Neither rats nor mice have a broad thermoneutral zone: implications for physiological studies

Unlike humans, small mammals dedicate considerable energy expenditure (EE) to maintaining core body temperature (Tb). We investigate the influence of ambient temperature (Ta) on energy homeostasis in single housed male mice and rats studied by indirect calorimetry and continuous Tb measurement at Ta ranging from 22 °C to 35 °C. With Tb increases at higher Ta, food intake and body weight decrease, demonstrating that the higher Tb signifies heat stress and not a benign adaptive response. Rats at 30 °C were at the edge of their thermal tolerance. Neither rats nor mice have a broad thermoneutral zone (TNZ). Rather, both have thermoneutral points (TNP), below which EE increases and above which Tb increases. In mice, the EE increase above the TNP is due to the higher Tb (van't Hoff Q₁₀ effect) and increased physical activity. Interestingly, the remaining EE is lower, demonstrating acute thermal adaptation with a reduced basal metabolic rate. In conclusion, rats are less cold-sensitive and more prone to overheating than mice. Despite their larger size, rats, like mice, have a TNP and not a broad TNZ. Studying rats and mice at a Ta of 28-29 °C is a sensible compromise that safely minimizes cold-induced thermogenesis while reducing the risk of heat stress.