The deer mouse (Peromyscus maniculatus) is one of the most widely distributed and abundant small mammals across North America, from Alaska down to Mexico. Given their small size and the harsh winters throughout much of their range, these mice do not engage in the deep, continuous dormancy of true hibernators. Instead, the deer mouse employs a flexible, short-term survival strategy known as torpor to navigate periods of extreme cold and food scarcity. This approach allows them to conserve energy without committing to a months-long, irreversible physiological shutdown.
Defining Hibernation Versus Torpor
The distinction between hibernation and torpor lies in the duration and depth of the metabolic suppression. True hibernation is a prolonged state of regulated hypothermia lasting weeks or months, such as that seen in groundhogs or certain bats. During this time, the animal’s body temperature can drop dramatically, often very close to the ambient temperature, and arousal from this deep state is a slow, energy-intensive process. Torpor, in contrast, is a state of decreased physiological activity that is short-term, typically lasting only a few hours or a few days. It is a flexible process that the animal can enter and exit quickly in response to immediate environmental conditions, such as a temporary cold snap or a shortage of food.
Behavioral Adaptations for Winter Survival
Beyond the physiological state of torpor, the deer mouse utilizes several behaviors to manage the energetic demands of winter. A primary strategy involves extensive food caching during the autumn months to ensure a readily available food supply when foraging becomes difficult. These omnivorous rodents will stockpile large quantities of seeds, nuts, and grains in hidden granaries within their nests or burrows.
The construction of deep, insulated nests, often made of shredded plant fibers, fur, and feathers, located in sheltered spots like tree cavities, under logs, or in abandoned burrows is also important. Communal nesting, where groups of up to ten or more individuals will huddle together, is highly effective. This collective behavior significantly reduces the surface area exposed to the cold, effectively conserving body heat and minimizing the energy each mouse must expend to stay warm.
The Physiological Mechanism of Torpor
The deer mouse’s adoption of torpor is a physiological response to environmental triggers like ambient temperatures below 10°C, particularly when combined with low food availability. Before entering a torpor bout, the mouse undergoes a metabolic shift, moving its primary fuel source from carbohydrates to fat reserves. This change is indicated by a drop in the respiratory quotient hours before the body temperature begins to fall.
Once initiated, torpor involves a decrease in the metabolic rate, which can be suppressed by up to 90% compared to the active state. Correspondingly, the mouse’s body temperature drops significantly, often falling to around 20°C, though some report temperatures as low as 18°C. This regulated hypothermia is maintained for a few hours, typically during the coldest part of the night for this nocturnal species.
The ability to spontaneously rewarm, or arouse, from torpor is a defining characteristic of this state. Deer mice can actively increase their body temperature using shivering, a process that expends energy but allows them to become fully active within a relatively short period, often less than two hours. This short-duration, reversible state allows them to save energy during cold periods while still permitting daily activity and access to their food caches.