Elephant seals dive asleep – and urchin death riddle solved

‘How are you sleeping at the moment?’ (William Warby}
‘How are you sleeping at the moment?’ (William Warby}

Elephant seals drift off to sleep and go into an unconscious spiral during deep dives, a new study by US scientists has discovered.

The brainwave patterns of tracked seals have revealed that they average only two hours of sleep daily during months-long foraging trips at sea, made up of a series of 10-minute power naps while diving. Yet at the surface on a beach, elephant seals will happily sleep for about 10 hours a day.

Whales, dolphins, fur seals and sea-lions are known to favour “unihemispheric sleep”, which means that one side of their brains always remains awake. In most other mammals such as humans and true seals, both hemispheres of the brain are asleep simultaneously. 

Elephant seal sleep diagram (UCSC)
Previously known sleep patterns (UCSC)

Elephant seals go into rapid eye-movement (REM) sleep during deep dives that can last up to 30 minutes, with sleep paralysis causing them to turn upside-down and drift downwards in a corkscrew “sleep spiral”, according to the study. This sometimes ends with them lying motionless on the seabed for minutes before they wake up again. Elephant seals feel safer from predators such as sharks and orca at depth.

The study marked the first time scientists have ever succeeded in recording brain activity in a free-ranging wild marine mammal.

Jessica Kendall-Bar, a postdoctoral fellow at UC San Diego’s Scripps Institution of Oceanography, led the study with ecology and evolutionary biology professors Daniel Costa and Terrie Williams of the University of California Santa Cruz, where she had been a graduate student.

Refined tags

“For years, one of the central questions about elephant seals has been: when do they sleep?” said Prof Costa, director of the UCSC Institute of Marine Sciences, which has studied elephant seals at Año Nuevo Reserve north of Santa Cruz for more than 25 years.

Increasingly refined tags have been used to track the movements and diving behaviour of the seals as they head into the northern Pacific for up to eight months at a time.

The elephant seal sleep spiral (Jessica Kendall-Bar)
An elephant seal’s logged sleep spiral (Jessica Kendall-Bar)

“The dive records show that they are constantly diving, so we thought they must be sleeping during what we call drift dives, when they stop swimming and slowly sink,” said Prof Costa. “But we really didn’t know.

“Now we’re finally able to say they’re definitely sleeping during those dives, and we also found that they’re not sleeping very much overall compared to other mammals.” While at sea, elephant seals rival African elephants for the record of mammals that get by on least sleep.

Kendall-Bar devised a system to record the elephant seals’ brain activity that employed EEG sensors and a data-logger on a neoprene headcap, all able to be retrieved once the animals returned to Año Nuevo. They also carried time-depth recorders, accelerometers and other instruments enabling their movements to be tracked and matched with their brain activity at any given moment.

Human freedivers

Seals taking shorter excursions out to sea from Año Nuevo were found to exhibit similar dive behaviour. With data on brain activity and dive behaviour from 13 juvenile female elephant seals, including a total of 104 sleep dives, Kendall-Bar developed an algorithm for identifying their time spent asleep.

More sustained sleep is possible on the beach (William Warby)
More sustained sleep is possible on the beach (William Warby)

Based on 25 years of data from Prof Costa’s Año Nuevo research, Kendall-Bar was able to extrapolate the results to more than 300 animals. She now plans to use similar methods to study brain activity in other seal species and sea-lions – as well as in human freedivers.

“It’s an amazing feat to pull this off,” said Prof Williams of Kendall-Bar’s work. “She developed an EEG system to work on an animal that’s diving several hundred metres in the ocean. Then she uses the data to create data-driven animations so we can really visualise what the animal is doing as it dives through the water column.”

The results could assist in conservation efforts by revealing a “sleepscape” of preferred resting areas, said Prof Williams. “Normally, we’re concerned about protecting the areas where animals go to feed, but perhaps the places where they sleep are as important as any other critical habitat.” The ground-breaking study has just been published in the journal Science.

Mystery killer unmasked after 40 years

Healthy hatpin urchins, at the mercy of a tiny killer (James St John)
Healthy hatpin urchins, at the mercy of a microscopic killer (James St John)

Meanwhile, the mystery perpetrator of a mass killing that almost wiped out long-spined sea-urchins in the Caribbean has been identified as a parasitic micro-organism – a ciliate.

The initial die-off of millions of Diadema antillarum or hatpin urchins occurred 40 years ago, when they suddenly started losing their spines, dying and vanishing in days from the reef. Within a year, 98% had been wiped out.

The urchins had made a slow recovery until last year, when the mystery killer struck again – this time wiping out up to 95% of the remaining Caribbean population.

Microbiologist Prof Ian Hewson of Cornell University in New York collected both healthy and diseased urchins from 23 reef sites and examined tissue samples in his laboratory, seeking evidence of viruses and pathogens.

Turning to genetic signals of micro-organisms such as fungi and ciliates, he found that one of the latter, Philaster apodigitiformis, was present only in sick urchins. Adding it to tanks containing healthy urchins caused 60% of them to lose their spines within days.

Related ciliates had been known to infect sharks but never before to kill sea urchins, having been thought of simply as consumers of bacteria and decaying tissue. Now the researchers know the culprit, they want want to learn what triggers such devastating attacks. Their findings have also just been published in Science.

Also on Divernet: Freedivers Beat Seals On Deep Dives, Grey Seals Clap For Victory, Seals Welcome Divers To The Farnes, Babbacombe Seal


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