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Elephant seals sleep dive into the depths to avoid predators

Elephant seals get the most sleep during breeding season, while they're lying on the beach. But scientists now have an idea of how they sleep while they're out at sea on monthslong foraging trips.

(CN) — Spinning like corkscrews as they dove to the depths of the ocean, the elephant seals donning headcaps drifted into a deep sleep — but only for 10 minutes at a time.

That's what researchers observed in the first recordings of brain activity in free-ranging, wild elephant seals that reveal their sleep habits during the months they spend at sea during foraging trips, according to a study published Thursday in the journal Science. While elephant seals spend about 10 hours a day sleeping on the beach during their breeding season, they average only two hours of sleep a day while at sea, and they sleep for only minutes at a time during 30-minute dives.

“For years, one of the central questions about elephant seals has been when do they sleep,” said University of California Santa Cruz Institute of Marine Sciences Director Daniel Costa.

"The dive records show that they are constantly diving, so 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 said.

Thanks to the initiative of lead author Jessica Kendall-Bar, who developed a system to reliably monitor brain activity in wild elephant seals, the dataset created by Costa over his 25 years of working with elephant seals, and the work of the other authors of the study, including Terrie Williams, the director of the Comparative Neurophysiology Lab at UC-Santa Cruz, that hypothesis has been proven to be accurate.

“It’s an amazing feat to pull this off,” Williams said. “[Kendall-Bar] developed an EEG system to work on an animal that’s diving several hundred meters in the ocean. Then she uses the data to create data-driven animations so we can really visualize what the animal is doing as it dives through the water column.”

The scientists attached waterproof neoprene headcaps to juvenile elephant seals and released them to the sea, at which point the headcaps would record the seals’ brain activity.

“We used the same sensors you’d use for a human sleep study at a sleep clinic and a removable, flexible adhesive to attach the headcap so that water couldn’t get in and disrupt the signals,” Kendall-Bar said.

She said the seals can hold their breath for around 10 to 30 minutes, depending on the age of the animal, and use that ability to swim and sleep depths away from the surface in order to avoid predators.

“Typical dives for an adult female last about 30 minutes, while juveniles have shorter dives," said Kendall-Bar, a graduate student at UC-Santa Cruz. "Their naps are also scaled proportionally such that average underwater sleep durations for juveniles lasted five to 10 minutes (with five to 10 minutes to descend and ascend) while adult females had 10-minute naps with about 10 minutes for descent and 10 minutes for ascent).”

Data loggers showed that the recorded seals maintained a controlled downward slide as they entered slow-wave sleep, and the study says that they turned upside down and drifted downwards during REM sleep. The seals are usually negatively buoyant at that point, passively falling into the depths at a corkscrew spiral.

Measuring this was easier with juvenile female or smaller juvenile elephant seals as the researchers could attach instruments to the smaller juveniles who they could transport to Monterey Bay, 40 miles away, with little issue, said Costa.

Adult female elephant seals are four times larger than juveniles and while Costa hopes future studies can measure them directly, he said that currently researchers have better success in retrieving sensors from juveniles who leave for a few days instead of adult females who leave for two and a half months.

Male elephant seals are not ideal test subjects because their higher drug sensitivity makes it harder to anesthetize them on the beach, according to Costa, who adds that males have a higher mortality rate at sea because they push themselves to mate while on the beach. Unlike males, females tend to return to the beaches where they birthed their pups, so researchers retrieve half of their sensors back from the males but retrieve 80% to 90% back from the females.

Along with future studies, Kendall-Bar believes their research can aid conservation.

“This study lays the groundwork for additional studies of sleep at sea or in the wild and provides a framework where a couple animals can reveal information at the population level with very broad implications,” says Kendall-Bar. “I think it would be interesting to apply what we know about sleep in northern elephant seals and use that to interpret patterns in other true seals that are of high conservation concern, like the endangered Hawaiian monk seal.”

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Categories / Environment, Science

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