ECTOTHERMY AND ENDOTHERMY 1 Many biological systems are based on the process of homeostasis, which means "steady state." Homeostasis is the ability to maintain balance. Homeostatic mechanisms enable animals to survive changes in their external environment by regulating conditions within their bodies. Conditions in the external environment, such as temperature, may vary widely, but conditions in the internal environment can vary only within a narrow range necessary for survival. 2 Temperature is a constraint for animals, all of which must maintain biochemical stability. When an animal’s body temperature drops too low, its metabolism slows, thus reducing the amount of energy the animal can use for activity. If body temperature rises too high, metabolic reactions become unbalanced, and enzyme activity is hindered. Animals can succeed only in a limited range of body temperatures, and for most, this is between 0 and 40 degrees Celsius. 3 One way to classify animals is to emphasize their source of body heat. For instance, "cold-blooded" animals are those that must warm their body with heat from the surrounding environment, and "warm-blooded" animals are those that can heat themselves. However, these traditional terms are inaccurate and misleading. Some "cold-blooded" animals, such as lizards, have higher body temperatures when active than many "warm-blooded" animals have when hibernating. Physiologists prefer the terms "ectotherm" and "endotherm" because they reflect the fact that an animal’s body temperature is a balance between heat loss and heat gain. 4 All animals produce heat from cellular metabolism, but in most the heat is conducted away as fast as it is produced, so the amount of heat obtained from metabolism is very small. In these animals, the ectotherms, body temperature is determined almost entirely by their surroundings. Most invertebrates, fishes, amphibians, and reptiles are ectotherms. In contrast, some animals are able to generate and retain enough heat from metabolism to elevate their own body temperature to the optimum level. These animals are called the endotherms because the source of their body heat is internal. Mammals, birds, some fishes, and numerous insects are endotherms. 5 Ectotherms warm their body mainly by absorbing heat from their environment. Ectotherms cannot control their body temperature physiologically, yet many are able to regulate it behaviorally by selecting areas of the environment with a more favorable temperature. Some, such as desert lizards, exploit hour-to-hour changes in solar radiation to keep their body temperature relatively constant. In the morning, the lizard absorbs the sun’s heat through its head, while keeping the rest of its body protected from the cool air. Later, the lizard will emerge to bask in the sun. At noon, with its body temperature high, it seeks shade under a rock. When the air temperature drops in the late afternoon, it emerges and lies parallel to the sun’s rays. 6 Endotherms, on the other hand, derive most or all of their body heat from their own metabolism. A consistently warm body temperature requires active metabolism, which includes oxidation of foods, cellular respiration, and muscular contraction. Conversely, a warm body temperature contributes to the high levels of metabolism required for extended periods of intense physical activity. This is one reason endotherms can generally endure vigorous activity longer than ectotherms. However, being endothermic is energetically expensive, especially in a cold environment. Because much of an endotherm’s daily intake of calories is used to generate heat, the endotherm must eat more food than an ectotherm of the same size. Endothermy allows birds and mammals to stabilize their internal temperature so that biochemical processes and nervous system functions can proceed at steady levels of activity. These animals maintain a constant body temperature through a delicate balance between heat production and heat loss. This is why endotherms can remain active in winter and exploit habitats denied to ectotherms. If the animal becomes too cool, it can generate heat by increasing muscular activity (exercise or shivering), or it can decrease heat loss by increasing insulation. In general, birds and mammals are warmer than their surroundings, but they also have mechanisms for cooling the body in a hot environment. If the animal becomes too warm, it decreases heat production and increases heat loss by evaporative cooling (sweating or panting).
A. endurance of intense physical activity B. survival in cold surroundings C. high level of intelligence D. ability to cool the body
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