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生存和成功的繁殖通常需要动物的活动与周围可预测的事件相协调。

Survival and successful reproduction usually require the activities of animals to be coordinated with predictable events around them.

因此，生物功能的时间和节奏必须与太阳日、潮汐、月亮周期和季节等周期性事件密切匹配。

Consequently, the timing and rhythms of biological functions must closely match periodic events like the solar day, the tides, the lunar cycle, and the seasons.

动物活动和这些周期之间的关系，特别是和日常节律的关系，已经引起了人们极大的兴趣和重视，以致于对它们进行了大量的研究，并且出现了时间生物学的专门研究领域。

The relations between animal activity and these periods, particularly for the daily rhythms, have been of such interest and importance that a huge amount of work has been done on them and the special research field of chronobiology has emerged.

通常情况下，动物的睡眠、进食、移动、繁殖、代谢和产生酶和激素的活动水平的不断变化与环境节奏很好地协调，但关键问题是，动物的活动日程是由外部信号驱动的，如日出还是日落，或者以某种方式依赖于内部计时器，它们自己产生观察到的生物节律。

Normally, the constantly changing levels of an animal’s activity—sleeping, feeding, moving, reproducing, metabolizing, and producing enzymes and hormones, for example—are well coordinated with environmental rhythms, but the key question is whether the animal’s schedule is driven by external cues, such as sunrise or sunset, or is instead dependent somehow on internal timers that themselves generate the observed biological rhythms.

几乎所有的生物学家都接受这样的观点：所有的真核生物（包括除细菌和某些藻类以外的大多数生物）都有内部生物钟。

Almost universally, biologists accept the idea that all eukaryotes (a category that includes most organisms except bacteria and certain algae) have internal clocks.

通过将生物体与外部周期信号完全隔离，生物学家了解到生物体具有内部时钟。

By isolating organisms completely from external periodic cues, biologists learned that organisms have internal clocks.

例如，当真菌Neurospora在航天飞机上环绕地球运行时，它被有意地与所有地球物理计时信号隔离开来，正常的每日生物活动周期似乎被维持了大约一个星期。

For instance, apparently normal daily periods of biological activity were maintained for about a week by the fungus Neurospora when it was intentionally isolated from all geophysical timing cues while orbiting in a space shuttle.

在没有外部提示的情况下，生物体中生物节律的延续证明了它有一个内部时钟。

The continuation of biological rhythms in an organism without external cues attests to its having an internal clock.

当小龙虾被连续地放在黑暗中，即使是4到5个月，它们的复眼也会根据白天和夜间的视力继续调整。

When crayfish are kept continuously in the dark, even for four to five months, their compound eyes continue to adjust on a daily schedule for daytime and nighttime vision.

研究发现，在黑暗中连续饲养一年的马蹄蟹，其大脑活动的持续节律与它们的眼睛每天适应明亮或微弱光线的节律类似。

Horseshoe crabs kept in the dark continuously for a year were found to maintain a persistent rhythm of brain activity that similarly adapts their eyes on a daily schedule for bright or for weak light.

就像几乎所有被剥夺了环境线索的动物的每日循环一样，在这些条件下对马蹄蟹的测量并没有精确到24小时。

Like almost all daily cycles of animals deprived of environmental cues, those measured for the horseshoe crabs in these conditions were not exactly 24 hours.

这种周期近似于——但不完全是——一天的节律被称为昼夜节律。

Such a rhythm whose period is approximately—but not exactly—a day is called circadian.

至于不同的马蹄蟹，它们的生理周期为22.2至25.5小时。

For different individual horseshoe crabs, the circadian period ranged from 22.2 to 25.5 hours.

一种特别的动物通常会在许多天内精确地保持其特有的周期。

A particular animal typically maintains its own characteristic cycle duration with great precision for many days.

的确，生物钟周期的稳定性是其主要特征之一，当生物体的环境受到诸如温度等因素的巨大变化时，这些因素预计会强烈影响生物活动。

Indeed, stability of the biological clock’s period is one of its major features, even when the organism’s environment is subjected to considerable changes in factors, such as temperature, that would be expected to affect biological activity strongly.

当通常的外部循环被改变时，进一步的证据就出现了，要么是实验性的，要么是通过快速的东西方向长距离移动来证明。

Further evidence for persistent internal rhythms appears when the usual external cycles are shifted—either experimentally or by rapid east-west travel over great distances.

通常，动物每天内部产生的活动循环没有变化。

Typically, the animal’s daily internally generated cycle of activity continues without change.

结果，它的活动相对于新环境的外部循环发生了转移。

As a result, its activities are shifted relative to the external cycle of the new environment.

外部时间线索和内部时间安排的不匹配所产生的迷惑效应可能会持续几天或几周，就像我们的时差反应一样，直到某些线索，如日光/黑夜循环，重新设置机体的时钟，使其与新环境的日常节奏同步。

The disorienting effects of this mismatch between external time cues and internal schedules may persist, like our jet lag, for several days or weeks until certain cues such as the daylight/darkness cycle reset the organism’s clock to synchronize with the daily rhythm of the new environment.

动物需要像日出这样的自然周期信号来维持一个正好是24小时的周期。

Animals need natural periodic signals like sunrise to maintain a cycle whose period is precisely 24 hours.

这种外部线索不仅使动物的日常节律与当地太阳日的特定特征相协调，而且——因为它通常日复一日地这样做——似乎使内部时钟周期与地球自转周期保持一致。

Such an external cue not only coordinates an animal’s daily rhythms with particular features of the local solar day but also—because it normally does so day after day—seems to keep the internal clock’s period close to that of Earth’s rotation.

