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  FEEDING HABITS OF EAST AFRICAN HERBIVORES

  Buffalo, zebras, wildebeests, topi, and Thomson’s gazelles live in huge groups that together make up some 90 percent of the total weight of mammals living on the Serengeti Plain of East Africa. They are all herbivores (plant-eating animals), and they all appear to be living on the same diet of grasses, herbs, and small bushes. This appearance, however, is illusory. When biologist Richard Bell and his colleagues analyzed the stomach contents of four of the five species (they did not study buffalo), they found that each species was living on a different part of the vegetation. The different vegetational parts differ in their food qualities: lower down, there are succulent, nutritious leaves; higher up are the harder stems. There are also sparsely distributed, highly nutritious fruits, and Bell found that only the Thomson’s gazelles eat much of these. The other three species differ in the proportion of lower leaves and higher stems that they eat: zebras eat the most stem matter, wildebeests eat the most leaves, and topi are intermediate.

  How are we to understand their different feeding preferences? The answer lies in two associated differences among the species, in their digestive systems and body sizes. According to their digestive systems, these herbivores can be divided into two categories: the nonruminants (such as the zebra, which has a digestive system like a horse) and the ruminants (such as the wildebeest, topi, and gazelle, which are like the cow). Nonruminants cannot extract much energy from the hard parts of a plant; however, this is more than made up for by the fast speed at which food passes through their guts. Thus, when there is only a short supply of poor-quality food, the wildebeest, topi, and gazelle enjoy an advantage. They are ruminants and have a special structure (the rumen) in their stomachs, which contains microorganisms that can break down the hard parts of plants. Food passes only slowly through the ruminant’s gut because ruminating—digesting the hard parts—takes time. The ruminant continually regurgitates food from its stomach back to its mouth to chew it up further (that is what a cow is doing when “chewing cud”). Only when it has been chewed up and digested almost to a liquid can the food pass through the rumen and on through the gut. Larger particles cannot pass through until they have been chewed down to size. Therefore, when food is in short supply, a ruminant can last longer than a nonruminant because it can derive more energy out of the same food. The difference can partially explain the eating habits of the Serengeti herbivores. The zebra chooses areas where there is more low-quality food. It migrates first to unexploited areas and chomps the abundant low-quality stems before moving on. It is a fast-in/fast-out feeder, relying on a high output of incompletely digested food. By the time the wildebeests (and other ruminants) arrive, the grazing and trampling of the zebras will have worn the vegetation down. As the ruminants then set to work, they eat down to the lower, leafier parts of the vegetation. All of this fits in with the differences in stomach contents with which we began.

  The other part of the explanation is body size. Larger animals require more food than smaller animals, but smaller animals have a higher metabolic rate. Smaller animals can therefore live where there is less food, provided that such food is of high energy content. That is why the smallest of the herbivores, Thomson’s gazelle, lives on fruit that is very nutritious but too thin on the ground to support a larger animal. By contrast, the large zebra lives on the masses of low-quality stem material.

  The differences in feeding preferences lead, in turn, to differences in migratory habits. The wildebeests follow, in their migration, the pattern of local rainfall. The other species do likewise. But when a new area is fueled by rain, the mammals migrate toward it in a set order to exploit it. The larger, less fastidious feeders, the zebras, move in first; the choosier, smaller wildebeests come later; and the smallest species of all, Thomson’s gazelle, arrives last. The later species all depend on the preparations of the earlier one, for the actions of the zebra alter the vegetation to suit the stomachs of the wildebeest, topi, and gazelle.

  Paragraph 1: Buffalo, zebras, wildebeests, topi, and Thomson’s gazelles live in huge groups that together make up some 90 percent of the total weight of mammals living on the Serengeti Plain of East Africa. They are all herbivores (plant-eating animals), and they all appear to be living on the same diet of grasses, herbs, and small bushes. This appearance, however, is illusory. When biologist Richard Bell and his colleagues analyzed the stomach contents of four of the five species (they did not study buffalo), they found that each species was living on a different part of the vegetation. The different vegetational parts differ in their food qualities: lower down, there are succulent, nutritious leaves; higher up are the harder stems. There are also sparsely distributed, highly nutritious fruits, and Bell found that only the Thomson’s gazelles eat much of these. The other three species differ in the proportion of lower leaves and higher stems that they eat: zebras eat the most stem matter, wildebeests eat the most leaves, and topi are intermediate.

  1.The word illusory in the passage is closest in meaning to

  ○definite

  ○illuminating

  ○misleading

  ○exceptional

  2.The word sparsely in the passage is closest in meaning to

  ○widely

  ○thinly

  ○clearly

  ○freshly

  3.Which of the following questions about Richard Bell’s research is NOT answered in paragraph 1?

  ○Which of the herbivores studied is the only one to eat much fruit?

  ○Which part of the plants do wildebeests prefer to eat?

  ○Where did the study of herbivores’ eating habits take place? ○Why were buffalo excluded from the research study?

