Thermal comfort in mice: Effects of behavioral thermoregulation on physiology

Mice under standard laboratory conditions are generally housed between 20 and 24°C, which are below their lower critical temperature of ≈ 30°C. When the ambient temperature falls below a mammal\u27s lower critical temperature, the metabolic rate is increased so that heat production by the body matches heat loss to the environment to maintain a constant body core temperature which results in increased metabolic rates. This alteration to physiology has the potential to alter scientific outcomes. Raising room temperatures is not a viable solution because increased aggressive interactions are seen at higher temperatures. Additionally, temperature preference differs with age, gender, and behavior, making it exceedingly difficult to identify an optimal temperature. A potential solution is to allow mice to build nests, which is an adaptive behavior that has been found to increase survival and reproduction at low temperatures. Therefore, the central hypothesis for this dissertation was that nest building, through improved insulation, would reduce thermal stress at typical laboratory temperatures. The goal of chapter 2 was to find the point at which preferences for temperature and nesting material are equal in three strains of mice. All mice preferred temperatures between 26-29°C and the shift from thermotaxis to nest building is seen between 6 and 10 g of material. These results suggest that mice need 6-10 g of nesting material to fully alleviate thermal stress under typical temperatures. If 6-10 g is preferred, it is assumed that thermal comfort is improved by these amounts. However, the extent to which nests alter physiological thermoregulation at a standard laboratory temperature is unknown. Thus, elucidating this quandary is the goal of chapter 3. We predicted that nesting material would insulate the mice, reduce heat loss, and decrease thermogenic processes. Dome-like nests radiated the least amount of heat and CD-1 mice and males showed the largest energetic reductions from nest building. BALB/c mice were the only strain to show a reduction in non-shivering thermogenesis. Reduced thermal stress, through nest building, results in energetic savings. Therefore in chapter 4 we predicted that this reduced energy expenditure would free resources for other physiological processes, such as reproduction. All mice provided nesting material had improved breeding performance at the same energy expenditure as controls. Therefore, improved insulation from nest building frees resources from thermogenesis, which are reallocated to improved reproduction. These experiments demonstrate that nest building behavior decreases heat loss and energy expenditure in 3 strains mice under typical laboratory temperatures. However, the extent to which nest building eliminates thermal stress varies between strains and sexes. Reduction in thermal stress, through control over a mouse’s microenvironment, will improve laboratory mouse welfare as well as increasing the external validity of the research they are used for