Deep Sleepers – Northern Elephant Seals Sleep Far Below Ocean Surface
6/13/2023
activewild.com
A recent study into the sleeping habits of northern elephant seals has found that the huge pinnipeds sleep for just 2 hours a day – less than almost any other mammal – & that sleep takes place deep underwater, where the seal is safe from predators such as sharks & killer whales.
Northern Elephant Seal Sleep Research
Researchers from Scripps Institution of Oceanography & UC Santa Cruz monitored the seals’ diving activities by attaching neoprene caps to the animals’ heads with a removable adhesive.
Using these & other sensors, the team were able to monitor numerous aspects of a seal’s dive, including duration, depth, & 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.
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 10 months of the year at sea, only hauling out to mate, give birth & molt.
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 & killer whales.
When the seals enter REM sleep, they stop swimming, turn upside-down, & 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.
Northern Elephant Seal
The northern elephant seal is the 2nd-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 & weigh between 1,500 & 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 & 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 & give birth.
Elephant seals are carnivorous & feed mostly in deep ocean waters. Their diet is diverse, consisting primarily of fish & squid, but it can also include sharks & 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 & 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 2 extended open-ocean voyages each year between their breeding & 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 & 4-5 months for males.
Males and females spend 80% & 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 & demonstrating an extraordinary capacity for diving & long-distance migration.
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