Passage 6
During the Pleistocene epoch, several species of elephants isolated on islands underwent rapid dwarfing. This phenomenon was not necessarily confined to the Pleistocene, but may have occurred much earlier in the Southeastern Asian islands, although evidence is fragmentary. Several explanations are possible for this dwarfing. For example, islands often have not been colonized by large predators or are too small to hold viable predator populations. Once free from predation pressure, large body size is of little advantage to herbivores. Additionally, island habitats have limited food resources, a smaller body size and a need for fewer resources would thus be favored. Interestingly, the island rule is reversed for small mammals such as rodents, for which gigantism is favored under insular conditions.
1. The primary purpose of the passage is to
A. question the plausibility of one explanation sometimes offered for the dwarfing of certain species living on islands
B. argue that dwarfing of certain species living on islands occurred prior to the Pleistocene
C. cite evidence suggesting that dwarfing may have adverse consequences for some species living on islands
D. present some possible explanations for the dwarfing of certain species living on islands
E. contrast the effects of insular conditions on species with large body size and species with small body
2. According to the passage, which of the following statements about body size in mammals is true?
A. A large body is unfavorable to mammalian species’ survival under most conditions.
B. A large body tends to benefit small mammals living on islands.
C. For most herbivorous mammals, a large body size is easier to sustain in the absence of large predators.
D. Under most conditions, a small body is less beneficial to herbivorous mammals than to nonherbivorous mammals.
E. Among nonherbivorous mammals, a small body is more beneficial on an island than on a mainland.
答案：D B
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