Temperature is a severe limiting factor for animals, all of which must maintain biochemical stability. Biochemical activities are sensitive to the chemical environment: biochemical reactions are very sensitive to temperature, if temperature goes above or below this optimum, the enzyme function is impaired. In instances where body temperature drops too low, metabolic processes slow, thus reducing the amount of energy the animal can produce for activity and reproduction. When body temperature rises too high, metabolic reactions become unbalanced and enzymatic activity is affected or in some cases, destroyed. Therefore, animals can succeed only in a restricted range of temperature, mostly between 0 to 40°C. If animals cannot find a place to live where they do not have to contend with extreme temperatures, they must find a way to stabilize their metabolism independent of extreme temperatures.
Animals can be classified according to their BEHAVIORAL or STRUCTURAL strategies for thermoregulation. Behavioral Strategies can be classified as either endothermic or exothermic. Endothermic animals or endotherms are animals that derive most of their body heat from their own internal metabolism. The source of their body heat is internal, thus the term endotherms. They are able to generate and retain enough heat to elevate their own body temperatures to a high but stable level. Examples of endotherms are all birds, mammals, a few fast-swimming fishes and reptiles, as well as certain insects. Being endothermic allows birds and mammals to stabilize their internal temperature so that the biochemical processes and nervous system functions can go on at high, steady levels of activity. In endotherms, high metabolic rates and heat production become important for thermoregulation. Ectothermic animals or ectotherms are animals that derive most of their body heat from external sources such as solar radiation.