在职申硕英语考试

【单选题】Birds are literally half-asleep-with one brain hemisphere alert and the other sleeping, according to a new study of sleeping ducks.
Earlier studies have documented half-brain sleep in a wide range of birds. The brain hemispheres take turns sinking into the sleep stage characterized by slow brain waves. The eye controlled by the sleeping hemisphere keeps shut, while the wakeful hemisphere’s eye stays open and alert.Birds also can sleep with both hemispheres resting at once.
Decades of studies of bird flocks led researchers to predict extra alertness in the more vulnerable, end-of-the-row sleepers. Sure enough, the end birds tended to watch carefully on the side away from their companions.Ducks in the inner spots showed no preference for gaze direction.
Also, birds dozing (打盹) at the end of the line resorted to single-hemisphere sleep, rather than total relaxation, more often than inner ducks diD、Rotating 16 birds through the positions in a four-duck row, the researchers found outer birds half-asleep during 32 percent of dozing time versus about 12 percent for birds in internal spots.
"We believe this is the first evidence for an animal behaviorally controlling sleep and wakefulness simultaneously in different regions of the brain," the researchers say.
The results provide the best evidence for a long-standing supposition that single- hemisphere sleep evolved as creatures scanned for enemies. The preference for opening an eye on the lockout side could be widespread, he predicts. He’s seen it in a pair of birds’ dozing side-by-side in the zoo and in a single pet bird sleeping by a mirror. The mirror-side eye closed as if the reflection were a companion and the other eye stayed open.
Useful as half-sleeping might be, it’s only been found in birds and such water mammals(哺乳动物) as dolphins, whales, and seals. Perhaps keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning.
Studies of birds may offer Unique insights into sleep. Jerome M. Siegel of the UCLA、says he wonders if birds’ half-brain sleep "is just the tip of the iceberg (冰山) ". He speculates that more examples may turn up when we take a closer look at other species.
According to the passage, birds often half sleep because ______.A．they have to watch out for possible attacks
B．their brain hemispheres take turns to rest
C．the two halves of their brain are differently structured
D．they have to constantly keep an eye on their companions

网考网参考答案：A
网考网解析：
根据第六段，“这些(研究)结果为长久以来就存在的一个猜想提供了最好的证据：鸟的单侧脑睡眠的习惯是由于警惕敌人而进化来的。scan一词这里是指扫描周围的风吹草动，以便能及时发现敌人。A选项中“它们不能不时刻当心，警惕可能受到的攻击”与此同义。 document.getElementById("warp").style.display="none"; document.getElementById("content").style.display="block"; 查看试题解析出处>>