公鸡为什么会在早上打鸣？
作者：
来源：《小天使·五年级语数英综合》2019年第10期
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鸡的大腦里有一个松果体，松果体可以分泌一种褪黑素的物质。褪黑素能直接控制鸟类的发声，因为鸡也属于鸟类，所以同样能够控制鸡的发声。但是如果有光射入眼睛，那么褪黑素的分泌便会被抑制。晨光乍现时，光线射入鸡的眼睛，褪黑素的分泌就受到了抑制，这时雄鸡便不由自主地司晨了。
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