Researchers have discovered that elephant seals only average 2 hours of sleep per day when they are at sea on long foraging journeys, with short naps happening throughout deep, 30-minute dives. The study, led by Jessica Kendall-Bar at UC Santa Cruz, is the first to record brain activity in a wild marine mammal, shedding light on their special sleep routines. Elephant seals sleep about 10 hours a day on the beach, but during months-long foraging journeys at sea they balance simply 2 hours of sleep per day. These 2-month-old northern elephant seals are sleeping on the beach at Año Nuevo State Park. Cetaceans (dolphins and whales) and otariids (fur seals and sea lions) keep one side of their brains awake while the other is sleeping (unihemispheric sleep).
Scientists have actually discovered that elephant seals just balance two hours of sleep daily when they are at sea on long foraging trips, with brief naps taking place throughout deep, 30-minute dives. The study, led by Jessica Kendall-Bar at UC Santa Cruz, is the very first to record brain activity in a wild marine mammal, shedding light on their distinct sleep routines. Elephant seals are vulnerable to predators at the ocean surface area, so they invest minimal time breathing there and rather fall under a deep sleep during their dives in deeper waters.
Brainwave patterns show elephant seals take brief naps while holding their breath on deep dives, balancing just 2 hours of sleep per day while at sea.
For the very first time, scientists have tape-recorded brain activity in a free-ranging, wild marine mammal, revealing the sleep routines of elephant seals throughout the months they spend at sea.
The brand-new findings, released on April 20 in the journal Science, reveal that while elephant seals may invest 10 hours a day sleeping on the beach throughout the breeding season, they balance just 2 hours of sleep per day when they are at sea on months-long foraging journeys. They sleep for about 10 minutes at a time throughout deep, 30-minute dives, frequently spiraling downward while quick asleep, and often lying motionless on the seafloor.
First author Jessica Kendall-Bar led the research study as a University of California, Santa Cruz (UCSC) graduate student working with Daniel Costa and Terrie Williams, both teachers of ecology and evolutionary biology at UCSC.
Elephant seals sleep about 10 hours a day on the beach, but during months-long foraging trips at sea they average simply 2 hours of sleep each day. These 2-month-old northern elephant seals are sleeping on the beach at Año Nuevo State Park. Credit: Photo by Jessica Kendall-Bar, NMFS 23188
” For years, among the central concerns about elephant seals has been when do they sleep,” stated Costa, who directs UCSCs Institute of Marine Sciences. Costas laboratory has actually led the UCSC elephant seal research program at Año Nuevo Reserve for over 25 years, using significantly advanced tags to track the movements and diving habits of the seals throughout their foraging migrations, when they head out into the North Pacific Ocean for as long as 8 months.
” The dive records reveal that they are constantly diving, so we believed they should be sleeping throughout what we call drift dives, when they stop swimming and slowly sink, but we really didnt know,” Costa said. “Now were lastly able to state theyre definitely sleeping throughout those dives, and we also found that theyre not sleeping quite general compared to other mammals.”
Data-driven animation showing the phases of a sleeping dive to 263 meters. Credit: Animation by Jessica Kendall-Bar
In truth, throughout their months at sea, elephant seals measure up to the record for the least sleep amongst all mammals, currently held by African elephants, which appear to sleep simply 2 hours each day based on their motion patterns.
” Elephant seals are unusual in that they switch in between getting a lot of sleep when theyre on land, over 10 hours a day, and two hours or less when theyre at sea,” said Kendall-Bar, who is presently a postdoctoral fellow at UC San Diegos Scripps Institution of Oceanography.
Elephant seals are most susceptible to predators such as sharks and killer whales when they are at the surface area outdoors ocean, so they only spend a minute or two breathing at the surface in between dives.
” Theyre able to hold their breath for a long time, so they can enter into a deep rest on these dives deep below the surface area where its safe,” Kendall-Bar stated.
Cetaceans (whales and dolphins) and otariids (fur seals and sea lions) keep one side of their brains awake while the other is asleep (unihemispheric sleep). In most other mammals, including phocids (true seals) and humans, both hemispheres of the brain are asleep at the very same time. Credit: Graphic by Jessica Kendall-Bar
Kendall-Bar established a system that can dependably record brain activity (as an electroencephalogram or EEG) in wild elephant seals during their regular diving habits at sea. With a neoprene headcap to secure the EEG sensors and a small data logger to tape-record the signals, the system can be recuperated when the animals go back to the beach at Año Nuevo.
” We utilized the same sensing units you d utilize for a human sleep research study at a sleep center and a removable, flexible adhesive to attach the headcap so that water could not get in and interrupt the signals,” Kendall-Bar said.
