Animals
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"aC, despite living in very different habitats. However if body temperature rises above its optimum level (usually around 40"aC in mammals) then the enzyme rate inside the body will go into sharp decline. Thi
Horned toad (Phrynosoma cornutum) These are short tailed, short-legged lizards in the iguana family. Reptiles Although reptiles are considered as ectotherms, they do regulate their own body temperatures to a surprising extent. 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. ' The tuber cinereum is stimulated either by nerve impulses from the cold receptors in the skin or by direct cooling. Long hair between the pads protects the bear's feet from the cold and provides traction on the ice. If it gets too hot, it may leave the rock or stand up slightly to allow some air to pass underneath it. Air can then be trapped in the spaces between the hairs and, being a poor conductor of heat, it serves as an insulatory layer round the animal. Moreover, its nostrils may be closed against flying dust, and its eyes are shielded by very long eyelashes and a third eyelid. "h The metabolic rate can be increased therefore heating the inside of the body. Another adaption is in the thickness of the sub-cutaneous fat. If the body temperature is too cold, the thermoregulator stimulates a thyroid secretion, which increases the rate of metabolism. It is usually assumed that reptiles have a body temperature roughly the same as the air around them. 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. but it has been shown that the temperature of reptiles is usually higher than that of the surrounding air. 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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