An elephant seal’s amazing sleep has been revealed

Extreme Relaxation: Elephant seals only sleep a record two hours a day during their months at sea. This has been documented by cleverly obtained recordings of marine mammal brain activity. Sleeping behavior is also surprising, the researchers report: During a ten-minute nap, the animals descend a spiral path to great depths, where they are safe from their enemies.

They lie lazily on the shore and stand a lot: on land, the largest representatives of the seals rest a lot. But how do seals sleep on their months-long fishing trips out to sea? Through these activities, this state, which is characterized by certain patterns of brain waves, cannot be recorded yet. Because marine mammals can only be equipped with recording devices that record their movement and diving behaviour. So far, this data has provided only the first clues about their sleeping behavior at sea: “It was suspected that the animals sleep in special dives where they stop swimming and slowly sink, but we didn’t know for sure,” says lead author Costa. von of the University of California, Santa Cruz (UCSC).

Elephant seals are equipped with EEG caps

To illustrate this, his research team has developed a device that can record the brain activity of elephant seals using electroencephalography (EEG) during their normal diving behaviour. The EEG sensors are housed under a type of neoprene covering that can be securely attached to the animals’ heads. “We used the same sensors used in human sleep studies,” says lead author Jessica Kendall-Barr of the University of California. The EEG data is stored on a device that can be read when the animals return to their original shore. In addition, the system includes tools that allow researchers to track movements along with corresponding brain activity. It has now been used in animals from an elephant seal colony on the coast of the Aino Nuevo Reserve north of Santa Cruz.

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The team reports that recordings from a total of 13 test animals documented how elephant seals reached a stage of deep sleep, and later entered a stage of REM sleep on the high seas. This happens within ten minutes in an approximately 30-minute dive. The animals sink downward in a spiral course and sometimes lie motionless on the sea floor until they float to the surface to catch their breath. “They enter slow-wave sleep while initially maintaining their position for a few minutes. Then, when they enter REM sleep, they lose control of the position and turn upside down,” says Kendall Barr. Often the whole thing takes place at depths of more than 200 metres. The researchers explained that the sleepers there are safe from enemies such as sharks and killer whales, which tend to hunt in the upper layers of the water.

In a very strange and short way

What distinguishes elephant seals from the high seas is their unusually short duration for a mammal: while elephant seals snooze ashore for up to ten hours a day, the bottom line is that they only spend about two hours soaring. Cruises, researchers showed ratings. Scientists say they and African elephants now hold the record for short-term sleep among mammals.

Using new data on brain activity and diving behavior, Kendall-Bar also developed a computational system for determining sleep periods based solely on elephant seal diving data. The researcher was then able to apply this to the comprehensive study data from the past. “In this way, our results were extrapolated to more than 300 animals and were able to take a closer look at the sleeping behavior of the population,” says Kendall Barr.

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The scientists now plan to use this concept to study brain activity in other seal and sea lion species in order to track their sleep behaviour. As they explain, the information is not only interesting from a biological point of view, but can also serve to protect species. Because if you know how, when and where marine mammals in the ocean sleep, their resting areas can be protected: “We usually take care of protecting the areas where animals eat, but perhaps just as important is where they sleep,” says co-author Terry Williams of the Scripps Institution of Oceanography in San Diego: “Like any other critical habitat.”

Source: UC Santa Cruz, professional article: Science, doi: 10.1126/science.adf0566

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