Endotherms are described as organisms capable of maintaining a stable internal temperature, regardless of external influence. This ability to control internal temperature is considered an important biological advance in evolutionary development because it allowed animal life to develop in environments with temperatures significantly different from those of the body. (9) It is the main characteristic of the life of mammals and birds that distinguishes them from reptiles and other vertebrates. Endothermy offers distinct psychological and ecological advantages, which enable the successful survival of mammals in a wide range of aerial, aquatic, and terrestrial environments. It allows them to maintain a level of activity that is beyond the capacity of endotherms. (6) Mammals maintain a constant body temperature in mainly two ways. First, by minimizing thermal conductance with fur or by storing large amounts of subcutaneous fat; Second, maintaining a metabolic heat production equivalent to the rate at which the body loses heat to the environment. They tend to significantly increase the amount of oxygen consumption in cold temperatures to compensate for the accelerated heat loss. Mammals also shiver in response to exposure to cold environments. However, there are also placental mammals that rely on nonshivering thermogenesis to increase the rate of metabolic heat production during long-term cold exposure. Brown adipose tissue is a specialized thermogenic tissue associated with hibernating, cold-adapted mammals. (4)It has been observed that the mechanisms used by homeothermic species to produce heat and maintain body temperature are not new. They precede homeothermy, which is supported by the presence of similar mechanisms below…middle of the paper…in anxiety. (7) It is possible that one or more theories providing explanations for the evolution of complex characteristics of endothermy may be correct, however there are insufficient fossil records to recover small changes associated with the evolution of fully expressed mammalian homeothermy. The correlated progression hypothesis supports that evidence from living amniotes or fossils may not be sufficient to reveal the sequence of mammalian endothermy because ectotherms show few relevant signs of early endothermy and mammals evolved from ancestors that had mechanisms fully evolved endothermic. There are several individual structures, processes and definable functions of endothermy, which have evolved in a related progression. Endothermy cannot be separated as a single function from its integration with the rest of the biological characteristics of an organism. (7)