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.

单选题 A new study on birds' sleep has revealed that ______.

A、 half-brain sleep is found in all kinds of birds
B、 half-brain sleep is characterized by accelerated brain waves
C、 birds can control their half-brain sleep consciously
D、 birds always sleep with the whole of their brain at rest

【正确答案】 C

【答案解析】由第一段第一句可知，“半睡的鸟(一半大脑警醒，一半大脑睡眠)能够控制哪一半大脑保持清醒，这是根据对睡鸭的新研究发现的”。因此可以说，鸟可以有意识地控制它们的半脑睡眠。

单选题 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

【正确答案】 A

【答案解析】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。

单选题 The example of a bird sleeping in front of a mirror indicates that ______.

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

【正确答案】 C

【答案解析】由第六段得知，在镜子边的鸟会将靠镜子一侧的眼睛合上，似乎镜中反射出来的鸟是它的同伴一样，而另一只不靠镜子一侧的眼睛却睁开着。可见，即使镜中自己的影像，一个假想的伙伴，也能给鸟带来安全感。

单选题 While sleeping, some water mammals tend to keep half-awake in order to ______.

A、 alert themselves to the approaching enemy
B、 emerge from water now and then to breathe
C、 be sensitive to the ever-changing environment
D、 avoid being swept away by rapid currents

【正确答案】 B

【答案解析】由第七段可知，像海豚、鲸鱼和海豹这类的哺乳动物也经常要保持一侧大脑的清醒，这样这些睡眠的动物就可以偶尔浮出水面，以免溺水。也就是说，他们要偶尔浮出水面进行呼吸(emerge from water now and then to breathe)。

单选题 By "just the tip of the iceberg" (Par

A、 8), 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 is a phenomenon that could exist among other species

【正确答案】

【答案解析】短语“the tip of the iceberg”意思是“(事物的)表面部分；端倪”。该短语前常搭配 merely和just(只是)这样的词。这里的意思是可以以小见大，推而广之，便可知半脑睡眠也是存在于其他物种中的普遍现象。