Study of sleep in a walrus

Abstract

Several behavioral and physiological adaptations have been developed in evolution of Pinnipeds allowing them to sleep both on land and in water. To date sleep has been examined in detail in eared and true seals (the families of Otariidae and Phocidae). The aim of this study was to examine sleep in another semiaquatic mammal - the walrus, which is the only extant representative of the family Odobenidae. Slow wave and paradoxical sleep (SWS and PS) in the examined walrus (2 year old female, weight 130 kg) averaged 19.4 ± 2.0 and 6.9 ± 1.1% of 24-h when on land, and 20.5 ± 0.8% of 24-h and 1.1 ± 0.6% when in water, respectively. The average duration of PS episode was 6.4 ± 0.6 min (maximum 23 min) when on land and 1.8 ± 0.1 min (maximum 3.3 min) when in water. In water, sleep occurred predominantly while the walrus submerged and lay on the bottom of the pool (89% of total sleep time). The walrus usually woke up while emerging to the surface for breathing. Most often EEG slow waves developed synchronously in both cortical hemispheres (90% of SWS time when on land and 97% when in water). Short episodes of interhemispheric EEG asymmetry usually coincided with brief opening of one eye. The pattern of sleep in the walrus was similar to the pattern of sleep in the Otariidae seals while on land (predominantly bilateral SWS, accompanied by regular breathing) and to the pattern of sleep in the Phocidae while in water (sleep during apneas both in depth and at the surface, interrupted by brief arousal when emerging for breathing).

Sleep in the northern fur seal

“…This hypothesis is also supported by the data on the pattern of sleep in water in the Phocidae seals [21-23] and the walrus [19]. They do not display USWS.…”

“…In addition to the northern fur seal, interhemispheric EEG asymmetry during SWS was also recorded in 3 other species of fur seals and sea lions [17,18], as well as in one young walrus (a pinniped belonging to the family Odobenidae; [19]) and in one Amazonian manatee (the order Sirenia; [20]). In contrast, no evidence for EEG asymmetry has ever been recorded in individuals of the 3 rd family of Pinnipeds, namely the Phocidae also called “earless seals” or “true seals” [21-23].…”

Sleep alterations in mammals: Did aquatic conditions inhibit rapid eye movement sleep?

“…REM sleep, but in water this significantly decreases to 1.1% [47] . This evidence thus suggests that REM sleep is not expressed at its full length if these animals sleep in water.…”

“…It can also be argued that animals that sleep less would exhibit less REM sleep. Although this could be true, an important interesting fact is that the majority of both [44][45][46][47][48] . Some pinnipeds, such as Caspian seals (Phoca caspica) [49] , harp seals (Pagophilus groenlandica) [50] and elephant seals (Mirounga angustirostris) [51] , do not exhibit inter-hemispheric asymmetry in slow-wave generation during NREM sleep, suggesting that unihemispheric sleep is typical of fur seals and sea lions but not of all semi-aquatic pinnipeds.…”

“…Among these, the sleep-wakefulness pattern has been studied in only 8, and REM sleep has been reported in all of these. Most of these animals [walrus [47] (Odobenidae), elephant seal [51] and harp seal [50] (Phocidae), and northern fur seal [55] (Otariidae)] exhibit either less REM sleep or shorter REM episodes in water than on land. In another four pinnipeds: gray seal [52] and…”