Why seals don’t drown – a fundamental question answered

New research from the University of St Andrews shows that seals are uniquely capable of cognitively sensing how much oxygen is in their blood –  shedding light on why marine mammals don’t drown when underwater for prolonged periods of time.

Numerous species of reptiles, birds and mammals breath-hold dive. Marine mammals spend prolonged periods of time under water, yet researchers at the University’s Sea Mammal Research Unit believe there is a key, undiscovered adaptation that is absolutely fundamental to staying alive in these conditions.

In a study published today in Science, researchers at the University of St Andrews Sea Mammal Research Unit have found that to be able to spend extended amounts of time under water, marine mammals have to be able to cognitively perceive oxygen levels in their blood in order to make diving decisions to not run out of oxygen and drown.

Not only that, but at the same time, these mammals have to be in some way insensitive to carbon dioxide in the blood  – most mammals, including humans, are very sensitive to even small changes in carbon dioxide which affects their physiology and behaviour.

Researchers worked with seals to experiment with varying levels of inhaled oxygen and carbon dioxide. The seals took part of their own free will in the University’s Pool Facility, diving underwater to get fish. The seals were exposed to air mixes that were altered to affect circulating levels of oxygen and carbon dioxide, and their diving behaviour was recorded.

Dive duration was positively correlated with circulating oxygen levels, but unaffected by carbon dioxide levels and blood pH.

Results showed that seals are uniquely capable of sensing how much oxygen is in their blood, and when there is less oxygen they dive for a shorter time, and when there is more oxygen they dive for longer.

Researchers also showed that carbon dioxide had no effect on how long the seals dived, suggesting they are aren’t sensitive to carbon dioxide changes in the way other mammals are.

This adaptation has been key in the evolution of marine mammals and potentially other breath-hold diving animals (like birds and reptiles) allowing them to spend most of their  lives without any access to air while diving, and therefore not running out of oxygen, losing consciousness, and drowning.

Dr Chris McKnight who led this research said “To find such a fundamental aspect of the evolution of marine mammals that is so central to a huge part of what they do – dive – is incredibly exciting. It feels like something that has been right under our nose all this time. 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.”

Dr Joanna Kershaw who co authored the study said “It was fantastic to team up with such a talented and dedicated group of colleagues while we studied the diving behaviour of six juvenile seals for almost a year. It’s been so exciting to work with these amazing animals and uncover one of the fundamental reasons they’re so perfectly adapted to their environment.”