Limits of Environmental Temperatures
The approach for thermal preference is highly related with metabolic activity, and its efficiency is optimal within a specific range of core body temperature (Tc). The cell integrity or function is affected when core temperature increases or decreases above the acceptable physiological limits.
The temperature preference seeking behaviour in animals reduces the temperature difference between the animal and its environment, thereby decreasing the temperature gradient. This approach persuades minimum metabolic energy expenditure in animals to maintain normothermia. However, there is an overlap between the range of preferred environmental temperature and the comfortable temperature zone. Further, the thermal preference varies among species depending upon the time of day due to daily variations in their requirement for body heat production. The day and night variations in preferred Ta are important in heterotherm animals where there is a great variation in the Tc. There is a necessity for the animal to choose appropriate temperature especially during the torpor phase wherein the animal which tries to save energy. The Ta is an important factor that influences thermoregulatory mechanism of animals at different levels.28.10.1 ThermoneutralZone
Thermoneutral Zone (TNZ) is defined as ‘the range of environmental temperature within which body temperature is maintained at a constant with minimal effort from thermoregulatory mechanism’. The TNZ can be differentiated by nature of blood vessels of the skin over the whole body which are neither all vasodilated nor vasoconstricted; evaporative heat loss is minimal, piloerection and behavioural responses to cold or heat are absent. TNZ varies with the age, species, breed, insulation, level of nutrition, earlier experience of temperature acclimation or acclimatization, production level, housing conditions, behavioural responses and time of the day.
Generally, TNZ is very narrow for the young animals in comparison to adult animals which varies widely within the same species. Further, it is the temperature zone at which the animal may perform at its maximum. TNZ and heat tolerance threshold level vary in sheep from 5 to 25 °C depending upon the breeds and climatic regions. When the environmental temperature reaches the limits of the TNZ in opposite directions, it approaches the lower and upper critical temperatures (UCT). The ambient temperature (Ta) below which the rate of heat production of a resting homeotherm increases to maintain thermal balance is known as the lower critical temperature (LCT). Hence, the normal metabolic rate is insufficient to re-establish homeostasis and the body has to generate additional heat while environmental temperature declines. Where, the metabolic rate of animal raise from the basal level to meet environmental demands for heat either by shivering or non-shivering thermogenesis. The UCT is the Ta above which the rate of evaporative heat loss of a resting animal is increased to sustain thermal balance. Endotherms have relatively low variation in UCT compared to the variation in the LCT. The UCT can be demarcated by increased metabolic rate and evaporative heat loss along with minimal tissue thermal insulation. The heat production is increased or decreased when temperature is below or above the critical limits.28.10.2 Lethal Body Temperature
Lethal body temperature, generally considered as the extreme Tc at which 50% of the experimental animals die. The lower lethal body temperature varies from 15 to 20 °C in majority of the species that is an average of 20 °C below the normal core temperature. The young animals have lower lethal temperatures than the adult animals of the same species. The upper lethal body temperature is life-threatening than the lower lethal temperature. Majority of the mammals die when their Tc reaches by 42-45 °C which is 3-6 °C above the normal body temperature.
28.10.3 Hypothermia
Hypothermia is an abnormally low Tc which develops as a consequence of continued exposure to cold along with an inability to conserve and enhance heat producing mechanisms. Hypothermia can be primary or secondary where primary hypothermia commonly occurs due to the exposure of animals possessing normal heat production to cold environment. Secondary hypothermia results as a consequence of alterations in heat production which may be due to illness, injury or drugs. Naturally, secondary hypothermia results in morbidity and mortality in particularly ill animals. The ability to withstand lowered body temperatures varies among species and is life threatening if not tackled effectively.
28.10.4 HypothermicSpiral
The thermoregulatory mechanism is impaired when Tc reduces lower than 94 °F (34.4 °C) and animals typically stop to shiver or seek heat. During such conditions, the peripheral vasodilation is predominated rather than vasoconstriction and results in continuous loss of core temperature with reduced metabolic rate. Simultaneously, severe hypothermia depresses the central nervous system making the hypothalamus less responsive to hypothermia, and cessation of thermoregulation ceases as the Tc falls below 88 °F (31.1 ° C).
28.10.5 Fever and Hyperthermia
Fever is an increase in Tc above the normal range which is caused by microorganisms and it is a beneficial effect which stimulates the body defence or immunological mechanisms. The temperature set point of the hypothalamus is enhanced to facilitate the body to deploy the heat conserving and producing mechanisms that may elevate temperatures of 41 °C (106 °F) in mammals. The point at which the body temperature reaches maximum and re-establishes the balance between heat loss and heat production, and temperature is precisely regulated at the new high level. The various heat loss mechanisms of the body are stimulated, and body temperature returns to its normal level at the end of fever.
Therefore, shivering and cold are characteristics of initial phase of fever and it is self-limiting. Hyperthermia is an increase in body temperature above normal range while body generates or absorbs more heat than it dissipates. The heat stroke, an indicator of hyperthermia, occurs when the heat production is higher than the evaporative capacity or evaporative mechanisms are impaired due to loss of body fluid and decreased blood volume.28.10.6 HibernationandEstivation
Hibernation and estivation are the important physiological and behavioural responses of homeothermic animals to avoid or escape from the extremes of environmental conditions. Hibernation refers to a cessation of coordinated locomotor activity and a reduction in body temperature, total metabolism, heart beat and respiration during winter. Further, the animals have the ability to spontaneously reoccur in normal homeothermic condition without any external heating. These hibernating animals awake from their dormant state periodically for urination as well as when body temperature reduces to level near freezing point. The brown fat is a connective tissue, and colour is due to presence of cytochrome pigments with higher level of mitochondria and usually found in hibernating animals. The new born animals of some species also pose brown fat which disappears within the few months of their life. Generally, brown fat accumulates in the subcutaneous region between the scapulae, kidneys and myocardium. Brown fat depots are the source of non-shivering thermogenic activities to generate more heat and being distinguished from white fat by its colour and metabolic effects. Further, activation of brown fat utilizes more amount of oxygen to generate high level of energy or heat. Estivation refers to a torpid sleeping state during the summer in harmony to hibernation during winter. The major advantage of estivation is to reduce the metabolic rate and Tc to prolong the period of survival of the animal with its energy reserves and preserves a significant quantity of water. However, the true torpor is not a state in which endothermy and thermoregulation are shunned but one in which they are regulates at a new level with a new critical minimum temperature. When environmental temperature drops below new critical temperature, the metabolic rate will be enhanced to maintain the critical body temperature. Animals such as the desert tortoise choose to sleep in their burrows when temperature is at its maximum and it will be active when the temperature drops down. Similarly, small endothermic animals of temperate climates also use torpor as an escape from transient seasonal or night low temperatures.
28.11