大多數(shù)動物都有可以生存的體溫上限。例如，在人類中，41攝氏度的體溫會導致蛋白質(zhì)功能喪失和神經(jīng)系統(tǒng)崩潰，而42至43攝氏度的體溫會導致是致命的。鳥類的靜息體溫略高于哺乳動物（大約為 40 至 41 攝氏度，而大多數(shù)哺乳動物為 35 至 38 攝氏度），它們無法在高于 46 至 47 攝氏度的體溫下生存。在高于 50 攝氏度的環(huán)境溫度下，幾乎所有動物都會死亡。然而，在溫度范圍的另一端，極冷的耐受性更好。例如，一些動物在解凍后可以冷凍并存活。冷凍/解凍通常是危險的，因為冰晶會在細胞和破裂膜內(nèi)形成。然而，許多昆蟲，如毛毛蟲，少數(shù)種類的兩棲動物，如林蛙，以及極少數(shù)爬行動物，如彩龜，都可以阻止其細胞中的晶體形成。他們通過從肝臟中大量流出葡萄糖來響應皮膚表面的冰塊來做到這一點。儲存在肝臟中的葡萄糖進入血液和細胞，降低它們的冰點，使細胞不會凍結(jié)成固體。這些動物可以長時間完全冷凍 65% 或更多的身體，只有在溫暖時期才解凍而不會產(chǎn)生有害影響。他們通過從肝臟中大量流出葡萄糖來響應皮膚表面的冰塊來做到這一點。儲存在肝臟中的葡萄糖進入血液和細胞，降低它們的冰點，使細胞不會凍結(jié)成固體。這些動物可以長時間完全冷凍 65% 或更多的身體，只有在溫暖時期才解凍而不會產(chǎn)生有害影響。他們通過從肝臟中大量流出葡萄糖來響應皮膚表面的冰塊來做到這一點。儲存在肝臟中的葡萄糖進入血液和細胞，降低它們的冰點，使細胞不會凍結(jié)成固體。這些動物可以長時間完全冷凍 65% 或更多的身體，只有在溫暖時期才解凍而不會產(chǎn)生有害影響。

過去，動物被分為兩類。冷血動物需要外部熱源（例如陽光）來溫暖自己。相比之下，溫血動物使用內(nèi)部熱量來維持體溫。然而，這些術(shù)語具有誤導性，因為許多冷血動物可以通過鍛煉骨骼肌產(chǎn)生大量熱量。事實上，許多人在白天的體溫至少與鳥類和哺乳動物等溫血動物的體溫一樣溫暖。

生物學家現(xiàn)在根據(jù)動物的熱源和維持體溫的能力對動物進行分類。等溫動物依靠外部熱源來溫暖他們的身體，而吸熱動物則使用自己代謝產(chǎn)生的熱量來溫暖自己。恒溫動物的體溫保持在一個狹窄的范圍內(nèi)，而異溫動物的體溫會隨著環(huán)境的變化而變化。大多數(shù)動物分為兩類。鳥類和哺乳動物是吸熱和恒溫的。而其他脊椎動物和大多數(shù)無脊椎動物是等溫的和異溫的。

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.

即使是吸熱恒溫動物也沒有真正恒定的體溫，而是在一個狹窄的體溫范圍內(nèi)，在極端氣候下運動期間甚至睡眠期間都會發(fā)生輕微的升高和降低。重要的特點是鳥類和哺乳動物可以快速調(diào)整身體保持或釋放熱量的機制，使體溫保持相對穩(wěn)定。這提供了一個優(yōu)勢，即即使在環(huán)境帶來極端挑戰(zhàn)時，身體化學反應也處于最佳水平。例如，靜止的哺乳動物的新陳代謝率大約是同等大小的爬行動物的六倍。突然醒來的哺乳動物即使在冬日也能進行激烈的活動，

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.

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