 Dear students, in this topic, we shall discuss and define the temperature-neutral zone, lower critical temperature and upper critical temperature for endotherms. The environmental temperatures affect the thermoregulatory activities of homeothermic endotherms. These environmental temperatures have three ranges. The thermal-neutral zone, lower critical temperature and upper critical temperature. The thermal-neutral zone is the range of temperatures at which the basal rate of heat production balances that of heat loss. In this range, heat production and heat loss are equal. There is no need to expend energy to maintain the body temperature within this range. In this range, whatever temperature needs to be regulated, that is just through alterations in the thermal conductance of the body surface. This thermal conductance of the body surface takes place through 1. Visomotor responses by constriction or dilation of blood vessels on the surface of the body. 2. Postural changes to alter the exposed areas. 3. Regulation of insulating effectiveness of pillage or plumage, for example, raising or lowering the hairs or feathers. Dear students, the lower critical temperature is the ambient temperature below which an endotherm's basal metabolic rate becomes insufficient to balance the heat loss. Below this, animal must increase heat production by thermogenesis to offset the heat loss. Dear students, Below the lower critical temperature, heat production rises linearly with decreasing temperature. The temperature which decreases, according to that, the process of thermogenesis is more of heat production. This zone is known as the zone of metabolic regulation. This has to be done through metabolic processes through heat generation. Dear students, if the environmental temperature falls below the zone of metabolic regulation, then the compensating mechanisms of animals, which are present for the heat generation and other mechanisms are present, if the zone of metabolic regulation or lower critical temperature goes below the environmental temperature, then such mechanisms fail. Then metabolic rate drops, body cools, animal enters a state of hypothermia and body cools progressively and finally animal dies. Dear students, the upper critical temperature is the temperature zone which activates evaporative heat dissipating mechanisms like sweating and panting. Above the upper critical temperature, the condition becomes hyperthermia. If the temperature increases from the upper critical temperature, then in that case animal body temperature starts to increase. This condition is called hyperthermia. In hyperthermia, heat production exceeds heat loss. If evaporative heat loss mechanism, which uses body temperature to control, fails then hyperthermia becomes lethal.