Sleeping dolphins can still tell when they are in danger: Researchers find only half animal's brain rests at a time

• Technique allows them to carry out echolocation constantly
• Believed to be similar to birds sleeping with one eye open

By Daily Mail Reporter

PUBLISHED: 20:07, 17 October 2012 | UPDATED: 01:08, 18 October 2012

Dolphins can stay alert indefinitely because they sleep with only half their brains, research has found.

It means they can continue to breathe at the surface of water, and maintain vigilance in case predators approach.

To do this they use echo-location, the clicking sonar system used to locate objects, with near-perfect accuracy even when asleep, say researchers from the California-based National Marine Mammal Foundation.

Researchers have discovered Bottlenose dolphins sleep with only HALf their brain, allowing them to stay awake for 15 days at a time

Brian Branstetter, who led the study, said: 'These majestic beasts are true unwavering sentinels of  the sea.

'The demands of ocean life on air-breathing dolphins have led to incredible capabilities, one of which is the ability to continuously, perhaps indefinitely, maintain vigilant behaviour through echo-location.'

The researchers studied two dolphins, one male and one female, and found that they were capable of echo-location with no signs of fatigue for five days.

The female dolphin carried on for 15 days but the team did not study how much longer she could have continued.

Sleeping with only half of the brain at a time is known as unihemispheric sleep. Birds also use unihemispheric sleep to stay alert and, like dolphins, they sleep with one eye open.

The discovery is believed to be similar to the way birds sleep with one eye open - and researchers found female dolphins are better than males at staying awake

Mr Branstetter said: 'The biological, sensory and cognitive ecology of these animals is relatively unique and demanding. If dolphins sleep like terrestrial animals, they might drown.

'If dolphins fail to maintain vigilance, they become susceptible to predation. As a result, the apparent “extreme” capabilities these animals possess are likely to be quite normal, unspectacular, and necessary for survival from the dolphin's perspective.

'Although much can be gained by observing dolphins in the field, complementary laboratory studies are necessary to document the full range of these animals' capabilities.'

The research is published today in the open access journal PLOS ONE.