Elephant seals nap while they dive, study finds, shedding new light on sleep

Elephant seals avoid predators by sleeping while they dive, a new study has found.

The research published Thursday in Science sheds new light on the universal mammal need for sleep and the species’ persistence through centuries of widespread hunting.

The large aquatic mammals spend up to seven months at sea at a time — most of it on repeated back-to-back dives hundreds of feet below an often-hazardous surface.

Daniel Costa of the University of California Santa Cruz told The Hill that this constant activity led to a consistent question.

“Whether they’re at sea for two and a half months or eight months, you look at the dive record, and it’s just one dive after another — 20 or 25 minutes underwater, then back to the surface, nonstop,” he said. 

“So: when do they sleep?”

To answer that question, the team fitted wild seals with brain-measuring electrodes (secured beneath a skin-tight custom swimmer’s cap designed by UC Santa Cruz biotechnologist and study coauthor Jessica Kendall-Bar).

They found that the seals take repeated power naps while diving — falling into deep sleep on their long, drifting descent to their undersea fishing grounds.

Then they wake up on the seafloor, catch fish until they are low on air, and surface to start the whole process again.

That means that what for humans is a potentially deadly condition is, for elephant seals, a lifesaving one: sleep apnea.

Sleep apnea is the name of a disorder for breath-holding during sleep — which can be fatal in humans. But in elephant seals it’s a normal part of development: By the time they are a few months old, young seals “routinely hold their breath when they sleep,” Costa said. 

“If we understand how the sleep cycle is regulated by what’s the fundamental components of how sleep functions normally in elephant seals, that might give us some insight into what goes wrong when humans go to sleep apnea,” he added.

In another way, the strange sleep patterns of elephant seals actually resemble those of humans more than other sea mammals.

The scientists found that the seals sleep so deeply in these dives that they hit REM sleep — the part of sleep marked in humans by both dreams and the physical paralysis that keeps us from acting them out.

Dolphins and smaller, fur-bearing seals cannot fully nap underwater. Instead, they use “hemispheric” sleeping, in which half of the brain sleeps at a time. 

Elephant seals, by contrast, sleep much more like people — when they’re out, they’re out. Why don’t they sleep in shifts more like dolphins and seals? 

“I suspect it’s because they don’t have to,” Costa said. Elephant seals are far better divers than those other species, he noted. “I also suspect going into depth is a much more efficient way of not being eaten.”

The seals’ unique sleeping habits point to the upside-down geography of the oceans as experienced by the aquatic mammals: one in which the ocean’s surface, to which they must continually return for air, is a place of danger.

That’s true even for elephant seals, who as the largest members of the seal family achieve sizes of up to two tons (4,000 pounds) — much of that composed of the blubber that keeps them warm during long fishing voyages.

That blubber makes them a rich prize for the pack-hunting predators that stake out the surface. These include pods of highly intelligent orca whales, roving great white sharks and — historically — shipborne crews of humans.

“If you think about the surface of the ocean and you’re an air-breathing critter — like a seal or a bird or a dolphin — you’ve always got to come back to the surface,” Costa said. 

That means an intelligent predator doesn’t have to chase its prey into the depths: It simply has to wait. “If it just sits at the surface or swims around. It’s gonna have a much greater chance of finding you — versus trying to swim through the water column and find you,” Costa said. 

For elephant seals, that makes the deep ocean a place of safety and a good location for sleeping off the effects of a big meal. 

By fitting the seals with accelerometers, the UC team found that after a series of jagged “feeding dives” — in which the seals traversed the seafloor’s peaks and valleys chasing fish — they would resurface and sink back down in long, slow “drift dives” back to the seafloor.

“We think that they are sleeping and processing their food, which makes sense, because what happens when you have a big meal? You want to take a nap,” Costa said.

The adaptation that allows the seals to take those naps underwater may have helped them squeak through a deadly evolutionary bottleneck. From the 1700s to the early 1900s, the waters of their North Pacific breeding grounds became the site of a killing frenzy.

Driven by commercial and industrial demands for fur and fat rendered from seafaring mammals like whales, dolphins, seals and otters, whaling and hunting expeditions drove species like the sea otter, right whale and elephant seal to the very brink of extinction.

Unlike sharks and killer whales, humans could land on seal colonies and kill the creatures at will — and by the 1920s, the elephant seal population hung on by a thread.

Just one colony of perhaps a few dozen individuals remained, taking refuge on an offshore island in the Mexican state of Baja California. 

But after the Mexican government passed laws protecting the mammals in 1922, the elephant seal population began to expand again almost immediately — with the U.S. following suit as seals began to be spotted in California waters, according to the state.

Further legislation in 1972 made it illegal to kill, harass or even annoy marine mammals deemed necessary to the functioning of their ecosystems — including northern elephant seals, which have since rebounded to a population of perhaps 200,000.

That’s a two-thousandfold increase from their lowest population levels and a sign that some endangered species need humans to step out of the way, Costa said.

In contrast to more finicky species like the California condor, whose recovery has relied on intensive captive breeding programs, “seals just needed a protector,” he said. 

“We didn’t have to do any management. All we did was stand back — and they were able to do this recovery on their own.”