Elephant seals sleep 1,200 feet below sea level to avoid predators

Elephant seals sleep in Antarctica.
Spinim Coskun/Anadolu Agency via Getty Images

  • A new study finds that elephant seals drift down into a “sleep vortex” when diving into the ocean depths.
  • Scientists believe that sleeping during deep dives allows seals to avoid predators.
  • Scientists recorded the brain waves of 13 young female seals in California as part of the study.

Elephant seals drift down into the ocean in a “sleep vortex” to catch up on their months-long foraging trips, but they are programmed not to drown, according to a new study.

Seals sleep while diving deep to 377 meters, about 1,235 feet, to avoid predators. They roll downward about 10 minutes at a time during half-hour dives, and sometimes even sleep on the sea floor, according to to the new results published in science.

The study marks the first time that scientists have studied the brain waves and sleeping habits of a free-range terrestrial marine mammal, according to the study. University of California, Santa Cruz.

The study examined the crucial nature of sleep for mammals, and noted that marine mammals “face particularly difficult conditions for sleep when they are at sea.”

“For years, one of the central questions about elephant seals has been when they sleep,” said Daniel Costa, director of the Institute of Marine Sciences at UCLA.

The lab used markers to track the movements of elephant seals in the Año Nuevo Sanctuary when the animals head out into the Pacific Ocean for months at a time.

Costa continued, “The diving records show that they dive constantly, so we thought they must be asleep during what we call drift dives, when they stop swimming and slowly sink, but we really didn’t know.”

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Professor Terry Williams, University of California, Santa Cruz, for BBC News It was “remarkable” for any mammal to fall asleep while drifting hundreds of feet below the water’s surface.

“This is not a light sleep but a true paralytic sleep, a deep sleep that would make humans snore. Remarkably, the seal’s brain reliably wakes them from it before they run out of oxygen.”

“Imagine waking up at the bottom of a pool—it sends a shiver down my spine,” said Williams.

African elephants currently hold the title of mammals that only sleep at least two hours a day, but these new discoveries show that elephant seals are “rivaling the record,” according to the UCSC.

Whales and killer sharks attack elephant seals when they’re at the ocean’s surface, which is why they spend very little time near the top and have little time breathing at the surface between dives, according to the UCSC.

“They’re able to hold their breath for a long time, so they can go into a deep slumber in these deep dives below the surface where it’s safe to be,” said Jessica Kendall Barr, who led the study.

The scientists fitted neoprene headgear with electroencephalography (EEG) sensors to record the brain activity of the 13 female seals.

“We used the same sensors that you use to study human sleep in the sleep clinic, and we used a flexible, removable adhesive to attach the headgear so water couldn’t get in and disrupt the signals,” said Kendall Parr, a postdoctoral fellow at the Scripps Institution of Oceanography at the University of California, San Francisco. California in San Diego.

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Experts found that the recordings showed the diving seals entering a stage of sleep known as “slow-wave sleep” before transitioning into REM sleep, resulting in a type of “sleep spiral,” or sleep paralysis.

Scientists said that elephant seals sleep a lot when they are on the ground – about 10 hours – which makes their sleep pattern “unusual”.

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