Among mammals, the peak-trough difference (PTD) of the circadian pattern of body temperature (Tb) drops very little with the increase in body mass (W), despite the large increase in heat capacitance and thermal inertia. We asked whether this might be contributed by systematic differences in the circadian pattern of breathing frequency (f) and skin temperature (Tskin), which are parts of the control mechanisms of heat loss. Measurements had been conducted on animals of eight species, chosen to cover a four-fold range in W, while resting and awake. The oscillation of f preceded that of Tb in 7 of the 8 species, and its acrophase did not correlate with W. The daily mean and PTD of f scaled with W in a similar manner (respectively, W-23 and W-0.29), the PTD averaging about 20% of the daily mean. The circadian oscillations of Tskin, measured in specimens of five species at three locations (abdomen, ear and thigh), were in phase with Tb. Neither the PTD nor the acrophase of Tskin changed systematically with W. The differences between Tb and Tskin (means, peaks and troughs) decreased significantly with W; on average, the Tb-Tskin difference scaled to W-0.19. In conclusion, the relative amplitudes and the acrophase of Tskin and f did not show systematic inter-species differences. The progressive increase of Tskin with W could be a factor in maintaining the PTD of T b within a narrow range among mammals of very different size.
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