Sleep in Cetaceans. physiological and behavioral adaptations for sleep in the aquatic environment
O.I. Lyamin¹,² and L.M. Mukhametov¹
¹Severtsov Inst. of Ecol. and Evol., Moscow, 117071, Russia.
²Dept. of Psychiatry, UCLA and Sepulveda VAMC, CA 91343.
While most studied Cetaceans appear to be able to sleep during swimming, other Cetaceans (e.g., the beluga whale, gray whale) can sleep motionless during long respiratory pauses both at and under the water surface. At least several species of Cetaceans (e.g., the white-sided dolphin, bottlenose dolphin, gray whale) are capable of sleeping with one eye open to survey the environment, which is also characteristic for some Pinnipeds as well as for birds. Unlike terrestrial mammals, which always experience slow wave sleep in the two brain hemispheres (according to electroencephalographic criteria), "unihemispheric slow wave sleep" has been found in all 4 electrophysiologically studied species of Odontoceti. Unihemispheric sleep could represent a neurophysiological mechanism that permits the animals to sleep while moving and maintain some contact with the environment. Paradoxical (rapid eye movement) stage of sleep is present in all studied terrestrial mammals, and is assumed to be absolutely necessary for the mammalian brain. However, this stage has not yet been found in its classical form, which includes skeletal muscle atonia, in Cetaceans. Recent data suggests, that paradoxical sleep might be present in Cetaceans but takes place without the typical features of this stage characteristic of terrestrial mammals. We conclude that the sleep patterns of Cetaceans are highly specialized for life in the aquatic environment and differ significantly from those seen in terrestrial mammals.
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