The incredibly little sleep that elephant seals get

Researchers have discovered how much elephant seals actually sleep each day, and under what circumstances.

While sleep is a fundamental part of life for all mammals, some live in environments where long periods of safe sleep are not possible. As a result, wild animals display a diversity of sleep strategies that balance the need for daily sleep with their ecology and risk of predation.

Marine mammals in the open ocean, where they coexist with their predators, face unique and challenging conditions. The way in which these creatures meet their sleep needs has been poorly understood.

Using a novel remote monitoring system, Jessica Kendall-Bar, of the Scripps Institution of Oceanography in the United States, and her colleagues assessed the sleeping behaviors of free-ranging northern elephant seals (Mirounga angustirostris), a species characterized by the large corpulence of its adult individuals and famous for undertaking foraging trips at sea that last up to seven months and have a range of up to more than 10,000 kilometers.

The system non-invasively recorded brain activity, heart rate, and three-dimensional spatial movement in northern elephant seals in a controlled laboratory setting and in wild conspecifics in Monterey Bay, California, United States.

A sea elephant. (Photo: Robert Schwemmer/CINMS/NOAA)

The mammals took short naps (often lasting less than 20 minutes) several hundred meters below the surface, depths below those occupied by their usual predators.

On these sleeping dives, the elephant seals transitioned from a drowsy state to slow-wave sleep. During this phase, the elephant seals maintained their upright posture, but as they transitioned from slow wave sleep to rapid eye movement (REM) sleep, sleep paralysis resulted in loss of postural control.

During REM sleep, the elephant seals turned upside down in a “sleep spiral.”

After these short sleep cycles, the animals woke up and returned to the surface.

This strategy allows northern elephant seals to enter full REM sleep, but at depths that reduce the risk of predation.

Combining these data, Kendall-Bar and colleagues were able to derive a “biomechanical signature for sleep” capable of identifying sleeping immersions in a 20-year data set of depth time records for 334 animals in the wild. Sleep patterns interpreted from these records reveal that elephant seals average only two hours of sleep per day during the 7 months they spend in the open sea.

The study is titled “Brain activity of diving seals reveals short sleep cycles at depth”. And it has been published in the academic journal Science. (Source: AAAS)