ZMT: Daily Fact: Why dolphins sleep with one eye open 🐬

Daily Fact: Why dolphins sleep with one eye open 🐬

Original photo by Andrea Izzotti/ iStock

Metallica lyrics notwithstanding, most humans would find it impossible to sleep with one eye open. Dolphins, on the other hand, can't sleep any other way. That's because, unlike us, they don't breathe automatically and have to remain conscious in order to do so actively. The solution to this predicament is called unihemispheric slow-wave sleep, in which one half of the brain rests while the other remains awake. When the right half of a dolphin's brain is sleeping, its left eye is closed, and vice versa; they periodically alternate which side of the brain is asleep in order to rest fully while retaining consciousness. While sleeping, dolphins may rest motionless or swim slowly and steadily near the water's surface.

Dolphins aren't the only species with strange sleeping patterns. A number of their fellow sea creatures, from beluga whales to sea lions, also exhibit unihemispheric sleep. On the extra adorable end of the spectrum are otters, who float belly-up and often hold hands with one another so as to not drift apart while snoozing. Great Frigatebirds, meanwhile, sleep in 10-second bursts while remaining airborne for as long as two months at a time. And, like dolphins, they do it with one eye open.

Numbers Don't Lie

Species of dolphins found exclusively in the Amazon River

Gestation period of dolphins, in months

9–17

Dolphins that can live together in a superpod

1,000+

Age of the world's oldest known wild dolphin at her death in 2017

Bottlenose dolphins call each other by unique names.

Bottlenose dolphins are talkative creatures. Their communication skills, which have been called "sophisticated" and "novel," include referring to each other by unique names. These "names" consist of learned, distinctive whistles that individual dolphins both broadcast and respond to, and they're smart enough to not respond to other whistles. Researchers have also noticed that individual bottlenose dolphins copy the signature whistles of others, as a way of finding lost friends and family in the sea.

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Daily Fact: Why dolphins sleep with one eye open 🐬