In the present study, environmental conditions indicative of HS for sheep in the tropics of Guerrero, Mexico were measured. Indeed, a high THI (65 to 79) and a heat tolerance coefficient of 6 units were found, indicating that the degree of HS is severe. Likewise, the physiological parameters (BF and RT) and skin temperatures (head, neck, scapula, RPF, abdomen, haunch, leg, udder, and vulva) were higher during the afternoon in the group of ewes lactating than during the morning. The above demonstrates the relationship that exists between the physiological response of ewes under HS environmental conditions.
4.1. Weather Conditions
The thermoneutral zone for hair sheep is between 15 and 30 °C and a THI < 74, which are classified as an absence of HS [
24]. In the present study, the maximum temperatures were 38.8 °C and the THI was 79, demonstrating that the ewes were under severe HS. For hair breed sheep, it is considered that animal welfare and HS are in conflict when the THI is ≥72, and BF and RT are increased to maintain the comfort zone [
25]. For example, sheep that remain in environments with HS increase the values of BF, RT, RH, and skin temperatures, altering their physiological functions [
26]. When RH is low, it favors heat exchange through respiratory or skin evaporation, maintaining the comfort of the ewes [
27]. This causes problems such as dryness of the mucous membranes and difficulty in the exchange of heat through non-evaporative (conduction, radiation, and convection) and evaporative (sweating and respiration) mechanisms, hindering the thermoregulation of these females [
26].
Results similar to those of the present study were reported by Seixas et al. [
24] under tropical conditions in northwest Brazil (monthly temperature, 31.6 to 35.5 °C, and THI, 76.8 to 80.4), demonstrating that the ewes were in HS conditions. However, the above intensifies according to the physiological stage of the females. In this regard, García y González et al. [
6] found HS in lambs and multiparous ewes intensified in the afternoon. In the present study, we worked with multiparous lactating and non-lactating ewes; although HS was found in both groups of females, in the lactating group it was more intense. In this regard, Macías-Cruz et al. [
11] found higher RT and BF during the afternoon in lactating ewes, and the same phenomenon occurred for the skin temperatures presented by these ewes.
In hot climates, lactating females can increase energy requirements for maintenance by between 7% and 25%, and, by inducing an increase in RT and BF, food consumption in HS ewes is decreased. Therefore, it is important to provide processed foods to reduce metabolic work and heat load. Fat and nitrogen reserves are used to provide energy through glucose biosynthesis at the expense of the active mammary gland [
28]. The ewes used in this study are not milk producers, but the milk they produce is sufficient to increase metabolic heat, which, when added to ambient heat and food intake, indicates an effect of HS [
16].
Some studies have reported that when the HTC indicator is 2 U, sheep do not present HS [
21,
22], and when it is increased to 3 U, the RT increases by 0.5 °C. In the present experiment, HTC was found during the morning to be 2.4 to 2.6 U, and during the afternoon it exceeded 6 U, which confirms that ewes from the Guerrero tropics suffer severe HS regardless of the group. However, this is more intense in lactating ewes than in non-lactating ewes. Consistent with these results, Serrano-Torres et al. [
25] and Souza et al. [
22], under tropical conditions, found HTC values similar to those reported in the present study. In another study, Mehaba et al. [
10] found higher RT and BF in lactating ewes under HS conditions. Likewise, García y González et al. [
13] found an increase in physiological parameters and skin temperatures in heat-stressed ewes during summer in the tropics.
4.2. Heat Tolerance Coefficient and Physiological Parameters
To maintain their productivity and, in some cases, guarantee their survival, sheep need to maintain their body temperature within the physiological limits for the species, that is, their homeothermy [
26]. The physiological parameters (BF and RT) are indicators of the animal’s degree of comfort and it is ability to tolerate environmental conditions [
5,
29]. The normal BF for sheep in the thermoneutral zone is 25 to 61 bpm [
30]. On the other hand, in severe HS conditions, it can rise to 300 or 400 bpm [
5]. Environmental temperature plays an important role in the increase in these parameters; for example, when environmental temperatures are above 35 °C, ewes increase heat loss through the respiratory tract by up to 60% [
31]. However, the increase in muscle activity due to the respiratory process can be disadvantageous due to the considerable amount of heat generated by the work performed by respiratory muscles [
32]. In this regard, Saldaña-Ríos et al. [
29] found a high BF in heat-stressed Dorper (116 ± 32 bpm), Katahdin (109 ± 38 bpm), and Pelibuey (86 ± 36 rpm) breed sheep in a humid tropical climate.
In the present study, lactating ewes maintained average BF during the morning of 37.7 bpm and in the afternoon of 115.4 bpm, which was higher than the ewes that were not lactating. This is logical since lactating ewes, due to the production of metabolic heat, try to dissipate heat through respiration. Indeed, in heat-stressed ewes, a decrease in BF by 20% was observed at midday, but in the afternoon, it increased to 66% [
11].
This adaptation mechanism that hair ewes present to reduce body water losses and avoid dehydration in conditions of HS was observed in both groups of ewes [
5], since during the afternoon the increase in BF was 68% in the ewes of the present study.
