- A new study has found that elephant seals move downward in a “sleep spiral” when diving deep into the ocean.
- Scientists believe that sleeping while deep diving allows seals to avoid predators.
- As part of the study, scientists recorded the brain waves of 13 young female seals in California.
According to a new study, elephant seals descend into the ocean in a “sleep spiral” to catch up on sleep during months-long foraging trips, but are programmed not to drown.
Seals sleep when they dive as deep as 377 meters, or about 1,235 feet, to avoid predators. During half-hour dives they spiral downward for about 10 minutes at a time, and they sometimes sleep briefly on the sea floor. For new findings published in science.
The study marks the first time scientists have recorded the sleep habits of a free-ranging, wild marine mammal by examining brain waves. University of California, Santa Cruz.
The study examined the importance of sleep to mammals, and pointed out that marine mammals “face particularly challenging conditions for sleep while at sea.”
“For many years the key question has been when elephant seals sleep,” said Daniel Costa, director of the UCSC Institute of Marine Sciences.
The lab used the tags to track elephant seals’ movements in the Ano Nuevo Reserve as the animals migrated to the Pacific Ocean for months.
“Dive logs show them diving continuously, so we thought they must be sleeping when they stop swimming and slowly sink, what we call drift dives, but we really don’t know,” Costa continued.
Professor Terry Williams, UC Santa Cruz, told BBC News It is “remarkable” that any mammal can sleep hundreds of feet below the water’s surface.
“It’s not light sleep, but the true paralytic, deep sleep that humans can snore through. 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 the spine,” Williams said.
African elephants currently hold the title of mammal that sleeps only two hours a day, but this new discovery shows that elephant seals “rival the record,” according to UCSC.
Killer whales and sharks attack elephant seals when they are at the ocean’s surface, which is why they spend so little time near the surface and breathe only briefly at the surface between dives at UCSC.
“They are able to hold their breath for long periods of time, so they can go into deep sleep in these dives deep below the surface where they are safe,” said Jessica Kendall-Barr, who led the study.
The scientists fitted neoprene headcaps with electroencephalogram (EEG) sensors to record the brain activity of 13 young female seals.
“We used the same sensors you use for human sleep studies in a sleep clinic, and attached the headcap with removable, flexible adhesives so water can’t get in and disrupt the signals,” said Kendall-Barr, a postdoctoral fellow. UC San Diego’s Scripps Institution of Oceanography, said.
The recordings showed that diving seals go through a sleep stage called “slow-wave sleep” before transitioning to REM sleep, which leads to a type of “sleep spiral” or sleep paralysis, the experts found.
Elephant seals get more sleep when they are on land – about 10 hours – scientists said, making their sleep patterns “unusual”.
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