Experts found out that marine mammals are sleeping by shutting down a half portion of their brain.
A study pointed out that if marine mammals want to sleep, they cannot just close their eyes and drift away for the night because of their need for air surface.
They could also not float at the water's surface and slumber because that exposes them to predators and heat loss.
Shutting Down One Half Of Brain
Researchers said one solution for the marine mammals is to shut down one half of their brain at a time. Called unihemispheric sleep, it is one way marine mammals, such as dolphins, can get some rest while they are in open water.
Experts explained that unihemispheric sleep is really valuable to these animals because it allows them to maintain a low level of activity while still sleeping half of their brain at a time.
They said that dolphins are the best-studied marine mammals that are capable of this style of sleeping.
Studies explained how brain scans on captive dolphins have shown that while one hemisphere is in slow-wave, deep sleep, the other hemisphere is on alert. This allows the animals to literally sleep with one eye open.
According to research, this style of sleeping is common in cetaceans or the group of mammals that includes dolphins, whales and porpoises.
Meanwhile, many bird species are also known to use unihemispheric sleep, which often allows them to doze while they are flying.
Experts said that birds and dolphins usually utilize their half-brain sleeping technique for different purposes.
When it comes to a flock of birds, many of the ones on the outside of the group usually keep the eye facing away from the flock open in order to watch for predators.
For its part, dolphins do the opposite as while they are sleeping, they tend to keep the eye open that faces the rest of the pod, likely to avoid becoming separated.
Experts said that it is likely that three life-sustaining factors have exerted important influences on the evolution of the patterns of sleep seen in present-day aquatic mammals.
These include the need to come to the water surface to breathe, more efficient monitoring of the environment, and the thermogenesis.
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Cetaceans
Unihemispheric sleep has allowed cetaceans to surface without awakening and for Otariidae seals to maintain a characteristic posture at the surface. Movement during unihemispheric sleep also facilitates muscle thermogenesis and postural thermoregulation.
This pattern can be achieved only in open or ice-free waters, which is the typical habitat for the majority of cetaceans and all otariids.
Sleeping at the surface requires maintaining continuous vigilance to minimize the risk of near-surface predators. Consequently, sleep in cetaceans is called unihemispheric slow-wave sleep (USWS). This state also represents unihemispheric waking.
Parallel recordings from symmetrical areas of the two cerebral hemispheres revealed that, regardless of regional differences between electroencephalogram (EEG) signals within one hemisphere, the differences between the two hemispheres were always more pronounced.
The study said that when the EEG was recorded simultaneously from the cortex and thalamus, EEG changes in the cortex were parallel with those in the ipsilateral thalamus, suggesting that USWS not only is a cortical phenomenon but also appears to involve, at a minimum, the entire cortical-thalamic system.
Experts noted that not all cetaceans seem to be capable of unihemispheric sleep.
They said that some use bihemispheric sleep, in which both hemispheres of the brain fall asleep, just like humans and most other mammals do.
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