Diurnal variation in rectal and cutaneous temperatures of horses housed under different management conditions.

Thermoregulation is an important mechanism to ensure thermal homeostasis. In horses, different housing conditions could influence this mechanism by exposing animals to different microclimates. The circadian rhythm of body temperature is well known. For this reason, we wanted to investigate the daily rhythm of cutaneous (jugular, shoulder, inner thigh and croup) temperature in comparison to rectal temperature in athletic horses kept under two different housing conditions. Ten clinically healthy Italian Saddle horses were divided into two equal groups: the first group was housed in individual boxes, and the second group was kept in paddocks. In all horses, rectal and cutaneous temperature measurements were performed every 4 h for 48 consecutive hours during different seasons (spring, summer, autumn and winter), respectively, by means of digital and infrared thermometers. Ambient temperature, relative humidity and ventilation were recorded simultaneously. A general linear model (GLM) was applied to the recorded temperature values displaying a significant effect of season, time of day, site of recording and housing condition (p < 0.001). No statistical differences were found between the two days of monitoring (p = 0.49). A trigonometric statistical model (single cosinor method) was applied to investigate the circadian rhythm of rectal and cutaneous temperatures in the two different groups. Our results showed circadian rhythmicity of rectal temperature during all seasons and in both groups. Cutaneous temperature shows daily rhythmicity that was different in the various regions and was influenced by housing conditions and seasons. Application of GLM also showed a statistically significant effect of season and site of recording (p < 0.0001) on all circadian parameters and of management condition on amplitude and robustness (p < 0.0001). No statistical differences between the two days of monitoring were observed (p = 0.68). These findings are probably due to the animals' thermoregulatory mechanisms ensuring heat exchange between the body and the environment, and that was influenced by the microclimate. In particular, the microclimate influenced the thermodispersion disrupting the daily rhythmicity of some body regions, in horses kept in boxes. Only the temperate climatic conditions observed in summer guarantee the expression of the daily rhythmicity of all body surfaces in both management conditions. These results contribute to the knowledge of the mechanisms of homeostasis and control of body temperature in the athletic horse kept in different housing conditions, with a chronophysiological interpretation that completes the proper management of thermal well-being in horses.