Responses of Animals to Heat
Animals exhibit varied behavioural, physiological, biochemical, cellular and molecular responses against heat stress. The shade seeking behaviour and reduced feed intake are the primary responses exhibited by an animal during heat stress condition.
Additionally, animals, especially buffaloes wallow often and/or standing near the water bodies to enhance the conductive heat loss to water in order to reduce heat stress. The increase in Tc increases the skin temperature by cutaneous vasodilatation and promotes a thermal exchange gradient to expedite heat loss by inhibiting of sympathetic vasoconstrictor tone. The temperature may decrease vasoconstrictor tone either via a rise in temperature at central nervous system (CNS) or through impulses mediated by thermoreceptors in the skin and other parts of the body. Above an environmental temperature of about 31 °C, skin vasodilatation no longer increases heat dissipation, and a rise in body temperature would be observed unless heat loss can be augmented by other assisted means. The evaporative heat loss is an effective means of heat dissipation mechanism in animals. One calorie is required to increase the temperature of 1 g of water by 1 °C whereas to evaporate the same quantity of water from the body needs around 600 cal. The heat generated in resting animals is dissipated by evaporation of water from the skin surface and respiratory tract which accounts for around 25% of heat dissipation at normal Ta and RH. The evaporative heat loss from cutaneous and respiratory system is constant at thermoneutral conditions. The increase in Ta enhances blood flow to the skin that leads to higher evaporative heat loss by sweating.28.15.1 Sweating
There are two types of sweat glands, eccrine and apocrine glands. The eccrine sweat glands open on the surface of the skin via duct and are supplied by cholinergic fibres present in sympathetic nerves.
The apocrine glands are associated with hair follicles and ducts open into the hair follicle which are not supplied by secretory nerves and are sensitive to epinephrine carried in the bloodstream. In many domestic animals, apocrine sweat glands are important for evaporative heat loss. The apocrine glands are controlled by the alpha-adrenergic system, in comparison to horses which are beta-adrenergic. The blood flow to capillary beds affects the rate of sweat production during heat stress in animals due to their close association. The sweat gland numbers vary regionally in the order of the neck, flank, back, thigh, forehead and abdominal regions. However, the evaporation of moisture from the skin is highly associated with the functionality of the glands rather than their distribution over the skin. Thermoregulatory sweating is achieved in two approaches, by the increased CNS temperature and spontaneous stimulation of warmth receptors in the skin and other parts of the body. The rate of sweating depends on the rate of increase in core body temperature and skin temperature. The degree of relevance of sweating as a heat loss mechanism varies among species where sweating is minimal and panting is more in dogs. The evaporative heat loss from the skin surface of cows is around 150 g/m2/h at an environmental temperature of 40 °C. Sweating rate is very low in sheep and maximum sweat secretion in shorn sheep is 32 g/m2/h during heat stress where evaporative heat loss is more important.28.15.2 RespiratoryFrequency
The major function of respiratory system is to eliminate carbon dioxide (CO2) from the tissues and supply of oxygen. Respiration rate is an indicator of heat stress or heat load on the animal during hot environmental conditions. The respiratory tract acts as a major heat loss route in heat stressed animals when the other heat dissipation mechanisms become inadequate. The normal respiration rate among domesticated animals ranges from 20 to 30 breathes/min at TNZ.
The increase in the respiration rate by 80-120 breathes/min in cattle indicates that they are under moderate to high thermal stress and above 120 breathes/min is designated as an excessive heat load. During heat stress, respiration is associated with the level of heat load in animals that increases ventilation of the dead space by increasing the frequency and reducing the tidal volume. When animals pant with closed mouth, body heat is exchanged via the upper respiratory tract which is transported to the nasal mucosa by blood. On dissipating heat via respiration, the cool blood drains into the venous sinuses at the base of the skull. The respiratory frequency reduces at the elevated body temperatures with increased tidal and minute volumes. The increase in minute volume establishes the continuous and enhanced respiratory evaporation even the increase occurs at the expense of an overventilation of the alveoli. The enhanced ventilation reduces CO2 level and results in increased blood pH. The transformation of respiration rate from rapid and shallow to slower and deeper breathing indicates that the physiological mechanism allows maximum evaporative cooling with least interruption in the blood gases. The last phase of respiratory frequency is open mouth panting with protrusion of tongue which is synchronized with highest respiratory frequency that is regulated by the airway resistance.28.16
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