Mice (genus Mus) are small endothermic mammals that generate internal heat to maintain a stable core body temperature. Their high surface-area-to-volume ratio means they lose heat to the surroundings at a rapid rate. This forces them to maintain a high mass-specific metabolic rate. Consequently, their survival is directly linked to continuous energy expenditure dictated by the ambient temperature. Maintaining thermal balance requires a delicate physiological and behavioral response, as extreme cold or heat can quickly lead to life-threatening conditions.
The Thermoneutral Zone Defining Comfort
The Thermoneutral Zone (TNZ) defines a mouse’s thermal comfort. It is the range of ambient temperatures where the animal maintains its core body temperature with the lowest possible energy expenditure, known as the basal metabolic rate. Within the TNZ, the mouse does not need to expend energy for heating or cooling. For laboratory mice, the TNZ is typically between 26°C and 34°C, though the ideal point shifts based on activity: near 29°C during the resting phase and toward 33°C during the active phase.
Temperatures within the TNZ are optimal for overall health because they minimize the cost of thermoregulation. Conserved energy can be allocated toward biological processes like growth, immune function, and reproduction. Housing mice below this range, such as at a typical room temperature of 22°C, causes constant, mild cold stress that significantly increases their daily energy expenditure.
Coping with Cold Stress and Survival
When temperatures drop below the TNZ, mice enter cold stress and activate mechanisms to generate or retain heat. The primary physiological response is increased metabolic heat production, which can elevate energy expenditure by 30% or more. The most important mechanism is non-shivering thermogenesis (NST), occurring mainly in Brown Adipose Tissue (BAT). BAT mitochondria generate heat directly by burning fat instead of producing ATP.
Mice also use behavioral strategies to reduce heat loss. Huddling with cage mates is highly effective, as it significantly reduces the group’s total exposed surface area, conserving heat collectively. Nest building is another adaptation, where mice construct insulated shelters to create a warmer microclimate.
A well-constructed nest reduces heat loss, compensating for low ambient temperatures. If physiological and behavioral defenses are insufficient, mice resort to shivering, an involuntary contraction of skeletal muscles to produce heat. Since these efforts demand continuous energy, survival in cold environments requires increased food consumption to fuel the elevated metabolic rate. Failure to secure adequate food rapidly depletes fat reserves, leading to hypothermia and death.
Navigating Heat Stress and Hyperthermia
When ambient temperatures exceed the TNZ, mice face heat stress and must actively dissipate excess internal heat. The primary physiological defense is evaporative cooling, achieved through rapid, shallow breathing known as panting. Since mice lack functional sweat glands, respiratory water loss via panting is the main cooling avenue. This mechanism, however, carries the risk of rapid dehydration during prolonged heat exposure.
The circulatory system aids cooling through vasodilation, where blood vessels near the skin surface widen. This allows warm blood to flow closer to the environment, facilitating heat transfer away from the body, especially in sparsely furred extremities like the tail. Behavioral responses are also important, as mice seek microclimates cooler than the surrounding air to escape the heat load.
This involves retreating into deep burrows or shaded areas, which offer a naturally cooler and more stable thermal environment. Mice also become strictly nocturnal, restricting activity to cooler night hours to avoid peak daytime heat. Due to their small size and limited cooling mechanisms, mice are highly susceptible to hyperthermia, or overheating. Sustained exposure to temperatures around 40°C can lead to lethal core body temperatures and death within hours.