2 hours of sleep a day? How this animal can survive is a mystery.
The surprisingly short shut-eye may be a way for the elephant seal to avoid predators—an intriguing find from the first-ever sleep study on marine mammals in the wild.
The African elephant holds the record for sleeping the least of any mammal—about two hours a day—but now, the elephant seal is giving its namesake a run for its money.
Recent experiments measuring brain waves of northern elephant seals in Monterey Bay, California, revealed the animals averaged only two hours of sleep per day during the seven months out of the year it spent at sea. That’s an impressive feat, considering gorillas sleep for 12 hours per day, dogs over 10, and lions up to 20.
Jessie Kendall-Bar, then a Ph.D. candidate at the University of California, Santa Cruz, discovered the elephant seals’ ability while researching how the large animals, whose males feature a trunk-like nose, sleep hundreds of feet below the ocean’s surface.
In the first-ever sleep study on marine mammals in the wild, she found in the open ocean, elephant seals sleep less than two hours per day—while on land, they doze more than 10 hours a day. (See 24 endearing photos of animals sleeping.)
"They're able to have these sort of dual lifestyles," says Kendall-Bar, a National Geographic Explorer who led a study on the phenomenon published this week in the journal Science. "As far as a mammal that has that degree of flexibility, that's pretty much unprecedented."
Testing out new headgear
Previous observations had shown that elephant seals in the open ocean surface for a couple of minutes at a time, between 10-to-30-minute dives. So scientists knew that they must be sleeping underwater. But still, “we know very little,” says study co-author Daniel Costa, a researcher at UC Santa Cruz and National Geographic Explorer. Roxanne Beltran, also a National Geographic Explorer at UC Santa Cruz, was an author on the paper.
To find out more, Kendall-Bar developed a headcap that contains the same type of sensors used to conduct sleep studies in humans. The apparatus is waterproof, able to withstand large amounts of water pressure, and sensitive enough to detect brain waves through a thick layer of blubber on the animal's head. (Read how a tiny number of northern elephant seal ‘supermoms’ produce the most pups.)
In 2019 at the Marine Mammal Center, an animal hospital in Sausalito, Kendall-Bar had the opportunity to try out her headcap on a sleeping elephant seal brought in for medical care, named Libelula. The device worked—"our first confirmation that we could actually detect something through centimeters of blubber," she says. Next stop: the open ocean.
Kendall-Bar tested three wild young female seals. Using an adhesive, she placed the headcap on one of them when they were laying out on the beach. It dove into the water and returned two days later, allowing her to remove the cap. The other two were equipped and translocated from Año Nuevo State Park to nearby Asilomar Beach in Monterey, and the instrumentation was recovered days later.
Sleep spiral
The headcaps gathered data about the animals’ brain waves, heart rates, dive depth, and movements to determine when they were sleeping. Kendall-Bar then used the data to extrapolate sleep patterns over time in adult seals.
She discovered that seals do not sleep in two-hour bursts, but rather, in a series of "catnaps" lasting less than 20 minutes each. Starting at the surface, adult seals take 10-minute dives at great depths, usually between 300 to a thousand feet.
At this point, the animal enters the first stage of sleep, or slow-wave sleep. Then, they drift into REM sleep, when they become paralyzed and their bodies turn upside down in what Kendall-Bar calls a "sleep spiral."
It would seem REM sleep at depth is risky, especially the inability to escape predators. "For an animal to do this underwater and just be in that paralytic state, to me that is just chilling," says study co-author Terrie Williams, a researcher at UC Santa Cruz. (Read more about the secrets of sleep.)
Predator avoidance
But what’s likely happening is the seals are sleeping at depths that their primary predators—sharks and orcas—rarely frequent.
"The elephant seal is basically using its ability to dive really deep as a protective mechanism," says Kendall-Bar, now a postdoctoral scholar at the Scripps Institution of Oceanography. "It doesn't have to keep one eye open, or stay awake. It's sleeping with its entire brain."
She speculates that this sleep behavior developed because of the seals' need to forage for long periods in the open ocean, as a means of supporting their large body weight of up to 4,500 pounds. "They're built to be at sea for a very long time.”
"The really cool thing about this study is that it is the first to examine brain-wave activity and other indicators of sleep in a free-ranging, deep-diving marine mammal,” says Jane Khudyakov, an associate professor in the Department of Biological Sciences at the University of the Pacific in Stockton, California, who wasn’t involved in the research.
“Most of the past studies of sleep in marine mammals were conducted in a captive setting [during submergence in pools] or on land."
Khudyakov also cautions that comparing an elephant seal’s two hours of shut-eye with, say, our seven hours, isn’t that linear—for one, the need for REM and non-REM sleep varies widely between mammals, even closely related ones.
"Maybe it seems little to us because we don't fully understand the diversity of sleep adaptations and function of sleep in different animals!” she says by email.
“A great privilege”
It’s still a mystery how elephant seals can survive in the open ocean on so little sleep.
Costa notes that large animals like African elephants have slower metabolic rates, which may mean they can get away with less sleep—possibly the case with elephant seals as well. Williams speculates the seals may postpone the "repair processes" that happen during sleep until they get to the beach and can rest in earnest. (Learn more about the possible origins of sleep.)
As for Kendall-Bar, rather than call her study a discovery, she considers it a great privilege. "Animals have been doing this for so long, it feels like an honor just to observe that and be let in on this secret.”
