Walruses are known to exhibit some of the strangest sleeping behaviour in the animal kingdom. Alongside their ability to sleep in rather odd positions in the ocean, they are known to remain active for extensive periods of time on no or very little sleep.
In a 2009 study published in the journal of Behavioural Brain Research, scientists studied the sleeping habits of walruses in the Utrish Marine Station of the Russian Academy of Sciences. Walruses were observed to sleep in varied positions including lying on the bottom of the water, floating on the surface, standing, leaning, and hanging out of ice floes by their tusks. Sleep taking place underwater was brief, as walruses can only hold their breath for around 4-5 minutes. In this short time, REM sleep (Rapid eye movement sleep- a unique phase of mammalian sleep characterized by random movement of the eyes, low muscle tone, and the ability to dream vividly.) was observed, albeit brief. When outside of water and thus able to breathe, walruses were observed to sleep for a significantly longer time of up to 19 hours. This allowed for longer periods of REM sleep, which was identified from several behaviours including rapid eye movements.
Such a long and deep sleep may be important for walruses due to another odd behaviour- the ability to swim continuously without sleep for up to 84 hours on a regular basis. It is thought that most if not all other animals engage in at least some sleeping activity each day. One behaviour which was observed and is known to also occur in many avian and aquatic animals is termed unihemispheric sleep. This is where one half of the brain effectively shuts down whilst the other remains alert and active. Additionally, one eye tends to be closed whilst the other remains open. This helps animals to avoid hazards/predation whilst sleeping, and in the case of aquatic endotherms, to continue moving and maintaining the correct body temperature. Scientists working on the walrus study do not however, believe that this behaviour could explain how walruses can remain active for so long as energetic swimming was only observed within the study. In the case of unihemispheric sleep, reduced movement is exhibited, with some animals including (aquatic species), physically stopping to rest.
A final unusual occurrence to be observed within the study was the difference in total sleep time between individuals, with some apparently needing much greater amounts of sleep that others. As in humans, most other species show a uniform total sleep time across individuals.
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