Anyone who has tried swimming laps without taking a breath, or having an underwater tea party as a kid, should respect crocodiles. Those thick-skinned reptiles can remain below the water’s surface for over an hour.

Researchers had known that when crocodiles hold their breath underwater, carbon dioxide builds up in their blood, dissolves, and forms bicarbonate ions. Those ions bind to amino acids in hemoglobin, the oxygen-carrying component of red blood cells. The bicarbonate ions cause the hemoglobin to release oxygen molecules, making them more readily available to tissue, N. Hennakao Komiyama of the Medical Research Council (MRC) in Cambridge, England, and his colleagues explain in the Jan. 19 Nature. » Read more: Improving humans’ blood with crocodiles’ – hemoglobin hybrid developed that binds bicarbonate ions