Why Deep-Diving Seals Don’t Drown

Marine mammals can dive for remarkably long periods because their bodies have evolved to adapt to the underwater environment. Elephant seals, in particular, can hold their breath for about two hours. Some might think seals can stay submerged for so long simply because they can control their breath. However, scientists have discovered another secret behind their diving abilities.

A new study from researchers at the University of St. Andrews’s Sea Mammal Research Unit in Scotland suggests that some seals can cognitively perceive their blood oxygen supply and plan their diving trips accordingly so they don’t suffocate while underwater.

Lead author J. Chris McKnight and colleagues studied wild-caught gray seals for nearly a year. The seals were kept in the University’s pool facility, where they could dive to get fish whenever they wanted. Meanwhile, the scientists filled their facility with four air mixes containing different concentrations of oxygen, carbon dioxide, and nitrogen to see how the seals adjusted their diving behaviors to various air exposures. The mammals’ dive lengths were positively correlated with blood oxygen levels, meaning they stayed underwater longer when they breathed more of the gas. These findings indicate that the gray seals could sense how much oxygen was in their systems and determine when they should surface for more air. The study also suggests that seals may be somewhat resistant to the effects of carbon dioxide in their blood, as their dive durations were unaffected by the gas. The findings appeared in the journal Science.

“As simple and as logical as it might seem at face value—that seals can sense oxygen, and that this helps them to make decisions so they don’t run out of oxygen and drown—evolutionarily, it puts them and their physiology amongst some of the most uniquely adapted animals,” McKnight said in a statement.

Earlier research showed that animals could determine carbon dioxide levels in their blood. With gray seals now demonstrating an ability to sense oxygen concentrations as well, McKnight and his colleagues predict that other sea mammals, like whales and dolphins, likely have the same ability. Further study is needed, however.

Marine mammals also conserve oxygen in various ways while diving. Whales and seals slow their heart rates and limit blood circulation to minimize oxygen consumption. Both also have high levels of myoglobin, a protein that stores oxygen in their blood, making their supply last longer. 

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