Elephant seals, the mammals that sleep the least: two hours a day in 10-minute naps

The monitoring of hundreds of these animals reveals how they enter the deepest phase of sleep, without falling into the jaws of orcas or sharks

When they’re on the beach, elephant seals – like these two-month-old calves – spend up to 14 hours sleeping each day
When they’re on the beach, elephant seals – like these two-month-old calves – spend up to 14 hours sleeping each dayJessica Kendall-Bar
Miguel Ángel Criado

Elephant seals are sleepyheads whenever they take a break on land: they can doze for up to 14 hours a day. But when they’re out in the sea, they are the mammal that sleeps the least of those that have ever been studied. They hardly spend two hours a day sleeping… and they don’t get all theat sleep in one shot. Instead, elephant seals nap in 10-minute intervals.

A recent study that examined hundreds of these mammals using a range of advanced technologies has shown that this extreme sleep pattern is the safest way that elephant seals have found to feed and rest, while simultaneously avoiding their only two notable predators: orcas and great white sharks.

Although it’s not entirely clear why we sleep, what does seem to be clear is that it’s a universal need within the animal kingdom. Practically all animals sleep. Some — such as the koala bear or the sloth — can sleep for more than 20 hours a day. It also seems clear that it’s impossible to live without sleep. And everything indicates that those hours of rest have a restorative effect, strengthening the immune system and helping the brain put things in order. Hence, biologists have great interest in understanding how much other species sleep. This knowledge can help them get to know animals better and learn more about this physiological obligation.

“Until now, it wasn’t known how and when marine mammals sleep in the sea and how our activities as humans might affect their ability to sleep,” says Jessica Kendall-Bar, a marine biologist at the Scripps Institution of Oceanography in San Diego. “Previous studies only analyzed their sleep in captivity, but it’s clear that this doesn’t accurately represent how they sleep in nature,” adds the scientist. Kendall-Barr is lead author of the latest research on this subject, which was recently published in the journal Science.

Less than 60 miles from Californian cities such as San Francisco or San José, you can find one of the largest communities of northern elephant seals (Mirounga angustirostris). Every year, more than 10,000 of these creatures arrive on the beaches of Año Nuevo State Park to mate and breed. From the phocid family, these seals are enormous, with the males weighing up to seven tons, rivalling elephants in size. Much is known about their behavior on land, but these mammals spend most of the year — up to eight months — on long sea voyages throughout the northern Pacific Ocean in search of food. Elephant seals spend most of this time diving, going deep into the sea to catch squid and fish, including small sharks. They only come to the surface for a minute or two to breathe and recharge their lungs. And, until recently, it wasn’t known how, when or where they slept.

“Normally, each adult female dive lasts about 30 minutes, although the [babies] have shorter dives,” Kendall-Bar explains. “Depending on the age of the animal, they sleep for about 10 minutes. Their naps are proportionally scaled, such that the average underwater sleep duration for juveniles was 5 to 10 minutes, while adult females had 10-minute-long naps with approximately another 10 minutes for descent and 10 minutes for ascent.” In a daily cycle, elephant seals sleep less than two hours a day, while young females sleep for even less.

To find all of this out, Kendall-Bar and her colleagues devised a complex tracking and monitoring system. They designed neoprene helmets, capable of performing both electroencephalograms — recordings of brain activity — and electrocardiograms, to track the heart rate. They also fitted some 340 adult seals with devices to record their position, depth or orientation. In this way, they were able to estimate what sleep was like in those short periods of time. Like humans and the vast majority of animals, these seals have polyphasic sleep. That is, they have at least two phases of sleep: a first called slow wave sleep (SWS) and a second, called rapid eye movement sleep (or REM phase).

The technology carried by the elephant seals made it possible to observe that, in some of the dives, they entered the SWS phase while descending, maintaining their position. But, after a while, they begin to fall in a spiral, slowly, like leaves falling from the tree, before finally ending up face up, or floating on the seabed. There, they enter REM sleep, the deepest sleep. After two or three refreshing minutes, they suddenly wake up and slowly rise to the surface to breathe.

A southern elephant seal yawns on  beach in Argentina's Patagonia.
A southern elephant seal yawns on beach in Argentina's Patagonia. DEA / P. JACCOD (De Agostini via Getty Images)

“They sleep at a depth of up to 1,200 feet below the surface. Generally, these naps begin around 200 to 300 feet below the photic zone (where sunlight reaches). Although killer whales and sharks can theoretically dive to these depths, they usually spend most of their time hunting marine mammals near the surface of the ocean, where they can see them best. Therefore, the dark depths — where elephant seals sleep — are likely to be safer.”

Daniel Costa, a biologist at the University of California at Santa Cruz and senior author of the study, notes that, in addition to security concerns, the seals need to nap after eating.

“In previous work, we saw that the same areas where elephant seals sleep are where they make the most successful dives to feed. Many of their sleeping dives are associated with taking time to digest their prey. During sleep drift dives, the seal is able to focus the flow of blood and oxygen into its gastrointestinal tract to process food, rather than using it to propel itself through the water in search of prey. These animals have a finite amount of oxygen in their bodies on each dive — they need to distribute it to be more efficient.”

Another piece of information highlights the role of predators in these sleep patterns. Using GPS, the researchers were able to see that the elephant seals slept less the closer they were to their base on Año Nuevo and the closer to shore their dives were. As they got further away, their sleep became longer and more frequent. “We suspect that it’s because there are more predators in the coastal zone. Great white sharks are more common near the Año Nuevo colony. In the middle of the Pacific, the risk of encountering a shark or an orca is much lower. The elephant seals are safer out there,” Costa explains.

The elephant seals beat (or at least tie) the animal that, until now, was believed to sleep least: the land elephant. Despite sharing a name, both species have little in common, beyond being mammals. The elephant seal bears the name because of its enormous size and because it has a kind of trunk. As for sleeping, there have been studies looking at elephants since the 1930s, but almost all of these observations have been of elephants in captivity. When captured, the data shows that the animals tend to sleep more. However, in 2017, a study followed two matriarchs from Botswana’s Chobe National Park. On average, it was found that, when able to roam free, they sleep for about two hours each day.

The work explains that the short sleep of elephants is due to the high risk of being attacked by lions, but also because of their enormous size and need to feed. Paul Manger, a professor of Anatomical Science at South Africa’s University of the Witwatersrand and author of the elephant study, emphasizes the size factor: “There’s a clear relationship between body size and sleep. The larger the body, the less the daily amount of sleep. Both elephant seals and African elephants are large animals, so they tend to sleep less,” he tells EL PAÍS via email. As for which creature sleeps less, Manger is clear: the land elephants.

“When the elephant seals are in the sea, these creatures sleep for two hours a day… but when they’re on land, they sleep for more than 10 hours! It’s very different from the African elephant, who only sleeps two hours a day overall.”

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