Dolphins Sleep With Half Their Brains Awake

Dolphins can stay sharp and alert, monitoring their environment for days on end without getting the least bit tired because they send half their brains to sleep while the other half remains conscious, researchers have learned.


Not only do dolphins have this clever trick for overcoming sleep deprivation, Sam Ridgway from the U.S. Navy Marine Mammal Program explained in a news statement, but they seem to be able to remain continually vigilant for sounds.

Ridgway and colleagues from San Diego and Tel Aviv wondered whether the dolphins’ unrelenting auditory vigilance tired them and took a toll on the animals’ other senses, according to the statement released by The Company of Biologists, a UK-based charity that promotes research in biology.

NGS picture of dolphins by Else Bostelmann

Ridgway and his team set about testing two dolphins’ acoustic and visual vigilance over a five-day period to find out how well they functioned after days without a break. The team publish their results on May 1, 2009 in the Journal of Experimental Biology.

First Ridgway and his colleagues, Mandy Keogh, Mark Todd and Tricia Kamolnick, trained two dolphins to respond to a 1.5-second beep sounded randomly against a background of 0.5 second beeps every 30 seconds, the news release said.

Ridgway explained that the sounds were low enough for the dolphins to barely notice them as they swam through their enclosure, but the animals sprung into action every time they heard the 1.5-second tone, even after listening to the sounds for five days without a break. Their auditory vigilance remained as sharp as it had been five days earlier.

“Next Allen Goldblatt and Don Carder designed a visual stimulus to test the dolphins’ vigilance while they continued listening to the repetitive beeps,” the statement added.

“Knowing that the dolphins’ binocular vision is limited because their eyes are situated on opposite sides of their heads, Kamolnick trained one of the dolphins, SAY, to recognise two shapes (either three horizontal red bars or one vertical green bar) with her right eye before training her to recognise the same shapes with the left eye, reasoning that if half of her brain was asleep during testing, the dolphin would only see the shapes through the eye connected to the conscious half of the brain.

“But the team were in for a surprise when they began training SAY’s left eye. She already recognised the shapes, even though her left eye had not seen them previously.”

Visual Information Is Transferred

The information must be transferred between the two brain hemispheres, Ridgway said. He suspects that the dolphin’s inter-hemispheric commissures, which connects the two halves, may transfer the visual information.

“Having trained both dolphins to recognise the shapes, the hard part began: monitoring and rewarding the dolphins continually over a five-day period while the team tested the animals’ responses to both the sound and visual stimuli,” the news statement continued.

“Amazingly, even after five days of listening out for 1.5-second beeps amongst the 0.5-second beep background, the dolphins were still responding as accurately as they had done at the beginning of the experiment.

“The team also enticed the dolphins into a bay at night where they could be shown the horizontal and vertical bar shapes, and found that the dolphins were as sharp at the end of the 120-hour experiment as they had been at the beginning.

“And when the team checked the dolphins’ blood for physical signs of sleep deprivation, they couldn’t find any.

“After five days of unbroken vigilance the dolphins were in much better shape than the scientists.”

Changing Planet

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More than forty years in U.S., UK, and South African media gives David Max Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. In his 22-year career at National Geographic he was VP and editor in chief of National Geographic Digital Media, and the founding editor of the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directed the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. A regular expert on National Geographic Expeditions, David also lectures on storytelling for impact. He has 120,000 followers on social media: Facebook  Twitter  LinkedIn