Dolphins sleep with only half of their brains at a time, bestowing them with the uncanny ability to stay constantly alert for at least 15 days in a row. Even in this seemingly sleep-deprived condition, the aquatic mammals can perform with near-perfect accuracy when using echolocation to identify targets and monitor their environment, according to new research from the journal PLoS One.

Just two dolphins—a male and female—took part in the study. The pair showed no signs of fatigue for the first five days of the experiment, and the female powered through additional tasks for the entire 15-day period. The researchers cut the study off at that point, so it’s possible that the two dolphins could have continued to perform normally for an indefinite period of time without a full-brain rest.

The researchers call the dolphins’ trick for staying alert unihemispheric sleep, or just shutting half of the brain down at a time. They think this technique evolved to allow dolphins to breathe at the surface of water, even when snoozing.

“These majestic beasts are true unwavering sentinels of the sea,” said lead researcher Brian Branstetter of the National Marine Mammal Foundation, in a statement. “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.”

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