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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 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 accompanied and the other eye stayed open.
Useful as half sleeping might be, it’s only been found in birds and such water mammals 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 speculates that more examples may turn up when we take a closer look at other species.
A new study on birds’ sleep has revealed that ________.
根据文章第一段和第三至六段内容可知，鸟类睡眠的最新研究表明，它们能够有意识地控制自己的半 脑睡眠。这几段举例说明了对这个问题实验的情况。选项B、C、D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，均不是正确答案。故A项为正确答案。
According to the passage, birds often half sleep because ________.
根据文章倒数第三段“The results provide the best evidence for a long-standing supposition that single hemisphere sleep evolved as creatures scanned for enemies.”可知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过程中进化形成的。故D项为正确答案。
The example of a bird sleeping in front of a mirror indicates that ________.
根据文章倒数第二段内容：鸟类单侧脑睡眠是在警惕敌人过程中进化而来，此结论还可推而广之。在 需要警惕的一侧，鸟儿喜欢睁着一只眼睛。一对动物园里的鸟儿挨着打瞌睡时是如此，宠物鸟靠镜子一侧 的眼睛闭着，好像镜子中的影子是一个伙伴，而另一只眼睛却睁着，也说明这种情况。故D项为正确答案。
While sleeping, some water mammals tend to keep half-awake in order to ________.
根据文章倒数第二段第二句“Perhaps keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning.”也许，保持一侧脑醒着可以使睡眠中的动物不时浮出水面来避免淹 死。故B项“emerge from water now and then to breathe”不时浮出水面来呼吸是正确答案。
By “just the tip of the iceberg” (Para. 8), Siegel suggests that ________.
根据文章最后的一段，也就是含有这个短语的上下文。对鸟类的研究可能提供对睡眠的独特认识。 他推测当我们对其他物种仔细观察时，就会发现更多的例证。因此D项所说“半侧脑睡眠这种现象可能存在 于其他物种”正是“just the tip of iceberg”所表达的意思。