If marine mammals want to sleep, they can’t close their eyes and drift off all night, because they need to rise to the surface to get air. Because of this, it cannot float on the surface of the water and sleep It exposes them to predators and heat loss. So how do marine mammals sleep without putting themselves in danger?
One solution is to shut down half of their brain at a time. It’s called unihemispheric sleep, and it’s one way Marine mammalsThey, like dolphins, can rest while in open water.
“Hemispheric sleep is of great value to these animals because it allows them to maintain a low level of activity while still sleeping half their brain at a time.” Patrick MillerThe biologist at the University of St Andrews in the United Kingdom told Live Science.
Dolphins are the best-studied marine mammal capable of this sleep pattern. Brain scan Studies of captive dolphins show that while one hemisphere is in a state of deep, slow sleep, the other hemisphere is in an alert state, allowing the animals to literally sleep with one eye open. This pattern of sleep is common in cetaceans — the group of mammals that includes dolphins, whales and porpoises — but it is not unique to them. Many bird species are known to use unihemispheric sleep, which often allows them to sleep in flight.
But according to Miller, birds and dolphins use their hemispheric sleep technology for different purposes. For example, in a flock of birds, many birds outside the group keep their eyes facing away from the flock open, to watch for predators. But dolphins do the opposite: While sleeping, they tend to keep their eyes open facing the rest of the group, potentially avoiding separation.
Related: Why do horses sleep standing?
Not all cetaceans seem to be able to sleep in the Western Hemisphere. Some use hemispheric sleep, in which both hemispheres sleep, just as humans and most other mammals do.
“It’s really hard to measure the brain activities of animals in the sea that you can’t capture, like a sperm whale or a blue whale or a humpback whale,” Miller said. “In this case, behavioral records are our best indicator of sleep behavior.”
In those cases, researchers can tag the animals to monitor their behavior. a 2008 study Led by Miller used suction tags attached to sperm whales (Big-headed physeter) to show that they sleep in the open ocean for short bouts. Whales dive shallowly below the surface, slow their swimming speed to a halt, and move lazily upward. Their upward movement is likely caused by the blooming oil, known as spermaceti, found in their heads.
While sleeping, entire groups of whales can be seen from above, just below the ocean surface. During this time, the animals are completely unresponsive, suggesting that they are in a state of deep sleep. However, the animals can only rest for 20 minutes underwater before they need to come back up for air. Once the whale catches its breath, it slides below the surface for more rest, and may continue this behavior for up to 3.5 hours.
northern elephant seal (Mirunga angustostris) They are also hemispheric sleepers and doze off in similarly short bouts. A 2023 Stady led by Jessica Kendall BarrA postdoctoral researcher at Scripps Institution of Oceanography, a division of the University of California, San Diego, has for the first time been able to monitor brain activity in a mammal sleeping at sea. Kendall Barr and colleagues found that seals dive to a depth of about 1,000 feet (300 metres), at which point their brains slow down and enter rapid eye movement (REM) sleep. Once in REM sleep, the seals turn upside down and spin in a slow circle while continuing to sleep.
“Seals sleeping in the ocean had 100% inverted REM sleep time, suggesting that just like us, they become paralyzed during REM sleep,” Kendall Parr said in an article. YouTube video Search description.
Possibly due to the risk of predation, elephant seals limit their total sleep time at sea to about two hours per day, “challenging the record for the least sleep of all mammals, which is Currently kept by the African elephant (about 2 hours daily),” the researchers wrote in the study.