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 exams on a couple of captive elephant seals, they hooked up this technique to eight wild feminine seals with a detachable adhesive. The staff didn’t do that with males as their a lot bigger dimension makes it tougher to connect any type of machine. Three of those wild seals frolicked within the open ocean whereas the gadgets had been hooked up, with the others remaining in shallower waters or on seashores.
The gadgets allowed the staff 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 might 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 research exhibits that females out within the open sea sleep for lower than 10 minutes at a time throughout dives and get solely round 2 hours sleep per day in whole, 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 might preserve their physique place,” says Kendall-Bar.
However when the animals go into REM sleep, they do lose management. Their our bodies flip the other 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 different seals do.
Different marine mammals have solved the how-to-sleep-at-sea downside differently. Solely half of the brains of whales and dolphins sleep at a time, permitting the opposite half to be careful for hazard and preserve respiratory on the floor.
Fur seals use the identical 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 now have beforehand reported that African elephants within the wild common 2 hours of sleep,” he says. His staff has additionally discovered that dolphins and killer whales don’t sleep in any respect for months after delivery.
“Sleep period is just not correlated with mind dimension or cognitive means,” says Siegel.
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