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  3. Humans and nearly all other animals vary their level of activity according to the time of day. Daily rhythms are not simply resp

Humans and nearly all other animals vary their level of activity according to the time of day. Daily rhythms are not simply resp

admin2009-04-23  69
问题     Humans and nearly all other animals vary their level of activity according to the time of day. Daily rhythms are not simply responses to daylight and darkness or to other signs of the time of day. They are generated by the animals themselves. This is shown by experiments in which animals live in artificial surroundings that do not vary with time of day. They experience no daylight, no darkness, no day-night swings of temperature, and no contact with other animals that "know" the time of day. Under such conditions, the animals continue to exhibit near-daily rhythms of activity and rest. These rhythms must be generated by the animals’ own inner "clock". Such a biological clock is normally reset every day, mainly by the cycle of daylight and darkness. An animal’s activity and rest periods are thereby kept "in step" with the environment and with the activity cycles of other animals.
    Daily periods of reduced activity serve at least two purposes. First, they allow the animal to save its resources for the time of day at which it is best suited to compete for food. Second, they make the animal less conspicuous to its enemies. The resting condition is reversible. Animals can become active at unusual times of day if they detect risks or if opportunities for feeding develop at times of shortage.
    Whether the resting state of an animal qualifies as "sleep", however, depends on the complexity of its nervous system. Mammals and birds are the animals in which true sleep, of the nature indicated above, has been observed. Other animals exhibit resting periods.
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答案animals have their own inner clock
解析 由题干experiments可定位至首段第四句：在人工环境下，动物仍然表现出平常的作息习惯，第七句做出推断说动物们一定有自己的内部时钟。
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大学英语四级

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