You are aware that almost all physiological processes are regulated by the action of specific enzymes, and that the rate of an enzyme mediated reaction is related to temperatures. Therefore, the temperature of an animal's my generally has profound effects on function. The cells, tissues and organs of all animals function within a narrow range of temperatures. However, outside the favourable temperature range, many animals can survive in an inactive or torpid state. In fact, some can survive freezing at extremely low temperatures. For instance, in polar regions numerous fish and invertebrates live in water at - 1.8?C. At the other extreme, in hot springs, a few animals can live at about 70?, and a few thermophilic bacteria thrive above the boiling point of water. Body temperature of most animals, particularly that of all aquatic invertebrates remains the same as that of their surroundings. Thus, the body temperature of these animals changes with the changing ambient temperature. Birds and mammals in contrast, usually maintain their body temperature nearly constant and independent of the environment. All the animals whose-body temperature fluctuates with that of the environment have traditionally been called poikilothermic (poikilo = changing).
To this category belong all the so-called cold blooded animals. The birds and mammals which maintain nearly constant body temperatures are classified as homeothermic or the so-called warm blooded animals, Nevertheless, it should be noted that there is no easy way to classify satisfactorily the various responses to the thermal environment. The above terminology is not very accurate. For instance, the blood of cold blooded animal is not always cold; or a tropical fish or a desert lizard or an insect sitting in the sun-may have a higher body temperature than a mammal. Furthermore, a few birds and mammals exhibit torpor or hibernation during which period their temperature decreases to near the freezing point of water.
These examples should serve to explain the inaccuracy of the terminology used. However, the basic difference between the so-called poikilothermic and homeothermic animals is that the homeothermic are able to maintain their body temperature by metabolic heat production whereas; the poikilothermic are unable to do so. Consequently, in recent years, the terms ectothermic and endothermic are used to denote the animals that depend on external heat sources (mainly solar radiation) and others which are able to maintain a high body temperature by endogenous heat production (endothermic) respectively. These definitions also have their limitations since several invertebrates and vertebrates can at times maintain a substantial difference between their own temperature and that of the surroundings. In the following section you will learn such varied relations and their physiological and ecological implications.