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 10 and meaningful over time, but popular terminology 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 blood As more species were studied, it became evident that this classification was inadequate. A fence lizard or a desert iguana-each cold 15 booded-usually hasabody temperature only a degree two below of humans and so is not cold, Therefore the next distinction was made between animals that maintain a constant body temperature, called homeotherms, and those whose body temperature varies with their environment, called poikilotherms. But this classification also proved inadequate because among mammals there are many 20 that vary their body temperatures during hibernation. Furthermore, many vertebrates that live in the depths of the ocean never experience a change in the chill of the deep water, and their body temperatures remain constant The current distinction is between animals whose body temperature is regulated chiefly by internal metabolic processes and those whose 25 temperature is regulated by, and who get most of their heat from the environment. The former are called endotherms. and the latter are called ectotherms. Most ectotherms do regulate their body temperature. and they do so mainly by locomoting to favorable sites or by changing their temperature by choosing favorable environments but primarily they regulate their temperature by making a variety of internal adjustments,