How can an air-breathing animal sleep safely in the open ocean, with killer whales and great whites prowling near the surface? For elephant seals, we now know the solution is to dive around 100 metres, then slowly sink up to a further 300 metres while they have a 10-minute power nap.
That is the finding of Jessica Kendall-Bar at the Scripps Institution of Oceanography in California and her colleagues, who have developed a system for recording the electrical brain activity of elephant seals, as well as their heart rates and movements. It consists of a headcap, plus several electrodes that can detect fin muscle contractions and motion trackers.
After tests on a few captive elephant seals, they attached this system to eight wild female seals with a removable adhesive. The team didn’t do this with males as their much larger size makes it harder to attach any kind of device. Three of these wild seals spent time in the open ocean while the devices were attached, with the others remaining in shallower waters or on beaches.
The devices allowed the team to identify how the seals move while in slow-wave, or deep, sleep, and also in rapid-eye-movement (REM), or dream, sleep. Using this, they could look back at previous time-and-depth recordings from more than 300 wild females and identify when they were sleeping and which sleep stage they were in.
The study shows that females out in the open sea sleep for less than 10 minutes at a time during dives and get only around 2 hours sleep per day in total, compared with around 10 hours when they are on land. At around 100 metres or so below the surface they go into slow-wave sleep and start drifting downwards. “I was really surprised by how much they could maintain their body position,” says Kendall-Bar.
But when the animals go into REM sleep, they do lose control. Their bodies turn upside down and sink in a characteristic spiral, with one animal going as deep as 377 metres. Then the seals wake and swim back to the surface. In waters less than 250 metres deep, the seals instead sleep motionless on the seafloor, like other seals do.
Other marine mammals have solved the how-to-sleep-at-sea problem in a different way. Only half of the brains of whales and dolphins sleep at a time, allowing the other half to watch out for danger and maintain breathing at the surface.
Fur seals use the same trick, reported Jerome Siegel at the University of California, Los Angeles, in 2018. Unusually, though, fur seals can do both half and whole-brain sleeping – when they are on land the proportion of whole-brain sleep increases.
Siegel says he isn’t surprised that elephant seals get by with just 2 hours of sleep. “We have previously reported that African elephants in the wild average 2 hours of sleep,” he says. His team has also found that dolphins and killer whales don’t sleep at all for months after birth.
“Sleep duration is not correlated with brain size or cognitive ability,” says Siegel.