elephant seals are the largest seals in the world. A male can be around six and a half meters long and weigh up to three and a half tons. However, the animals are not named because of their size – the adult males also have distinctive trunk-like enlarged noses.

Months of sea voyages

Those distributed on the west coast of North America Northern elephant seals have another special feature: They are the only mammals in the world that undertake two long journeys every year.

After the mating season, which lasts from December to March and which they spend on the Californian and Mexican coasts, the animals head north to hunt in the Pacific at their usual feeding grounds. In between, the elephant seals are also drawn back to warmer waters, because they are next to the monk seals the only seal species that molt, losing the outer layer of fur in the process. Once that’s done, the elephant seals swim back to their northern feeding grounds until the next mating season drives them back to the California and Mexican shores. The northern elephant seals cover more than 10,000 kilometers in around seven months.

Riddles regarding sleep behavior

However, the elephant seals also have to sleep at some point on their long journeys and replenish their energy reserves. How they do this in the middle of the sea was not entirely clear until now. A US research team led by the marine biologist Jessica Kendall-Bar from the University of California in San Diego and Santa Cruz has therefore examined the sleeping behavior of the elephant seals more closely. The result The researchers are currently presenting this in the journal “Science”.

To get analysable data, the team fitted thirteen northern elephant seals with special devices to measure the brain activity, heart rate and movement of the seals. The devices were designed not to disturb the animals or affect their behavior. The investigations took place both in the wild and in an artificial laboratory environment.

Short naps on the open sea

It was found that during their long journeys and generally when foraging in the open sea, the northern elephant seals take several short underwater naps, often lasting less than 20 minutes. According to the researchers, the short duration of the recovery breaks is partly due to the potential presence of predators.

While northern elephant seals don’t have many enemies due to their size, they do have to be wary of great white sharks and killer whales. While other seal species can keep one eye open while sleeping in the sea, elephant seals cannot. They are therefore dependent on other measures to make their recovery phases as safe as possible. Short naps allow the elephant seals to keep an eye on their surroundings and to react to potential predators.

sleeping in the depths

The place where they take their naps is also important for the protection of the animals. They also benefit from their size and the associated potential to stay under water for a long time and to dive very deep. Since their predators usually stay near the surface of the water, the elephant seals dive several hundred meters or even onto the seabed before they go to sleep, in order to relax in comparatively safe waters.

On the open sea, the animals only make a handful of such dives – the researchers were able to demonstrate around five of them per day in one of the elephant seals examined. In slightly more sheltered waters, say near one continental shelves On the other hand, some of the animals rested under water more than 30 times a day.

Spiral dive

For a while, elephant seals even manage to float upright in the water while they are sleeping. Once they get into the REM sleep phase when they come, they lose all control of their bodies, roll onto their backs, and begin to descend even further in a spiraling pattern. The researchers were able to locate sleeping elephant seals at a depth of up to 400 meters. After a short time, however, the animals wake up once more and return to the water surface to breathe.

Shortest sleep in mammals

The research team also compared the new findings with data already collected from more than 300 elephant seals and calculated that the animals only sleep around two hours a day on their journeys. That even puts them in the running for the title of having the shortest sleep among mammals—only real ones African elephants rest for regarding two hours a day, just as short as the northern elephant seals.

On land, however, the elephant seals are not easily disturbed. The lack of predators often allows the animals to sleep more than five times longer on land than on their long voyages across the open sea.