In addition to the EEG system, the seals carried time-depth recorders, accelerometers, and other instruments that allowed the scientists to track the seals motions together with the matching brain activity. The recordings reveal diving seals going into the deep sleep stage called slow-wave sleep while keeping a controlled slide downward, then transitioning into rapid-eye-movement (REM) sleep, when sleep paralysis causes them to turn upside down and drift downwards in a “sleep spiral.”
” They go into slow-wave sleep and keep their body posture for several minutes before they shift into REM sleep, when they lose postural control and turn upside down,” Kendall-Bar stated.
At the depths at which this occurs, the seals are generally adversely resilient and continue to fall passively in a corkscrew spiral “like a falling leaf,” Williams stated. In shallower waters over the continental shelf, elephant seals sometimes sleep while resting on the seafloor.
When elephant seals go into rapid-eye-movement (REM) sleep throughout deep dives, sleep paralysis causes them to turn upside down and wander downwards in a “sleep spiral.” This data-driven graphic programs sleeping postures every 20 seconds, with accompanying 30-second segments of EEG traces in the background. Credit: Graphic by Jessica Kendall-Bar
” It doesnt seem possible that they would really go into paralytic REM sleep during a dive, but it tells us something about the decision-making procedures of these seals to see where in the water column they feel safe adequate to go to sleep,” stated Williams, who directs the Comparative Neurophysiology Lab at UCSC.
In developing the new EEG instrument, Kendall-Bar initially released it on elephant seals housed briefly in the marine mammal facilities at UCSCs Long Marine Laboratory. The next step was to deploy it on animals in the elephant seal colony at Año Nuevo Reserve north of Santa Cruz, where researchers could observe the animals on the beach.
” I spent a great deal of time watching sleeping seals,” Kendall-Bar stated. “Our group kept track of instrumented seals to ensure they were able to reintegrate with the nest and were acting naturally.”
Some of those seals took short excursions into the water, however to observe diving habits the researchers used a translocation treatment developed by Costas lab. Juvenile female elephant seals equipped with the EEG sensing units and trackers were transported from Año Nuevo to Monterey and launched on a beach at the southern end of Monterey Bay. Over the next couple of days, the animals would swim back to Año Nuevo across the deep Monterey Canyon, where their dive behavior is very comparable to that seen throughout a lot longer foraging trips outdoors ocean.
With data on brain activity and dive behavior from 13 juvenile female elephant seals, consisting of an overall of 104 sleep dives, Kendall-Bar developed a highly accurate algorithm for recognizing periods of sleep based upon the dive data alone. This enabled her to approximate sleep quotas for 334 adult seals using dive information recorded over a number of months throughout their foraging trips.
” Because of the dataset that Dan Costa has actually curated over 25 years of dealing with elephant seals at Año Nuevo, I was able to extrapolate our outcomes to over 300 animals and get a population-level take a look at sleep habits,” said Kendall-Bar, who now prepares to utilize similar approaches to study brain activity in other types of seals and sea lions and in human freedivers.
Williams called Kendall-Bars deal with the project a tour de force. “Its an amazing feat to pull this off,” she said. “She developed an EEG system to deal with an animal thats diving a number of hundred meters in the ocean. She utilizes the data to develop data-driven animations so we can actually picture what the animal is doing as it dives through the water column.”
The results may be handy for conservation efforts by revealing a “sleepscape” of preferred resting locations, Williams said. “Normally, were concerned about securing the areas where animals go to feed, but maybe the locations where they sleep are as crucial as any other critical environment,” she stated.
Reference: “Brain activity of diving seals reveals short sleep cycles at depth” by Jessica M. Kendall-Bar, Terrie M. Williams, Ritika Mukherji, Daniel A. Lozano, Julie K. Pitman, Rachel R. Holser, Theresa Keates, Roxanne S. Beltran, Patrick W. Robinson, Daniel E. Crocker, Taiki Adachi, Oleg I. Lyamin, Alexei L. Vyssotski, Daniel P. Costa, 20 April 2023, Science.DOI: 10.1126/ science.adf0566.
In addition to Kendall-Bar, Costa, and Williams, the coauthors of the paper include Daniel Lozano, Rachel Holser, Theresa Keates, Roxanne Beltran, Patrick Robinson, and Taiki Adachi at UC Santa Cruz; Ritika Mukherji at University of Oxford; Julie Pitman at Sleep Health MD in Santa Cruz; Daniel Crocker at Sonoma State University; Oleg Lyamin at UCLA; and Alexei Vyssotski at the University of Zurich and Swiss Federal Institute of Technology. This work was moneyed in part by the National Science Foundation and the Office of Naval Research.