Sleeping elephant seals fall through ocean's depths, and some even nap on the sea floor

By Nick Kilvert, ABC science and environment reporter

The ocean is full of weird and wonderful things, and as a general rule, the deeper you go, the weirder it gets.

Thanks to research published in Science on Friday, we can add freefalling elephant seals into the mix.

Researchers fitted elephant seals (Mirounga angustirostris) in Monterey Bay, California with electroencephalogram (EEG) sensors - the same type used to measure human brain activity.

They also added a few other measuring devices like accelerometers, heart monitors, and time and depth recorders.

Then they let the seals go on their way, and recovered the recording equipment when the seals returned to land.

The data showed that while spending long periods at sea, the elephant seals would make their way down to deep, dark water, and nod off. When they did, they'd enter a kind of freefall toward the ocean floor.

In the slow-wave sleep phase, they'd fall with a rigid body posture, straight down, but after entering REM (rapid eye-movement) sleep, they experienced body paralysis and spiralled toward the bottom like a falling leaf.

The seals would sleep less than 20 minutes at a time, sinking to depths of up to 377 metres.

Sometimes they'd even hit the sea floor and go on sleeping, according to study lead author, PhD candidate Jessica Kendall-Bar from the University of California Santa Cruz.

"Seals approaching the continental shelf - about 200m deep - would sometimes wake up upon coming into contact with the bottom," Kendall-Bar said.

"But more often they continued to sleep as they settled on the bottom on their bellies or backs."

Some marine animals like dolphins are capable of "unihemispheric sleep", where they keep one side of the brain awake and one eye open.

But elephant seals are more like us, in that they go into total torpor.

Scientists suspected elephant seals must sleep underwater because the animals spend so much time at sea.

The northern elephant seals in this study go on foraging trips for seven months or more, covering more than 10,000km. They are capable of diving to around 2000m.

Earlier studies had fitted elephant seals with tracking instruments that showed they were making repetitive, slow dives.

But measuring the animals' brain activity has added another layer to the story, according to study co-author Professor Terrie Williams of the University of California Santa Cruz.

"Given the duration of time the seals are at sea they had to sleep sometime," Williams said.

"By measuring brain activity, we now know they're sleeping. And not any old sleep, but deep, paralytic sleep - that is when a human would start snoring."

Behaviour likely to help avoid sharks, killer whales

The study found that, on average, the seals managed about two hours sleep a day over a seven-month period.

That means they rival the African elephant for the record of lowest amount of sleep for a mammal. African elephants sleep about two hours a day all year round.

Though some seals were found to be sleeping on the sea floor in quite shallow water, the shallowest instance of the spiral-shaped falling during REM sleep was measured at 82m. The deepest was 377m.

Their predators such as killer whales and great white sharks tend to hunt in shallower water, explained co-author Daniel Costa of the University of California Santa Cruz.

"Where is the one place that all marine mammals have to go? The surface to breathe. So if you are a predator looking for prey, [hunting at the surface is] going to be much more effective than trying to find prey in the water column," Professor Costa said.

"So elephant seals spend as little time at the surface as possible."

Logically it is safest to nap, forage, even feed your babies, away from your predators.

Elephant seals come ashore for brief periods to mate, moult and give birth.

They also spend a lot of this shore leave catching up on lost sleep.

The researchers attached their custom-made monitoring equipment to seals on beaches in California.

Elephant seals have high site fidelity, meaning they'll come back to the same beach over and over, Kendall-Bar said.

"Even after trips to sea that last multiple days, they come right back to the beach, where a team of specialists remove the equipment."

Macquarie University marine ecologist Professor Robert Harcourt said the data could also be used to figure out what else was going on deep in the ocean.

"When they go to sea they've either had a baby or the males have been breeding. So they're skinny," said Harcourt, who was not involved with the study.

"As they hit patches of food, they get fatter. And what that means is that they switch over from being negatively buoyant to positively buoyant and so they drift slower down."

In other words, by recording the rate at which the seals sink, or don't, and where, we can tell how much weight they've gained.

"Which is phenomenal because ... we can use that to work out where all the food is in the Southern Ocean and north-eastern Pacific.

"It's a really amazing study. They've got an amazing dataset."

-ABC