【正确答案】
B
【答案解析】[听力原文] 31-40
Welcome to Magic Animal Time. Today I'd like to give you an introduction to a magic animal called sloth. Native to Central and South American rain forest, the sloth is the slowest animal Q31 on the earth. It needs 12 seconds to finish one step even slower than tortoise. Algae grows on its brown furry back because its inactive lifestyle. Even the fur of sloths is adapted to their lifestyle. Sloths usually hang upside down so their stiff, wiry fur grows differently from the coats of most mammals. Horses' hair, for example, grows from the back, so rainwater will run off. Below this tough top layer, a dense downy layer of hair provides additional protection from pesky insects. The greenish cast of the sloth's shaggy fur is caused by symbiotic algae that grow in grooves running along the length of each of the mammal's outer hairs. The sloth hosts the algae, and the algae return the favour by providing nutrition Q32 that the sloth either ingests by licking its fur or absorbs through its skin. The grayish green colour makes the sloth look just like a clump of dry leaves hanging from a branch—the perfect jungle camouflage. And the longer a sloth lives, the greener it gets.
Sloths have a short and flat head, a snout that is short in length, big round eyes, long tree-climbing legs, a stubby Q33 tail and small ears and though some have called them ugly, they have the cutest face that looks like they are always smiling just like koala in Australia.
Sloths spend most of the day sleeping. Q34 Curling up in a tree notch or hanging from a branch with all four legs closed together, their heads tuck between their front legs. People seldom see them much move even when they are awake. Only at feeding time, sloths move slowly and carefully, hand over and through the treetops, searching for leaves, fruits, and twigs. They even mate and give birth while suspended from their long, curved claws. Sometimes a sloth will die hanging upside down from a tree limb and remain so after death, until they decompose or are forcibly removed.
Why do sloths have such an unusual lifestyle? It's the strategy they evolved to survive as tree- dwelling plant eaters in a place with many predators.
Most domestic herbivores, such as cattle, horse, and sheep, graze all day long. Because they feed on nutrient poor vegetation, they must eat almost constantly. Sloths are also classified as herbivores, Q35 and their diets are also low in nutrition; but rather than munching all day long, they have become masters at conserving energy.
As any athlete knows, maintaining muscle requires large quantities of food energy—far more than a sloth's vegetarian diet can provide. Sloths are relatively inactive and usually spend most of their day hanging upside down, so they can get by with haft as much muscle mass as similar sized mammals. Hence, they don't need to eat too much. In addition, because of heavy muscle sloths weigh far less than other mammals. This makes it possible for them to climb on thin branches high in the tropical forest canopy, where they can more easily find food and avoid heftier predators.
Keeping a high body temperature also takes energy. Sloths fall this need by maintaining a lower average body temperature than other mammals. Dogs, cats, horses, sheep, and cows all have average body temperatures between about 100 and 103°F, but a sloth's average body temperature is about 93°F. Q36
Equally important, a sloth's body temperature fluctuates with the surrounding temperature. Q37 Like snakes and frogs which are "cold-blooded" creatures, a sloth's body temperature is highest on warm, sunny days and lowest at night and on rainy days. Sometimes a sloth takes a sunbath in the morning to warm up. Then, during the hottest time of the day, it hides in the shade so it won't overheat. During a 24-hour period, a sloth's body temperature may vary as much as 10°F. If the body temperature of a person, a cat, or a dog varies just 5°F, it can be life threatening.
Because sloths have difficulty moving over land, they spend most of their time in the trees. They can get just about everything they need high above the forest floor—even water, Q38 which comes from eating juicy leaves and licking up drops of morning dew. Short bursts of feeding followed by long periods of inactivity make sloths less vulnerable to large raptors such as harpy eagles and other carnivores such as ocelots. As long as sloths remain perfectly quiet and still, they're nearly impossible to detect.
At feeding time, a sloth reaches out, grabs an overhead branch with its flexible feet, and tugs until the food is within reach of its long tongue. After pulling the vegetation into its mouth, the sloth clips the leaves with its hard, tough lips and slowly grinds them with large, peg-like teeth. A sloth digests its food just as it does everything else—very slowly. People usually digest their food in about a day, but sloths can take about a month Q39 to finish the process. As soon as a sloth swallows a mouthful of pulverized plants, the bacteria Q40 in its stomach and intestines begin breaking down the food. It takes many hours for the bacteria to digest the tough plant material that makes up a sloth's diet. Only then can the sloth absorb the nutrients it needs to live and grow. So do you have a new understanding about the slow but not lazy animal? Next week we will talk about... (fade)
定位词是slowest，考的是哪种动物最慢?