5月7日托福考试阅读第三篇背景知识解读

参加了5月7日的托福考试，而且正好又在考前完整做过小站托福官方真题Official的战友，一定会发现三篇文章的题材都似曾相识，虽然肯定都不是大段大段出现相同材料，但隐约中似乎都很肯定这些话题都熟悉的不能再熟悉了。其中第三篇文章中沙漠中两栖动物的话题我们曾在小站托福官方真题Official阅读材料中多次遇到。(详见文末资源链接)

托福阅读基础知识

托福考试阅读部分约为60分钟(不含加试)，包括3篇文章，每篇650至750个单词，对应11至13道试题。题目类型包括：图表题，篇章总结题，变换措辞题，词汇题，指代关系题，简化句子题，插入文本题，事实信息题，推断题，修辞目的题以及否定排除题。在完成答题的过程中，考生可以使用“复查”功能瞬间找出没有回答的题目，而不必每道题都过一遍。阅读部分的总分是42至45分。

5月7日托福考试阅读第二篇

真题回忆

讲的是两种两栖动物，蟾蜍(toad)和—种虾(desert shrimp)，如何在沙漠缺水的环境里生存。主要是讲它们利用很短的湿润季节进行繁殖来维持种族繁衍;旱季时，通过各种保护行为生存下来。蟾蜍是长期躲地下，然后在合适的时候出来产卵繁衍。还说了未成年幼体的应对，比如水快没了的一些应对。蟾蜍存活率很低，因为缺水的话它们就会长得很小，然后toad内部有弱肉强食，外部又有其他捕食者。还讲了有水的时候才能活的一种虾，这种虾也是在盆地底层一直冬眠很久，直到合适环境迅速孵化，一般是冬天，因为冬天水会比较多，在十天左右的时间，然后迅速繁衍。

背景知识解读(Wikipedia)

Desert Animal Survival/Adaptations of Desert Animals

沙漠动物生存/沙漠动物的适应

Lack of water creates a survival problem for all desert organisms, animals and plants alike. But animals have an additional problem -- they are more susceptible to extremes of temperature than are plants. Animals receive heat directly by radiation from the sun, and indirectly, by conduction from the substrate (rocks and soil) and convection from the air.

The biological processes of animal tissue can function only within a relatively narrow temperature range. When this range is exceeded, the animal dies. For four or five months of the year, the daily temperatures in the desert may actually exceed this range, called the range of thermoneutrality. Combined with the scarcity of life-sustaining water, survival for desert animals can become extremely tenuous.

Fortunately, most desert animals have evolved both behavioral and physiological mechanisms to solve the heat and water problems the desert environment creates. Among the thousands of desert animal species, there are almost as many remarkable behavioral and structural adaptations developed for avoiding excess heat.

Equally ingenious are the diverse mechanisms various animal species have developed to acquire, conserve, recycle, and actually manufacture water.

Avoiding Heat躲避高温

Behavioral techniques for avoiding excess heat are numerous among desert animals. Certain species of birds, such as the Phainopepla, a slim, glossy, black bird with a slender crest, breed during the relatively cool spring, then leave the desert for cooler areas at higher elevations or along the Pacific coast. The Costa's Hummingbird, a purple-crowned and purple-throated desert species, begins breeding in late winter, then leaves in late spring when temperatures become extreme. Many birds are active primarily at dawn and within a few hours of sunset, retiring to a cool, shady spot for the remainder of the day. Some birds, such as the kingbird, continue activity throughout the day, but always perch in the shade.

Many animals (especially mammals and reptiles) are crepuscular, that is, they are active only at dusk and again at dawn. For this reason, humans seldom encounter rattlesnakes and Gila Monsters. Many animals are completely nocturnal, restricting all their activities to the cooler temperatures of the night. Bats, many snakes, most rodents and some larger mammals like foxes and skunks, are nocturnal, sleeping in a cool den, cave or burrow by day.

Some smaller desert animals burrow below the surface of the soil or sand to escape the high temperatures at the desert surface. These include many mammals, reptiles, insects and all the desert amphibians. Rodents may plug the entrances to their burrows to keep out hot, desiccating air.

A few desert animals, such as the Round-tailed Ground Squirrel, a diurnal mammal, enter a state of estivation when the days become too hot and the vegetation too dry. They sleep away the hottest part of the summer. (They also hibernate in winter to avoid the cold season.)

Some desert animals such as Desert Toads, remain dormant deep in the ground until the summer rains fill ponds. They then emerge, breed, lay eggs and replenish their body reserves of food and water for another long period. Some arthropods, such as the fairy shrimps and brine shrimps, survive as eggs, hatching in saline ponds and playas during summer or winter rains, and completing their life cycles.

Certain desert lizards are active during the hottest seasons, but move extremely rapidly over hot surfaces, stopping in cooler "islands" of shade. Even their legs may be longer so they absorb less surface heat while running.

