Birds that are half-asleep-with one brain hemispherealert and the other sleeping

Birds thatare half-asleep—with one brain hemisphere（半球）alert and the other sleeping—controlwhich side of the brain remains awake, according to a new study of sleepingducks．

   Earlier studies have documented half-brain sleep in a wide range of birds．The brain hemispheres take turns sinkinginto the sleep stage characterized by slow brain waves．The eye controlled by the sleepinghemisphere keeps shut, while the wakeful hemisphere’s eye stays open and alert．Birds also can sleep with bothhemispheres resting at once．

   Decades of studies of bird groups led researchers to predict extra alertness inthe end-of-the-row sleepers which tend to be attacked more easily．Sure enough, the end birds tended towatch carefully on the side away from their companions．Ducks in the inner spots showed nopreference 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 positionsin a four-duck row, the researchers found that compared with 12 percent forbirds in internal spots, outer birds half-asleep during some 32 percent ofnapping time．

   “We believe this is the first evidence for an animal behaviorally controllingsleep and wakefulness at the same time in different regions of the brain,” theresearchers say．

   The results provide the best evidence for a long-standing assumption that single-hemispheresleep evolved（发展）ascreatures scanned for enemies．Thepreference for opening an eye on the lookout side could be widespread, hepredicts．He’s seenit in a pair of birds napping side-by-side in the zoo and in a single pet birdsleeping by a mirror．Themirror-side eye closed as if the reflection were a companion and the other eyestayed open．

   Useful as half-sleeping might be, it’s only been found in birds and such wateranimals as dolphins, whales, and seals．Perhaps keeping one side of the brain awake allows a sleepinganimal 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 ifbirds’ half-brain sleep “is just the tip of the iceberg．” He supposes that more examples mayturn up when we take a closer look at other species．

1.Accordingto the passage, birds often half sleep because ______．

       A．they have to watch out for possibleattacks

       B．their brain hemispheres take turns torest

       C．the two halves of their brain aredifferently structured

       D．they have to constantly keep an eye ontheir companions

2.What isimplied about the example of a bird’s sleeping in front of a mirror?

       A．An imagined companion gives the bird asense of security．

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

       C．The phenomenon of birds napping in pairsis widespread．

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

3.Whilesleeping, some water animals tend to keep half awake in order to ______．

A．alertthemselves to the approaching enemy

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

       C．be sensitive to the ever-changingenvironment

       D．avoid being swept away by rapid currents

4.Bysaying “just the tip of the iceberg”, Siegel suggests that ______．

       A．half-brain sleep has something to dowith icy weather

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

       C．most birds living in cold regions tendto be half sleepers

       D．half-brain sleep may exist among otherspecies

试题答案

【答案】

1.A

2.A

3.B

4.D

【解析】略