By: Melody Dou
Scientists recently discovered that northern elephant seals only get around 2 hours of sleep every day when migrating thousands of miles in the Pacific Ocean to search for food such as fish and squid. They can swim 5,000 miles off shore and would only come back twice a year to molt and mate. While at sea, the seals would dive more than 3,000 feet underwater.
So how do northern elephant seals sleep?
To answer this question, researchers put caps and motion sensors on two northern elephant seals to monitor their brain waves and movements while they slept. The seals were taken from Año Nuevo State Park in California, put on a truck, and released approximately 60 km south in Monterey.
“They’ll have this strong urge to come back to the colony,” said Jessica Kendall-Bar, one of the researchers involved in this study. To return home, the seals must cross the Monterey Canyon, like canyons in the Pacific Ocean.
After reviewing the data, researchers found that the seals would first dive 60 to 100 meters (200 to 360 feet), switch to a glide, and begin to fall asleep while holding itself upright. This would be “a time when the brain activity slows down and becomes more synchronous,” said Kendall-Bar. About a minute or so later, the seal would transition into a deep stage of sleep known as REM (rapid eye-movement) sleep.
“They flip upside down and they start this sleep spiral where they spin 360 degrees while sinking towards the ocean floor just like a falling leaf,” Jessica Kendall-Bar said. A northern elephant seal can go hundreds of meters below sea level during a nap. Sleeping deep in the ocean also helps seals avoid predators like sharks and killer whales who hunt closer to the surface.
Finally after a 5-10 minute nap, a seal would go back to the surface.
Researchers then observed tracked motions of 334 other northern elephant seals to conclude that they sleep for an average of 2 hours a day during these ocean voyages. However, the seals sleep nearly 11 hours a day on land.
It was found that the seals like to sleep near the coast. “We can build protected areas that conserve not only the areas that are really important for the animals to feed, but also those areas that they might go to sleep,” said Kendall-Bar. This way northern elephant seals could have a safe space to take naps.
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