Why do seals breathe out when they dive?

Jon Dunn discovers why seals expel air when they are about to go underwater.

Grey seal

Grey seal diving in the Farne Islands, UK © Barnard Radvaner / Getty


When we dive, we fill our lungs with air and then hold our breath. Seals, in contrast, empty their lungs before submerging themselves. If you ever happen to be close to an individual that’s about to go under, you may well hear the snorting expulsion of breath from its nostrils.

Exhaling may seem counter-intuitive, but seals control their oxygen levels underwater far more efficiently than we do, enabling them to stay underwater for amazingly long periods and dive to incredible depths.

Most of the oxygen required to sustain a seal on a dive is dissolved in its blood, meaning it can avoid dragging a buoyant lungful of air down on the descent. Seals have fewer and larger red blood cells than terrestrial mammals, with higher concentrations of oxygen-storing haemoglobin.

Seal blood also contains high levels of a compound known as myoglobin, which helps the animals to tolerate the build-up of carbon dioxide as they descend.

Seals have another respiratory trick up their sleeves. All mammal muscles operate aerobically (when they are oxygenated). But seal muscles continue to work – for a time at least – with reduced oxygen, prolonging dive time further. 


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