Tree squirrels, common in backyards and parks, often lead people to wonder how they survive winter. While many associate winter survival with deep, prolonged sleep, tree squirrels do not truly hibernate. Instead, they employ behavioral and physiological adjustments to endure colder months, remaining active. These adaptations allow them to navigate fluctuating temperatures and scarce food without entering complete dormancy.
Winter Survival Strategies of Tree Squirrels
Tree squirrels prepare for winter by gathering and storing food, known as caching. They primarily use “scatter hoarding,” burying individual nuts and seeds in numerous shallow pits across their territory rather than consolidating them. This strategy ensures a diverse food supply and protects against total loss if a cache is discovered. Squirrels possess impressive spatial memory and a keen sense of smell, enabling them to locate hidden provisions, even under snow.
In addition to caching, squirrels build fat reserves during warmer months by eating extensively. This stored fat serves as an energy source and insulation against the cold. They also construct insulated nests, called dreys, high in tree branches using twigs, leaves, and moss, or find shelter in tree cavities. These shelters can maintain an internal temperature significantly warmer than the outside air, sometimes by 20 to 30 degrees Celsius.
During severe cold spells, tree squirrels reduce activity, spending more time inside their dreys to conserve energy. They may enter torpor, a temporary reduction in body temperature, heart rate, and metabolic rate, usually lasting a few hours or overnight. This differs from true hibernation as they easily awaken from torpor and emerge on warmer winter days to forage for cached food.
Understanding True Hibernation
True hibernation represents a profound physiological transformation, far more extreme than tree squirrel torpor. Animals that truly hibernate undergo a significant drop in body temperature, often nearing ambient temperature, sometimes even falling below freezing. Their heart rate slows dramatically, sometimes to a few beats per minute, and breathing becomes shallow and infrequent, with some species pausing respiration for extended periods.
This metabolic suppression allows hibernators to conserve immense energy during food scarcity and extreme cold, typically lasting weeks or months. Unlike torpor, arousal from true hibernation is a slow, energy-intensive process taking several hours. While hibernators remain largely inactive, some species periodically wake up briefly to excrete waste or consume stored food before re-entering their dormant state.
Other Squirrel Species and Hibernation
While tree squirrels do not truly hibernate, the misconception often arises because other squirrel species, like ground squirrels, do engage in true hibernation. Ground squirrels, such as the Arctic ground squirrel, spend most of their time underground in burrows, providing a stable environment for their long winter sleep. Their burrowing lifestyle, rather than living in trees, contributes to their ability to undergo such a prolonged dormant period.
Arctic ground squirrels are obligate hibernators, capable of hibernating for seven to eight months, and are unique among warm-blooded animals for surviving body temperatures below freezing. Their ability to withstand such extreme conditions highlights a distinct evolutionary path compared to their arboreal cousins. This difference in survival strategy underscores the diversity within the squirrel family.