Elephant seals and Weddell seals can go without breathing for well over an hour. During their dives Weddell seals may reach depths of 860 metres, while southern elephant seals have been known to go more than 1,500 metres down.
All life began in the sea. But despite having evolved originally from fish, land-based vertebrates face a major problem if they return to a life in the sea and act like a fish again. As air breathers, they cannot take up dissolved oxygen from water like a fish. So they can only dive for as long as they can store oxygen breathed in at the surface. Consequently, these animals developed special adaptations for storing oxygen. This was the case with the ancestors of seals.
In animals such as humans, oxygen is carried from the lungs to the body’s organs and muscles via the blood. But in a seal’s muscles there are much higher concentrations of a protein called myoglobin, which can also bind to oxygen. This turns its muscles into oxygen stores that can power the metabolism while it is diving. As well, compared to land-based mammals, seals have much more blood, a higher proportion of red blood cells, the ability to reduce their heart rates while diving, and much greater control over the distribution of blood within the body.
Diving to such extraordinary depths creates its own problems because of the enormous pressure. Humans are likely to suffer the bends from much shallower dives, and sometimes need treatment in a recompression chamber. But unlike us, seals do not use any air remaining within the lungs once they have dived to about 70 metres. The lungs collapse, and the seal relies on oxygen stores in the blood and muscle.