Extreme relaxation: elephant seals only sleep a record-breaking two hours a day during their months at sea. This is documented by cleverly obtained recordings of the marine mammals’ brain activity. The sleeping behavior is also amazing, the researchers report: During their ten-minute naps, the animals descend on a spiral course to great depths, where they are safe from their enemies.
They lie around lazily on the beach and kip a lot: On land, the largest representatives of the seals rest a lot. But how do elephant seals sleep on their months-long hunting tours through the open sea? With these activities, this condition, which is characterized by certain brain wave patterns, could not be recorded so far. Because the marine mammals could only be equipped with recording devices that record their movement and diving behavior. These data have so far provided only the first clues about their sleeping behavior at sea: “It was suspected that the animals sleep on special dives where they stop swimming and slowly sink, but we didn’t know for sure,” says senior author Costa von from the University of California at Santa Cruz (UCSC).
Elephant seals equipped with EEG caps
To clarify this, his research team has developed a device that can record the brain activity of elephant seals using electroencephalography (EEG) during their normal diving behavior. The EEG sensors sit under a type of neoprene cap that can be firmly attached to the animals’ heads. “We used the same sensors that are used for human sleep studies,” says lead author Jessica Kendall-Bar of UCSC. The EEG data is stored on a device that can be read when the animals return to their home beach. Additionally, the system includes instruments that allow researchers to track the movements along with the corresponding brain activity. It has now been used in animals from an elephant seal colony on the coast of the Año Nuevo Reserve north of Santa Cruz.
The team reports that the recordings of a total of 13 test animals documented how the elephant seals reach a deep sleep stage and later enter the REM sleep phase on the high seas. This occurs within ten minutes on approximately 30 minute dives. The animals sink downwards on a spiral course and sometimes even lie motionless on the seabed until they surface to catch their breath. “They enter slow-wave sleep while initially maintaining their posture for a few minutes. Then, when they enter REM sleep, they lose postural control and roll upside down,” says Kendall-Bar. The whole thing often takes place at a depth of up to more than 200 meters. There, the sleepers are safe from enemies such as sharks and orcas, which tend to hunt in the higher water layers, the researchers explain.
In a bizarre way and extremely short
What is also special about the elephant seal sheep on the high seas is the unusually short duration for mammals: while elephant seals snooze on the beach for up to ten hours a day, the bottom line is that they only spend about two hours on their high sea tours, the researchers showed evaluations. Together with the African elephants, they now hold the short-term sleep record among mammals, the scientists say.
Using the new data on brain activity and diving behavior, Kendall-Bar also developed a computational system to identify sleep periods based solely on elephant seal diving data. The researcher was then able to apply this to the extensive study data from the past. “In this way, I extrapolated our results to over 300 animals and was able to take a look at the sleeping behavior of the population,” says Kendall-Bar.
The scientists are now planning to use the concept to study brain activity in other seal and sea lion species in order to track their sleeping behavior. As they explain, the information is not only interesting from a biological point of view, but could also serve to protect species. Because if you know how, when and where marine mammals sleep in the ocean, their quiet zones can be protected: “Usually we take care of the protection of the areas where the animals eat, but maybe the places where they sleep are the same important as any other critical habitat,” says co-author Terrie Williams of the Scripps Institution of Oceanography in San Diego.
Source: University of California – Santa Cruz, professional article: Science, doi: 10.1126/science.adf0566