  Paragraph 2: How are we to understand their different feeding preferences? The answer lies in two associated differences among the species, in their digestive systems and body sizes. According to their digestive systems, these herbivores can be divided into two categories: the nonruminants (such as the zebra, which has a digestive system like a horse) and the ruminants (such as the wildebeest, topi, and gazelle, which are like the cow). Nonruminants cannot extract much energy from the hard parts of a plant; however, this is more than made up for by the fast speed at which food passes through their guts. Thus, when there is only a short supply of poor-quality food, the wildebeest, topi, and gazelle enjoy an advantage. They are ruminants and have a special structure (the rumen) in their stomachs, which contains microorganisms that can break down the hard parts of plants. Food passes only slowly through the ruminant’s gut because ruminating—digesting the hard parts—takes time. The ruminant continually regurgitates food from its stomach back to its mouth to chew it up further (that is what a cow is doing when “chewing cud”). Only when it has been chewed up and digested almost to a liquid can the food pass through the rumen and on through the gut. Larger particles cannot pass through until they have been chewed down to size. Therefore, when food is in short supply, a ruminant can last longer than a nonruminant because it can derive more energy out of the same food. The difference can partially explain the eating habits of the Serengeti herbivores. The zebra chooses areas where there is more low-quality food. It migrates first to unexploited areas and chomps the abundant low-quality stems before moving on. It is a fast-in/fast-out feeder, relying on a high output of incompletely digested food. By the time the wildebeests (and other ruminants) arrive, the grazing and trampling of the zebras will have worn the vegetation down. As the ruminants then set to work, they eat down to the lower, leafier parts of the vegetation. All of this fits in with the differences in stomach contents with which we began.

  4.The word associated in the passage is closest in meaning to

  ○obvious

  ○significant

  ○expected

  ○connected

  5.The author mentions the cow and the horse in paragraph 2 in order to

  ○distinguish the functioning of their digestive systems from those of East African animals

  ○emphasize that their relatively large body size leads them to have feeding practices similar to those of East African mammals

  ○illustrate differences between ruminants and nonruminants through the use of animals likely to be familiar to most readers

  ○emphasize similarities between the diets of cows and horses and the diets of East African mammals

  6.According to paragraph 2, which of the following herbivores has to eat large quantities of plant stems because it gains relatively little energy from each given quantity of this food?

  ○The gazelle

  ○The wildebeest

  ○ The zebra

  ○ The topi

  7.Paragraph 2 suggests that which of the following is one of the most important factors in determining differences in feeding preferences of East African herbivores?

  ○The availability of certain foods

  ○The differences in stomach structure

  ○The physical nature of vegetation in the environment

  ○The ability to migrate when food supplies are low

  8.According to paragraph 2, all of the following are true of East African gazelles EXCEPT:

  ○They digest their food very quickly.

  ○Microorganisms help them digest their food.

  ○They are unable to digest large food particles unless these are chewed down considerably.

  ○They survive well even if food supplies are not abundant.

  provided that

  Paragraph 3: The other part of the explanation is body size. Larger animals require more food than smaller animals, but smaller animals have a higher metabolic rate. Smaller animals can therefore live where there is less food,such food is of high energy content. That is why the smallest of the herbivores, Thomson’s gazelle, lives on fruit that is very nutritious but too thin on the ground to support a larger animal. By contrast, the large zebra lives on the masses of low-quality stem material.

  provided that

  9.The phrasein the passage is closest in meaning to

  ○as long as

  ○unless

  ○as if

  ○even though

  Paragraph 4: The differences in feeding preferences lead, in turn, to differences in migratory habits. The wildebeests follow, in their migration, the pattern of local rainfall. The other species do likewise. But when a new area is fueled by rain, the mammals migrate toward it in a set order to exploit it. The larger, less fastidious feeders, the zebras, move in first; the choosier, smaller wildebeests come later; and the smallest species of all, Thomson’s gazelle, arrives last. The later species all depend on the preparations of the earlier one, for the actions of the zebra alter the vegetation to suit the stomachs of the wildebeest, topi, and gazelle.

  10.The word fastidious in the passage is closest in meaning to

  ○rapid

  ○determined

  ○flexible

  ○demanding

  11.According to paragraph 4, which of the following mammals exhibits a feeding behavior that is beneficial to the other herbivores that share the same habitat?

  ○Topi

  ○Zebra

  ○Wildebeest

  ○Gazelle

  12.According to the passage, which of the following is true of wildebeests?

  ○They eat more stem matter than zebras do.

  ○They are able to digest large food particles if the food is of a high quality.

  ○They tend to choose feeding areas in which the vegetation has been worn down.

  ○They are likely to choose low-quality food to eat in periods when the quantity of rainfall is low.

  Paragraph 4: The differences in feeding preferences lead, in turn, to differences in migratory habits.■ The wildebeests follow, in their migration, the pattern of local rainfall. ■ The other species do likewise. ■ But when a new area is fueled by rain, the mammals migrate toward it in a set order to exploit it. ■ The larger, less fastidious feeders, the zebras, move in first; the choosier, smaller wildebeests come later; and the smallest species of all, Thomson’s gazelle, arrives last. The later species all depend on the preparations of the earlier one, for the actions of the zebra alter the vegetation to suit the stomachs of the wildebeest, topi, and gazelle.

  13.Look at the four squares [■] that indicate where the following sentence could be added to the passage.

  The sequence in which they migrate correlates with their body size. Where would the sentence best fit?

  14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

  East African herbivores, though they all live in the same environment, have a range of feeding preferences.

  Answer Choices

  1.The survival of East African mammals depends more than anything else on the quantity of highly nutritious fruits that they are able to find.

  2.A herbivore’s size and metabolic rate affect the kinds of food and the quantities of food it needs to eat.

  3.Zebras and wildebeests rarely compete for the same food resources in the same locations.

  4.The different digestive systems of herbivores explain their feeding preferences.

  5.Migratory habits are influenced by feeding preferences.

  6.Patterns in the migratory habits of East African herbivores are hard to establish.

  参考答案

  1.○3

  2.○2

  3.○4

  4.○4

  5.○3

  6.○3

  7.○2

  8.○1

  9.○1

  10.○4

  11.○2

  12.○3

  13.○4

  14.○245

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