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新托福TPO40阅读原文(三):Amphibian Thermoregulation

编辑:share 来源: 美联乐闻 发布时间:2016-06-16

文章摘要: 美联乐闻为考生们整理了新托福TPO阅读原文和译文,供大家参考。本文是新托福TPO40阅读原文(三):Amphibian Thermoregulation(两栖类动物的体温调节)。

新托福TPO40阅读原文(三):Amphibian Thermoregulation。《文勇新托福黄金阅读真题》文章全部来源于ETS考试中心官方推出的托福TPO阅读,美联乐闻为考生们整理了新托福TPO阅读原文和译文,供大家参考。本文是新托福TPO40第三篇文章的原文,新托福TPO40阅读原文(三):Amphibian Thermoregulation(两栖类动物的体温调节)。

TPO40-3:Amphibian Thermoregulation

In contrast to mammals and birds, amphibians are unable to produce thermal energy through their metabolic activity, which would allow them to regulate their body temperature independent of the surrounding or ambient temperature. However, the idea that amphibians have no control whatsoever over their body temperature has been proven false because their body temperature does not always correspond to the surrounding temperature. While amphibians are poor thermoregulators, they do exercise control over their body temperature to a limited degree.

Physiological adaptations can assist amphibians in colonizing habitats where extreme conditions prevail. The tolerance range in body temperature represents the range of temperatures within which a species can survive. One species of North American newt is still active when temperatures drop to -2°C while one South American frog feels comfortable even when temperatures rise to 41°C—the highest body temperature measured in a free-ranging amphibian. ■Recently it has been shown that some North American frog and toad species can survive up to five days with a body temperature of -6°C with approximately one-third of their body fluids frozen. ■The other tissues are protected because they contain the frost-protective agents glycerin or glucose. ■Additionally, in many species the tolerance boundaries are flexible and can change as a result of acclimatization (long-term exposure to particular conditions). ■

Frog species that remain exposed to the sun despite high diurnal (daytime) temperatures exhibit some fascinating modifications in the skin structure that function as morphological adaptations. Most amphibian skin is fully water permeable and is therefore not a barrier against evaporation or solar radiation. The African savanna frog Hyperolius viridiflavus stores guanine crystals in its skin, which enable it to better reflect solar radiation, thus providing protection against overheating. The tree frog Phyllomedusa sauvagei responds to evaporative losses with gland secretions that provide a greasy film over its entire body that helps prevent desiccation (dehydration).

However, behavior is by far the most important factor in thermoregulation. The principal elements in behavioral thermoregulation are basking (heliothermy), heat exchange with substrates such as rock or earth (thigmothermy), and diurnal and annual avoidance behaviors, which include moving to shelter during the day for cooling and hibernating or estivating (reducing activity during cold or hot weather, respectively). Heliothermy is especially common among frogs and toads: it allows them to increase their body temperature by more than 10°C. The Andean toad Bufo spinulosus exposes itself immediately after sunrise on moist ground and attains its preferred body temperature by this means, long before either ground or air is correspondingly warmed. A positive side effect of this approach is that it accelerates the digestion of the prey consumed overnight, thus also accelerating growth. Thigmothermy is a behavior present in most amphibians, although pressing against the ground serves a dual purpose: heat absorption by conductivity and water absorption through the skin. The effect of thigmothermy is especially evident in the Andean toad during rainfall: its body temperature corresponds to the temperature of the warm earth and not to the much cooler air temperature.

Avoidance behavior occurs whenever physiological and morphological adaptations are insufficient to maintain body temperature within the vital range. Nocturnal activity in amphibians with low tolerance for high ambient temperatures is a typical thermoregulatory behavior of avoidance. Seasonal avoidance behavior is extremely important in many amphibians. Species whose habitat lies in the temperate latitudes are confronted by lethal low temperatures in winter, while species dwelling in semi- and regions are exposed to long dry, hot periods in summer.

