Invertebrates are at the mercy of the environment as they generally cannot adjust their body temperatures. Vertebrates have evolved mechanisms for balancing heat production and heat loss.
Reptiles, amphibian and fish are called “cold blooded” (poikilothermic), and have relatively rudimentary mechanisms for adjusting temperature, so that their body temperature fluctuates over a considerable range.
Mammals are “warm blooded” (homeothermic), requiring a nearly constant internal body temperature and core temperature is one of the most closely guarded physiological parameters. Because the speed of chemical reactions varies with temperature and because the enzyme systems of the body have narrow temperature ranges in which their function is optimal, normal body function depends upon a relatively constant body temperature.
Although core temperature varies daily with circadian rhythm and monthly in women, body temperature does not deviate more than a few tenths of a degree either side of normal. A group of reflex responses that are primarily integrated in the hypothalamus operate to maintain body temperature within a narrow range in spite of wide fluctuations in environmental temperature.