A younger northern elephant seal (Mirounga angustirostris) within the Pacific Ocean
Nature Image Library/Alamy
How can an air-breathing animal sleep safely within the open ocean, with killer whales and nice whites prowling close to the floor? For elephant seals, we now know the answer is to dive round 100 metres, then slowly sink as much as an additional 300 metres whereas they’ve a 10-minute energy nap.
That’s the discovering of Jessica Kendall-Bar on the Scripps Establishment of Oceanography in California and her colleagues, who’ve developed a system for recording {the electrical} mind exercise of elephant seals, in addition to their coronary heart charges and actions. It consists of a headcap, plus a number of electrodes that may detect fin muscle contractions and movement trackers.
After assessments on just a few captive elephant seals, they hooked up this technique to eight wild feminine seals with a detachable adhesive. The workforce didn’t do that with males as their a lot bigger measurement makes it more durable to connect any form of system. Three of those wild seals frolicked within the open ocean whereas the units had been hooked up, with the others remaining in shallower waters or on seashores.
The units allowed the workforce to determine how the seals transfer whereas in slow-wave, or deep, sleep, and in addition in rapid-eye-movement (REM), or dream, sleep. Utilizing this, they may look again at earlier time-and-depth recordings from greater than 300 wild females and determine once they had been sleeping and which sleep stage they had been in.
The examine reveals that females out within the open sea sleep for lower than 10 minutes at a time during dives and get solely round 2 hours sleep per day in complete, in contrast with round 10 hours when they’re on land. At round 100 metres or so beneath the floor they go into slow-wave sleep and begin drifting downwards. “I used to be actually stunned by how a lot they may keep their physique place,” says Kendall-Bar.
However when the animals go into REM sleep, they do lose management. Their our bodies flip the wrong way up and sink in a attribute spiral, with one animal going as deep as 377 metres. Then the seals wake and swim again to the floor. In waters lower than 250 metres deep, the seals as an alternative sleep immobile on the seafloor, like other seals do.
Different marine mammals have solved the how-to-sleep-at-sea drawback differently. Solely half of the brains of whales and dolphins sleep at a time, permitting the opposite half to be careful for hazard and keep respiration on the floor.
Fur seals use the same trick, reported Jerome Siegel on the College of California, Los Angeles, in 2018. Unusually, although, fur seals can do each half and whole-brain sleeping – when they’re on land the proportion of whole-brain sleep will increase.
Siegel says he isn’t stunned that elephant seals get by with simply 2 hours of sleep. “We’ve got beforehand reported that African elephants within the wild common 2 hours of sleep,” he says. His workforce has additionally discovered that dolphins and killer whales don’t sleep in any respect for months after beginning.
“Sleep length isn’t correlated with mind measurement or cognitive potential,” says Siegel.
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