first frost, during a period of intense feeding known as hyperphagia. Animals proactively consume excessive amounts of food in the late summer and fall to accumulate the massive fat reserves needed for the long sleep. These reserves are stored in two distinct types of fat tissue, each serving a specific physiological purpose.
White Adipose Tissue (WAT) is the primary energy source for sustaining life throughout the torpor bouts, providing the necessary fuel for the minimal metabolic activities. This fat is metabolized slowly over the months of inactivity, releasing energy and water as a byproduct. Brown Adipose Tissue (BAT) serves a different function, acting as a specialized heat generator.
BAT is highly concentrated in the neck and shoulder regions and is rich in mitochondria, which allows for non-shivering thermogenesis, or the rapid production of heat. This tissue is essential for the process of periodic arousal, providing the initial burst of heat that raises the body temperature by over 30°C in a matter of hours.
Distinguishing Hibernation from Torpor and Winter Sleep
The question of whether animals eat during dormancy is often complicated by the different types of winter resting strategies employed by various species. True hibernation is the most extreme form, but it should be clearly distinguished from daily torpor and the “winter sleep” of larger mammals. Daily torpor is a short-term, shallow reduction in body temperature and metabolism that lasts only a few hours, often seen in smaller animals like hummingbirds, mice, or hamsters. These animals can enter and exit this state quickly, often on a daily cycle, and will feed during the active periods.
The concept of “winter sleep” applies to mammals like bears, which are not considered true hibernators because their body temperature only drops by a modest 3 to 5°C. They maintain a relatively high body temperature and are easily aroused, especially compared to the deep, unresponsive state of a ground squirrel.
Despite their higher body temperature, bears also do not eat, drink, urinate, or defecate for months, relying on their massive fat stores. This ability to maintain a near-normal body temperature while suppressing metabolism allows bears to give birth and care for cubs during their denning period. Their strategy is one of deep rest and fasting rather than the profound metabolic collapse seen in true hibernation. This is why a bear may be seen moving near its den on a warmer day, whereas a true hibernator remains deeply dormant.