Squirrels have developed strategies to endure harsh winter conditions. Hibernation is a physiological adaptation, allowing some species to survive periods of extreme cold and food scarcity. While animals “sleeping through winter” is widely known, the specific biological changes, particularly concerning body temperature, are complex.
Core Body Temperature During Hibernation
Hibernating squirrels undergo a significant reduction in their core body temperature, a controlled process that distinguishes true hibernation from ordinary sleep. For many ground squirrel species, body temperature can drop to 0-5°C (32-41°F). The 13-lined ground squirrel, for instance, typically maintains 4-8°C during hibernation.
Some species, like the Arctic ground squirrel, exhibit an even more extreme adaptation, with their core body temperature plummeting to as low as -2.9°C to -3°C (26.8-27°F). This ability to supercool their blood, preventing ice crystal formation, is a unique survival trait among mammals. In contrast, an active squirrel’s body temperature is around 37°C (99°F).
Physiological Adaptations for Hibernation
To sustain low body temperatures, hibernating squirrels undergo physiological transformations. Their metabolic rate, the speed at which their bodies use energy, can decrease to as little as 1-2% of their active state. This slowing of metabolism allows them to conserve energy for months without eating. Heart rate slows considerably, from hundreds of beats per minute to just 3-10 beats per minute, and in Arctic ground squirrels, it can be as low as one beat per minute. Breathing also becomes suppressed, occurring in episodic bursts, with some species taking only 4-6 breaths per minute.
During hibernation, squirrels enter torpor, a regulated hypothermia, not simply sleep. This state is periodically interrupted by brief arousal periods, where the squirrel’s body temperature rapidly returns to near-normal levels for a few hours before re-entering torpor. Brown adipose tissue (BAT), a specialized fat, plays a significant role in rewarming. BAT generates heat through non-shivering thermogenesis, allowing the squirrel to quickly raise its body temperature without muscular activity.
Environmental Cues for Hibernation
External factors serve as important signals, prompting squirrels to initiate hibernation. Decreasing daylight hours, known as photoperiod, is a primary cue that signals the approaching winter. This reduction in light triggers hormonal changes. A dwindling food supply further reinforces the need to conserve energy.
These environmental triggers stimulate squirrels to increase food intake in late summer and fall, building fat reserves. This pre-hibernation fattening provides energy to sustain them through months of inactivity. Genetic predisposition also influences when and for how long individual squirrels hibernate.
Different Squirrel Species and Their Hibernation
Not all squirrel species engage in the deep, prolonged physiological changes of true hibernation. True hibernators, such as many ground squirrels, marmots, and chipmunks, undergo significant metabolic and temperature reductions. These species spend months underground, relying on stored fat reserves.
Tree squirrels, including gray and red squirrels, do not truly hibernate. They remain active or semi-active throughout winter, relying on food caches. While they may enter short bouts of torpor, involving a less profound and shorter decrease in body temperature and metabolic rate, they do not undergo the deep, sustained hibernation of ground squirrels. These tree-dwelling squirrels often build insulated nests (dreys) or find tree cavities for shelter, sometimes huddling for warmth.