Elephant seals sleep underwater for only two hours per day

In a groundbreaking study, scientists have successfully recorded brain activity in a free-ranging, wild marine mammal, shedding light on the sleep habits of elephant seals during their months-long foraging trips at sea. 

The study, led by first author Jessica Kendall-Bar, a graduate student from UC Santa Cruz, in collaboration with Professors Daniel Costa and Terrie Williams, was published in the journal Science.

Elephant seals spend a significant amount of time on land during the breeding season, averaging around 10 hours of sleep per day. However, during their foraging trips at sea, which can last up to 8 months, the seals manage to sleep for just 2 hours per day. 

These brief, 10-minute sleep intervals occur during deep, 30-minute dives, often with the seals spiraling downward while fast asleep and sometimes resting motionless on the seafloor.

Daniel Costa, the director of UCSC’s Institute of Marine Sciences, has led the university’s elephant seal research program at Año Nuevo Reserve for over 25 years. His lab has employed increasingly advanced tracking technology to monitor the seals’ movements and diving behavior during their migrations into the North Pacific Ocean. 

“For years, one of the central questions about elephant seals has been when do they sleep,” Costa said. “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.”

Elephant seals sleep less than most animals

The recent findings have confirmed the team’s suspicions that the seals indeed sleep during those drift dives. Surprisingly, the study also revealed that elephant seals have one of the lowest sleep durations among mammals. Their sleep patterns rival the record currently held by African elephants. Based on their movement patterns, we believe they sleep only two hours per day.

Kendall-Bar, now a postdoctoral fellow at UC San Diego’s Scripps Institution of Oceanography, commented on the unusual nature of the seals’ sleep habits: “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.”

While at sea, elephant seals are most vulnerable to predators such as sharks and killer whales when they surface to breathe, which is why they only spend a minute or two at the surface 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,” said Kendall-Bar.

Jessica Kendall-Bar, utilized a neoprene headcap to secure electroencephalogram (EEG) sensors and a small data logger to record the signals. The researchers retrieved them later when the animals returned 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 were fitted with time-depth recorders, accelerometers, and other instruments, enabling the researchers to track the animals’ movements in tandem with their brain activity.

Analyzing the research data

The recorded data revealed that diving seals enter a deep sleep stage known as slow-wave sleep while maintaining a controlled glide downward. This is followed by a transition into rapid-eye-movement (REM) sleep, during which 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,” explained Kendall-Bar. 

Terrie Williams, who directs the Comparative Neurophysiology Lab at UCSC, added that at the depths where this occurs, the seals are usually negatively buoyant and continue to fall passively in a corkscrew spiral “like a falling leaf.” Researchers have observed elephant seals sleeping while resting on the seafloor in shallower waters over the continental shelf.

“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,” Williams said.

Creating a new instrument for the study

To develop the new EEG instrument, Kendall-Bar initially deployed it on elephant seals temporarily housed in the marine mammal facilities at UCSC’s Long Marine Laboratory. They subsequently deployed the instrument on animals in the elephant seal colony at Año Nuevo Reserve north of Santa Cruz. There, 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.”

After collecting 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 to identify periods of sleep based solely on the dive data. This allowed 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,” Kendall-Bar said. She now plans to apply similar methods to study brain activity in other species of seals, sea lions, and human freedivers.

Researchers express great pride in the study’s results

Williams lauded Kendall-Bar’s work on the project as a “tour de force,” stating, “It’s an amazing feat to pull this off. 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.”

These findings may prove valuable for conservation efforts by identifying a “sleepscape” of preferred resting areas. “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.

The study provides fascinating insights into the adaptive sleep patterns of these remarkable marine mammals and opens new avenues for further research on sleep in other aquatic species.

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. The National Science Foundation and the Office of Naval Research partially funded this work.

More about elephant seals

Elephant seals are large marine mammals belonging to the family Phocidae, which includes two species: the northern elephant seal (Mirounga angustirostris) and the southern elephant seal (Mirounga leonina). 

These fascinating creatures display unique characteristics and behaviors that set them apart from other marine mammals.

Size and appearance

Elephant seals are among the largest seals, with adult males being significantly larger than females. Northern elephant seal males can weigh up to 4,500 pounds (2,000 kg) and reach lengths of up to 16 feet (5 meters), while southern elephant seal males can weigh up to 8,800 pounds (4,000 kg) and measure up to 20 feet (6 meters) long. The males of both species have a proboscis-like, elongated nose, which is the origin of their name.

Sexual dimorphism and mating

Elephant seals exhibit considerable sexual dimorphism, with males being much larger and having more distinctive features than females. During the breeding season, males compete fiercely for access to females, often engaging in violent fights to establish dominance and secure mating rights.

Breeding and molting

Elephant seals give birth to their pups and mate during the breeding season, which occurs between December and March for northern elephant seals and between August and November for southern elephant seals. Their annual catastrophic molting is also a well-known characteristic of them. This is when they shed the entire outer layer of their skin and fur in a relatively short period.

Diving and foraging

Their exceptional diving abilities have made elephant seals renowned. They can dive to depths exceeding 5,000 feet (1,500 meters) and hold their breath for over an hour, which allows them to forage for food in deep ocean waters. Their diet mainly consists of fish and squid.

Migration and habitat

Both northern and southern elephant seals undertake long-distance migrations twice a year between their feeding and breeding grounds. Elephant seals are found along the coasts of the eastern Pacific Ocean, from Baja California in Mexico to Alaska, including Northern regions. Southern elephant seals inhabit the sub-Antarctic and Antarctic regions.

Conservation status

Elephant seal populations faced significant threats in the past. This is primarily due to commercial hunting for their blubber, which was used to produce oil. However, with the implementation of protective measures, their numbers have rebounded. Northern elephant seals are currently listed as “Least Concern” by the International Union for Conservation of Nature (IUCN). Southern elephant seals are classified as “Lower Risk.”

Adaptations

Elephant seals have several physiological adaptations that enable them to thrive in their deep-diving, cold-water environments, including high levels of oxygen-storing myoglobin in their muscles, a flexible ribcage that can withstand high pressure, and an impressive ability to conserve oxygen by reducing their heart rate and diverting blood flow to essential organs during deep dives.

Research on elephant seals continues to reveal fascinating insights into their behavior, physiology, and ecological role, helping scientists better understand these unique marine mammals and informing conservation efforts to protect their populations.

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