普特英语听力网在线听力：海洋生物有趣的睡觉习惯

Marine Animals Have Interesting Sleeping Habits

Many people know that marine mammals, like whales, porpoises and dolphins, breathe air, like we do. But, many people don’t know that marine mammals have developed some interesting sleeping habits in order to accommodate their regular trips up to the surface for air. Today, A Moment of Science looks at what its like to sleep with the fishes.

No Sleep Tonight

Unlike our own breathing, which does not require conscious effort, marine mammals must consciously go to the surface of the water to breathe through a blowhole on top of their heads. Because of this, marine mammals don’t sleep for a solid 8 hours a night, the way we do.

Some marine mammals sleep while swimming. Whales nap for brief periods while swimming alongside an alert friend who guides them. In fact, young whales must sleep alongside their mother because the mother’s slipstream actually pulls the calf along: it will sink if the mother stops.

Dolphin

Other marine mammals, like dolphins, sleep while floating on the surface of the water or take short catnaps while floating below the surface. Researchers claim that while marine mammals sleep their brain is always partially alert. Aside from these brief naps, dolphins are active most of the night.

Of course, marine mammals are equipped to handle underwater sleeping. They have proportionately larger lungs, exchange air more fully at each breath, and carry more oxygen in their blood than we do, so they can hold their breath long enough to sleep with the fishes.