Birds that are half-asleep — with one brain hemisphere (半球) alert(警觉) and the other sleepi

Birds that are half-asleep — with one brain hemisphere (半球) alert(警觉) and the other sleeping — control which side of the brain remains awake, according to a new study of sleeping ducks.
Earlier studies have recorded 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 (打盹) 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 inner 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 developed over time 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 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.
【小题1】According to the passage, birds often half sleep because .A．they have to watch out for possible attacksB．their brain hemispheres take turns to restC．the two halves of their brain are differently organizedD．they have to constantly keep an eye on their companions【小题2】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.【小题3】It can be inferred that .A．Birds never sleep with both hemispheres resting at once.B．Inner ducks depend on single-hemisphere sleep, rather than total relaxation, as often as birds napping at the end of the line doC．It is not the first time for scientists to research on birds’ half-brain sleepD．Birds hardly sleep without a companion【小题4】By saying “just the tip of the iceberg”, Siegel suggests that .A．half-brain sleep has something to do with icy weatherB．the mystery of half-brain sleep is close to being solvedC．most birds living in cold regions tend to be half sleepersD．half-brain sleep may exist among other speciesA

(答案→)A 解析：【小题1】细节理解题。结合single-hemisphere sleep developed over time as creatures scanned for enemies（半睡眠是动物为防范敌人进化而来的）可知答案。【小题2】推理判断题。结合第六段后两句，即鸟在镜子前睡觉的例子是对本段前两句的进一步佐证。The preference for opening an eye on the lookout side... 是为了对敌人的进攻保持警觉，而The mirror-side eye closed是因为as if the reflection were a companion(好像镜中的影像是它的同伴一样) 为鸟提供安全感。【小题3】推理判断题。结合Earlier studies have recorded half-brain sleep in a wide range of birds可知答案。【小题4】推理判断题。根据最后一段可知“对鸟类的研究或许能使人们以独特的视角了解睡眠。M. Siegel说，他认为鸟类的半睡眠‘只是冰山浮出水面的那一角’，并推断当我们更深入地研究其他物种时还会发现更多类似的例子。”故D项为正确答案。

谢谢了