Mice Do Not Have a Thermoneutral 'Zone': Revising the Definition of Thermoneutrality

Abstract

The thermal physiology of humans and mice is radically different due to their 3000-fold difference in body size. In a cool environment, mice expend energy to maintain body temperature, while this is limited in adult humans. Here we map the mouse thermoneutral zone (TNZ) using indirect calorimetry with continuous body temperature monitoring. Unexpectedly, we found no range of environmental temperatures over which both metabolic rate and body temperature were constant. Rather, body temperature increased when the ambient temperature was warm enough that cold-induced thermogenesis was no longer required. This result was reproduced in male, female, C57BL/6J, 129, chow-fed, diet-induced obese, and ob/ob mice as well as Trpv1-/-;Trpm8-/-;Trpa1-/- mice with thermal sensory channel deletions. We propose a new upper limit of the TNZ in mice, the point at which body temperature increases, and a new general definition of the TNZ. Thus, the mouse TNZ is not the “zone” of textbooks, but rather a thermoneutral point of 29 °C in the light phase. These observations suggest changes for how mice are used to model human energy physiology and drug development.