Elephant seals drift off to sleep while diving far below the ocean surface

For the first time, scientists have recorded brain activity in a free-ranging, wild marine mammal, revealing the sleep habits of elephant seals during the months they spend at sea.

The new findings, published April 20 in Science, show that while elephant seals may spend 10 hours a day sleeping on the beach during the breeding season, they average just 2 hours of sleep per day when they are at sea on months-long foraging trips. They sleep for about 10 minutes at a time during deep, 30-minute dives, often spiraling downward while fast asleep, and sometimes lying motionless on the seafloor.

First author Jessica Kendall-Bar led the study as a UC Santa Cruz graduate student working with Daniel Costa and Terrie Williams, both professors of ecology and evolutionary biology at UCSC.

"For years, one of the central questions about elephant seals has been when do they sleep," said Costa, who directs UCSC's Institute of Marine Sciences. Costa's lab has led the UCSC elephant seal research program at Año Nuevo Reserve for over 25 years, using increasingly sophisticated tags to track the movements and diving behavior of the seals during their foraging migrations, when they head out into the North Pacific Ocean for as long as 8 months.

"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. "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."

In fact, during their months at sea, elephant seals rival the record for the least sleep among all mammals, currently held by African elephants, which appear to sleep just two hours per day based on their movement patterns.

"Elephant seals are unusual in that they switch between getting a lot of sleep when they're on land, over 10 hours a day, and two hours or less when they're at sea," said Kendall-Bar, who is currently a postdoctoral fellow at UC San Diego's Scripps Institution of Oceanography.

Elephant seals are most vulnerable to predators such as sharks and killer whales when they are at the surface in the open ocean, so they only spend a minute or two breathing at the surface in between dives.

"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," Kendall-Bar said.

Kendall-Bar developed a system that can reliably record brain activity (as an electroencephalogram or EEG) in wild elephant seals during their normal diving behavior at sea. With a neoprene headcap to secure the EEG sensors and a small data logger to record the signals, the system can be recovered when the animals return to the beach at Año Nuevo.

"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.

In addition to the EEG system, the seals carried time-depth recorders, accelerometers, and other instruments that allowed the researchers to track the seals' movements along with the corresponding brain activity. The recordings show diving seals going into the deep sleep stage known as slow-wave sleep while maintaining a controlled glide 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 maintain their body posture for several minutes before they transition into REM sleep, when they lose postural control and turn upside down," Kendall-Bar said.

At the depths at which this happens, the seals are usually negatively buoyant and continue to fall passively in a corkscrew spiral "like a falling leaf," Williams said. In shallower waters over the continental shelf, elephant seals sometimes sleep while resting on the seafloor.

"It doesn't seem possible that they would truly go into paralytic REM sleep during a dive, but it tells us something about the decision-making processes of these seals to see where in the water column they feel safe enough to go to sleep," said Williams, who directs the Comparative Neurophysiology Lab at UCSC.

In developing the new EEG instrument, Kendall-Bar first deployed it on elephant seals housed temporarily in the marine mammal facilities at UCSC's 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 lot of time watching sleeping seals," Kendall-Bar said. "Our team monitored instrumented seals to make sure they were able to reintegrate with the colony and were behaving naturally."

Some of those seals took short excursions into the water, but to observe diving behavior the researchers used a translocation procedure developed by Costa's lab. Juvenile female elephant seals outfitted with the EEG sensors and trackers were transported from Año Nuevo to Monterey and released on a beach at the southern end of Monterey Bay. Over the next few days, the animals would swim back to Año Nuevo across the deep Monterey Canyon, where their dive behavior is very similar to that seen during much longer foraging trips in the open ocean.

With data on brain activity and dive behavior from 13 juvenile female elephant seals, including a total of 104 sleep dives, Kendall-Bar developed a highly accurate algorithm for identifying periods of sleep based on the dive data alone. This enabled her to estimate sleep quotas for 334 adult seals using dive data recorded over several months during their foraging trips.

"Because of the dataset that Dan Costa has curated over 25 years of working with elephant seals at Año Nuevo, I was able to extrapolate our results to over 300 animals and get a population-level look at sleep behavior," said Kendall-Bar, who now plans to use similar methods to study brain activity in other species of seals and sea lions and in human freedivers.

Williams called Kendall-Bar's work on the project a tour de force. "It's an amazing feat to pull this off," she said. "She 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 results may be helpful for conservation efforts by revealing a "sleepscape" of preferred resting areas, Williams said. "Normally, we're concerned about protecting the areas where animals go to feed, but perhaps the places where they sleep are as important as any other critical habitat," she said.

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Bird flu associated with hundreds of seal deaths in New England in 2022

Researchers at Cummings School of Veterinary Medicine at Tufts University found that an outbreak of highly pathogenic avian influenza (HPAI) was associated with the deaths of more than 330 New England harbor and gray seals along the North Atlantic coast in June and July 2022, and the outbreak was connected to a wave of avian influenza in birds in the region.

The study was published on March 15 in the journal Emerging Infectious Disease.

