Seals dozing on the beach may appear to be enjoying the ultimate life of leisure. However, groundbreaking research has revealed that for most of their lives elephant seals sleep just two hours daily in a series of short naps while performing deep dives.
The findings, revealed in the first study to record brain activity in a free-ranging, wild marine mammal, show that during the months they spend at sea, elephant seals rival the record for the least sleep among all mammals, currently held by African elephants. The seals were found to typically sleep in 10-minute bursts during deep, 30-minute dives, often spiralling downwards while dreaming, and occasionally lying down for a nap on the seafloor.
“For years, one of the central questions about elephant seals has been when do they sleep,” said Prof Daniel Costa, a marine ecologist at the University of California Santa Cruz and senior author. “We thought they must be sleeping during what we call drift dives, when they stop swimming and slowly sink, but we really didn’t know.”
Costa’s lab has been tracking elephant seals at the Año Nuevo reserve for more than 25 years, using increasingly sophisticated tags to track the movements and diving behaviour of the seals during their foraging migrations, when they head out into the north Pacific Ocean for as long as eight months.
“Now we’re finally able to say they’re definitely sleeping during those dives, and we also found that they’re not sleeping very much overall compared to other mammals,” Costa added.
The scientists believe that sleeping while diving allows the seals to avoid predation: they are most vulnerable to predators, including sharks and killer whales, while at the ocean surface, so they only spend a few minutes there to breath between dives.
“They’re able to hold their breath for a long time, so they can go into a deep slumber on these dives deep below the surface where it’s safe,” said Jessica Kendall-Bar, the paper’s first author, who developed the system as a graduate student at UC Santa Cruz and who now works at UC San Diego.
In the latest study, 13 young female seals were fitted with a neoprene headcap to secure sensors on the head that could reliably record brain waves in a scan known as an electroencephalogram or EEG. Time-depth recorders, accelerometers, and other instruments were also used to track the seals’ movements.
“I spent a lot of time watching sleeping seals,” Kendall-Bar said. “Our team monitored instrumented seals to make sure they were able to reintegrate with the colony and were behaving naturally.”
The recordings, collected from the seals during more than 100 dives, revealed that as they descended, seals entered a deep sleep stage known as slow-wave sleep while maintaining a controlled glide downward and then entered rapid-eye-movement (REM) sleep, when sleep paralysis causes them to turn upside down and drift downwards in a “sleep spiral”.
At the depths at which this happens, the seals are usually negatively buoyant and continue to fall passively in a corkscrew spiral like a falling leaf. In shallower waters over the continental shelf, elephant seals sometimes sleep while resting on the seafloor.
“They go into slow-wave sleep and maintain their body posture for several minutes before they transition into REM sleep, when they lose postural control and turn upside down,” Kendall-Bar said.
The team now plans to use similar methods to study brain activity in other species of seals, sea lions and also human free divers.
