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The secret to the superhero animal feat is elevated levels of special oxygen-carrying proteins found in their brains, a new study reveals. But the research leaves puzzles.

Scientists have long wondered why marine mammals, such as dolphins, whales, Weddell seals and sea otters, are so tolerant of such low oxygen conditions. The simplest explanation had been that they evolved adaptations to boost oxygen delivery to the brain. But studies have shown that the oxygen levels in their blood vessels plummeted within minutes of dipping beneath the water's surface.

"The kinds of levels we were seeing in arterial blood would have caused us to black out under water," said lead study author Terrie Williams, professor of ecology and evolutionary biology at the University of California, Santa Cruz.