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Most animals have an upper limit of body temperature at which they can survive.In humans, for example, a body temperature of 41 degrees Celsius causes loss of protein function and breakdown of the nervous system, and a body temperature of 42 to 43 degrees Celsius is fatal. Birds, which have slightly higher resting body temperatures than mammals(approximately 40 to 41 degrees Celsius compared with 35 to 38 degrees Celsius for most mammals), cannot survive at body temperatures above 46 to 47 degrees Celsius. At environmental temperatures greater than 50 degrees Celsius, nearly all animals die. At the other end of the temperature spectrum, though, extreme cold is better tolerated. Fo example, Some animals can freeze and survive after thawing. Freezing/thawing is normally dangerous because ice crystals form inside cells and rupture membranes. However, many insects, such as the woolly caterpillar, a few species of amphibian such as the wood frog, and a very small number of reptiles such as the painted turtle, can block crystal formation in their cells. They do this by responding to ice on their skin surfaces with an enormous outpouring of glucose from the liver. The glucose stored in the liver enters the blood and the cells, lowering their freezing point so that the cells do not freeze solid. These animals can have 65 percent or more of their bodies completely frozen for long periods, only to thaw during warm periods without harmful effects.

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In the past, animals were classified into two categories. Cold blooded animals require an external heat source such as sunlight to warm themselves. By contrast, warm-blooded animals use internal heat to maintain their body temperature. These terms are misleading, however, because many cold-blooded animals can generate considerable heat by exercising their skeletal muscles. Indeed, many have a body temperature during daylight hours that is at least as warm as that of warm blooded animals like birds and mammals.

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Biologists now classify animals according to both their source of heat and their ability to maintain body temperature. Ectotherms depend on external heat sources to warm their bodies, while endotherms use their own metabolically generated heat to warm themselves. Homeotherms maintain their body temperature within a narrow range, while heterotherrms have body temperatures that vary with the environment. Most animals fall into two categories. Birds and mammals are endothermic and homeothermic. while other vertebrates and most invertebrates are ectothermic and heterothermic.

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Not all animals, however, can be neatly classified into two categories at all times.Hibernating mammals, for example, are endotherms. They are homeothermic, but during the winter their body temperature drops dramatically as their metabolism slows to conserve energy for the winter. Hibernators behave like heterotherms during the transition from fall to winter and again from winter to spring, During the winter, however, they are homeothermic except for brief periods of arousal, but at a lower body temperature than at other times of the year. Similarly, a fish swimming in deep ocean waters is an ectotherm but also homeothermic because the temperature of the water-and therefore of its body-is essentially constant. Fish that live in waters with fluctuating temperatures, by contrast are ectothermic and heterothermic.

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Even endothermic homeotherms do not have truly constant body temperatures but rather a narrow range of body temperatures within which slight increases and decreases occur in extreme climates during exercise, or even during sleep. The important feature is that birds and mammals can quickly adjust the body’s mechanisms for retaining or releasing heat such that body temperature remains relatively stable. This provides the advantage that the body chemical reactions are at optimal levels even when the environment imposes extreme challenges. The metabolic rate of a resting mammal, for example, is roughly six times greater than that of a comparably sized reptile. A suddenly awakened mammal is capable of intense activity even on a winter day, but an icy-cold reptile could be at the mercy of a predator because of the time required to warm itself and flee.

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Endothermy does have two major disadvantages, however. First, to produce sufficient heat by metabolic processes, endotherms must consume larger amounts of food. Small endotherms, such as shrews, must eat almost continually and may die if deprived of food for as little as a day. By contrast, many ectotherms, such as snakes can go for weeks without eating. Second, endotherms run the risk of overheating during periods of intense activity, even in cold weather.

1.Most animals have an upper limit of body temperature at which they can survive.In humans, for example, a body temperature of 41 degrees Celsius causes loss of protein function and breakdown of the nervous system, and a body temperature of 42 to 43 degrees Celsius is fatal. Birds, which have slightly higher resting body temperatures than mammals(approximately 40 to 41 degrees Celsius compared with 35 to 38 degrees Celsius for most mammals), cannot survive at body temperatures above 46 to 47 degrees Celsius. At environmental temperatures greater than 50 degrees Celsius, nearly all animals die. At the other end of the temperature spectrum, though, extreme cold is better tolerated. Fo example, Some animals can freeze and survive after thawing. Freezing/thawing is normally dangerous because ice crystals form inside cells and rupture membranes. However, many insects, such as the woolly caterpillar, a few species of amphibian such as the wood frog, and a very small number of reptiles such as the painted turtle, can block crystal formation in their cells. They do this by responding to ice on their skin surfaces with an enormous outpouring of glucose from the liver. The glucose stored in the liver enters the blood and the cells, lowering their freezing point so that the cells do not freeze solid. These animals can have 65 percent or more of their bodies completely frozen for long periods, only to thaw during warm periods without harmful effects.