gwd-5-12 小树懒提问

       animal native to Nepal, exhibits only one

       behavior that is truly distinct from that of

Line       other bear species:  the females carry

  (5)      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

 (10)      (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

 (15)      walking between food patches.  How-

ever, although polar bears’ locomotion

is similarly inefficient, polar bear cubs

walk along with their mother.  Further-

more, the daily movements of sloth

 (20)      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

 (25)      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

 (30)      and polar bears, which regularly inhabit

treeless environments, rely on aggres-

sion to protect their cubs.  Like brown

       bears and polar bears (and unlike other

myrmecophagous mammals, which are

 (35)
                           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.

 (40)     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

 (45)      persist today on the Indian subcontinent,

this type of habitat was once wide-

spread there.  Grasslands support

high densities of tigers, which fight and

sometimes kill sloth bears; sloth bears

 (50)      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

 (55)      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,

 (60)      they may have adopted the extra pre-

caution of carrying their cubs.  Although

cub-carrying may also be adoptive for

myrmecophagous foraging, the behavior

of sloth bear cubs, which climb on their

 (65)      mother’s back at the first sign of danger,

suggests that predation was a key

stimulus.

1. They tend to become aggressive when provoked.
2. They live almost exclusively in treeless environments.
3. They are preyed upon by animals that can climb or topple trees.
4. They are inefficient in their locomotion.
5. They have relatively large canine teeth.

标准答案是A，出处是红色的部分。但是从兰色部分不能推出B选项吗？