Natures wonders: seals sleep with half of brain at sea (Introduction)

by David Turell , Friday, June 08, 2018, 01:30 (2359 days ago) @ David Turell

Not surprising since they breathe air:

https://www.newscientist.com/article/2171076-seals-only-sleep-with-half-their-brain-whe...

"One species of seal sleeps in a way that has never been seen in any other animal. Their odd habits may help explain the function of “REM” sleep, the form of sleep in which we have our most vivid dreams.

"REM is short for “rapid eye movement”, because humans in REM sleep move their eyes back and forth even though their eyelids are shut. REM sleep seems to be essential for most mammals’ health. If rats are deprived of REM sleep, they lose weight, suffer hypothermia, and eventually die.

"Cetaceans like whales and dolphins are an exception. They sleep half of their brain at a time, so they can remain vigilant by keeping half the brain awake. Studies have failed to find evidence for REM sleep in cetaceans.

"To see if there were more exceptions, Jerome Siegel of the University of California, Los Angeles and his colleagues studied northern fur seals, which live in the north Pacific. They are semi-aquatic, living mostly on land during the breeding season but spending most of their lives in the sea.

"The team implanted electrodes into the brains of four captive juvenile seals and recorded their brain activity. The seals had access to a dry platform for some of the time, but this was removed for periods of 10 to 14 days to simulate time spent at sea.

"On land, the seals’ sleep consisted of both REM sleep and slow-wave (non-REM) sleep, with 80 minutes of REM sleep a day. In the water, their average amount of REM sleep fell to just 3 minutes a day. That’s less than the rats got during experiments on REM sleep deprivation.

"Unlike any animal studied before, the fur seals spent some time sleeping with their whole brain, and some time sleeping with half the brain like dolphins. The proportion of half-brain sleep increased from 62 per cent on land to 94 per cent in water. They may do this to watch out for predators like sharks and orcas, and to keep their nostrils above water to breathe."

Comment: An other weird adaptation required for sea-going mammals.