- A new study has found that elephant seals curl downward in a “sleep spiral” while diving deep in the ocean.
- Scientists believe that seals can avoid predators by sleeping while deep diving.
- Scientists recorded the brain waves of 13 young female seals in California as part of the study.
Elephant seals drift downstream in the ocean in “sleep spirals” and sleep during months-long foraging trips, but are programmed not to drown, according to a new study.
Seals fall asleep during deep dives of up to 377 meters, which is about 1,235 feet, to avoid predators. According to new findings published in Science, they spiral downward for about 10 minutes at a time during a half-hour dive, and they sometimes even take a nap on the sea floor.
According to the University of California, Santa Cruz, the study is the first time scientists have studied the brain waves and recorded the sleep habits of free-ranging, wild marine mammals.
The study examined the critical nature of sleep for mammals, and pointed out that marine mammals “face particularly challenging conditions for sleep when at sea.”
“For years, one of the central questions about elephant seals has been when they sleep,” said Daniel Costa, director of the UCSC Institute of Marine Science.
Ano Nuevo uses lab tags to track the movements of elephant seals in the reserve as the animals migrate to the Pacific Ocean for months at a time.
“Dive records show that they dive continuously, so we thought they must be sleeping during what we call drift dives, when they stop swimming and slowly sink, but we really didn’t know,” Costa continued.
UC Santa Cruz professor Terry Williams told BBC News it was “remarkable” that any mammal would sleep hundreds of feet below the water’s surface.
“This is not light sleep but actual paralyzing, deep sleep in which humans would be snoring. Remarkably, the seal’s brain reliably expels oxygen before it runs out.
“Imagine waking up at the bottom of a pool — it sends shivers down the spine,” Williams said.
—Jessica Kendall-Barr (@jmkendallbar) April 20, 2023
African elephants currently hold the title of mammal that sleeps the least at just two hours a day, but this new discovery shows that elephant seals “beat the record,” according to UCSC.
Killer whales and sharks attack elephant seals when they are at the surface of the ocean, which is why they spend so little time near the surface and take little time to breathe at the surface between dives, according to UCSC.
“They’re able to hold their breath for longer periods of time, so they can go into a deep sleep on these dives where it’s safe,” said Jessica Kendall-Barr, who led the study.
Scientists fitted neoprene headcaps with electroencephalogram (EEG) sensors to record the brain activity of 13 young female seals.
“We used the same sensors that you use for human sleep studies in a sleep clinic and used a removable, flexible adhesive to attach the headcap so that water can’t get in and disrupt the signals,” Kendall-Barr, a postdoctoral fellow. said a colleague at UC San Diego’s Scripps Institution of Oceanography.
The recordings showed diving seals go through a sleep stage known as “slow-wave sleep” before transitioning to REM sleep, leading to a type of “sleep spiral” or sleep paralysis, the experts found.
When elephant seals are on land they get a lot of sleep – about 10 hours – scientists said, which makes their sleep patterns “unusual”.
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