In amphibians hibernation occurs in mud or deep holes away from frost. North of the Pyrenees Mountains, the natterjack toad offers a good example of hibernation, passing the winter dug deep into sandy ground. Conversely, natterjacks in southern Spain remain active during the mild winters common to the region and are instead forced into inactivity during the dry, hot summer season. Summer estivation also occurs by burrowing into the ground or hiding in cool, deep rock crevasses to avoid desiccation and lethal ambient temperature. Amphibians are therefore hardly at mercy of ambient temperature, since by means of the mechanisms described above they are more than) exercise some control over their body temperature.

TPO40-3译文:两栖类动物的体温调节

与哺乳动物和鸟类相比,两栖类动物不能通过自身的代谢活动产生热能,这使得它们可以调节自身体温而不受周围环境温度的限制。然而,两栖类动物不能控制自己体温的说法已被证明是假的,因为它们的体温并不总是与周围环境的温度一致。尽管两栖类动物调节体温的能力差,但是还是有一定的控制能力的。

生理适应性可以帮助两栖动物在条件极端的栖息地生存。体温的耐受范围代表着一个物种可以生存的温度范围。当温度降到零下2°C时,北美蝾螈依然活跃;而即使温度上升到41°C——这是自由生长的两栖动物的最高体温,南美洲青蛙依然感觉舒适。最近证实,一些北美的青蛙和蟾蜍物种可以在体温零下6°C的情况下生存五天,而它们约三分之一的体液会冻结。其他组织会得到保护,因为它们含有抗冻保护剂甘油或葡萄糖。此外,有许多物种的容忍范围是多变的,可以为了适应环境而变化(长期暴露于特定的条件)。

在白天的高温下仍可以暴露在太阳下的青蛙物种在皮肤结构上有一些有趣的变化,这些变化是形态上的适应。大多数两栖动物的皮肤是完全透水性的,因此不能阻挡蒸发或太阳辐射。在非洲大草原的青蛙hyperolius viridiflavus皮肤中储存有鸟嘌呤晶体,这使它能够更好地反射太阳辐射,从而在过热时提供保护。树蛙phyllomedusa sauvagei的腺分泌物在其整个身体布满一层油脂薄膜以对抗水分蒸发,从而帮助防止出现干燥(脱水)。

然而,行为调节是迄今为止在体温调节中最重要的因素。行为温度调节主要包括晒太阳(日光浴),与岩石或土壤之类的基质的热交换(接触热源),以及每天或每年的躲避行为,这包括转移到遮蔽处去乘凉、冬眠或夏眠(分别在寒冷或炙热的天气减少活动)。日光浴在青蛙和蟾蜍中特别常见:这样它们能将自己的体温提高超过10°C。安第斯蟾蜍bufo spinulosus在太阳从潮湿的地面上升起后迅速出现,并以此方式达到想要的体温,而那时候地面或空气还没那么温暖。这种方法的一个积极的副作用是,它加速了夜间食用的猎物的消化,从而加速蟾蜍的生长。接触热源是会出现在大多数两栖动物中的行为,虽然压在地上有双重目的:通过热量传输来吸热,以及通过皮肤吸收水。对在安第斯蟾蜍来说,接触热源在降雨效果尤其明显:体温与温度较高的地面而不是温度较低的空气一致。

生理和形态的适应不足以维持生存温度时,就会产生躲避行为。对高环境温度容忍性差的两栖动物会选择夜间活动,这就是一个典型的躲避行为。季节性躲避行为在许多两栖类动物中极为重要。栖息地在温带地区的物种在冬季会面临致命的低温,而半干旱地区的物种在夏季会暴露于干燥、炎热的环境中。

两栖动物冬眠发生在泥或没有霜冻的深洞。比利牛斯山脉以北的黄条蟾蜍冬眠便是一个很好的例子:它们挖向沙地的深处,在深入度过冬季。相反,西班牙南部的黄条蟾蜍在该地区常见的暖冬依然活跃,但却不得不在干燥炎热的夏季夏眠。这种夏眠也是要挖到地下或躲到凉爽幽深的岩体裂隙中来避免干燥和高温。因此,两栖类动物是很难受环境温度影响的,因为通过上述机制它们会控制自身的体温。

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