精选优质文档-倾情为你奉上第四部分 动物世界（一）1. Why Are The Rats Traveling?老鼠为何迁徙北欧一些国家有一种老鼠，不仅繁殖能力强，而且特别喜欢迁徙，其频率之高、速度之快，令人们瞠目而困惑。为何这种老鼠不像我们的家鼠一样喜欢呆在一个特定的地方，而不辞辛苦地挪窝呢？请看这篇文章，它会告诉你什么。In some North European countries such as Norway and Sweden, there is a kind of rats called traveling rats, which produce their young seven or eight times a year. The young will grow up in about six months and produce their own young. So, in a few years these rats increase in great numbers. Usually they live together in hol
