Like all mammals, whales need to sleep in order to survive. But they are also air breathers, meaning that they cannot sleep in the water all the time. There are around 90 different types of whale, which can hold their breath for about an hour, depending on the species. However, they usually travel up to the surface to breathe every 15 minutes.
Some scientists have found that whales cannot become fully unconscious or they would drown. To deal with living in the water full-time while having to breathe air, these air-breathers have evolved into voluntary ones, as a way to prevent accidentally breathing in water at inopportune moments. Therefore, they have to be awake and alert at all times to prevent themselves from drowning.
Whales have some of the largest brains on the planet. They can actively decide which part of their brains to use at a given time. This particular way of sleeping can be seen most clearly in captive (圈养的) whales. When whales are sleeping, they can be seen keeping one eye closed while the other remains open.
Some species, like sperm whales, enter a deeper sleep where they hang in groups, vertically, not too far below the surface for just over an hour. Other species, such as the humpback whales, have been observed resting motionless at the surface of the water for only 30 minutes. And, killer whales, which are very socially complex, never swim too far from the other members of their pod, sleeping in close groups.
A. So how exactly do they get any sleep?
B. They continue to swim slowly and regularly.
C. They won't lose their body temperature by staying still.
D. It's an interesting dilemma for wholly ocean air breathers.
E. This sleeping technique varies slightly among species, however.
F. Whales consciously control their blowholes with powerful muscles.
G. Sperm whales in particular have the biggest brain of any living mammal.
