Heat tolerance in hair sheep: individual differences on physiological, endocrine, and behavioral responses

Heat stress is a major factor affecting animal productivity in tropical countries, with effects on physiological, hormonal, and behavioral responses. This study aimed to assess the differences in these responses between heat-tolerant and less heat-tolerant hair sheep during heat stress. Twenty-four Santa Ines sheep were selected from a group of 80 sheep, with 12 identified as heat-tolerant and 12 as less heat-tolerant based on thermotolerance assessment. The animals were exposed to heat stress in a climatic chamber at an average temperature of 36 °C (1000–1600 h) for 8 days and maintained at 28 °C (1600–1000 h). The rectal temperature, respiration rate, sweat rate, ocular surface temperature, body surface temperature, tympanic temperature, triiodothyronine level, and insulin level were measured. Skin samples were collected on the last day of the cycle for histological analysis. The results showed that the less heat-tolerant sheep had higher rectal and body surface temperatures (P < 0.05). Although no differences in skin morphology were observed between the groups, less heat-tolerant sheep continued to sweat for a longer period after the end of the thermal challenge to lose heat (P < 0.05). Less heat-tolerant animals also presented higher rectal temperatures during cooler hours and required more time to dissipate the excess heat. These findings suggest that there are individual differences in the thermoregulatory responses within the same breed under the same environmental conditions, and that breeding programs could be employed to produce more heat-tolerant, but still productive animals in tropical conditions.