Physiologic responses of the horse to a hot, arid environment.

Field investigations were conducted under natural environmental conditions to determine the physiologic responses of rested, hydrated horses (Equus caballus) to the very hot, dry weather characteristic of the summer season in southern Arizona. The emphasis of the investigation was placed on those thermoregulatory mechanisms which are involved in the maintenance of homoiothermy. Rectal temperature of the horses studied remained relatively stable throughout the day, during both cool and hot weather seasons. However, when horses were exposed to hot summer temperatures, rectal temperature (heat stressed) was significantly higher than during exposure in cool winter temperatures (nonheat stressed). Skin temperature was also significantly higher during hot weather, and was directly related to ambient temperature. Heart rate and respiratory rate did not change significantly with exposure to the higher ambient temperatures of summer. Sweating rate increased markedly with higher ambient temperatures. Plasma chloride, serum potassium, serum sodium, and total protein values remained constant throughout the year (that is, they were not affected by hot, summer temperatures). Hemoglobin decreased significantly during hot weather and while packed cell volume declined, the decline was not statistically significant.