ANIMAL THERMOTOLERANCE
The thermal environment is the most important ecological factor as normal body functions require relatively constant temperature and any deviation on either side alters the chemical processes and efficient functioning of the body.
Based on the stability of the body temperature, animals are classified as homeotherms and poikilotherms. The homeo- therms, also known as warm-blooded animals, can control their body temperature despite varying external temperatures. These groups of animals are called temperature regulators, e.g., mammals and avian species. Contrarily, poikilotherms, or cold-blooded animals, cannot maintain a constant body temperature as their body temperature varies with the temperature of the environment. These groups of animals are called temperature conformers, e.g., reptiles, amphibians, fishes, and invertebrates. The homeotherms behave as endotherms and they generate their own heat through metabolic heat production and do not depend on the environment for their heat thus can maintain their body temperature above the ambient temperature. In addition, they employ substantial physiological and biochemical adjustments like changes in blood flow or respiratory rate, an increase in shivering or nonshivering thermogenesis, or the production of heat-shock proteins (HSP), aquaporins, and antifreeze molecules.The capacity to manage body temperature is an evolutionary adaptation seen in warm-blooded animals, enabling the use of temperature as a signal to regulate internal processes. Thermosensitive receptors in the body, both in peripheral areas and within the central nervous system, detect temperature changes and trigger appropriate responses. The peripheral and core receptors transmit nerve impulses to the specific centres in the hypothalamus involved in feed intake, enzymatic activities, hormone synthesis and heat loss or production.
For livestock, maintaining a stable body temperature relies on balancing heat generation and dissipation, primarily achieved through three key pathways: adjusting the body’s temperature to the surrounding environment through behavioural thermoregulation; dispersing heat through processes like vasodilation, conduction, convection, radiation, and evaporation; and by making alterations in metabolic rate. Accordingly, these pathways of heat exchange are categorised into non-evap- orative or sensible heat exchange which includes conduction, convection and radiation, and evaporative or insensible heat exchange (loss) as evaporation of water from the skin and respiratory passages. The various factors which affect this heat exchange include physical and optical properties of hair coat, length and density of hair follicles, hair colour, properties of the skin, etc. Similarly, when environmental temperature decreases, animals prevent a decline in their body temperature by employing physical regulatory mechanisms to minimise heat loss. If these physical regulatory methods become insufficient in maintaining the body temperature, animals resort to increasing heat production through chemical regulatory processes as a defensive measure. In domestic animals, the reduction of heat loss is taken care of by behavioural response, circulatory adjustment, piloerection and increased fur insulation.At an animal level, we primarily consider microenvironments, or microhabitats, or microclimates because it define the actual experience by the animal on an appropriate spatial and temporal scale. The range of temperatures as depicted in Figure 24.3, within which the animal uses no additional energy to maintain its body temperature is called the zone of thermal comfort, a subset within the thermoneutral zone. This zone represents the temperature range where the physiological costs of animals are minimal and productivity is maximum. Within this zone, fluctuations in environmental temperature do not impact metabolic heat production, and circulatory adjustments effectively sustain a consistent body temperature. Beyond this thermoneutral range, circulatory adaptations are insufficient to maintain heat equilibrium, and this range is influenced by factors such as age, body mass, insulation, acclimatisation, evaporative cooling capacity, feed intake and activity of the body.
24.5