Dolphins sleep with only one half of their brains at a time, and according to new research published Oct. 17 in the open access journal PLOS ONE, this trait allows them to stay constantly alert for at least 15 days in a row（连续）. Brian Branstetter from the National Marine Mammal Foundation and colleagues found that dolphins can use echolocation with near-perfect accuracy continuously for up to 15 days, identifying targets and monitoring their environment. The researchers studied 2 dolphins, one male and one female, and found that they were capable of this task with no signs of fatigue for 5 days. The female dolphin performed additional tasks for a 15-day period. How much longer they could have continued was not studied.

Sleeping with only one half of the brain at a time, or unihemispheric sleep, was believed to have evolved in dolphins to enable them to breathe at the surface of water even when half-asleep. This new research suggests that the need to remain vigilant（警惕的） may also have played a role in the evolution of this sleeping behavior.

"These majestic beasts are true unwavering sentinels（哨兵） 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 behavior through echolocation" says Branstetter.