In the case of RT, in sheep the normal average is 39° C and it ranges between 38 and 40 °C [
30]. The temperature obtained in the study by Saldaña-Ríos et al. [
29] was higher than 38.7 °C, which is similar to that reported by Quesada et al. [
31] (38.8 °C) in ewes under HS conditions. This same phenomenon was found in ewes of the Dorper (39 ± 0.45 °C), Katahdin (39 ± 0.37 °C), and Pelibuey (39 ± 0.66 °C) breeds under humid tropical conditions [
29]. In the present study, non-lactating ewes presented a higher RT at both times of the day, in the morning with 38 °C and in the afternoon with an average of 38.9 °C. In this regard, Ruiz-Ortega et al. [
5] suggests that an increase in RT demonstrates that the heat release mechanisms are not efficient. In this sense, Saldaña-Ríos et al. [
29] observed in climates with an average ambient temperature of 38.5 °C and 80% RH, a maximum average RT of 41.6 °C, specifying that the range was outside normality for ewes. Therefore, the thermoregulation mechanisms were not sufficient, since the high percentage of RH, environmental temperature, and solar radiation favor HS, decreasing the gradient of body heat dissipation. Macías-Cruz et al. [
11] published changes in RT, BF, and skin temperatures in the summer, correlated with changes in environmental temperature and THI throughout the day. This same phenomenon was published in regions of the Guerrero tropics in multiparous ewes and lambs under HS during the summer [
5,
6]. Other investigations agreed by obtaining similar results in their experiments, where the physiological parameters increased with the increase in environmental temperature, both under HS and thermoneutrality conditions [
24,
27,
33].
4.3. Skin Temperatures
The exchange of heat between the body and the environment through the skin is also considered important in the thermoregulation of ewes in climates that cause HS [
27,
34]. A skin temperature higher than the ambient temperature causes a release of body heat, but when it is lower it causes a gain in environmental heat [
34]. When measuring the surface temperature on the back, hip, side, forehead, and base of the skull, a direct effect of solar radiation on the surface located towards the dorsal region has been reported; compared to the forelimb and hindlimb, together, in the rostral region a selective cooling mechanism of the brain has been reported in sheep [
35], associated with greater activity of the superficial veins to maintain brain temperature below the average body temperature [
32].
In an experiment conducted in northwest Mexico in desert climates, temperatures of different body areas were highest (
p < 0.05) at midday and lowest (
p < 0.05) between midnight and mornings during the summer [
11]. However, the temperature of the head was not affected (
p > 0.05) at midnight or in the morning, but the temperatures of the right paralumbar fossa, scapula, and haunch were higher (
p < 0.05). In the case of surface temperatures, all body regions were higher (
p < 0.05) at midday [
11]. In this same work, the skin temperatures (scapula, haunch, and right paralumbar fossa) showed that, during the month of August when compared with the other summer months, the dissipation of body heat was greater during the mornings, in the afternoon there was a gain in heat, and at midnight the body and environmental temperatures balanced. In another study carried out in sheep under tropical conditions, Santos et al. [
36] reported body temperatures of 39–39.2 °C in the morning and 39.6–39.7 °C in the afternoon.
In the present investigation, lactating ewes presented higher surface temperature values in the afternoon; however, significant differences were found by group. The temperature was higher in the afternoon in the lactating group in the head (36.6 °C), neck (36.5 °C), scapula (36 °C), abdomen (37.2 °C), udder (35.4 °C), leg (37.3 °C), and vulva (38.6 °C). In the case of the right paralumbar fossa zone and the rump, it was higher for the group of non-lactating ewes (36.7 °C) than the lactating group (35.9 °C) in the afternoon. Some research suggests that the values of the thermal gradient do not present a significant effect (
p > 0.05) in the hours of 1:00 p.m. and 2:00 p.m., but a significant effect (
p < 0.05) has been observed at 3:00 p.m., where RT, environmental temperature, and skin temperatures have higher values [
26]. Furthermore, it is observed that, at 3:00 p.m., the gradient between RT and surface temperature is low, so it is difficult for the sheep to eliminate excess body heat. This phenomenon can be explained by the presence of the short wool layer on its body [
11]. Recent studies corroborate that BF, RT, and THI are predictors of HS in ewes [
6]. Under tropical conditions, the recording of skin temperatures allows a rapid evaluation of HS conditions [
32]. The study by Reyes et al. [
32] indicates that by taking the skin temperatures on the back (≥41.2 °C), hip (≥39.0 °C), and side (38.2 °C), the RT can be ≥39.6 °C. In this sense, in another study carried out in an arid region, they found higher skin temperatures in ewes in the morning, noon, and afternoon during the month of August, which is when the summer heat is most intense; but they also observed greater dissipation of body heat through the scapula, haunch, and right paralumbar fossa [
11]. The high correlations found between BF, RT, and temperature in different body parts of ewes with the THI indicate that they are excellent predictors of HS [
25]. Most of the thermoregulation in heat-stressed ewes occurs through the respiratory tract (60%), and the rest occurs through the skin, regardless of the physiological stage.
Currently, due to the phenomenon of global warming, production animals suffer from HS [
2]; therefore, it is important that the general population becomes aware of the care of the environment and animals. The current environmental conditions result in serious consequences for production (meat and milk), health, animal welfare, and the economy of farmers [
11]. Faced with this challenge, it is necessary to implement teaching strategies in veterinary medicine study programs to train students and new veterinarians so that they can face this problem, which is becoming more serious every day [
37,
38]. Researchers must consider one of the basic axes of the concept of animal protection in their research: the principle of the three Rs, i.e., replacement, reduction, and refinement [
37]. This will allow the number of animals to be reduced, especially in HS work with ewes, to avoid the manipulation of many animals during the experimental phase, which, combined with the stress caused by the environment, could increase the animals’ stress [
5]. Finally, it is necessary to advise producers to implement HS mitigation strategies considering the physiological stage (empty, pregnant, and lactating ewes) [
2]. Therefore, the triad of education of students, farmers, and researchers will contribute to implementing research, management, and production strategies for species of zootechnical interest in the face of the growing environmental threat to the production of protein of animal origin.