Thermoregulation
The mammals and birds maintain relatively a constant Tc to prevent the alternations in the physiological and chemical responses. In general, Tc is maintained within a narrow range of 36-39 °C depending upon the specific mammalian species such as cats and dogs 37-38 °C; sheep and goats 39 ° C and cattle 38.4 °C.
The constant core temperature is maintained by many homeostatic mechanisms when the TNZ of specific species is altered. The reduction in core and skin temperatures activates heat production by shivering or activation of brown adipose tissue, vasoconstriction to conserve heat and warmth-seeking behaviour for thermal comfort. In contrast, as soon as the core or skin temperature increases, heat dissipation mechanism is activated through evaporative cooling, sweating, panting and skin vasodilatation. Further, the reliability of Tc is not absolute where temperature homeostasis may be compromised in fever and hibernation to support survival by eliminating pathogens and preserving nutrients, respectively.The thermal receptors for cold and warm are present throughout the body, and the signals are traversed via A-delta and C fibres for cold and warm receptors, respectively. The thermoregulatory process occurs by three pathways, afferent thermal sensing from the periphery, central regulation in the hypothalamus and efferent responses. The peripheral body temperatures are continuously fluctuating, whereas the posterior hypothalamic thermoregulatory centre sustains a quite constant Tc. The cellular metabolism enhances the heat increment in the body which is transferred to conductive tissues then to the skin and is subsequently lost to the environment. In general, heat is transferred from the animal’s body core to the skin via a network of blood vessels that are controlled by the autonomic nervous system.
The rate of blood flow through these arteriovenous anastomoses fluctuates during climatic extremes wherein the vasodilation (during warm environment) facilitates the heat loss by increased blood flow and vasoconstriction (during cold environment) restores the Tc with decreased blood flow.The body functions are primarily regulated by Tc where the increase or decrease in temperature alters the biochemical processes that occur in the body. Thus, it is essential to maintain a relatively constant Tc for the optimal function of all the tissues in the body. The animals are classified as homeotherms (or) warm blooded animals and poikilotherms (or) cold blooded animals based on the constancy of the Tc. Homeotherms are the animals having the capability of regulating their body temperature within a limited range nearly around 37-40 °C even when the external temperature varies (mammals and birds). The level of heat generation is 8-10 times greater in homeotherms than poikilotherms of the similar body size and temperature. Poikilotherms are the animals that do not have a control over their body temperature and varies with the environmental temperature. They are also called as temperature conformers (reptiles, amphibia, fishes and invertebrates).
Animals are also classified based on the source of body heat production as endotherms, ectotherms and heterotherms. Endotherms are the animals which could generate their own heat through metabolic heat production (birds, mammals) and maintain their body temperature above Ta with well isolated fur, feathers or fat which make them to conserve heat against high temperature (lower vertebrates and insects). They are having higher resting metabolic rate of five times greater in comparison to ectotherms of identical body size and temperature. Whereas, ectotherms are the animals that absolutely depend on the environment for their heat production. These animals are low metabolic heat producers with features of high thermal conductance and regulate their body temperature by absorbing heat from environment (crab).
The heterotherms are animals capable of varying their degree of endothermic heat production. They do not regulate body temperature in the narrow limits (monotremes-egg laying mammals and few large fishes).28.11.1 Selective Brain Cooling
Most of the domestic mammals (except horse) are highly competent to decrease the brain temperature lower than Tc when threshold temperature of brain exceeds which is called as selective brain cooling (SBC). The SBC is achieved through a specialized anatomical feature wherein the carotid rete, passing through the cavernous sinus, receives cooler venous blood from the nose (provided the hemodynamic conditions in the effluent veins are appropriate). This allows heat to be exchanged from arterial blood (carotid rete) to the cooler venous blood (cavernous sinus), thus cooling the incoming carotid arterial blood before entering the circle of Willis to supply the brain. Therefore, SBC is a protective mechanism which maintains brain temperature against the elevated Tc during hot environment and fever.
28.11.2 Role of Animal Skin in Thermoregulation
The integumentary system is the set of organs which forms the external covering of the body and protects the animal against various hazards such as invasion by microorganisms, desiccation, abrasion, chemicals and environmental factors. The integumentary system/skin is important in the regulation of body homeostasis through controlling thermoregulation, sensory reception, biochemical synthesis, absorption and protection. The cold and heat sensory receptors are present in the skin to perceive thermal variations in the environment. As the body temperature increases, the hypothalamus directs neuronal signal to the sweat glands in the skin to secrete sweat for cooling the body. In addition, hypothalamus facilitates the dilation of the peripheral blood vessels of skin to accommodate high blood flow which favours heat convection from the skin surface. In contrary, when body temperature decreases, the sweat glands constrict to reduce the production of sweat.
