How Do Dolphins and Whales Sleep? 6 Answers
A dolphin sleeps by switching off only half of its brain. Here are six answers about whales and dolphins: how they rest, how they call one another, why they sing, how much they eat and what happened in the ocean when millions of whales were hunted.
How a Dolphin Sleeps Without Drowning
We humans breathe automatically. A dolphin does not: every breath is voluntary. If it fell completely asleep, it could stop coming up to the surface.
That is why it lets one hemisphere of its brain sleep while the other stays active. The two sides take turns, like two drivers swapping at the wheel without stopping the car. In addition, the eye controlled by the awake side stays open.
How Whales Sleep
It was discovered almost by chance. In 2008, a British and Japanese team came across sperm whales hanging motionless in a vertical position just below the surface. Most likely, they were asleep.
Tags attached to other sperm whales revealed naps lasting ten to fifteen minutes. In total, these add up to barely seven percent of the day: less than two hours.
Do Dolphins Have Names?
Bottlenose dolphins, in a sense, do. During its first year, each one develops a whistle of its own, known as a signature whistle. The others imitate it to call that dolphin.
In 2013, a team from St Andrews played copies of wild dolphins' whistles underwater in Scotland. Each dolphin responded mainly to the copy of its own whistle, just as someone hears their name in a crowded room.
The memory is long-lasting. In captivity, a female dolphin recognized the whistle of a former companion after about twenty years apart.
Why Whales Sing
Long, complex songs are characteristic of humpback whales. Only the males sing, mainly in breeding areas and during the mating season. The most widely accepted explanation is that it is a display: to attract females or to compete with other males.
The surprising part is that the song is passed on. In the South Pacific, a new song spreads from one population to another, from eastern Australia to French Polynesia, in about two years. And it replaces the previous one.
How Much a Blue Whale Eats
The blue whale can reach up to thirty meters in length, yet it feeds on prey just a few centimeters long. It does not chew. It speeds up, throws its mouth open and the pleats in its throat expand like an accordion. In a single gulp, it can take in more water than the volume of its own body.
It then pushes the water out through its baleen, plates made of keratin, the same material as fingernails. The krill stays inside, as if caught in a sieve.
A figure circulating online says it eats about four tonnes of krill a day. However, a study published in Nature in 2021, based on tags and sonar, puts the figure at around sixteen tonnes a day during the feeding season for blue whales in the eastern North Pacific.
Why Fewer Whales Means Less Krill
Millions of whales were hunted in the 20th century. Logically, krill should have increased after losing its large predators. That is not what happened: the data indicate that it declined in the Southern Ocean.
The strongest explanation lies in their feces. In that region, what limits the microscopic algae that krill feed on is a lack of iron. Whales take in iron when they eat and release it at the surface when they defecate. Fewer whales mean less iron, fewer algae and less krill.
The 2021 study estimates that, before industrial whaling, whales in the Antarctic ate about 430 million tonnes of krill a year, twice as much as all the krill found there today. Whales did not just eat krill: they also helped it grow.
Frequently asked questions
Do dolphins sleep with one eye open?
Yes. They sleep with one hemisphere of the brain while the other stays active, and the eye controlled by the awake side remains open.
How much do sperm whales sleep?
They take naps of ten to fifteen minutes in a vertical position. In total, they sleep around seven percent of the day, less than two hours.
Which whales sing?
Long, complex songs belong to humpback whales, and only the males sing, mainly in breeding areas and during the mating season.
How much krill does a blue whale eat a day?
According to a study published in Nature in 2021, about sixteen tonnes a day during the feeding season, not the four that is often repeated.
Informative article written with the help of AI tools from the verified script of the video. The video is in Spanish.