HPAI is more commonly known as bird flu, and the H5N1 strain has been responsible for about 60 million poultry deaths in the U.S. since October 2020, with similar numbers in Europe. The virus was known to have spilled over from birds into mammals, such as mink, foxes, skunk, and bears, but those were mostly small, localized events. This study is among the first to directly connect HPAI to a larger scale mortality event in wild mammals.

The co-first authors on the paper -- virologist and senior scientist Wendy Puryear and post-doctoral researcher Kaitlin Sawatzki, who both work in the Runstadler Lab at Cummings School -- have been researching viruses in seals for years. They credit their findings in the new study to a unique and robust data set made possible by a collaboration with wildlife clinics and rehabilitation and response organizations in the region, in particular with Tufts Wildlife Clinic and director Maureen Murray, V03, associate clinical professor at Cummings School, and an author on the paper.

"We have a better resolution and greater depth of detail on this virus than before because we were able to sequence it and detect changes almost in real time," said Puryear. "And we have pairings of samples, sometimes literally from a bird and a seal on the same beach."

The clinic has been conducting avian influenza surveillance on birds and some mammals since January 2022, shortly after this strain of avian influenza took a trans-Atlantic journey from Europe into the U.S. Through this testing, the team found a wide range of flu viruses, including at least three strains that crossed the Atlantic, and they witnessed consistent waves of infection in birds.

At the same time, in collaboration with NOAA's Greater Atlantic Region Marine Mammal Stranding Network, they were able to screen nearly all seals that came through the network, whether or not the animal appeared sick. The stranding network is composed of experts from state and federal wildlife and fisheries agencies, non-profit rehabilitation and response facilities, aquariums, and academic institutions who respond to strandings.

"Because of the genetic data that we gathered, we were the first to see a strain of the virus that's unique to New England. The data set will allow us to more meaningfully address questions of which animals are passing the virus to which animals and how the virus is changing," said Sawatzki.

How HPAI Is Transmitted

In addition to poultry, H5N1 also has had a huge impact on wild birds, especially sea birds. Multiple locations around the globe have experienced large die-offs, such as recently in Peru, where the virus killed 60,000 pelicans, penguins, and gulls.

At the time of the seal mortality event in New England, the virus was hitting gulls particularly hard, the researchers found. There are lots of ways gulls and other birds may transmit the virus to seals, they said. Seals and sea birds are coastal animals living in the same areas that have environmental contact, if not direct contact, since they share the same water and shoreline. A seal may contract the virus if it comes in contact with a sick bird's excrement or water contaminated by that excrement, or if it preys upon an infected bird.

The accepted knowledge is that H5N1 is nearly 100% fatal for domestic and wild birds other than waterfowl, and the same is proving true when it comes to spillover in wild mammals. All the seals that tested positive for HPAI were deceased at the time of sampling or succumbed shortly after. None of the animals that tested positive recovered. However, it's possible some asymptomatic or recovered cases never came into the stranding networks.

In addition to the seal mortality event in New England, which was the first time H5N1 was detected in marine mammals in the wild, other locations have lost marine mammals to the virus. Peru announced about 3,500 sea lions died from the virus, Canada reported a seal mortality event along the St. Lawrence Estuary, and there was a similar event with seals in the Caspian Sea, according to news reports from Russia.

A hotly debated topic among scientists is whether there has been mammal-to-mammal transmission of HPAI between seals.

"It's not surprising that you might have transmission between the seals, because it has happened with low pathogenic avian influenza," said Puryear. "However, we can't say definitively whether or not there has been mammal-to-mammal transmission of HPAI."

"To get strong evidence of mammal-to-mammal transmission, you need two things: lots of infected animals and time," explained Sawatzki. "Time for the virus to mutate, and time for the mutated virus to be transmitted to another seal. As the virus acquires mutations, we can see shared mutations in the sequences that are specific only to mammals and that haven't been seen in a bird before. We had the numbers, but this outbreak didn't last long enough to provide evidence for seal-to-seal transmission."

The research team found evidence that the virus mutated in a small number of seals. But fortunately, they have not seen a case of bird flu in seals along the Atlantic coast since the end of last summer. However, stranding season is about to start for harbor seals and gray seals, so they are bracing themselves for what might happen.

Prevention and Risk to Humans

The risk to the public remains low, according to the Centers for Disease Control and Prevention. Since December 2021, less than 10 human cases of H5N1 have been reported globally, and those cases occurred in people with direct exposure to infected poultry. There are no documented cases of human transmission for this variant.

However, there is the possibility it could become a larger issue for human health. Avian influenza emerged in 1996, and since 2003, 868 cases of human infection with H5N1 have been reported worldwide, according to the World Health Organization. Of those, 457 were fatal, roughly a 50% fatality rate.

"And that's why people get nervous about it," Puryear said.

There is a single-dose vaccine available for poultry, but it's not currently administered on a large scale -- in part because of cost and logistics, and in part because there's some concern it may make future surveillance of the virus more difficult. There's not much that can be done in terms of responding to the virus for wildlife, particularly given the scale at which infection is occurring.

Biosecurity is important in limiting the ways in which the virus can spread between and within species, the researchers said. For example, wild birds should be kept separate from domestic birds, such as backyard chickens. In addition, thorough and timely surveillance of domestic animals and wildlife is key to understanding how the virus is evolving to prepare the best possible vaccines and treatments.

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