Ground squirrels are one of nature’s most impressive examples of a mammal that truly hibernates. While some animals, like bears, enter a state of winter dormancy where their body temperature drops only slightly, ground squirrels undergo a profound physiological transformation that defines true hibernation. This deep, controlled state of metabolic suppression allows them to survive for months in environments where food is unavailable and temperatures are extreme.
The Physiological State of True Hibernation
True hibernation is characterized by an extreme reduction in nearly all physiological functions. The most dramatic change is the core body temperature, which plummets from a normal range near 37°C to just a few degrees above the surrounding soil temperature. Arctic ground squirrels, in particular, can achieve the lowest recorded body temperature for a mammal, dropping to as low as -2.9°C in their supercooled state without their body fluids freezing.
This massive drop in temperature is accompanied by a near-total collapse of the metabolic rate, which can be suppressed to between 1% and 3% of the normal resting rate. Consequently, the heart rate slows drastically from an active rate of 200–300 beats per minute to only 3–4 beats per minute. Breathing is also dramatically reduced, with the ground squirrel taking only one breath every few minutes. This state allows the animal to conserve the energy stored in its fat reserves, effectively putting life on hold for up to eight months.
Preparing for the Long Sleep
Preparation for the long sleep begins months before the first snow falls. Ground squirrels engage in a period of intense feeding known as hyperphagia, where they consume large amounts of food to build up substantial fat reserves. This process can cause their body weight to increase by up to 40% to 60%, with fat mass increasing seven to eight-fold. The stored energy is accumulated in both white adipose tissue, which provides the main fuel source, and brown adipose tissue, which is necessary for rewarming.
The timing of hibernation is precisely regulated by an internal clock and environmental cues, though it varies by sex and species. For example, adult female Arctic ground squirrels often enter their burrows in early August, while males follow later in September. Before entering the long sleep, the squirrel constructs or seals its hibernaculum deep underground, which offers protection from predators and helps maintain a stable, cold temperature.
The Energy Cost of Periodic Waking
Ground squirrels do not remain in a continuous deep sleep for the entire winter but instead undergo periodic arousals, returning to a normal body temperature of around 37°C for a short period. These interbout arousals typically occur every one to three weeks and last for about 12 to 16 hours, after which the animal rapidly recools and re-enters torpor. The process of rewarming is incredibly energy-intensive, requiring the animal to increase its metabolic rate up to 235 times in just a few hours.
Despite the short duration, these periodic waking events consume a significant portion of the total energy budget for the hibernation season, accounting for 60% to 80% of the stored fat in some species. The reason for these energy-costly arousals remains a subject of scientific research. Hypotheses suggest they may be necessary to maintain the health of the immune system, which is largely suppressed during the cold torpor state. Other theories suggest that waking is required to fulfill a mounting “sleep debt,” as the brain is too cold to achieve restorative sleep during deep torpor, or that the warm-up periods are needed for DNA repair or to correct metabolic imbalances that accumulate during the prolonged state of suspended animation.