Exposure to heat is a common phenomenon for all creatures in the hot semi-arid region. The lambs of the T group were in heat exposed as clearly indicated by higher temperatures inside the climate chamber.
Exposure to simulated summer increases the water intake of lambs. The higher water intake in T group lambs might be due to higher water requirements for the evaporative cooling mechanism through the respiratory tract and skin surface to cope with the high-temperature exposure (De et al., 2017a). However, comparable dry matter intake and ADG of both the groups reflected the adaptability of Malpura lambs to the summer environment of the semi-arid region.
The RR and PR indicate the animals' thermal comfort and adaptability of the animal to harsh conditions (Dias e Silva et al., 2016). The higher respiration rate is a thermolysis response to maintain the homeothermy in exposure to higher environmental temperature (McManus et al., 2009). The increase in RR in T group lamb was a natural response to cope with higher temperatures during summer through evaporative heat dissipation (De, Sharma, Kumawat, Kumar, & Sahoo, 2020). The metabolic status and homeostasis in circulation are reflected through PR (De et al., 2014). The increase in PR in the T group plausibly increases blood flow to the surface from the core to elevate the heat loss from the body (Mohapatra et al., 2019). A similar increase in PR was also reported by (Indu et al., 2015) in sheep in heat exposure. The higher RT value of lambs in the T group indicated exposure of lambs to a higher temperature and in such conditions, the respiratory evaporative cooling mechanism for thermolysis was not sufficient to maintain their body temperature (De et al., 2017a). In concordance with our findings (Fadare et al., 2012) also reported an increase in RT to occur when the physiological activities fail to nullify the excess heat load. However, the ability to conserve the body heat within normal range and improvement of comparable bodyweight with C group reflected the adaptability of the lambs to harsh summer of semi-arid environment. The higher ST in the lambs of the T group plausibly ascribed to vasodilation to redistribute the heat load in the skin capillary through increase blood flow for heat dissipation from the skin surface (Alhaidary, 1560; De et al., 2019). Along with that, the higher environmental temperature around the lambs may transfer heat to the animal is a normal thermo-dynamic process that can contribute to the higher ST in T group lambs.
The behavioral alteration in animals is used to evaluate the welfare of the animals (Pascual-Alonso et al., 2015). However, heat stress may alter the behavior, welfare and production of sheep (Bernabucci et al., 2010). Sheep alter their behavioral responses in higher environmental temperatures (De et al., 2017b). Behavioral response is an indicator of farm animal welfare (Piccione et al., 2011). Higher temperature reduces the appetite (Polsky and von Keyserlingk, 2017) to reduce the heat production (Sevi et al., 2001) of the animal. However, in the present study, the lambs of the T group did not reduce their feed intake although their feeding time reduced significantly. This might be due to their most of the intake in the cooler part of the day i.e. in the evening hours so that they can generate heat as per their body requirement to adjust with the microclimatic heat load. Their voluntary feed intake gets depressed during day time as an adaptive strategy, to produce less metabolic heat in the day time when the microclimatic temperature was high. In the conference of our study, (Spiers et al., 2004) reported reduced feed intake in higher temperature in dairy cattle and (Alvarez et al., 2013) in goat. Furthermore, van Wettere et al., 2021) also reported grazing time decreased in sheep during summer.
In the present study, the rumination time reduced in T as compared to C, which might be an effort of T group lamb to produce less metabolic heat (Rashid et al., 2013) during the day time to support the reduced heat load in high-temperature exposure which helped the animal to maintain body temperature (Soriani et al., 2013). In accordance with our observations; researchers also reported reduced rumination in ruminants under heat stress (Hirayama et al., 2004; Moallem et al., 2010). Rumination generally generates heat through bacterial fermentation (Piccione et al., 2014) and it is a good indicator of animal welfare (Gregorini et al., 2012).
Panting is a counter mechanism of animals in response to higher environment temperature (Ghassemi Nejad and Sung, 2017) and therefore panting is used to assess the heat stress of sheep (De et al., 2020). When the environmental temperature is higher than the core temperature, then the dissipation of body heat through convection and radiation is not sufficient to maintain the heat load; at that situation, sheep compelled to start panting for immediate evaporative heat loss (De, Kumar, Saxena, et al., 2017a) which might be the plausible reason of higher panting in the exposure of higher environmental temperature.
The lying behavior generally imparts rest to the animal and enhances animal welfare (Leme et al., 2013). In adult crossbreed sheep we found lower idle lying and higher idle standing time under higher ambient temperature (De et al., 2017b). Other previous studies also support the higher standing and lower-lying time in sheep (Dikmen et al., 2011; Kanjanapruthipong et al., 2015; Legrand et al., 2011). However, in the present study, the comparable changes of the idle lying and standing time indicated efficiency of adaptability of local Malpura lambs to the summer of the hot semi-arid region without affecting their comfort much with only modification of the physiological responses and feeding behavior.
Commonly the endocrine profile alters under heat stress in sheep and is involved with thermal adaptation and can be an indicator for assessment of heat stress (Joy et al., 2020). The GH and IGF-1 are involved in the growth and peripheral tissue development of lambs (Matteri et al., 2000). However, diversion of energy expenditure for respiratory and evaporative cooling reduces the energy available for growth that might ultimately lead to lower GH and IGF-1 (De et al., 2017c). A similar finding was reported by (Aggarwal and Upadhyay, 2013) in lambs during the summer months. Furthermore, high temperatures suppress the thyroid hormone gland activity and reduce the thyroid hormone (Al-Dawood, 2017; Ross et al., 1985). Cortisol, a stress hormone increased during heat stress in sheep reported in early studies (Ghassemi Nejad et al., 2014). However, the similar endocrine level of T and C group lambs reflects the metabolic and endocrine adaptability of native lambs to the high temperature of the semi-arid region. The similar growth showed that their growth did not affect high temperature, which might manifest energy sufficiency as the C group might explain the unchanged GH and IGF-1. The comparable level of T3, T4 and cortisol express the metabolic adaptability of the lambs to cope up with the summer temperature of the hot semi-arid region.