Official 40 Passage 2


Amphibian Thermoregulation


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正确答案: C

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  • 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 -2ºC while one South American frog feels comfortable even when temperatures rise to 41ºC-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 -6ºC 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 10ºC. The Andean toad Bufo spinulosusexposes 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 arid and semi-arid 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 temperatures. Amphibians are therefore hardly at the mercy of ambient temperatures, since by means of the mechanisms described above they are more than able to exercise some control over their body temperature.

  • 与晡乳动物和鸟类相反,两栖类动物不能通过它们的代谢活动产生热能,代谢活动允许鸟类和哺乳动物脱离周围环境温度调节体温。 然而,两栖类动物完全不能控制他们的体温的说法已被证明是假的,因为他们的体温并不总是对应于周围的温度。 尽管两栖类在温度调节上比较薄弱,他们会在有限的程度上控制他们的身体温度。

    生理适应性可以帮助两栖动物在极端条件的栖息地生存。 体温的耐受范围代表一个物种可以生存的温度范围。 当温度降到-2°C时,一种北美蝾螈依然活跃,而在即使温度上升到41°C,南美洲青蛙依然感觉舒适,这是在自由放养的两栖动物中测试到的最高体温。 最近有证据表明,一些北美青蛙和蟾蜍的物种可以在体温-6°C并且大约三分之一的体液冻结的情况下生存五天。 它们的其他组织受到保护,因为这些组织包含防冻剂甘油或葡萄糖。 此外,许多物种的忍受极限是弹性的,可以适应变化而变化(长期暴露于特定的条件下)。

    尽管白天温度高,暴露在太阳下的青蛙物种在皮肤结构中产生了一些有趣的改变,其作用是形态适应。 大多数两栖动物的皮肤是完全透水性的,因此不是一种阻挡蒸发或太阳辐射的屏障。 非洲大草原的青蛙Hyperolius viridiflavus在其皮肤里储存嘌昤晶体,使它能够更好地反射太阳辐射,从而提供对过高温度的保护。 树娃Phyllomedusasauvager 应对蒸发损失的方法是通过腺分泌物在其整个身体提供油脂薄膜,有助于防止脱水(脱水)。

    然而,行为是迄今为止在体温调节中最重要的因素。 行为的体温调节中最主要元素的是取暖(heliothermy),与基材如岩土热交换(thigmothermy),和每日或每年的逃避行为,包括移动到庇护地去冷却和冬眠或夏眠(分别在过冷或热的天气减少活动)。 heliothermy在青娃和蟾蜍中是特别常见的:这使得他们能够提高体温超过10°C。 安第斯蟾蜍spinulosus在日出后立即将自身暴露在潮湿地面上,早早在地面或空气温度升高之前,通过这种方式达到其喜好的体温。 这种方法的一个正面的副作用是,它加速了晚上对猎物的消化,从而也加速增长。 thigmothermy是大多数两栖动物常见的行为,虽然压在地上有双重目的:通过导电性吸热和通过皮肤吸收水分。 在降雨的时候,thigmothermy的效果在安第斯蟾蜍尤其明显:它依据温暖的地面温度调节体温,而不是依据更冷的空气温度。

    避免行为会在生理和形态的适应不足以将身体的温度保持在重要范围内时发生。 对周围温度有着比较低的耐受性的两栖动物的夜间活动是一种典型的逃避式体温调节行为。 季节性回避行为在许多两栖类动物中极为重要。 温带地区的物种的栖息地在冬季会面临着致命的低温,而居住在干旱和半干旱地区的物种会暴露于长期干燥炎热的夏天。

    两栖动物的冬眠发生在泥里或深洞这种远离霜冻的地方。 比利牛斯山脉以北,natterjack蟾蜍为冬眠提供了一个很好的例子,通过挖掘到沙地深处度过冬季。 相反,在西班牙南部的natterjacks在该地区常见的暖冬仍然活跃着,但是不得不在干燥、炎热的夏季处于休眠状态。 夏眠也是通过钻入地下或隐藏在阴凉、深处的岩体裂隙来实现,这样能避免干燥和致死温度。 因此,两栖类动物是不受环境温度控制的,因为通过上述的机制,他们对他们身体的温度进行了控制。
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    题干分析:词汇所在句attains its preferred body temperature”即“达到想要的体温attains, 达到,获得,实现。

    选项分析: raises, 提高,养育,筹集。lowers, 减弱,降低。reaches, 达到,延伸。regulates, 调节,规定。选项C符合题干词意。