Dissipating Heat

Some animals dissipate heat absorbed from their surroundings by various mechanisms. Owls, Poorwills and nighthawks gape open-mouthed while rapidly fluttering their throat region to evaporate water from their mouth cavities. (Only animals with a good supply of water from prey can afford this type of cooling, however.) Many desert mammals have evolved long appendages to dissipate body heat into their environment. The enormous ears of jackrabbits, with their many blood vessels, release heat when the animal is resting in a cool, shady location. Their relatives in cooler regions have much shorter ears.

New World vultures, such as the Turkey and Black Vultures, are dark in color and thus absorb considerable heat in the desert. But they excrete urine on their legs, cooling them by evaporation, and circulate the cooled blood back through the body. This behavior, called urohydrosis, is shared with their relatives the storks, successful birds of the African deserts. Both vultures and storks may escape the hot midday temperatures of the desert by soaring effortlessly, high on thermals of cooler air.

Many desert animals are paler than their relatives elsewhere in more moderate environments. Pale colors may be seen in feathers, fur, scales or skin. Pale colors not only ensure that the animal takes in less heat from the environment, but help to make it less conspicuous to predators in the bright, pallid surroundings.

Retaining Water存储水分

The mechanisms some desert animals have evolved to retain water are even more elaborate. They range from simple to physiologically complex. Some retain water by burrowing into moist soil during the dry daylight hours (all desert toads). Some predatory and scavenging animals can obtain their entire moisture needs from the food they eat (e.g., Turkey Vulture) but still may drink when water is available. Reptiles and birds excrete metabolic wastes in the form of uric acid, an insoluble white compound, wasting very little water in the process. Mammals, however, excrete urea, a soluble compound that accounts for considerable water loss. Most mammals, therefore, need access to a good supply of fresh water, at least every few days, if not daily.

Acquiring Water寻找水分

Desert creatures derive water directly from plants, particularly succulent ones, such as cactus. Many species of insects thrive in the deserts this way. Some insects tap plant fluids such as nectar or sap from stems, while others extract water from the plant parts they eat, such as leaves and fruit. The abundance of insect life permits insectivorous birds, bats and lizards to thrive in the desert.

Some desert creatures utilize all of these physical and behavioral mechanism to survive the extremes of heat and dryness. Certain desert mammals, such as Kangaroo Rats, live in underground dens which they seal off to block out midday heat and to recycle the moisture from their own breathing.

These ingenious rodents (there are a number of species) also have specialized kidneys with extra microscopic tubules to extract most of the water from their urine and return it to the blood stream. And much of the moisture that would be exhaled in breathing is recaptured in the nasal cavities by specialized organs.

If that weren't enough, Kangaroo Rats, and some other desert rodents, actually manufacture their water metabolically from the digestion of dry seeds. These highly specialized desert mammals will not drink water even when it is given to them in captivity!

These are just a few examples of the ingenious variety of adaptations animals use to survey in the desert, overcoming the extremes of heat and the paucity of water.

简要比对中文信息：

沙漠中有鲜鱼活虾，这并非“天方夜谭”。

西部是片大沙漠，干旱少雨，水坑中即使有水，含酸量也极高，很少有生物能够生存。但是，就在这片沙漠里，却奇迹般生活着一种“蝾螈鱼”。原来，当水坑干涸时，“蝾螈鱼”全部钻进地下较湿的地方，待水坑里有水后，再出来活动，为了适应环境，它能用皮肤呼吸，并磨炼出了长期忍饥挨饿的超凡本领。

不仅有鱼还有虾。美国科学家在美国的一个大沙漠中发现了一种淡水虾，它的虾卵能在沙漠干燥的缝隙中存活20—50年。每逢沙漠降下特大暴雨(这种机会往往要几年甚至几十年才遇上一次)时，受雨水湿润的虾卵便迅速被孵化成小虾。此后，这些小虾还会四处寻找水塘、水洼作为栖身地，并快速生长。

人惊奇的是，一些水生生物脱离了原来水生的环境，将家安置在沙漠中。在澳洲有一种穴居的螃蟹，竟选择了干旱的大沙漠作为自己的栖身地，它的耐旱本领，连以耐旱著称的沙漠飞蝗也望尘莫及。为何它们要离开湖海——对它们来说那才是优越的生活环境呀，而迁居沙漠自讨苦吃?科学家分析，也许是由于长期沧桑之变，河湖干枯了，为了生存下去，蟹只好在险恶的环境中施展应变本领，于是慢慢适应了干旱的沙漠气候，成为“生活的强者”。

官方真题Official相应题目扩展阅读：

官方真题Official-40 Amphibian Thermoregulation

官方真题Official-26 Survival of Plants andAnimals in Desert Conditions

http://bbs.zhan.com/thread-54370-1-1.html

分学科托福单词：

http://bbs.zhan.com/thread-81784-1-1.html

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