First of all we need to understand what ectothermic and endothermic animals are. Animals differ in their abilities to regulate body temperature (thermoregulation). We sometimes use the terms "cold-blooded" or "warm-blooded." Most reptiles feel cold to the touch, while mammals and birds often feel warm. Somewhat more precise descriptions can be made by using the terms poikilothermic and homoiothermic. The body temperature of poikllotherms is relatively variable, while that of homeotherms is relatively constant. Even more useful terms are Ectothermic or Endothermic, which suggest two different mechanisms of thermoregulation. Ectotherms generally obtain heat from their external surroundings. Their body temperature varies, corresponding at any time with the temperature of their external environment. Endothermic animals, on the other hand, have relatively constant body temperatures. Their body temperature is independent of that of their external environment. Monkeys and walruses, for example, both have body temperatures of about 38â€žaC, despite living in very different habitats. However if body temperature rises above its optimum level (usually around 40â€žaC in mammals) then the enzyme rate inside the body will go into sharp decline. This is because enzymes are proteins, and become denatured. One of the first organs to be affected is the brain. Since the brain controls breathing and the circulation, the rise in body temperature disrupts the normal functioning of these important systems. If the body temperature decreases dramatically (hypothermia) then this will slow metabolic activity and impairs brain function. Here is a graph to show the relationship between the body temperature and environmental temperature for a cat (endotherm) and a lizard (ectotherm) Also we need to clarify what is meant by an extreme climate. In this investigation I will be using two different climates, The Desert and The Arctic When an endotherm is subjected to severe cold it is liable to lose heat energy but this can be counteracted in a number of ways; â€žh It could raise the hairs into a more vertical position by the contraction of the erector pili muscles.