Daily rhythmicity of core and surface temperatures of sheep kept under thermoneutrality or in the cold

We compared the daily rhythms of body core temperature and surface temperature in 10 Comisana ewes housed at thermoneutrality (25 °C) and in the cold (9 °C). Core temperature was measured with a rectal probe, whereas surface temperature (in the eye, forehead, and foot) was measured by infrared thermog- raphy, at 3 h intervals for 27 consecutive hours under each condition. We found a robust rhythm of core temperature, whose amplitude doubled from 0.2 °C at thermoneutrality to 0.4 °C in the cold. The surface temperatures were all lower than core temperature and exhibited weaker but significant daily rhythmic- ity. At thermoneutrality, foot temperature (an indicator of vasodilation in the service of heat loss) peaked 3 h before rectal temperature. Studies on other species are needed to clarify whether the latency between the peaks of the oscillations in foot and core temperature vary in a species-specific manner and are responsible for the generation of the core temperature rhythm.