The ability to regulate body temperature is critical for animals. This is especially true for small mammals as they have a larger surface area resulting in more heat loss to the environment than larger animals. Animals that stay active during the winter likewise have to spend more energy to stay warm. To do this well, they need to balance heat loss with heat generation through shivering as well as non-shivering thermogenesis. To create heat (i.e. thermogenesis) animals must burn fuels. Shivering is typically fueled by burning fats and carbohydrates whereas non-shivering thermogenesis is fueled mostly by fats. With extended exposure to cold, an animal needs to choose the right foods to support the prolonged energy expenditure needed to stay warm.
Interestingly, studies examining the VO2max (maximum rate of oxygen use during aerobic exercise) of small cold-acclimated mammals found that VO2max levels can often be higher during thermogenesis than during exercise. During exercise, fat use at VO2max typically plateaus or even drops at which point mammals switch to burning more carbohydrates.
A new study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology compared fuel use by deer mice, which are found in both high and low altitude areas with white-footed mice that are only found at low-altitudes. They did this to compare how the animals living at high altitude may have adapted to thrive and stay active during prolonged cold exposure and hypoxia. They exposed the animals either to a warm environment with normal levels of oxygen (23°C, 760 mmHg) or to a simulated high-altitude environment (5°C, 480 mmHg (mimics ~4300 m above sea level)) for 6-8 weeks.
White-footed mice, which only live at low altitudes lost more heat to the environment than the deer mice. In addition, while all animals increased VO2max with cold exposure, both populations of deer mice were found to have greater increases in VO2max than the white-footed mice. In addition, both species of animals greatly increased their use fats for fuel during simulated high-altitude exposure. Moreover, the ability of deer mice to burn fats was the highest among land mammals of similar mass and 5 times higher than levels measured during exercise.
SA Lyons, KB Tate, KC Welch, GB McClelland. Lipid Oxidation during Thermogenesis in High-Altitude Deer Mice (Peromyscus maniculatu). American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. In Press.