Elephant seals take a nap underwater as they slowly sink hundreds of feet under the waves

At sea, elephant seals sleep in 10-minute naps underwater and drift gently at depths of nearly 400 meters – sometimes coming to rest on the sea floor.

Researchers discovered the remarkable behavior after fitting the animals with neoprene caps, similar to the caps humans use to measure brain activity. The results showed that they also fall into paralyzed REM sleep – the time when people have their most vivid dreams.

Studying wild northern elephant seals showed that they only needed an average of two hours of sleep per day while foraging at sea, a number that rivals their terrestrial namesake African elephants, which also rarely sleep.

For elephant seals, their sleeping patterns vary greatly depending on the season.

“Elephant seals are unusual in that they alternately sleep a lot when they’re on land, more than 10 hours a day, and two hours or less when they’re at sea,” said lead author Jessica Kendall-Bar of the Scripps. Institute of Oceanography. “This flexibility in sleep duration is unprecedented in mammals and rivals the mammal record for least sleep.”

At sea – for up to eight months at a time – elephant seals are most at risk of predation from surface sharks and killer whales, so spend just a minute or two between dives. Each dive lasts up to 30 minutes and may include a nap.

While they sleep, the seals roll onto their backs and spiral down to depths of up to 377 meters.

“They can hold their breath for long periods of time so they can fall into a deep sleep during those dives deep below the surface where it’s safe,” said Kendall-Bar, who completed the study at UC Santa Cruz.

“By learning more about where, when and how animals sleep at sea, we can better manage and protect their critical resting places.”

In addition to the electroencephalogram monitors in the tank, the seals wore time-depth recorders and accelerometers to better understand their movements and behavior at sea and identify “dormant landscapes.” The team tracked 334 wild northern elephant seals off the Pacific Northwest coast of the United States.

Previous research followed the seals on regular long dives, but couldn’t determine where and when the marine mammals slept.

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Other seal species sleep at sea in an upright position called “bottling”, with only their heads above the water.

Co-author and professor of Ecology and Evolutionary Biology at UCSC Terrie Williams said, “We usually worry about protecting the areas where animals forage, but maybe the places where they sleep are just as important as any other important habitat.”

The study is published in the journal Science.