Seals cannot breathe underwater. As air-breathing mammals, they rely on atmospheric oxygen, just like humans. Their extended underwater excursions are not due to an ability to extract oxygen from water, but rather specialized adaptations that allow them to hold their breath for impressive durations.
Mammalian Respiration Underwater
Like all mammals, seals possess lungs for respiration. Their respiratory system draws air into the lungs for gas exchange, transferring oxygen into the bloodstream and removing carbon dioxide. This distinguishes them from fish, which use gills to extract dissolved oxygen from water. Each seal breath is highly efficient, exchanging approximately 90% of the air in their lungs, compared to roughly 20% for humans.
Specialized Diving Adaptations
Seals have evolved physiological adaptations for prolonged and deep dives. Their bodies efficiently store oxygen, with blood volume up to 15% of their body mass, compared to 5-7% in humans. They also have a higher concentration of oxygen-carrying red blood cells. Their muscles are rich in myoglobin, an oxygen-binding protein that stores oxygen directly within the tissue, often at levels 5 to 12 times greater than in humans. This internal oxygen reserve is a cornerstone of their diving capability.
When a seal dives, involuntary responses known as the “dive reflex” activate to conserve oxygen. Bradycardia, a dramatic slowing of the heart rate, is a key component, dropping from 75-120 beats per minute to as low as 4-10 in some species. This reflex is often triggered by facial immersion in cold water. Simultaneously, peripheral vasoconstriction restricts blood flow to less oxygen-sensitive areas like limbs and digestive organs. Blood is redirected to vital organs such as the brain, heart, and lungs, ensuring their function during oxygen deprivation.
Another adaptation involves their lungs, which undergo a controlled collapse at certain depths (25 to 70 meters). This minimizes nitrogen absorption into the bloodstream, reducing the risk of decompression sickness (“the bends”), which can affect human divers. Seals often exhale considerable air before diving, further reducing buoyancy and available nitrogen. This combination of oxygen storage, conservation, and pressure management allows seals to thrive underwater.
Navigating Depths and Durations
A seal’s dive duration and depth vary widely by species, influenced by activity and condition. Elephant seals, renowned for their diving prowess, can hold their breath for up to two hours, regularly descending over 1,700 meters, with some dives reaching nearly 2,400 meters. Weddell seals are also exceptional, staying submerged over 90 minutes and reaching depths of 600 to 900 meters.
Other species, like harbor seals, typically make shorter dives (3-5 minutes), though they can remain underwater for 30 minutes and dive over 200 meters. Seals undertake extensive dives primarily for foraging, pursuing prey that often resides in deeper waters, or to evade predators. They also use deep dives for travel between areas.
The Essential Breath
After their underwater excursions, seals must return to the surface to breathe. They exhibit a rapid and efficient breathing pattern upon surfacing, often “porpoising” or taking quick, powerful breaths to rapidly exchange gases. This allows them to quickly offload accumulated carbon dioxide and replenish oxygen stores. Their heart rate accelerates briefly to aid in recovery.
A recovery period is necessary after long dives to restore oxygen and clear metabolic byproducts. Unlike humans, whose urge to breathe is primarily triggered by rising carbon dioxide levels, seals directly sense blood-oxygen levels. This allows them to precisely regulate dive durations, preventing dangerous oxygen depletion. While formidable underwater, their time at the surface for breathing can make them temporarily more susceptible to predators.