Mice, like many small mammals, have a high surface-area-to-volume ratio, which means they gain and lose heat quickly, making thermoregulation a constant challenge. Unlike humans, who rely heavily on sweating to dissipate heat, mice do not possess the widespread sweat glands necessary for this type of evaporative cooling. When ambient temperatures rise above their comfort zone, known as the thermoneutral zone, mice must employ a different set of physiological and behavioral strategies to maintain a stable internal body temperature. These alternative mechanisms are less efficient than human sweating, which explains why mice are particularly susceptible to heat stress and hyperthermia.
Glands Used for Grip and Marking
Mice do have eccrine sweat glands, but their location and function are highly specialized, bearing no resemblance to the cooling system in humans. These glands are primarily restricted to the plantar surfaces, meaning the soles or pads of their feet. This limited distribution prevents them from using the moisture for whole-body evaporative cooling, which requires widespread skin coverage.
The moisture produced by these eccrine glands serves two main non-thermoregulatory purposes: increasing friction for grip, allowing mice to climb effectively, and contributing to scent marking for communication. Therefore, these glands do not enable a mouse to “sweat” for temperature regulation.
Cooling Through Respiration and Saliva
When the temperature rises, mice primarily switch to behavioral and respiratory methods to induce evaporative cooling. One of the most recognizable responses is an increase in the rate of breathing, commonly known as panting. This rapid, shallow breathing draws air over the moist surfaces of the upper respiratory tract, including the mouth and lungs, causing water to evaporate.
Evaporation from these moist surfaces removes latent heat, cooling the circulating blood. Panting is less effective than sweating because it is limited by the surface area of the mouth and respiratory passages. A second behavioral strategy is saliva spreading, where the mouse grooms itself and spreads saliva over its fur. As the saliva evaporates, it draws heat away from the skin, providing a temporary cooling effect.
Vascular Heat Exchange in Appendages
Mice utilize a physiological process involving their less-furred appendages to dissipate excess heat. This relies on dry heat loss, where heat is transferred directly to the cooler surrounding air. The tail and the ears serve as specialized heat radiators due to their sparse fur and high surface-area-to-volume ratio.
To facilitate heat loss, the sympathetic nervous system triggers vasodilation, the widening of blood vessels in these appendages. This increases the flow of warm blood from the core close to the skin surface of the tail and ears. The heat transfers from the blood into the environment, cooling the circulating blood before it returns to the core. While the tail is a useful biomarker for vasodilation, its contribution to total heat dissipation is modest, typically accounting for about 5 to 8% of whole-body heat loss.
Recognizing and Preventing Heat Stress
The reliance on less-efficient cooling mechanisms means mice are highly susceptible to overheating, a dangerous condition called hyperthermia. Because of their small size, mice gain heat quickly, and their thermoregulatory system can fail rapidly when ambient temperatures are high. For example, a mouse exposed to 40°C can die from heat stress within a few hours.
Observable signs of heat stress include incessant panting, lethargy, weakness, and collapse. In severe cases, body temperatures exceeding 40°C can lead to irreversible damage and death. Prevention requires simple measures, such as ensuring constant access to fresh water to prevent dehydration. Providing cool surfaces, like ceramic tiles, and ensuring a shaded, well-ventilated environment are effective ways for the mouse to behaviorally regulate its temperature.