Sleep in the northern fur seal

Abstract

The pattern of sleep in the fur seal, a semiaquatic pinniped, has several striking behavioral and physiological adaptations that allow this species to inhabit both the land and water environment. These features include unihemispheric slow wave sleep (USWS, also being unihemispheric waking), the ability to maintain movement for stabilization of the sleep posture and to briefly open one eye while having a sleep electroencephalogram (EEG) in one hemisphere. In vivo microdialysis studies suggest that acetylcholine release is required for cortical activation during USWS, and that monoamines are not required for USWS. The need to breathe, to maintain efficient thermoregulation, and to avoid predation have shaped the sleep patterns in semiaquatic fur seals as in fully aquatic cetaceans.

Fur Seals Suppress REM Sleep for Very Long Periods without Subsequent Rebound

“…SWS in seawater was predominantly USWS (94% ± 1% of all SWS compared to 61% ± 4% of SWS when on land; see Figures 1E-1G and Table S2). There was a clear association between the lateralization of EEG slow waves in fur seals sleeping in seawater and the asymmetric lateral posture and eye opening [8] ( Figure 1F).…”

“…It can sleep on land and in seawater. The fur seal is unique in showing both the bilateral SWS seen in most mammals and the asymmetric sleep previously reported in cetaceans [8]. Here we show that when the fur seal stays in seawater, where it spends most of its life [7], it goes without or greatly reduces REM sleep for days or weeks.…”

“…Aspiration of seawater during sleep is also a considerable risk since the nostrils must be held above the waves. Thermoregulation is impaired in REM sleep [18], and long episodes in cold water might cause hypothermia [8,11]. Phocidae seals (the other group of pinnipeds) deal with some of these issues by holding their breath and hiding in the seawater's depths during which REM sleep of normal duration may occur [19][20][21].…”

“…The behavior of seals was scored in 20 s epochs. Behavioral features of active waking (AW), quiet waking (QW), slow wave sleep (SWS) and REM sleep in fur seals while on land have been described in our prior publications [8][9][10]38].…”

Cyclic nature of the REM-like state in the cuttlefish Sepia officinalis

“…For example, most species show a circadian periodicity of REM; however, marine mammals, such as pinnipeds, show significantly decreased durations of REM sleep while in the water as opposed to when on land (O. Lyamin et al, 2008;O. I. Lyamin & Mukhametov, 1998) and migrating birds decrease REM sleep while in flight (Rattenborg, 2006;Rattenborg et al, 2016).…”