...困难以外，哺乳动物是温血，需要很多的氧气，水里的氧气太少，只能满足鱼这样的冷血动物,生活于5000万年前的走鲸（Ambulocetus）：,鲸鱼的祖先是陆生动物，它现存最近的亲戚是河马
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I mean, Ambulocetus is a mammal that shows a sort of bridge between walking on land and swimming.
我的意思是，陆行鲸是一种哺乳动物，可以指明一个路上行走动物和水中游泳动物的清晰联系。
On land, where it probably bred and gave birth, Ambulocetus may have moved around very much like a modern sea lion.
它们很可能在陆地上进行生育繁殖，而且，走鲸在陆地上移动时非常像现代海狮。
The structure of the backbone shows, however, that Ambulocetus swam like modern whales by moving the rear portion of its body up and down, even though a fluke was missing.
不过，走鲸的脊椎结构显示，即使没有尾片，它们在水中同样能够像现代鲸鱼那样通过身体后面部分的上下摆动来游动。
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