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 38C, despite living in very different habitats.

However if body temperature rises above its optimum level (usually ar

 Sweating occurs and the evaporation of the water from the skin cools the skin and blood.

It is usually assumed that reptiles have a body temperature roughly the same as the air around them. but it has been shown that the temperature of reptiles is usually higher than that of the surrounding air. In fact, the body temperatures of lizards may be as much as 40C or 50C higher than the air temperature. For example a lizard lying on a black rock on a cool but sunny day will take on the temperature of the rock rather than that of the air.

Books: Biology, Principles and processes

 The metabolic rate can be increased therefore heating the inside of the body. A general increase in the metabolic rate is brought about by the hormones adrenaline and thyroxin which are produced in large amount during cold conditions. There is then a general increase in muscle tone, which is then followed by spasmodic contractions (shivering)

The tuber cinereum is stimulated either by nerve impulses from the cold receptors in the skin or by direct cooling. It is sensitive and will react to temperature changes of only a few tenths of a degree. If the body temperature is too cold, the thermoregulator stimulates a thyroid secretion, which increases the rate of metabolism. If the body temperature continues to drop, the work of internal organs like the liver may be increased and muscle activity in the form of shivering may begin, for muscular activity is the main source of heat available to animals.

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