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Birds that are literally half-asleep—with one brain hemisphere alert and the other sleeping—control which side of the brain rema

admin2016-04-30  130
问题     Birds that are literally half-asleep—with one brain hemisphere alert and the other sleeping—control which side of the brain remains awake, 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 doping(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 some 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 lookout 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 _____.
选项 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
答案A
解析 第6段首句提到“新的研究成果为长期以来的一种设想提供了最好的证据，即半脑睡眠机制是动物为防范敌人进化而来的”，A与文章观点一致；B只是讲述了半脑睡眠是怎么回事，并没有告知原因；C文中没有提及：D与文中内容不符。
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