(a) A lizard regulates its body temperature throughout the day, basking in the sun to warm and seeking shade to prevent overheating. If its body temperature rises too high, the lizard may pant to accelerate heat loss (b). The graph shows an early-morning increase in the lizard's body temperature. The body temperature fluctuates only slightly during the remainder of the day, despite wide fluctuations in ground temperature.
The control of body temperature is known as thermoregulation. Vertebrates regulate their body temperature in two different ways. An ectotherm warms its body by absorbing heat from its surroundings. Reptiles, fishes, and amphibians are ectotherms. Endotherms, such as mammals and birds, have a rapid metabolism, which generates heat needed to warm the body. Most endotherms have insulation, such as hair, feathers, or fat, to retain heat. The body temperatures of many aquatic ectotherms, such as fishes and amphibians, remain close to the temperature of their surroundings. Terrestrial ectotherms, such as lizards and snakes, usually keep their body temperatures about the same as the body temperatures of endotherms.
Most reptiles live in warm climates and regulate their body temperature by controlling how much heat they absorb. For example, when a lizard emerges from its nest after a cool night, its body temperature is low and must be raised before it can become active. The lizard warms itself by basking in the sun, as shown in Figure 41-11a. The lizard's warm blood is diverted from the skin to the interior of the body. As the graph in Figure 41-11b shows, a lizard can maintain its body temperature within a narrow range despite variations in air temperature. The lizard uses a variety of behaviors to accomplish this.
Because their metabolism is very slow, ectotherms require very little energy and need only about one-tenth as much food as an endotherm of the same size. Ectotherms cannot live in very cold climates, and they can survive temperate climates only by becoming dormant during the coldest months. Furthermore, ectotherms can run or swim at maximum speed only for short periods of time. Ectothermic metabolism cannot provide enough energy for sustained exertion.
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