It is not clear whether cetaceans undergo dream sleep.

鲸类动物是否经历梦睡眠还不清楚.

There are plenty of stories of cetaceans saving humans.

鲸类动物救人的故事有很多。

Any of various similar cetaceans, such as the killer whale.

须鲸类;露脊鲸;长须鲸; 蓝鲸;座头鲸.

Cetaceans reduce the number of breaths they take during rest periods.

鲸类动物在休息时减少它们的呼吸次数.

Stem cetaceans are quite diverse and well documented in the fossil record.

基干鲸类的多样性很高，化石记录也很完整。

It is this essential mystery that leads us on in our conflicted attitude toward cetaceans, named for the Greek for sea monster.

正是这种神秘感将我们引向对鲸类的矛盾态度。希腊语中鲸鱼是海中怪兽的意思。

The findings might someday help rescuers test whether the lost cetaceans might be deaf before releasing them back into the wild.

也许有一天这个研究能帮助研究人员发现为什么一些被救助的鲸豚类生物在被放归大海前就失聪的原因。

Roughly speaking, these animals filled the ecological niches now occupied by toothed cetaceans such as dolphins and killer whales.

粗略地讲，这些动物在生态系统中所占的位置现在被海豚和杀手鲸一类的齿鲸类占据。

Originally land mammals, the first cetaceans entered the water about 55 million years ago as large predators with sharp teeth.

它们最初是陆地上的哺乳动物，第一只鲸目动物大约在5500万年前进入水中，它们长出利齿，成为大型捕食者。

And the nasal opening migrated back from the tip of the snout in Pakicetus to the top of the head in modern cetaceans , forming the blowhole.

巴基鲸的鼻腔开口从口鼻尖端移到头顶，成为现代鲸豚的喷气孔。