The question of whether squirrels hibernate in California depends on the species, as the term “squirrel” covers many types with distinct survival strategies. Many people define hibernation simply as a long period of deep winter sleep. However, this overlooks the complex physiological adaptations squirrels use to survive lean or cold seasons. The behavior of ground-dwelling species differs fundamentally from that of tree-dwelling species. Understanding the scientific distinctions between true dormancy states is necessary to determine how each squirrel type manages California’s varied climate.
Defining True Hibernation and Torpor
Hibernation describes a state of prolonged metabolic depression lasting for weeks or months. During true hibernation, an animal’s heart rate, breathing, and metabolism slow dramatically, sometimes dropping to one or two percent of the active rate. The body temperature decreases significantly, often falling to just a few degrees above freezing. This allows the animal to conserve the maximum amount of energy from stored body fat.
Torpor, in contrast, is a shallower, shorter state of decreased physiological activity, typically lasting less than 24 hours. Animals in torpor reduce their body temperature and metabolic rate, but not to the extreme levels seen in true hibernation. This short-term dormancy is often used daily by smaller mammals to survive a cold night or brief food scarcity, allowing them to quickly wake and become active.
A third form of dormancy is aestivation, which is essentially a summer version of hibernation. Aestivation is triggered by hot, dry conditions or a lack of food and water, rather than cold temperatures. These specialized states of inactivity are distinct survival mechanisms used by certain California squirrels.
The Hibernators: California Ground Squirrels
The California Ground Squirrel (Otospermophilus beecheyi) engages in true hibernation, especially in inland areas where temperatures fluctuate widely. These squirrels prepare for their long sleep by consuming large amounts of food in the late summer and fall to build up fat reserves. Their winter dormancy involves long bouts of inactivity where their body temperature drops close to the ambient temperature of their burrow.
In some populations, adults begin entering a dormant state as early as summer, known as aestivation, when the dry landscape offers little forage. This summer dormancy helps them survive drought and food scarcity before transitioning into winter hibernation. Squirrels in colder, high-altitude locations spend more time hibernating. However, those in mild, coastal areas may remain active throughout the year. Males typically emerge from burrows first, with activity resuming between late January and March depending on the local climate.
The Non-Hibernators: Tree Squirrel Survival
California is home to several tree-dwelling squirrels, such as the Western Grey Squirrel (Sciurus griseus) and the introduced Eastern Fox Squirrel (Sciurus niger). Neither of these species are true hibernators; they remain active throughout the winter. They rely on behavioral adaptations rather than a deep, prolonged metabolic shutdown, surviving by reducing activity and seeking shelter in insulated nests, called dreys, or in tree cavities.
Their primary strategy for winter survival is scatter hoarding, which involves burying individual food items like nuts and seeds across a wide area during the fall. They rely on spatial memory and smell to locate these buried caches when food becomes scarce. On the coldest days, tree squirrels may enter a short, shallow state of torpor. However, they must wake up every few days to retrieve food and maintain their energy balance, meaning they are a familiar sight even during the coldest parts of the year.