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How long can you hold your breath? Two minutes is tough for most people, but for pinnipeds—including seals and sea lions—two minutes is a breeze. In fact, the longest recorded breath hold for a pinniped was completed by a southern elephant seal that stayed submerged for nearly two hours.

Some seals and sea lions regularly hold their breath for more than 10 minutes as they forage for food along the ocean floor, and a new study indicates that the key to their lengthy breath holds lies in their hearts.

Previous anatomical studies have found that pinnipeds have a structure attached to their hearts called an aortic bulb. This enlarged artery brings more oxygen-rich blood to the rest of the animal’s body. Scientists believe these balloonlike structures allow the animal to maintain constant blood flow, which, combined with a lower heart rate, conserves oxygen and allows for longer dives. So deep dives are in their blood, so to speak.

To see if a larger aortic bulb corresponds with dive duration, scientists from the University of British Columbia gathered existing data on the heart size and dive durations of crabeater, leopard, harbor, and Weddell seals and used ultrasound to measure heart size in northern sea lions and northern fur seals.