Leopard seals, like all seals, rely on underwater predation for most of their diet. Yet when a seal takes a lengthy swim to hunt its prey, it does not have access to oxygen; unlike fish, seals do not have gills and cannot absorb oxygen from the water. While seals cannot breathe underwater, they do have an extremely high capacity for storing oxygen in their blood and muscles. In fact, seals can carry more than three times as much oxygen as humans on a per weight basis. This means that leopard seals can go without a breath for almost ten minutes; other species of seals have been clocked staying underwater for more than an hour.
Underwater predation poses another challenge for seals: They must find a way to stop water from filling their lungs. When a seal dives into the sea, its nostrils automatically shut in response to the pressure of the water. Upon opening its mouth to eat prey, the tongue and soft palate block the back of the mouth, keeping water out of its lungs.