Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only within a narrow range around 37C；a departure of a few degrees from this value seriously impairs their functioning. Even though cells can survive wider fluctuations the integrated 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 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-blooded. As more species were studied， it became evident that this classification was inadequate. A fence lizard.