A recent study into the sleeping habits of northern elephant seals has found that the huge pinnipeds sleep for just two hours a day – less than almost any other mammal – and that sleep takes place deep underwater, where the seal is safe from predators such as sharks and killer whales.
Northern Elephant Seal Sleep Research
Researchers from Scripps Institution of Oceanography and UC Santa Cruz monitored the seals’ diving activities by attaching neoprene caps to the animals’ heads with a removable adhesive.
Using these and other sensors, the team were able to monitor numerous aspects of a seal’s dive, including duration, depth and stroke rate. Critically to the study, the sensors were also able to monitor the seal’s sleep state.
The researchers then combined their own data with existing data drawn from over half a million northern elephant seal dives compiled by the Costa lab at UC Santa Cruz.
Results
The team found that the northern elephant seal – while at sea* – sleeps for an average of just 2 hours per day. The only mammal known to spend as little time sleeping is the African elephant.
* Elephant seals spend up to ten months of the year at sea, only hauling out to mate, give birth and molt (see below).
Another interesting aspect of the northern elephant seal’s sleep patterns is that sleep occurs far below the surface.
This is presumably an anti-predation adaptation; while at or near the surface, the seals are at their most vulnerable from attacks by sharks and killer whales.
When the seals enter REM sleep, they stop swimming, turn upside-down, and gradually spin downwards. This suggests that, like ours, the seals’ movement becomes inhibited during this phase of sleep.
Data such as that revealed by the study is important in building a complete picture of the northern elephant seal’s behavior, thereby helping preserve the population of this characterful ocean mammal.
The original paper can be read here (article behind paywall).
Northern Elephant Seal
The northern elephant seal is the second-largest seal species; only the closely related southern elephant seal is larger. Male northern elephant seals can grow up to 4-5 meters in length and weigh between 1,500 and 2,300 kg – that’s significantly heavier than a typical compact car.
Northern elephant seals are typically found in the eastern Pacific Ocean, ranging from the Gulf of Alaska down to Baja California in Mexico.
During the breeding season, the seals migrate to select rookeries on offshore islands and certain mainland beaches.
A significant rookery for the species is the Año Nuevo State Park in California, where each winter they come ashore in large numbers to mate and give birth.
Elephant seals are carnivorous and feed mostly in deep ocean waters. Their diet is diverse, consisting primarily of fish and squid, but it can also include sharks and rays.
The seals are known to forage at depths of around 300 to 600 meters, but they are capable of diving up to 1500 meters deep and can remain submerged for more than an hour, making them one of the deepest diving pinnipeds.
How Long Do Northern Elephant Seals Spend At Sea?
Northern elephant seals spend the majority of their life at sea, undertaking two extended open-ocean voyages each year between their breeding and molting periods on land.
After the breeding season, they remain at sea for an average of 2-3 months before returning to shore for molting.
Following this, they embark on a longer post-molting migration that lasts 7-8 months for females and 4-5 months for males.
Males and females spend 80% and 90% of their lives in the ocean, respectively. This makes them one of the most aquatic of all pinniped species, spending up to 10 months of the year in the ocean and demonstrating an extraordinary capacity for diving and long-distance migration.
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