Ground squirrels (genus Spermophilus or Urocitellus) are small rodents that employ true hibernation to survive winter when food is scarce and temperatures are low. Hibernation is a highly regulated physiological state called torpor, characterized by a dramatic, reversible depression of metabolism and body temperature. This process allows them to endure prolonged environmental stress by relying solely on stored energy reserves. The success of this dormancy depends entirely on a specialized, secure underground location.
The Specialized Hibernation Burrow
The answer to where ground squirrels hibernate is a meticulously constructed, solitary chamber deep within the earth, known as the hibernaculum. This structure is often distinct from the complex, multi-chambered burrow systems they use for daily life during the active season. The hibernaculum is typically situated below the frost line to maintain a stable, near-freezing temperature that is not susceptible to surface fluctuations.
For many species, this dedicated chamber is dug to a depth of 3 to 6 feet (around 1 to 2 meters) below the surface, though some species dig even deeper. The precise depth is crucial because it ensures the ground temperature remains stable and above freezing, protecting the squirrel from lethal cold. The hibernaculum is a single, enlarged chamber, often lined with insulating materials like dried grass, leaves, or fur collected during the preceding active season.
Once settled in the chamber, the ground squirrel seals off the entrance tunnel from the rest of the burrow system using a plug of packed soil. This soil plug serves multiple purposes: it helps regulate the chamber’s humidity and temperature, protects the squirrel from predators, and physically isolates the individual for the long period of torpor. A dedicated drain tunnel is often part of the architecture to carry away any moisture that might accumulate.
Seasonal Triggers for Entering the Burrow
The decision to retreat into the hibernaculum is governed by a combination of environmental cues and internal physiological changes. As summer transitions into autumn, the decreasing amount of daylight, known as the photoperiod, acts as a primary external signal. This reduction in light exposure alerts the squirrel’s internal clock that winter is approaching, initiating the shift toward dormancy.
The drop in ambient air temperature further reinforces the environmental signal for hibernation onset. However, the ultimate trigger for the final sealing of the burrow is a sufficient accumulation of body fat. The timing and duration of hibernation vary significantly based on species and geographic location; for instance, Arctic ground squirrels may hibernate for seven to eight months.
Species like the California ground squirrel, found in warmer regions, may not hibernate at all, or they may enter a period of summer dormancy called estivation during times of extreme heat and drought. Even within a single population, adult males often enter the burrow and begin hibernation several weeks earlier than females and juveniles. Males are typically the first to emerge in the spring, sometimes 10 to 14 days before the females, to establish territories.
Pre-Hibernation Preparation
Before entering the hibernaculum, ground squirrels undergo a period of intense feeding behavior known as hyperphagia. This is a deliberate, rapid accumulation of energy stores that sustains them through the months of metabolic suppression. During this phase, their body weight can increase by up to 40% as they accumulate triglycerides in white adipose tissue.
The stored fat is a necessary precursor to survival in the sealed burrow, and they accumulate both white adipose tissue (WAT) for long-term fuel and brown adipose tissue (BAT). Brown fat is utilized to generate heat during the periodic arousals from torpor, which occur roughly every two weeks.
The physiological process is so extreme that ground squirrels develop a transient, natural form of insulin resistance during this pre-hibernation phase to facilitate rapid fat storage. The completion of this fat accumulation is the final internal determinant that dictates when the squirrel can safely retreat underground.