Northern elephant seals sleep in depth to avoid predators

Northern elephant seals sleep while drifting hundreds of meters below the surface of the sea – at depths where their predators would not normally lurk.

American researchers tracked the animals, recording their brain activity while seals swim thousands of kilometers.

The mammals, which reach depths of up to 2,500 feet (760 metres), sleep for just two hours a day in what the researchers describe as “nap-like sleep dives.”

The results are published in the journal Science.

University of California Santa Cruz researcher Jessica Kendall Barr and her colleagues developed a non-invasive adhesive tag to track and monitor brain activity in wild northern elephant seals off the coast of California.

They followed eight wild mammals on their foraging travels, which lasted about seven months and spanned more than 6,200 miles. They recorded the animals’ brain activity, heart rate, movement, and body position.

“We have developed a scientific ‘sleep signature’ by studying their behavior and physiology for many years,” explained Ritika Mukherjee of the University of Oxford, who was involved in the study.

It revealed that, at depths of more than 984 feet, seals will sleep and descend in what scientists describe as “sleep snails” for 20 minutes at a time. “It looks like fallen leaves,” said Ms. Mukherjee.

Lead researcher Terry Williams, from the University of California, Santa Cruz, told BBC News: “The thing I find fascinating is that any mammal will fall asleep while drifting hundreds of meters below the water’s surface.

“This is not a light sleep but a true paralytic sleep, a deep sleep that would make humans snore. Remarkably, the seal’s brain reliably wakes them from it before they run out of oxygen.”

“Imagine waking up at the bottom of a pool—it sends shivers down your spine.”

The scientists say their study mapped the sea seals’ “nap maps” – suggesting that sleeping areas may be just as important as these animals’ active hunting grounds.

“They show us what their world looks like and help us understand what they do and when they do it, so we can understand how to avoid getting in their way,” explained Ms Mukherjee.