How Does an Elephant Seal Sleep While Still Awake Under Water?

Elephant seals spend the majority of their time sleeping at sea, yet for the first time, brain activity in a free-ranging, wild marine mammal has been observed.

The latest research, which was released on April 20 in Science, demonstrates that elephant seals sleep on the beach for up to 10 hours every day during the breeding season, but only get an average of 2 hours each day when they are at sea on months-long foraging expeditions.

Elephant Seals Drift Off To Sleep

(Photo : PATRICK HERTZOG/AFP via Getty Images)

During deep, 30-minute dives, they sleep for about 10 minutes at a time, frequently spiraling downward while dozing off and occasionally lying still on the ocean floor, as per Phys.org.

Elephant seals spend the majority of their time sleeping at sea, yet for the first time, brain activity in a free-ranging, wild marine mammal has been observed.

The latest research, which was released on April 20 in Science, demonstrates that elephant seals sleep on the beach for up to 10 hours every day during the breeding season, but only get an average of 2 hours each day when they are at sea on months-long foraging expeditions.

During deep, 30-minute dives, they sleep for about 10 minutes at a time, frequently spiraling downward while dozing off and occasionally lying still on the ocean floor.

Jessica Kendall-Bar, the study's first author, conducted the research as a graduate student at UC Santa Cruz under the supervision of Terrie Williams and Daniel Costa, who are both department chairs for ecology and evolutionary biology.

When elephant seals sleep has been a major topic of discussion for many years, according to Costa, director of the UCSC Institute of Marine Sciences.

Since more than 25 years ago, Costa's lab has been in charge of the UCSC elephant seal research program at the Ano Nuevo Reserve.

During the foraging migrations, which can last up to eight months, the lab uses increasingly sophisticated tags to track the movements and diving behavior of the seals.

Elephant seals only spend a minute or two breathing at the surface between dives since this is when they are most exposed to predators like sharks and killer whales.

When wild elephant seals dive normally at sea, Kendall-Bar created a method that can accurately monitor brain activity (as an electroencephalogram, or EEG).

The device can be recovered when the animals return to the beach at Ano Nuevo thanks to a neoprene head cap that secures the EEG sensors and a tiny data logger that records the signals.

The seals also carried time-depth recorders, accelerometers, and other equipment that allowed the researchers to monitor the seals' movements and related brain activity in addition to the EEG system.

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Going REM?

The recordings depict diving seals entering the stage of deep sleep known as slow-wave sleep while continuing a controlled glide downward, then changing to rapid-eye movement (REM) sleep when sleep paralysis leads them to turn upside down and float below in a "sleep spiral", as per The Guardian.

Some of those seals made brief forays into the ocean, but the researchers utilized a translocation technique created by Costa's lab to study diving behavior.

Female juvenile elephant seals were brought from Ano Nuevo to Monterey where they were fitted with EEG sensors and trackers before being released on a beach at the mouth of Monterey Bay.

The animals would then swim across the deep Monterey Canyon to return to Ano Nuevo during the next three days.

Here, their diving behavior is strikingly comparable to that observed during far longer foraging expeditions in the open ocean.

Kendall-Bar created a very accurate algorithm for identifying sleep periods based on the dive data alone using data on brain activity and dive behavior from 13 adolescent female elephant seals, including a total of 104 sleep dives.

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