Chapter 1Temperature Regulation in AnimalsThere are, broadly speaking, two kinds of animals with regard to bodytemperature: exothermic (cold-blooded) and endothermic (warm-blooded)animals. The exothermic animals, such as reptiles, do not supply body heatby metabolic conversion of food to heat. Reptiles allow their surroundingsto determine their body temperature. They lie out in the sun to warm theirbody. 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 betweenrocks or hide in leaf cover. Reptiles avoid the extremes of temperature. Whenreptiles become cool, their movements slow down, and chemical processesin their bodies, such as digestion, are inhibited. Predators, such as hawksand eagles, find it easier to prey on lizards and snakes in cooler weather. Thedistribution 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? Whenthe lizard is in a cool environment and cannot find a warmer spot, its bodysimply cools to the temperature of the surroundings. It is not necessary forthe exothermic lizard to generate heat to increase its body temperature.This means that the lizard uses less energy and does not have to eat asmuch. As the lizard cools its digestion, breathing rate and heart rate slow,saving energy. A disadvantage occurs when the cool lizard is attacked bya predator. If warm, he could run fast and have a much better chance of1 St. Patrick did not chase the snakes out of Ireland. Ireland was already completely free ofsnakes. St. Patrick was instrumental in converting pagans to Christianity. Since the snake wasa symbol used in pagan rituals, St. Patrick was influential in ridding Ireland of the ritual useof symbolic snakes.10 Verne A. Simonevading capture. A warm lizard being chased by a predator can move quitefast for a short distance, but like other exotherms, lacks endurance andsoon tires. When the exotherm is running fast, its effort is anaerobic, thatis, is not using oxygen, and lactic acid is building up in its body. It soontires and is unable to exert itself. It must recover by taking in oxygen torid the body of lactic acid. Another disadvantage of exothermic life is thatcold climates are not available as habitat. If there is a sudden climatechange, an exothermic animal wouldnt be able to mount the sustainedeffort needed to migrate to a better environment. The exothermic creaturemight simply perish.About 180 million years ago, mammals appeared. Mammals areendothermic (warm-blooded) and are able to maintain a nearly constant bodytemperature regardless of the temperature of their surroundings within widelimits. Their bodies will not tolerate too high or too low a temperature. If thesurroundings are too hot or cold, causing the body temperature to exceedallowed limits, the animal will die. Mammals have furry coats to help themtolerate low temperatures. Sea-dwelling mammalswhales, seals, andwalrushave thick layers of blubber for insulation. Birds are endothermic andhave feathers to protect them from the cold. Many types of birds and mammalssurvive in cold climates. Emperor penguins even live in the Antarctic, in thecoldest climate on earth. Under normal circumstances, mammals and birdsmanage to keep this very nearly constant body temperature regardless ofthe temperature of their surroundings. Mammals are characterized by havingbody hair and suckling their young. This latter behavior gives the class itsname; mammals must have mammary glands. A second advantage is thatendothermic animals are not limited to activity only in daylight hours. Inmany locations, it is too cold at night for exotherms to be active. Even verycold temperatures do not exclude endothermic animals such as mammalsand birds from nocturnal activity. Exothermic animals are not normally foundin cold climates, though there are a few exceptions.