A．alert themselves to the approaching enemyB．emerge fro……
TEXT C
Birds that are literally
half-asleep-with one brain hemisphere alert and the other sleeping-control which
side of the brain remains awake, ac cording 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 hemi
sphere 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, cud-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-hemi sphere sleep, rather
than total relaxation, more often than inner ducks did. Rotating 16 birds
through the positions in a four-duck row, the re searchers 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 wide spread, 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 as
dolphins, whales, and seals. Perhaps keeping one side of the brain awake allows
a sleeping animal to surface occasion ally 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.
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
