The energy budget, thermogenic capacity and behavior in Swiss mice exposed to a consecutive decrease in temperatures

The limitation on sustainable energy intake (SusEI) is important because it establishes the upper energetic limit on the ability of animals to disperse, survive and reproduce. However, there are still arguments about what factors impose that limitation. Thermoregulation in cold environments imposes great energy demands on small mammals. A cold-exposed animal has been suggested to be a model suitable for testing these factors. Here, we examined the changes in food intake and digestible energy intake (DEI) as measures of SusEI, thermogenic capacity and behavioral patterns in Swiss mice exposed to consecutively lower ambient temperatures from 23 to -15°C. Cold-exposed mice showed significant decreases in body mass, fat content of the carcass and body temperature, and increases in DEI compared with controls. The time spent on feeding significantly increased with decreasing temperatures, and time spent on general activity decreased following cold exposure. Resting metabolic rate, nonshivering thermogenesis and serum tri-iodothyronine levels significantly increased in mice exposed to lower temperatures in comparison with controls, whereas these thermogenic variables were not significantly different between 0 and -15°C. It might suggest that SusEI in cold exposed Swiss mice was constrained peripherally by the capacity to produce heat and also by the ability to dissipate body heat, but to a different extent. Moderate cold exposure might result in a relaxation of the heat dissipation limit (HDL), allowing the animals to increase food intake to meet cold stress. When animals are exposed to severe cold, the thermogenenic capacity might reach a ceiling, failing to compensate for the heat loss and which would finally result in lower body temperature and considerable weight loss. This might indicate that the HDL was set at a higher level than peripheral limits for Swiss mice exposed to a consecutive decrease in ambient temperatures.