Thermal neutral zone

Endothermic organisms known as homeotherms maintain internal temperatures with minimal metabolic regulation within a range of ambient temperatures called the thermal neutral zone (TNZ). Within the TNZ the basal rate of heat production is equal to the rate of heat loss to the environment. Homeothermic organisms adjust to the temperatures within the TNZ through different responses requiring little energy.

Environmental temperatures can cause fluctuations in a homeothermic organism's metabolic rate. This response is due to the energy required to maintain a relatively constant body temperature above ambient temperature by controlling heat loss and heat gain.^[1] The degree of this response depends not only on the species, but also on the levels of insulative and metabolic adaptation.^[2] Environmental temperatures below the TNZ, the lower critical temperature (LCT), require an organism to increase its metabolic rate to meet the environmental demands for heat.^[3] The Regulation about the TNZ requires metabolic heat production when the LCT is reached, as heat is lost to the environment. The organism reaches the LCT when the T_a (ambient temp.) decreases.

When an organism reaches this stage the metabolic rate increases significantly and thermogenesis increases the T_b (body temp.) If the T_a continues to decrease far below the LCT hypothermia occurs. Alternatively, evaporative heat loss for cooling occurs when temperatures above the TNZ, the upper critical zone (UCT), are realized (Speakman and Keijer 2013). When the T_a reaches too far above the UCT, the rate of heat gain and rate of heat production become higher than the rate of heat dissipation (heat loss through evaporative cooling), resulting in hyperthermia.

It can show postural changes where it changes its body shape or moves and exposes different areas to the sun/shade, and through radiation, convection and conduction, heat exchange occurs. Vasomotor responses allow control of the flow of blood between the periphery and the core to control heat loss from the surface of the body. Lastly, the organism can show insulation adjustments; a common example being "goosebumps" in humans where hair follicles are raised by pilomotor muscles, also shown in animals' pelage and plumage.^[4]