Birds that are half-asleep—with one brain hemisphere（半球）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 groups led researchers to predict extra alertness in the end-of-the-row sleepers which tend to be attacked more easily．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（注视）direction．
Also, birds napping at the end of the line depend on single-hemisphere sleep, rather than total relaxation, more often than inner ducks did．Turning 16 birds through the positions in a four-duck row, the researchers found that compared with 12 percent for birds in internal spots, outer birds half-asleep during some 32 percent of napping time．
“We believe this is the first evidence for an animal behaviorally controlling sleep and wakefulness at the same time in different regions of the brain,” the researchers say．
The results provide the best evidence for a long-standing assumption that single-hemisphere sleep evolved（发展）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 napping 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 animals 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（深刻的理解）into sleep．Jerome M．Siegel of the UCLA says he wonders if birds’ half-brain sleep “is just the tip of the iceberg．” He supposes 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

What is implied about the example of a bird’s sleeping in front of a mirror?

A．An imagined companion gives the bird a sense of security．

B．Birds prefer to sleep in pairs for the sake of their security．

C．The phenomenon of birds napping in pairs is widespread．

D．A single pet bird enjoys seeing its own reflection in the mirror．

While sleeping, some water animals tend to keep half awake in order to ______．

A．alert themselves to the approaching enemy

B．emerge（浮现）from water now and then to breathe

C．be sensitive to the ever-changing environment

D．avoid being swept away by rapid currents

By saying “just the tip of the iceberg”, Siegel suggests that ______．

A．half-brain sleep has something to do with icy weather

B．the mystery of half-brain sleep is close to being solved

C．most birds living in cold regions tend to be half sleepers

D．half-brain sleep may exist among other species

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