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　　公鸡总是以报晓声来迎接朝阳。但在其它时刻它们也会鸣叫。那么它们是在响应光吗?还是它们知道天什么时候亮?新的研究应证了后一说法：公鸡打鸣是因为“内时钟”。这项研究发表在《当代生物学》期刊上。

　　Scientists controlled the light levels in rooster habitats. For two weeks, the birds experienced12 hours of light followed by 12 hours of dim light. Consistent with the pre-dawn noisesobserved in wild fowl, the roosters began to crow about two hours before their rooms lit up.

　　科学家们控制了公鸡住所的光照强度。连续两个星期，先让公鸡们经历了12个小时的光照，接着是12个小时的微弱光线。公鸡在房间被照亮前2个小时就开始鸣叫了，这与在野外观察到的、公鸡黎明前报晓的时间一致。

　　Then, for two weeks, the roosters lived in constant dim light. Yet they continued to crowabout once a day—at intervals of 23.7 hours to be precise. Even without morning light, theircircadian rhythms told them when dawn should be breaking.

　　然后，让公鸡持续在微弱的光线中度过两个星期。然而，它们仍然坚持每隔一天，确切来讲是23.7个小时报晓一次。即使没有晨光，它们的生物钟也会告诉它们什么时候天亮。

　　The birds also crowed in response to sudden light, and to the sounds of other roosters. Butthey were more likely to react when those stimuli occurred near dawn. Showing that you can'treally keep a rooster in the dark about the time.

　　为了响应突然亮起来的光线和其它公鸡的叫声，公鸡也会鸣叫。然而，当这些刺激发生在黎明，公鸡响应的可能性会更大。这说明人们不可能向公鸡隐瞒时间。

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