But why were they named elephant seals? Was it because the scientific name Mirounga did not please Darwin? Maybe it is because with a weight of about 2 tons, the male outweighs all the other pinnipeds by far? Because the “dominant” ones develop a nose shaped like a trunk? On Friday, April 21 an article published in the journal Science, added a new answer to the catalog: It is because, with two hours of sleep per night during the seven months it spends at sea, the northern elephant seal (Mirounga angustirostris) join the African elephant at the top of the list of mammals that sleep the least.
A team from the University of California, Santa Cruz (UCSC) owes this discovery to the development of a revolutionary sensor. Invented by Alexei Vyssotski, from the University of Zurich in Switzerland, to record the brain waves of birds, it was adapted to the marine world by Jessica Kendall-Bar. Using a neoprene cap and gold sensors, the UCSC researcher was able to track the sleep of 13 specimens, five in the laboratory and eight in the wild, in the Año Nuevo Island Reserve in California.
The results stounded her: While the animal sleeps 10 hours a day during the five-month breeding season on land, its sleep duration plunges to two hours a day for the rest of the year it spends in the water. And still: “It comes to them with small naps of about 10 minutes, during their half-hour dives to the bottom of the ocean,” said Kendall-Bar. It is in the deep sea, up to 2,000 meters in depth, that the animal finds food and security. Conversely, the two minutes or so that it spends on the surface between two dives place it at the mercy of orcas and white sharks.
By analyzing the recordings, the researchers observed both the slow waves of deep sleep and the faster ones of REM sleep – like humans. But they noted a particularly long duration of the latter in the animal (up to 25% of the total sleep cycle, compared to less than 10% in most species). Above all, thanks to the parallel measurement of the animal’s movements, also carried out by the sensors, they were able to follow the physical behavior of the animals in the two different states. In deep sleep, the pinniped stops swimming and moves about in a normal position, belly down, in an oblique, more or less rectilinear, trajectory of descent. But when it switches to REM sleep, it loses control of its body, turns over, belly up, and drops in a spiral towards the bottom.
The team then went back to the swimming profile data collected over the last 25 years by Daniel Costa’s marine biology laboratory at UCSC. Through the data of 334 specimens and more than 3 million dives, they verified the “extraordinary” character of the elephant seal’s sleep cycle. The largest of the pinnipeds, the one that dives in the deepest seas for the longest periods of time, is also the one that sleeps the least, while still resting soundly. Fur seals, by contrast, do without REM sleep and let only one of their two cerebral hemispheres doze, the other one remaining on standby.
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