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”. “Our team monitored instrumented seals to make sure they were able to reintegrate with the colony and were behaving naturally.” “I spent a lot of time watching sleeping seals,” Kendall-Bar said. ![]() Time-depth recorders, accelerometers, and other instruments were also used to track the seals’ movements. ![]() 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. “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. 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. “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. Tapirs inhabit jungle and forest regions of South and Central America and Southeast Asia. “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.”Ĭosta’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. Tapirs (/ t e p r, t e p r, t p r / TAY-pr, TAY-peer, t-PEER) are large, herbivorous mammals belonging to the family Tapiridae.They are similar in shape to a pig, with a short, prehensile nose trunk. “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.
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