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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.
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, 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.
9. The phrase provided that in 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
参考译文：
野牛，斑马，角马，转角牛羚和汤氏羚这些群居动物占据了非洲东部塞伦盖蒂平原的总哺乳动物的数量的90%。它们都是草食动物(以吃植物为生的动物)，并且看似有着相同的日常饮食：草，香草，和小的灌木。不过，这个现象是假的。在生物学家Richard Bell和他的同僚分析5种物种其中的4个(他们没有研究野牛)的胃内含量时，他们发现其实每个物种所食用的植物部位是不同的。这些不一样的植物部分是区分于它们的食物质量：下部的是多汁又营养的树叶;上面的部分则是更坚硬的茎杆。Bell还在汤氏羚的胃里发现了一些分布稀少的高营养含量的水果，不过只有汤氏羚吃这些。其他三个物种是因为所食用的低树叶和高的茎杆的比例不同而区别的：斑马主要吃茎杆部分，角马主要吃树叶，转角牛羚则一半一半。
那么我们怎样来理解他们这些不同的食物选择呢?答案就在所有物种的两个相互关联的差异：他们的消化系统和体型大小。这些草食动物可根据他们的消化系统而分为两类：非反刍动物(比如说有着和马类似消化系统的斑马)和反刍动物(比如角马，转角牛羚，和小羚羊，他们的则和奶牛的相似)。非反刍动物并不能够从植物的坚硬部分提取出很多能量;不管怎样，能有这些能量已经不错了，因为这些是相对于食物是以一个非常快的速度进入肠胃的情况产生的。因此，当只有供应不足的质量低劣的食物时，角马，转角牛羚和小羚羊享有了优势。因为他们是反刍动物，而反刍动物的胃部含有能够分解食物坚硬部分的微生物的特殊结构(瘤胃)。食物只很慢的在反刍动物的肠胃里传递，因为反刍的过程—消化坚硬的部分—需要一定时间。反刍动物不断地将胃里的食物返回嘴里继续咀嚼(这就是奶牛在“反刍”时所做的)。只有当食物在经过咀嚼和消化的过程变成近似液体的时候，它才可能通过瘤胃并进入和通过肠胃。比较大的颗粒在被咀嚼成小块之前，是不能通过的。所以，当食物供不应求时，一个反刍动物可以比一个非反刍动物活的时间更长，因为它能从同样的食物中提取到更多的能量。这个差异部分的解释了塞伦盖蒂草食动物的饮食习惯。斑马选择的是有更多低质量食物的区域。它首先迁移到未被开垦的区域，并在继续迁移前，食用掉当地充足的低质量食物。斑马是一个新陈代谢很快的进食者，这一结论依据于它们的大量的排泄物都是那些没有被完全消化的食物。当角马(或其他反刍动物)到来时，斑马的牧草和踩踏已经把当地的植被进行耗损筛选了。所以当这些反刍动物开始行动时，它们吃的是植物较矮的叶状的部分。所有这些答案都符合了我们最开始提到的胃含量的差异。
另一方面的解释则是体型的大小。体型较大的动物相对于较小的需要更多的食物，而小型动物具有更高的代谢率。所以更小的动物可以居住在有少量食物的地方，如果这种食物是具有高能量的话。这就是为什么，具有最小体型的汤氏羚，可以以水果这样一个很有营养，但是对于支撑大型动物来说过于单薄的食物生存下去。相反，大斑马是居住在具有大量低质量茎杆的地方。
依次下来，食物选择的差异进而造成了迁移习性的不同。角马的迁移遵循的是当地的降雨类型。其他物种的做法也与其相似。但当一个新的地点被发现降水量充足时，哺乳动物以一定的先后顺序向此地迁徙的。较大的，不那么挑剔的进食者斑马最先移入;比较挑剔的稍小的角马第二个;汤氏羚，作为这些当中最小的物种，则是最后。就像斑马给角马，转角牛羚和汤氏羚的食物进行了筛选一样，后进来的物种是要依赖于前面物种给它们所做的准备的。
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