Chapter 1 Temperature Regulation in Animals There are, broadly speaking, two kinds of animals with regard to body temperature: exothermic (cold-blooded) and endothermic (warm-blooded) animals. The exothermic animals, such as reptiles, do not supply body heat by metabolic conversion of food to heat. Reptiles allow their surroundings to determine their body temperature. They lie out in the sun to warm their body. If they are too hot, they seek the shade or even burrow into the ground. At night they hide from the cold in burrows or squeeze into cracks between rocks or hide in leaf cover. Reptiles avoid the extremes of temperature. When reptiles become cool, their movements slow down, and chemical processes in their bodies, such as digestion, are inhibited. Predators, such as hawks and eagles, find it easier to prey on lizards and snakes in cooler weather. The distribution of reptiles is somewhat limited by their exothermic character. They do not thrive in cold climates1. What are the advantages and disadvantages in being exothermic? When the lizard is in a cool environment and cannot find a warmer spot, its body simply cools to the temperature of the surroundings. It is not necessary for the exothermic lizard to generate heat to increase its body temperature. This means that the lizard uses less energy and does not have to eat as much. As the lizard cools its digestion, breathing rate and heart rate slow, saving energy. A disadvantage occurs when the cool lizard is attacked by a predator. If warm, he could run fast and have a much better chance of 1 St. Patrick did not chase the snakes out of Ireland. Ireland was already completely free of snakes. St. Patrick was instrumental in converting pagans to Christianity. Since the snake was a symbol used in pagan rituals, St. Patrick was influential in ridding Ireland of the ritual use of symbolic snakes. 10 Verne A. Simon evading capture. A warm lizard being chased by a predator can move quite fast for a short distance, but like other exotherms, lacks endurance and soon tires. When the exotherm is running fast, its effort is anaerobic, that is, is not using oxygen, and lactic acid is building up in its body. It soon tires and is unable to exert itself. It must recover by taking in oxygen to rid the body of lactic acid. Another disadvantage of exothermic life is that cold climates are not available as habitat. If there is a sudden climate change, an exothermic animal wouldn't be able to mount the sustained effort needed to migrate to a better environment. The exothermic creature might simply perish. About 180 million years ago, mammals appeared. Mammals are endothermic (warm-blooded) and are able to maintain a nearly constant body temperature regardless of the temperature of their surroundings within wide limits. Their bodies will not tolerate too high or too low a temperature. If the surroundings are too hot or cold, causing the body temperature to exceed allowed limits, the animal will die. Mammals have furry coats to help them tolerate low temperatures. Sea-dwelling mammals-whales, seals, and walrus-have thick layers of blubber for insulation. Birds are endothermic and have feathers to protect them from the cold. Many types of birds and mammals survive in cold climates. Emperor penguins even live in the Antarctic, in the coldest climate on earth. Under normal circumstances, mammals and birds manage to keep this very nearly constant body temperature regardless of the temperature of their surroundings. Mammals are characterized by having body hair and suckling their young. This latter behavior gives the class its name; mammals must have mammary glands. A second advantage is that endothermic animals are not limited to activity only in daylight hours. In many locat