Many wonder how squirrels survive colder months, often assuming they disappear into a long winter slumber. However, their winter survival strategies are more varied and complex than a single, continuous sleep. Understanding these adaptations reveals how different squirrel species navigate dwindling food and freezing temperatures.
Understanding Squirrel Hibernation
The term “hibernation” often brings to mind a deep, continuous sleep, but for squirrels, this state varies significantly by species. True hibernation, characterized by prolonged inactivity and dramatically reduced metabolic function, is primarily observed in ground squirrels. During this deep sleep, a ground squirrel’s body temperature can drop to near freezing (sometimes as low as 0°C). Its heart rate can slow from 200-300 beats per minute to just 2-4, and metabolism can decrease by as much as 90-99%, allowing them to conserve energy for many weeks or months.
Tree squirrels, such as the common Eastern gray squirrel, do not undergo true hibernation. Instead, they enter a less profound state known as torpor or winter lethargy. During torpor, their metabolism, body temperature, heart rate, and breathing rate decrease, though not as drastically as in true hibernators. This state allows them to save energy while remaining capable of waking up quickly to forage for stored food on milder winter days.
The Annual Cycle of Hibernation
The timing of winter inactivity for squirrels is closely linked to environmental cues and species-specific adaptations. Ground squirrels typically begin true hibernation in late summer or early fall, often around September, and may remain in their burrows until March or April. This prolonged period is triggered by decreasing ambient temperatures, shortening daylight hours, and reduced food sources. The exact duration can vary, with some adult Richardson’s ground squirrels hibernating for up to nine months.
Tree squirrels, utilizing torpor, exhibit a more flexible schedule. They may enter short bouts of inactivity for a few days, particularly when temperatures drop below 10°C. These periods of reduced activity allow them to conserve energy during harsh weather, emerging to find food when conditions improve. The duration of torpor depends on factors like food availability and winter severity, with some skipping it entirely in mild climates.
Preparing for Winter Rest
As summer transitions into fall, squirrels engage in behaviors to prepare for the lean winter months. A primary preparation is hyperphagia, a period of increased eating, to build significant fat reserves. For true hibernators like ground squirrels, this fat accumulation is necessary, as they rely on these internal stores to survive their dormant period. Arctic ground squirrels, for instance, can increase body mass by 42% (males) and 62% (females), primarily through an eight-fold increase in fat mass.
Alongside building fat reserves, squirrels also secure shelter. They construct or prepare dens and burrows that offer protection from the cold. Tree squirrels build insulated nests called dreys, often high in trees, using leaves, twigs, and other natural materials. Many squirrel species also engage in food caching, burying nuts and seeds in various locations to serve as accessible food sources during milder winter days.
Waking Up and Spring Activity
The emergence from hibernation or torpor signals the arrival of spring and a renewed period of activity. For ground squirrels, arousal from deep hibernation involves a rapid increase in body temperature and metabolic rate. They typically emerge in early spring, often in March, when rising temperatures and increasing food availability provide favorable conditions. This rewarming is energetically costly, consuming a significant portion of their winter energy budget.
Upon waking, squirrels immediately begin foraging to replenish energy lost during their inactive period. They are often quite lean after months without eating, having used their fat reserves as fuel. This period also marks the beginning of the breeding season for many squirrel species, with activities like mating rituals and nest building becoming prominent. For tree squirrels, who may have been intermittently active throughout winter, spring simply brings a return to consistent daily foraging and social behaviors.