Endothermic Response to Temperature

Endothermic homeotherms alter their metabolic rate in response to environmental temperature (Figure 2). The response to temperature depends in part on how precisely or not endotherms maintain body temperature. Over a range of intermediate temperatures, called the thermal neutral zone (TNZ), metabolic rate remains constant. If body temperature and metabolic rate are constant, but the thermal gradient between the animal and the environment is changed, then the insulation (or its reciprocal conductance) of the animal must change. A typical endothermic home-otherm would have maximal conductance at the upper end of its TNZ and minimal conductance at the lower end of its TNZ. Above the upper end of the TNZ, metabolic rate and body temperature increase, and the animal may undertake such behaviors as panting to increase evaporative heat loss. Below the lower end of the TNZ, endotherms increase heat production so that they can offset increased heat loss in colder environments; thereby, they balance heat production and heat loss to maintain a constant body temperature (Figure 2).

Endotherms that live in cold environments (e.g., aquatic environments, where the thermal conductivity of the water is many times greater than that of air) tend to be well insulated, and aquatic endotherms tend to be large. Insulation in vertebrates can take the form of fur, feathers,

\

Thermoneutral

\

zone (TNZ)

y

Environmental temperature

Environmental temperature

Figure 2 Change in metabolic rate of endotherms in response to environmental temperature. Other response patterns are possible, but this pattern is roughly typical for endotherms that are strict thermoregulators.

or fat layers. As insulation increases, the TNZ widens and the lower end of the TNZ occurs at lower temperatures. Animals that are very well insulated (the arctic fox is a classic example) have thermal neutral zones that may extend to extremely cold temperatures (e.g., they may experience temperatures as low as -40 ° C or perhaps even lower without having to increase their basal metabolic rate).

High temperatures can present problems for endotherms. When the thermal environment is hot, endotherms must increase evaporation to avoid overheating. Hence, hot environments can be challenging, particularly if ample water is not available.

to ectothermy is not as great as at cold environmental temperatures (Table 3).

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