Sleeping while diving: seals at sea drift in and out of sleep at depth

Abstract

In mammals, sleep is critical and ubiquitous. However, in marine mammals that spend months or entire lifetimes at sea, sleep can limit access to air and foraging opportunities and leave animals vulnerable to predation. Large, deep-diving northern elephant seals (Mirounga angustirostris) travel thousands of kilometers and spend up to 7 months at sea in search of food. We posited that combined demands to feed and avoid predators would strongly restrict aquatic sleep in seals. We built a portable, non-invasive system to simultaneously monitor electroencephalogram (EEG), electrocardiogram (ECG), three-dimensional motion, and swimming behavior. EEG activity of 13 young (< 3-years-old) female northern elephant seals (N = 5 in the lab, N = 8 in the wild [N = 3 at sea]) revealed that total sleep time was indeed strongly restricted at sea compared to on land (> 6-fold difference; 10.8 ± 3.0 h/day on land, 1.7 ± 0.7 h/day at sea). Seals at sea slept during short naps (< 20 min) while resting on the ocean floor (64-249 m deep) or drifting through the water column (82-377 m deep). REM sleep in mammals co-occurs with sleep paralysis. Our data support the presence of sleep paralysis through a persistent supine inversion (belly-up posture) whenever the seal was in REM (100% of REM sleep in the open ocean). Unexpectedly, elephant seals at sea exhibited a large proportion of REM sleep (29.1 ± 4.3% of at-sea total sleep time), compared to aquatic sleep in captive Caspian seals, harp seals, walruses, and fur seals (11%, 6%, 5%, and 1% respectively). This suggests that sleep at depth allows seals to drift safely in and out of sleep paralysis. However, time for sleep is limited by near-constant foraging to support high energetic demands due to their large body size. The extreme flexibility in sleep duration exhibited by northern elephant seals is unprecedented among mammals – approaching that which is observed in migrating birds. These insights into sleep for a large, highly mobile marine mammal can contribute to our understanding of sleep’s function and pathology across mammals, including humans. Funding: NSF Graduate Resesarch Fellowship, Laurel Clark Sea to Space Physiology Research Grant, ARCS Fellowship, Sooy Graduate Fellowship, National Geographic Early Career Grant, Special Research Grant – UC Santa Cruz, Joint Industry Project (JIP2207-23) [DPC], Office of Naval Research (Grants # N00014-08-1-1195 [DPC], N00014-18-1-2822 [DPC], N00014-00-1-0880 [DPC], N00014-03-1-0651 [DPC], N00014-08-1-1195 [DPC], N00014-20-1-2762 [TMW], N00014-19-1-2178 [TMW]) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.