Cells and Temperature

Cells cannot remain alive outside certain limits of temperature, and much narrower limits mark the boundaries of effective functioning.

systems of mammals and birds are most efficient only within a narrow range around 37℃; a departure of a few degrees from this value seriously their functioning. Even though cells can survive wider fluctuations, the actions of bodily systems are impaired. Other animals have a wider tolerance for changes of bodily temperature.

For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become more accurate and meaningful over time, but popular still reflects the old division into "warm blooded" and "cold blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was inadequate. A fence or a desert -- each cold-blooded -- usually has a body temperature only a degree or two below that of humans and so is not cold.

Therefore the next distinction was made between animals that maintain a constant body temperature, called , and those whose body temperature varies with their environment, called . But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during . Furthermore, many that live in the depths of the ocean never experience changes in the of the deep water, and their body temperatures remain constant.