Science
Elephant seals drift off to sleep while diving far below the ocean surface
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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 eight 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.
More information:
Jessica M. Kendall-Bar, Brain activity of diving seals reveals short sleep cycles at depth, Science (2023). DOI: 10.1126/science.adf0566. www.science.org/doi/10.1126/science.adf0566
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The James Webb Space Telescope has found traces of water vapor in the atmosphere of a super-hot gas giant exoplanet that orbits its star in less than one Earth day.
The exoplanet in question, WASP-18 b, is a gas giant 10 times more massive than the solar system‘s largest planet, Jupiter. The planet is quite extreme, as it orbits the sun-like star WASP-18, which is located some 400 light-years away from Earth, at an average distance of just 1.9 million miles (3.1 million kilometers). For comparison, the solar system’s innermost planet, Mercury, circles the sun at a distance of 39.4 million miles (63.4 million km).
Due to such close proximity to the parent star, the temperatures in WASP-18 b’s atmosphere are so high that most water molecules break apart, NASA said in a statement. The fact that Webb managed to resolve signatures of the residual water is a testament to the telescope’s observing powers.
“The spectrum of the planet’s atmosphere clearly shows multiple small but precisely measured water features, present despite the extreme temperatures of almost 5,000 degrees Fahrenheit (2,700 degrees Celsius),” NASA wrote in the statement. “It’s so hot that it would tear most water molecules apart, so still seeing its presence speaks to Webb’s extraordinary sensitivity to detect remaining water.”
WASP-18 b, discovered in 2008, has been studied by other telescopes, including the Hubble Space Telescope, NASA’s X-ray space telescope Chandra, the exoplanet hunter TESS and the now-retired infrared Spitzer Space Telescope. None of these space telescopes, however, was sensitive enough to see the signatures of water in the planet’s atmosphere.
“Because the water features in this spectrum are so subtle, they were difficult to identify in previous observations,” Anjali Piette, a postdoctoral fellow at the Carnegie Institution for Science and one of the authors of the new research, said in the statement. “That made it really exciting to finally see water features with these JWST observations.”
In addition to being so massive, hot and close to its parent star, WASP-18 b is also tidally locked. That means one side of the planet constantly faces the star, just like the moon‘s near side always faces Earth. As a result of this tidal locking, considerable differences in temperature exist across the planet’s surface. The Webb measurements, for the first time, enabled scientists to map these differences in detail.
The measurements found that the most intensely illuminated parts of the planet can be up to 2,000 degrees F (1,100 degrees C) hotter than those in the twilight zone. The scientists didn’t expect such significant temperature differences and now think that there must be some not yet understood mechanism in action that prevents the distribution of heat around the planet’s globe.
“The brightness map of WASP-18 b shows a lack of east-west winds that is best matched by models with atmospheric drag,” co-author Ryan Challener, of the University of Michigan, said in the statement. “One possible explanation is that this planet has a strong magnetic field, which would be an exciting discovery!”
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To create the temperature map, the researchers calculated the planet’s infrared glow by measuring the difference in the glow of the parent star during the time the planet transited in front of the star’s disk and then when it disappeared behind it.
“JWST is giving us the sensitivity to make much more detailed maps of hot giant planets like WASP-18 b than ever before,” Megan Mansfield, a Sagan Fellow at the University of Arizona and one of the authors of the paper describing the results. said in the statement. “This is the first time a planet has been mapped with JWST, and it’s really exciting to see that some of what our models predicted, such as a sharp drop in temperature away from the point on the planet directly facing the star, is actually seen in the data.”
The new study was published online Wednesday (May 31) in the journal Nature.
When astronomers discovered WASP-18b in 2009, they uncovered one of the most unusual planets ever found. It’s ten times as massive as Jupiter is, it’s tidally locked to its Sun-like star, and it completes an orbit in less than one Earth day, about 23 hours.
Now astronomers have pointed the JWST and its powerful NIRSS instrument at the ultra-Hot Jupiter and mapped its extraordinary atmosphere.
Ever since its discovery, astronomers have been keenly interested in WASP-18b. For one thing, it’s massive. At ten times more massive than Jupiter, the planet is nearing brown dwarf territory. It’s also extremely hot, with its dayside temperature exceeding 2750 C (4900 F.) Not only that, but it’s likely to spiral to its doom and collide with its star sometime in the next one million years.
For these reasons and more, astronomers are practically obsessed with it. They’ve made extensive efforts to map the exoplanet’s atmosphere and uncover its details with the Hubble and the Spitzer. But those space telescopes, as powerful as they are, were unable to collect data detailed enough to reveal the atmosphere’s properties conclusively.
