Elephant seals rest at sea in downward-diving “sleep spirals”

Scientists have scanned the brains of 13 elephant seals and discovered how they sleep when they’re foraging in the ocean.

They’ve found that the seals function on two hours of sleep per day out at sea, which they grab in 10-minute bursts on deep dives, where they spiral gently towards the sea floor.

This makes them some of the least sleepy mammals in the world. The African elephant, currently the record holder for mammal needing the least sleep, also only gets two hours of shut-eye a day.

“For years, one of the central questions about elephant seals has been when do they sleep,” says Professor Daniel Costa, director of the Institute of Marine Sciences at the University of California – Santa Cruz (UCSC), US.

850 sleep spiral
Sleep spiral: This graphic shows sleeping postures in elephant seals in REM sleep every 20 seconds, with accompanying 30-second segments of EEG traces in the background. Credit: Graphic by Jessica Kendall-Bar

When they’re on land in breeding season, elephant seals can sleep for more than 10 hours per day, but they can spend as much as eight months at a time on foraging trips in the Pacific Ocean.

“The dive records show that they are constantly diving, 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,” says Costa.

“Now we’re finally able to say they’re definitely sleeping during those dives, and we also found that they’re not sleeping very much overall compared to other mammals.”

The researchers developed an electroencephalograph, or EEG, system that could track the seals’ brainwaves while they were at sea.

“We used the same sensors you’d use for a human sleep study and a removable, flexible adhesive to attach the headcap so that water couldn’t get in and disrupt the signals,” says lead author Dr Jessica Kendall-Bar, who did the research as a graduate student at UCSC.

They tested these neoprene headcaps first with five captive seals in their marine laboratory.

“I spent a lot of time watching sleeping seals,” says Kendall-Bar.

“Our team monitored instrumented seals to make sure they were able to reintegrate with the colony and were behaving naturally.”

They then fitted them on eight wild seals, along with depth recorders and accelerometers to track the seals’ movements.

They collected data from 104 sleep dives, showing that the seals go into “slow-wave sleep” – a deep sleep stage – while gliding downward, then switching to rapid eye-movement (REM) sleep.

Diagram showing cetaceans have unhemispheric sleep, while otariids have both unhemispheric and bilateral sleep and phocids and humans have bilateral sleep
Cetaceans (whales and dolphins) and otariids (fur seals and sea lions) keep one side of their brains awake while the other is asleep (unihemispheric sleep). In most other mammals, including phocids (true seals, including elephant seals) and humans, both hemispheres of the brain are asleep at the same time. Credit: Graphic by Jessica Kendall-Bar

“They go into slow-wave sleep and maintain their body posture for several minutes before they transition into REM sleep, when they lose postural control and turn upside down,” says Kendall-Barr.

They’re deep enough at this point that they tend to keep sinking, drifting downwards in a corkscrew “sleep spiral”, and occasionally ending up resting gently on the sea floor.

“It doesn’t seem possible that they would truly go into paralytic REM sleep during a dive, but it tells us something about the decision-making processes of these seals to see where in the water column they feel safe enough to go to sleep,” says Professor Terrie Williams, director of the Comparative Neurophysiology Lab at UCSC.

The surface is where elephant seals are most vulnerable to predators, which is why they only spend a minute or two there to breathe.

The EEG “sleepscape” might be useful for showing researchers where to direct conservation efforts.

“Normally, we’re concerned about protecting the areas where animals go to feed, but perhaps the places where they sleep are as important as any other critical habitat,” says Williams.

The results are published in Science.

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