The California ground squirrel (Otospermophilus beecheyi) enters a state of deep dormancy, including true hibernation, though its activity patterns are highly variable across its range. This species exhibits a complex annual cycle, adapting its periods of inactivity to both cold winter temperatures and the intense summer heat and drought characteristic of California’s climate. The extent and timing of their dormancy are directly influenced by local environmental conditions, elevation, and food availability, allowing them to survive when resources are scarce.
The Mechanism of Dormancy
The winter dormancy involves a physiological shift known as torpor. During a torpor bout, the squirrel’s normal body temperature (typically around 98°F) drops significantly to nearly match the ambient temperature of its burrow, often hovering just above freezing. This suppression reduces the animal’s heart rate to only a few beats per minute and lowers the metabolic rate to as little as one to nine percent of its active rate.
The squirrel cycles through torpor bouts, which can last for days or weeks, punctuated by periodic arousal episodes. These brief, internally driven warm-ups return the animal’s body temperature to near-normal levels for about 12 to 24 hours. A specialized tissue called brown adipose tissue (BAT) is activated to generate the heat needed for this rewarming process. BAT allows the squirrel to efficiently burn fat reserves to emerge from torpor before re-entering the dormant state.
Seasonal Timing and Duration
The timing for entering and exiting winter dormancy depends on the local climate and elevation. Adult ground squirrels in colder, inland areas typically begin hibernation in late summer or early fall after accumulating sufficient fat reserves. They generally emerge from their burrows in late winter or early spring, with some populations appearing as early as January and others not until March.
Differences exist between the sexes and age groups regarding the hibernation schedule. Adult males often emerge from dormancy a week or two before the females, likely to establish territories and prepare for breeding. Juvenile squirrels, especially those born in the spring, may remain active longer into the fall and, in mild climates, may not truly hibernate during their first winter. Populations in mild coastal regions may remain active year-round or only experience short periods of reduced activity during the winter months.
The Unique Phenomenon of Estivation
Beyond winter hibernation, the California ground squirrel also exhibits a distinct period of summer dormancy called estivation. This state is triggered by the scarcity of resources, such as dry vegetation and lack of green food, combined with high summer temperatures. Estivation serves as a survival mechanism against the harsh, arid conditions typical of a California summer.
Estivation is a reaction to high temperatures and the resulting lack of nutritious forage. Adults may enter this summer dormancy period as early as July, a time when juveniles often remain active above ground. Some adult squirrels may enter estivation immediately after the vegetation dries out and remain dormant until the following spring, effectively linking their summer and winter dormancy periods. This behavior means an adult squirrel can spend up to six to eight months continuously sealed within its burrow, surviving entirely on stored body fat.