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If marine mammals want to sleep, they cannot close their eyes and drift overnight, as they have to surface mid-way in search of air. They also cannot sleep floating on the surface of the water. exposed to predators and heat loss. So how do marine mammals close their eyes without putting themselves at risk?
One solution is to shut down half the brain at once.Called hemispheric sleep, it’s one-way marine mammalsDolphins and others can rest while in open water.
“Hemispheric sleep is invaluable for these animals because it allows them to maintain low levels of activity while keeping half their brains asleep at once.” patrick millerbiologists from the University of St Andrews in the UK told Live Science.
Dolphins are the most well-studied marine mammal capable of this style of sleep. brain scan Studies of dolphins in captivity have shown that while one hemisphere is in deep, slow-wave sleep, the other hemisphere is awake, allowing them to literally sleep with one eye open. Although this sleeping style is common in cetaceans (a group of mammals that includes dolphins, whales, and porpoises), it is not unique to cetaceans. Many birds are known to have hemispheric sleep, and often doze off during flight.
But Miller says birds and dolphins use half-brain sleep technology for different purposes. For example, in a flock of birds, many of the birds on the outside of the flock keep their eyes on the other side of the flock to watch for predators. But dolphins are the opposite. While sleeping, they tend to keep their eyes open towards the rest of the herd to avoid getting separated.
Related: Why do horses sleep standing up?
It appears that not all cetaceans can sleep in one hemisphere. Some animals, like humans and most other mammals, utilize bihemispheric sleep, where both hemispheres of the brain sleep.
“It’s very difficult to measure brain activity in uncatchable marine animals, such as sperm whales, blue whales, and humpback whales,” Miller said. “In that case, behavioral records are the best indicator of sleep behavior.”
In such cases, researchers can attach tags to animals to monitor their behavior.a 2008 survey Researchers led by Miller used suction tags attached to sperm whales (physeter macrocephalus) indicates sleeping in the open ocean for short matches. The whale burrows shallowly under the water, slows down and stops swimming, and slowly moves upwards. Their upward pitch is probably due to buoyant oil in their heads known as whale sacs.
While sleeping, entire pods of whales can be seen nose-up just below the surface of the ocean. During this time, the animal does not react at all. This suggests that they are in a deep sleep. However, the animals can only rest in the water for about 20 minutes before having to come up for air. Once the whale has caught its breath, it burrows below the surface for further rest, a behavior that can last up to 3.5 hours.
Northern elephant seal (milounga angustirostris) are also hemispheric sleepers and doze off in similar short bouts. 2023 study guided by jessica kendall barrA postdoctoral fellow at the Scripps Institution of Oceanography, a faculty at the University of California, San Diego, he was the first to successfully monitor the brain activity of a sleeping mammal at sea. Kendall-Bar and colleagues found that when seals dive to a depth of about 1,000 feet (300 meters), at which point their brains slow down and enter rapid eye movement (REM) sleep. When entering REM sleep, seals turn upside down and slowly rotate as they continue to sleep.
“Seals sleeping in the ocean spend 100% of their REM sleep upside down, suggesting that paralysis occurs during REM sleep, just like in us,” Kendall Barr said in the paper. YouTube video Explaining the research.
Possibly due to predation risk, elephant seals limit their total sleep time at sea to only about two hours per day, “compared to the shortest recorded sleep duration of any mammal.” Currently owned by African elephants (approximately 2 hours per day),” the researchers said in the study.
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