Emperor penguins may have a supercharged form of a blood protein that allows them to dive underwater for more than 20 minutes on a single breath, a new study suggests.

The research showed that penguins in Antarctica return from long fishing excursions under the sea ice with the lowest blood oxygen levels ever recorded in wild animals.

With such depleted reserves, experts say, other creatures would black out and suffer tissue damage.

The finding suggests that emperors—the largest of all penguin species—may have a hyped-up version of hemoglobin, the blood protein that carries oxygen from the lungs to the body.

Other species are unable to use all the oxygen in their lungs—even when starved for air—because their hemoglobin cannot efficiently bind with and carry oxygen in low concentrations.

But penguin hemoglobin appears to be extra-sensitive, scooping up the last remaining oxygen in the birds' air sacs and delivering it to vital organs.