The skin also functions as a mini- excretory system for urea, salts and water in addition to synthesizes of vitamin D.The physiological properties and functions of the skin are varying among the different breeds of animals. The physical, environmental, physiological factors and hair coat’s optical properties are actively involved in skin temperature dynamics and are affected by evaporative cooling. The sweating rates are varied significantly within and between breeds which is attributed to genetic variation in thermoregulation among the individuals. The evaporative cooling is highly influenced by wind velocity, RH and solar radiation. In addition, the physical and optical characteristics of hair coat such as hair coat density and thickness, hair length and colour also influence evaporative cooling. Black/dark coloured skin and hair enhance solar absorption and elevate heat load on the skin surface. Further, hair coat density hinders evaporation of water from the skin surface by covering a thin film of water at the skin-hair coat interface. The sweating rates also differ between high-producing dairy and feedlot animals in their natural habitats.
28.11.3 ThermoregulationinBirds
Birds are endothermic and depend on high heat production to maintain the higher core body temperature to which birds are forced to distribute significantly more energy into thermoregulatory process. The insulating features in birds play vital role in conservation of internally produced heat and facilitate the thermal conductance which is an energy demanding process. The birds maintain a significantly constant body temperature of 41-42 °C at rest and inactive phase in a wide range of environmental conditions (tropical, temperate and polar) through physiological, morphological and behavioural responses. The endothermic characteristics facilitate the survival of birds in different environmental situations such as aerial, aquatic and terrestrial. The Tc of birds is high with 3-4 °C above mammalian body temperature.
The lack of sweat glands and the high efficiency of the thermal insulation via feathers protect the birds against cold but they are more vulnerable to heat stress. In birds, the air sacs are extensions of the lungs that extend into the body cavities. The air of pulmonary ventilation cools the body of birds more than that of mammals due to the larger gradient and closeness of the air sacs to the vital organs.The effect of Ta on avian body temperature regulation is significant and reported across seasons and different life stages. During winter, birds have to generate more heat so as to maintain normal body temperature. In the summer and spring, they have to dissipate excessive heat, especially during the breeding season. The thermoregulatory system maintains a constant body temperature with fluctuating environmental temperatures in birds. This thermoregulatory mechanism is comprised of a sensory part which senses the variations in environment and an integrating part, the thermoregulatory centre in preoptic anterior hypothalamus which perceives temperature fluctuations through peripheral thermoreceptors. Therefore, the Tc is maintained by heat generation or heat loss mechanisms in relation to thermal status of bird. In addition, the assertive part consisting of neuroendocrine signals, controls the downstream mechanisms to sustain the body temperature by shivering and non-shivering thermogenesis, evaporative heat loss, peripheral vasoconstriction or vasodilation and behavioural changes. Shivering thermogenesis is the major activity in contribution of thermogenesis in the birds. The heat production below LCT, increased through shivering thermogenesis, is characterized by asynchronous muscle contractions via production of chemical energy through the hydrolysis of ATP as heat instead of kinetic energy. In addition, contraction or tremor amplitude during shivering is low which prevents the convective heat loss. The non-shivering thermogenesis also contributes towards heat production in birds.
However, when environmental temperature is above UCT, heat needs to be dissipated to restore normal body temperature which is achieved primarily through evaporation. The birds are well adapted to hot and arid environments and dissipate excessive heat through evaporation of water via cutaneous surfaces or gular fluttering. Majority of passerines dissipate heat by respiratory evaporative heat loss which is energy demanding process in contrast to cutaneous evaporation. Hyperthermia augments the risk of physiological damage, and hypothermia could enhance predation risk in certain conditions.The thermal gradient between the body and environment during hot conditions hinders the heat dissipation from the body. Additionally, the availability of feed and water is highly essential during hot environment for birds due to the high energy demanding activity of panting and to facilitate or potentiate the major heat dissipation mechanism of evaporative cooling. However, during the dry season in arid regions, water is in short supply, thus making the birds prone to hyperthermia. Therefore, the birds focus to save water which is size dependent and its efficiency reduces with body size, particularly for long bouts of heat stress. Water loss in large sized birds exceeds 1 kg during hyperthermic conditions.
The maintenance of constant body temperature is an energy demanding process and to reduce the energy cost of thermoregulation, birds use a variety of morphological and behavioural characters to modify the rates of heat loss and heat gain. The unfeathered body surface areas are the major important site for heat exchange with the environment. During cold to minimize heat loss, the arteries and veins in the unfeathered areas play a crucial role of counter-current heat exchange system to retain heat. Standing on one leg while tucking the other among its breast feathers, reduces exposure of the limb, thereby reducing heat loss from the body. Further, birds’ fluffs out their feathers that increases the thickness of their coat, thereby effectively enhancing their insulation. Additionally, they also alter their posture or orient towards the sun, thereby reducing heat loss from the body. The birds are capable of sustain their body temperature at lower level when they are inactive, and this regulated hypothermia facilitating a significant energy savings. Further, hummingbirds, swifts and poorwills enter a state of torpor wherein their body temperature may drop as much as 50 °F (10 °C) for several hours during the night or days in extreme weather conditions.
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