然而，尽管这种内部循环的周期是同步的，这种动物的计时器本身仍然有自己的基因内置周期，接近但不同于24小时。

Yet despite this synchronization of the period of the internal cycle, the animal’s timer itself continues to have its own genetically built-in period close to, but different from, 24 hours.

如果没有外部的提示，差异就会累积，因此生物日的内部调节活动就会相对太阳日不断地漂移，就像潮汐一样。

Without the external cue, the difference accumulates and so the internally regulated activities of the biological day drift continuously, like the tides, in relation to the solar day.

这种漂移已经在许多动物和生物活动中得到了广泛的研究，从果蝇卵的孵化到松鼠跑轮。

This drift has been studied extensively in many animals and in biological activities ranging from the hatching of fruit fly eggs to wheel running by squirrels.

光线对时钟的设定有很大的影响。

Light has a predominating influence in setting the clock.

在持续的黑暗中，即使是15分钟的光亮也能重新设定动物的昼夜节律。

Even a fifteen-minute burst of light in otherwise sustained darkness can reset an animal’s circadian rhythm.

正常情况下，内部节律与正常的环境循环保持一致。

Normally, internal rhythms are kept in step by regular environmental cycles.

例如，如果一只信鸽要用它的太阳罗盘导航，它的时钟必须根据日光/黑夜循环提供的线索进行适当的设置。

For instance, if a homing pigeon is to navigate with its Sun compass, its clock must be properly set by cues provided by the daylight/darkness cycle.

In paragraph 1, the experiment on the fungus Neurospora is mentioned to illustrate

According to paragraph 1, all of the following are generally assumed to be true EXCEPT:

Why does the author mention that the period for different horseshoe crabs ranges “from 22.2 to 25.5 hours”?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

According to paragraph 2, which of the following is true about the circadian periods of animals deprived of environmental cues?

According to paragraph 2, what will an animal experience when its internal rhythms no longer correspond with the daily cycle of the environment?

In paragraph 2, the author provides evidence for the role of biological clocks by

The word “it” in the passage refers to

Look at the four squares  that indicate where the following sentence could be added to the passage. Because the internal signals that regulate waking and going to sleep tend to align themselves with these external cues, the external clock appears to dominate the internal clock.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Biological Clocks Survival and successful reproduction usually require the activities of animals to be coordinated with predictable events around them. Consequently, the timing and rhythms of biological functions must closely match periodic events like the solar day, the tides, the lunar cycle, and the seasons. The relations between animal activity and these periods, particularly for the daily rhythms, have been of such interest and importance that a huge amount of work has been done on them and the special research field of chronobiology has emerged. Normally, the constantly changing levels of an animal’s activity—sleeping, feeding, moving, reproducing, metabolizing, and producing enzymes and hormones, for example—are well coordinated with environmental rhythms, but the key question is whether the animal’s schedule is driven by external cues, such as sunrise or sunset, or is instead dependent somehow on internal timers that themselves generate the observed biological rhythms. Almost universally, biologists accept the idea that all eukaryotes (a category that includes most organisms except bacteria and certain algae) have internal clocks. By isolating organisms completely from external periodic cues, biologists learned that organisms have internal clocks. For instance, apparently normal daily periods of biological activity were maintained for about a week by the fungus Neurospora when it was intentionally isolated from all geophysical timing cues while orbiting in a space shuttle. The continuation of biological rhythms in an organism without external cues attests to its having an internal clock. When crayfish are kept continuously in the dark, even for four to five months, their compound eyes continue to adjust on a daily schedule for daytime and nighttime vision. Horseshoe crabs kept in the dark continuously for a year were found to maintain a persistent rhythm of brain activity that similarly adapts their eyes on a daily schedule for bright or for weak light. Like almost all daily cycles of animals deprived of environmental cues, those measured for the horseshoe crabs in these conditions were not exactly 24 hours. Such a rhythm whose period is approximately—but not exactly—a day is called circadian. For different individual horseshoe crabs, the circadian period ranged from 22.2 to 25.5 hours. A particular animal typically maintains its own characteristic cycle duration with great precision for many days. Indeed, stability of the biological clock’s period is one of its major features, even when the organism’s environment is subjected to considerable changes in factors, such as temperature, that would be expected to affect biological activity strongly.Further evidence for persistent internal rhythms appears when the usual external cycles are shifted—either experimentally or by rapid east-west travel over great distances. Typically, the animal’s daily internally generated cycle of activity continues without change. As a result, its activities are shifted relative to the external cycle of the new environment. The disorienting effects of this mismatch between external time cues and internal schedules may persist, like our jet lag, for several days or weeks until certain cues such as the daylight/darkness cycle reset the organism’s clock to synchronize with the daily rhythm of the new environment. Animals need natural periodic signals like sunrise to maintain a cycle whose period is precisely 24 hours.  Such an external cue not only coordinates an animal’s daily rhythms with particular features of the local solar day but also—because it normally does so day after day—seems to keep the internal clock’s period close to that of Earth’s rotation.  Yet despite this synchronization of the period of the internal cycle, the animal’s timer itself continues to have its own genetically built-in period close to, but different from, 24 hours.  Without the external cue, the difference accumulates and so the internally regulated activities of the biological day drift continuously, like the tides, in relation to the solar day.  This drift has been studied extensively in many animals and in biological activities ranging from the hatching of fruit fly eggs to wheel running by squirrels. Light has a predominating influence in setting the clock. Even a fifteen-minute burst of light in otherwise sustained darkness can reset an animal’s circadian rhythm. Normally, internal rhythms are kept in step by regular environmental cycles. For instance, if a homing pigeon is to navigate with its Sun compass, its clock must be properly set by cues provided by the daylight/darkness cycle.