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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

admin2015-04-30  22
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 dozing 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.  
The example of a bird sleeping in front of a mirror indicates that_____.
选项 A、the phenomenon of birds dozing in pairs is widespread
B、birds prefer to sleep in pairs for the sake of security
C、even an imagined companion gives the bird a sense of security
D、a single pet bird enjoys seeing its own reflection in the mirror
答案C
解析 细节题。本题定位到第六段。该段首先提到鸟类单侧脑睡眠是在警惕敌人过程中进化而来，接着指出两只鸟一起打盹儿和单个宠物乌在镜子前睡觉时也是这样。当单个乌在镜子前睡觉时会将镜子中的影像当成一个同伴，所以靠近镜子那侧的眼睛会闭着，可见一个影像也会使鸟拥有安全感。故选C。
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