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Why must whales regularly surface to breathe?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Whales must regularly surface to breathe because, unlike terrestrial mammals, breathing is not an automatic reflex for cetaceans. If a whale loses consciousness, it will not breathe and risks drowning.12

Because they must remain conscious to regulate their respiration, whales have evolved a unique process called unihemispheric slow-wave sleep, where one half of the brain remains active to manage breathing and maintain awareness while the other half rests.2

When they surface, whales utilize a blowhole—a specialized nostril located on the top of the head—to perform an explosive ventilation known as a blow. This process is highly efficient; while humans typically use only 20 percent of their lung volume in a single breath, cetaceans can utilize up to 80 percent. The frequency of these surface intervals varies by species and activity level, ranging from seconds for active swimmers to over an hour for deep-diving species like sperm whales.1345