Now that the JWST is in full swing, it was inevitable that someone’s request to point it at WASP-18b would be granted. Who in the Astronomocracy would say no?
In new research, a team led by a Ph.D. student at the University of Montreal mapped WASP-19b’s atmosphere with the JWST. They used the NIRISS instrument, one of Canada’s contributions to the JWST. The paper is “A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b.” It’s published in Nature, and the lead author is Louis-Philippe Coulombe.
The researchers trained Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) on the planet during a secondary eclipse. This is when the planet passes behind its star and emerges on the other side. The instrument measures the light from the star and the planet, then during the eclipse, they deduct the star’s light, giving a measurement of the planet’s spectrum. The NIRISS’ power gave the researchers a detailed map of the planet’s atmosphere.
With the help of NIRISS, the researchers mapped the temperature gradients on the planet’s dayside. They found that the planet is much cooler near the terminator line: about 1,000 degrees cooler than the hottest point of the planet directly facing the star. That shows that winds are unable to spread heat efficiently to the planet’s nightside. What’s stopping that from happening?
“JWST is giving us the sensitivity to make much more detailed maps of hot giant planets like WASP-18 b than ever before. This is the first time a planet has been mapped with JWST, and it’s really exciting to see that some of what our models predicted, such as a sharp drop in temperature away from the point on the planet directly facing the star, is actually seen in the data!” said paper co-author Megan Mansfield, a Sagan Fellow at the University of Arizona.
The lack of winds moving the atmosphere around and regulating the temperature is surprising, and atmospheric drag has something to do with it.
“The brightness map of WASP-18 b shows a lack of east-west winds that is best matched by models with atmospheric drag,” said co-author Ryan Challener, a post-doctoral researcher at the University of Michigan. “One possible explanation is that this planet has a strong magnetic field, which would be an exciting discovery!”
In our Solar System, Jupiter has the strongest magnetic field. Scientists think that swirling conducting materials deep inside the planet, near its bizarre liquid, metallic hydrogen core generates the magnetic fields. The fields are so powerful that they protect the three Galilean moons from the solar wind. They also generate permanent aurorae and create powerful radiation belts around the giant planet.
But WASP-18 b is ten times more massive than Jupiter, and it’s reasonable to think its magnetic fields are even more dominant. If the planet’s magnetic field is responsible for the lack of east-west winds, it could be forcing the winds to move over the North Pole and down the South Pole.
The researchers were also able to measure the atmosphere’s temperature at different depths. Temperatures increased with altitude, sometimes by hundreds of degrees. They also found water vapour at different depths.
At 2,700 Celsius, the heat should tear most water molecules apart. The fact that the JWST was able to spot the remaining water speaks to its sensitivity.
CREDIT: NASA/JPL-CALTECH/R. HURT
“Because the water features in this spectrum are so subtle, they were difficult to identify in previous observations. That made it really exciting to finally see water features with these JWST observations,” said Anjali Piette, a postdoctoral fellow at the Carnegie Institution for Science and one of the authors of the new research.
But the JWST was able to reveal more about the star than just its temperature gradients and its water content. The researchers found that the atmosphere contains Vanadium Oxide, Titanium Oxide, and Hydride, a negative ion of hydrogen. Together, those chemicals could combine to give the atmosphere its opacity.
All these findings came from only six hours of observations with NIRISS. Six hours of JWST time is precious to astronomers, and that’s all the researchers needed. That’s not only because the JWST is so powerful and capable, but also because of WASP-18 b itself.
At only 400 light-years away, it’s relatively close in astronomical terms. Its proximity to its star also helped, and the planet is huddled right next to its star. Plus, WASP-18 b is huge. In fact, it’s one of the most massive planets accessible to atmospheric investigation.
The planet’s atmospheric properties also provide clues to its origins. Comparisons of metallicity and composition between planets and stars can help explain a planet’s history. WASP-18 b couldn’t have formed in its current location. It must have migrated there somehow. And while this work can’t answer that conclusively, it does tell us other things about the giant planet’s formation.
“By analyzing WASP-18 b’s spectrum, we not only learn about the various molecules that can be found in its atmosphere but also about the way it formed. We find from our observations that WASP-18 b’s composition is very similar to that of its star, meaning it most likely formed from the leftover gas that was present just after the star was born,” Coulombe said. “Those results are very valuable to get a clear picture of how strange planets like WASP-18 b, which have no counterpart in our Solar System, come to exist.”
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