单选题 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 (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.

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

【正确答案】 C

【答案解析】[解析] 细节题。利用定位关键词可以定位到第一段“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”，即“根据一项对睡眠中的鸭子的新研究，真正半睡的鸟(即一个脑半球警觉，另一个脑半球睡眠)能控制哪一侧大脑保持清醒”，由此可知答案为选项C(禽类能有意识地控制它们半脑的睡眠)；选项A(许多禽类被发现都有半脑睡眠的情况)、B(半脑睡眠的特征就是缓慢的脑波)没有提到，D(禽类在睡眠时很少整个大脑都处于休息状态)虽在文章第二段中也提到了，但那是文献记载的以前研究的情况(Earlier studies have documented...)，故均不是正确答案。

单选题 According to the passage, birds often half sleep because ______.

【正确答案】 A

【答案解析】[解析] 细节题。本题无明显关键词，可以尝试利用选项进行答题。利用选项A的关键词可以定位到第三段The results provide the best evidence for along standing supposition that single hemisphere sleep evolved as creatures scanned for enemies，即“这些研究结果为一个长久以来的假设提供了最好的证据，即单侧脑睡眠是在生物警惕敌人的过程中进化形成的”，原文中的“scanned for enemies”与选项A中的“watch out for possible attack”为同义改写；选项B(两个脑半球轮流休息)不能解释半脑睡眠的原因；利用选项C(两个半脑的结构不同)的关键词定位，原文没有相关内容；利用选项D的关键词可以定位到第二段“the end birds tended to watch carefully on the side away from their companions”，即“在末尾的禽类往往小心翼翼地看没有同伴的那一侧”，可知D(禽类一直盯着它的同伴)与原文信息不符。因而，该题的正确答案为A。

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

【正确答案】 C

【答案解析】[解析] 细节题。利用定位关键词可以定位到第三段“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”，即“他看到过这种现象，公园里一对紧挨着打盹的鸟是这样，在镜子旁睡觉的单个宠物鸟也是如此。靠镜子一侧的眼睛闭着，好像镜子里的映像是伙伴，而另一只眼睛却睁着”，由此可知选项C(假想的同伴也能给鸟儿安全感)是正确答案；其他选项A(禽类打盹的现象是普遍的)、B(为了安全禽类喜欢结伴而睡)、D(一只孤单的鸟喜欢看镜子里自己的镜像)都与原文信息不符。

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

【正确答案】 B

【答案解析】[解析] 细节题。利用定位关键词可以定位到第四段“Perhaps keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning”。即“也许，保持一侧脑醒着可以使睡眠中的动物不时浮出水面而避免淹死”，由此可知选项B(不时浮出水面来呼吸)是正确答案，其他选项A(警惕靠近的敌人)、C(保持对环境变化的敏感性)、D(避免被激流冲走)都与原文信息不一致。