GWD 6-10,11

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 walking between food patches.  However, although polar bears’ locomotion is similarly inefficient, polar bear cubs walk along with their mother.  Furthermore, the daily movements of sloth bears 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 black bear 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 on aggression to protect their cubs.  Like brown bears and polar bears (and unlike other myrmecophagous mammals, which are noted for their passivity), sloth bears are easily provoked 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, sloth bears may have evolved in an environment with few trees.  They are especially attracted to food-rich grasslands; although few grasslands persist today on the Indian subcontinent, 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 with and have been killed by tree-climbing leopards, and are often confronted and chased by rhinoceroses and elephants, which can topple trees. Collectively these factors probably selected 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 dangerous animals, they may have adopted the extra precaution of carrying their cubs.  Although cub-carrying may also be adoptive for myrmecophagous foraging, the behavior of sloth bear cubs, which climb on their mother’s back at the first sign of danger, suggests that predation was a key stimulus.(67)

Q10:The author mentions rhinoceroses and elephants (lines 52-53) primarily in order to

About Q 11, see ankeke mm's explaination in the following link. I think she explained the answer very well.

http://forum.chasedream.com/dispbbs.asp?BoardID=25&ID=84287

line 18--26: Furthermore, the daily movements of sloth bears 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 black bear cubs are able to keep up with their mother, so too should sloth bear cubs.

choice E: The habitat of black bears consists of terrain that is significantly more varied than that of the habitat of sloth bears.

Don't equal home ranges to terrain. That's the key point of ruling out choice E.

这道题是削弱题，我认为D,E都作为他因可以用来削弱。但在这篇文章里，只可以选E。因为D作为削弱的可能性已经在 L16-19否定掉了。

However, although polar bears’ locomotion is similarly inefficient, polar bear cubs walk along with their mother.

也就是说，是否inefficient不能作为熊妈妈背熊宝宝的依据。

这跟terrain和home ranges是否一致没关系。不一样，可以是好的他因削弱。如果一致，反而不能削弱。因为假设A说C对，B说C不对，这样能说B是A的削弱吗？

我觉得E不是削弱了作者的观点，而是加强，所以不可能是答案。The habitat of black bears consists of terrain that is significantly more varied than that of the habitat of sloth bears，意思是黑熊的居住区域比sloth bear的居住区域有更多变化，居住区域有更多变化大概意思是有一些小山、河流什么的，使得这个区域不象平地那样毫无变化（这是我对terrian is more varied的理解）。地域多变化自然就难走，如果小黑熊在地域变化较大的地方都可以走，那么sloth bear cubs就更没有理由在地域变化较小的地方还需要妈妈背着。所以这支持了作者的locomotion不是决定sloth bear cubs不独立行走的因素的观点，而不是削弱。

欢迎指教。

shuodehao