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  3. For most species of animals, the number of individuals in the species is inversely proportional to the average body size for mem

For most species of animals, the number of individuals in the species is inversely proportional to the average body size for mem

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问题 For most species of animals, the number of individuals in the species is inversely proportional to the average body size for members of the species: the smaller the body size, the larger the number of individual animals. The tamarin, a small South American monkey, breaks this rule. Of the ten primate species studied in Peru’s Manu National Park, for example, the two species of tamarins, saddle-backed and emperor, are the eighth and ninth least abundant, respectively. Only the pygmy marmoset, which is even smaller, is less abundant. The tamarin’s scarcity is not easily explained; it cannot be dismissed as a consequence of diet, because tamarins feed on the same mixture of fruit, nectar, and small prey as do several of their more numerous larger counterparts, including the two capuchins known as the squirrel monkey and the night monkey. Although the relative proportions of fruits consumed varies somewhat among species, it is hard to imagine that such subtle differences are crucial to understanding the relative rarity of tamarins.
To emphasize just how anomalously rare tamarins are, we can compare them to the other omnivorous primates in the community. In terms of numbers of individuals per square kilometer, they rank well below the two capuchins, the squirrel monkey and the night monkey. And in terms of biomass, or the total weight of the individuals that occupy a unit area of habitat, each tamarin species is present at only one-twentieth the mass of brown capuchins or one-tenth that of squirrel monkeys. To gain another perspective, consider the spatial requirements of tamarins. Tamarins are rigidly territorial, vigorously expelling any intruders that may stray within the sharply defined boundaries of their domains. Groups invest an appreciable part of their time and energy in patrolling their territorial boundaries, announcing their presence to their neighbors with shrill, sweeping cries. Such concerted territoriality is rather exceptional among primates, though the gibbons and siamangs of Asia show it, as do a few other New World species such as the titi and night monkeys. What is most surprising about tamarin territories is their size.
Titi monkeys routinely live within territories of 6 to 8 hectares, and night monkeys seldom defend more than 10 hectares, but tamarin groups routinely occupy areas of 30 to 120 hectares. Contrast this with the 1 to 2 hectares needed by the common North American gray squirrel, a nonterritorial mammal of about the same size. A group of tamarins uses about as much space as a troop of brown capuchins, though the latter weighs 15 times as much. Thus, in addition to being rare, tamarins require an amount of space that seems completely out of proportion to their size.
The author regards the differences between the diets of the tamarins and several larger species as
选项 A、generally explicable in terms of territory size
B、apparently too small to explain the rarity of tamarins
C、wholly predictable on the basis of differences in body size
D、a result of the rigid territoriality of tamarins
E、a significant factor in determining behavioral differences
答案B
解析 Supporting Idea
This question depends on recognizing that the passage rejects the idea that any differences between the diets of tamarins and those of certain larger animals are large enough to explain tamarins’ relative rarity. The passage points out that these animals feed on the same fruits, nectar, and small prey, and claims that though the proportions of the fruits consumed varies somewhat, this variation is not sufficient to explain the tamarins rarity.
A The author does not seek to explain why these differences in diet—which the passage indicates are minimal—exist. Given that the author indicates that differences in territory size are large and differences in diet are small, it is unlikely, in any case, that the author would regard the former as explaining the latter.
B Correct. The passage indicates that the differences in diet among these animals are too small to explain the rarity of tamarins.
C The author does not give any indication that the differences in the diets of these animals are predictable based on differences in body size.
D The author does indicate that tamarins are rather unusual among primates in their rigid territoriality, but there is no indication that this rigid territoriality explains the small differences in diet among tamarins and certain larger animals.
E The author mentions differences in diet merely to rule out that these differences are large enough to explain tamarins’ rarity; these differences are not mentioned as a factor in determining behavioral differences.
The correct answer is B.
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