Small ruminants kept in stressful environments, with high AT and low RH, increase their endogenous and superficial heating, causing behavioral, metabolic, physiological and reproductive dysfunctions, which compromise the maintenance of homeostasis (Darcan & Silanikove, 2018, Sejian et al. 2019), leading to increase in physiological variables, which may favor the reduction of production and production efficiency (Furtado et al. 2020, Marcone et al. 2021), demonstrating the difficulty of animals in dissipating heat, so peripheral vasodilation, respiratory rate elevation and sweating are activated to dissipate body heat (Fonseca et al. 2019, Aboul Naga et al. 2021). Low RH may favor heat exchange by evaporation, causing less stress in the animals (Façanha et al. 2020), however, very low values can dry out the mucous membranes, hinder heat exchange through non-evaporative mechanisms (conduction, radiation and convection) and evaporative mechanisms (sweating and respiration) (Sejian et al. 2019).
The normal variation of RT for the species ranges from 38.5 to 39.7°C, reaching the minimum in the morning and the maximum in the afternoon (Shilja et al. 2016), and, even under stressful environmental conditions, these values remained within normal limits, demonstrating their adaptability to the climatic conditions of the Brazilian semi-arid region, resulting from the natural selection pressure to which they were subjected during their formations (Fonseca et al. 2019, Furtado et al. 2020) and the tolerance to the hot and dry climate of the Brazilian semi-arid region.
TS is one of the most sensitive variables to determine the effects of the environment on the acclimatization of the animal (Pequeno et al. 2017), which may exceed the normal range under the influence of several factors (Shilja et al. 2016), such as AT and coat color, which can influence the magnitude of the absorbed and reflected radiation and, consequently, the amount of heat transfer between the surrounding environment and the animal’s body (Stuart-Fox et al. 2017, Al-Haidary et al. 2021).
The properties of adaptation to heat may depend on the morphological characteristics of the skin (color, thickness, sweat glands), as well as on the characteristics of the coat (fur thickness, length, diameter and density) that determine the ability of animals to dissipate heat through sensible and latent mechanisms (Amorim et al. 2019, Al-Haidary et al. 2021). The thermal effects of coat color depend more on long wavelength absorbance near infrared than on the short visible wavelengths of direct sunlight (Stuart-Fox et al. 2017).
TS is a sensitive variable easily influenced by the increase in AT, it is also affected by the time of day, as AT increases, there is a reduction in the thermal gradient between the animal and the medium, and the animals trigger a series of thermal mechanisms, using them more intensely, also increasing the RT.
For domestic animals, the ideal is that there is a thermal gradient around 6°C between body core and skin surface temperatures (Medeiros et al. 2015, Silva et al. 2019), so the body heat flow will be directed spontaneously to the extremities of the body, and metabolic reactions can be performed without heating the internal organs of the animal (Sejian et al. 2019, Souza et al. 2019).
The means of the RTST gradient were below 2°C for the species, genders and periods, under all conditions evaluated, which can be considered low, hampering the exchange of heat from the body core to the skin of the animals and from their skin to the environment. Peripheral vasodilation increases blood flow to the body surface, increasing the surface temperature of the skin, facilitating the exchange of heat between the animal and the environment through sensible mechanisms (Pequeno et al. 2017, Torres et al. 2017, Ribeiro et al. 2018).
The means of the RTAT gradient for sheep were close to 6°C under all evaluated conditions (Table 5), considered favorable for the exchange of heat from the body core to the skin of the animals, and from their skin to the environment. The increase in blood flow in the body surface facilitates the exchange of heat by the animal through sensible mechanisms (Pequeno et al. 2017, Torres et al. 2017, Amorim et al. 2019).
At high ambient temperatures, the body core temperature flows to the body surface and to the limbs of the animals, in the opposite situation, the skin temperature gradients extend along the limbs, and the central temperature is restricted to the trunk and head (Medeiros et al. 2015, Silva et al. 2019). Thermal conduction is important in the heat dissipation process, from the central core to the external surface of the animal, as well as from the surface to the surrounding environment (Fonseca et al. 2019, Aboul Naga et al. 2021).
Hot and dry environments can hamper the performance of animals, because they result in low gradients, making it difficult to dissipate heat. Thus, it is possible to explain the increase in RT and ST and the low thermal gradient, since there is an accumulation of endogenous heat, requiring the activation of evaporative mechanisms of thermoregulation (Stuart-Fox et al. 2017, Silva et al. 2019, Leite et al. 2021).