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本题由lindy提供

The sloth bear, an insect-eating animal native to Nepal, exhibits only one behavior that is truly distinct from that of

other bear species: the females carry their cubs (at least part-time)until the cubs are about nine months old, even though the cubs can walk on their own at six months. Cub-carrying also occurs among some other myrmecophagous (ant-eating)mammals; therefore, one explanation is that cub-carrying is necessitated by myrmecophagy, since myrmecophagy entails a low metabolic rate and high energy expenditure in walkingbetween food patches. However, although polar bears’ locomotion is similarly inefficient, polar bear cubs walk along with their mother. Furthermore, the daily movements of slothbears and American black bears—which are similar in size to sloth bears and have similar-sized home ranges—reveal similar travel rates and distances, suggesting that if blackbear cubs are able to keep up with their mother, so too should sloth bear cubs.

An alternative explanation is defense from predation. Black bear cubs use trees for defense, whereas brown bears and polar bears, which regularly inhabit treeless environments, rely onaggression to protect their cubs. Like brown bears and polar bears (and unlike other myrmecophagous mammals, which are noted for their passivity), sloth bears are easilyprovoked to aggression. Sloth bears also have relatively large canine teeth, which appear to be more functional for fighting than for foraging. Like brown bears and polar bears, slothbears may have evolved in an environment with few trees. They are especially attracted to food rich grasslands; although few types of grassland persist today on the Indiansubcontinent, this type of habitat was once widespread there. Grasslands support high densities of tigers, which fight and sometimes kill sloth bears; sloth bears also coexist withand have been killed by tree-climbing leopards, and are often confronted and chased by rhinoceroses and elephants, which can topple trees. Collectively these factors probablyselected against tree climbing as a defensive strategy for sloth bear cubs. Because sloth bears are smaller than brown and polar bears and are under greater threat from dangerousanimals, they may have adopted the extra precaution of carrying their cubs. Although cub carrying may also be adaptive for myrmecophagous foraging, the behavior of sloth bear cubs,which on their mother's back at the first sign of danger, suggests that predation was a key stimulus.

The author mentions rhinoceroses and elephants (see highlighted text) primarily

in order to

    A. explain why sloth bears are not successful foragers in grassland habitats
    B. identify the predators that have had the most influence on the behavior of sloth bears
    C. suggest a possible reason that sloth bear cubs do not use tree-climbing as a defense
    D. provide examples of predators that were once widespread across the Indian subcontinent
    E. defend the assertion that sloth bears are under greater threat from dangerous animals than are other bear species

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答案:
C
第一结构:
The sloth bear... (1)Cub-carrying also occurs... (2)However, although polar... (3)Furthermore, the daily movements of sloth bears and American black bears... 第二结构:An alternative explanation is defense from predation. (1)Black bear cubs use trees... (2) Sloth bears also have relatively large canine... (3) Like brown bears and polar bears, sloth bears may have evolved in ... (4) Although cub-carrying may also be adoptive... The author mentions rhinoceroses and elephants (highlight) primarily in order to(一般细节题,第2结构第3层次) A. explain why sloth bears are not successful foragers in grassland habitats(层次未提及) B. identify the predators that have had the most influence on the behavior of sloth bears(绝对词) C. suggest a possible reason that sloth bear cubs do not use tree-climbing as a defense(rhinoceroses and elephants 是捕食sloth bears的一个例子,这些动物会撬树,所以对于本身栖息地树就不多,而且有那么多会撬树的捕食者在,躲树上捕食sloth bear的有效选择,所以带着幼崽走) D. provide examples of predators that were once widespread across the Indian subcontinent(与题目逻辑方向不符,题目问的是主要目的,这些动物存在与这个环境并不是主要目的,而是有这些存在,导致爬树上的保护措施对于sloth bear来说是没啥用的, 而且once windspread,现在也很多这些捕食者) E. defend the assertion that sloth bears are under greater threat from dangerous animals than are other bear species(无关比较)

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