Seals are marine mammals that cannot breathe underwater. Like all mammals, they possess lungs and require atmospheric air. Despite this, seals exhibit an extraordinary ability to remain submerged for extended periods, holding their breath for remarkable durations. This capability stems from specialized physiological adaptations that maximize their time below the surface.
How Seals Maximize Underwater Time
Seals have evolved several distinct physiological mechanisms to sustain long dives. One significant adaptation is their capacity for oxygen storage, considerably greater than that of land mammals. Seals possess a higher concentration of hemoglobin in their blood and a high volume of blood relative to their body size. Elephant seals, for instance, have a blood volume around 22% of their body weight (compared to 8% in humans) and hemoglobin concentrations ranging from 20-24 gm/dl. Their muscles also contain high levels of myoglobin, an oxygen-storing protein, which can be more than eight times more concentrated than in human muscle.
During a dive, seals activate bradycardia, where their heart rate slows dramatically. Some seals decrease their heart rate from 100-120 beats per minute to as few as 4-6. This slowed heart rate conserves oxygen by reducing the metabolic demand of the heart and the rate at which oxygen is circulated and consumed. The slowing of the heart rate contributes to an overall reduction in the animal’s metabolic rate, further extending their underwater endurance.
Another key adaptation is peripheral vasoconstriction, selectively redirecting blood flow. During a dive, blood vessels constrict in less oxygen-sensitive areas, such as the limbs, skin, and digestive system, shunting oxygenated blood primarily to organs that are highly dependent on a continuous oxygen supply, like the brain and heart. This ensures sensitive tissues receive adequate oxygen while less critical areas temporarily function with reduced blood flow. The spleen also plays a role by releasing red blood cells into the circulation, increasing the blood’s oxygen-carrying capacity.
Seals also manage the air in their lungs to prevent decompression sickness. As they dive to significant depths, their lungs can partially collapse, typically at depths ranging from 30 to 70 meters, and potentially deeper depending on the species. This collapse prevents nitrogen from dissolving into their bloodstream, which could lead to “the bends” upon ascent. The remaining air is held in the rigid upper airways, preserving a small oxygen reservoir for the ascent.
The Breathing Process on Land and at Surface
When seals are not submerged, their breathing patterns are similar to other air-breathing mammals, yet optimized for their semi-aquatic lifestyle. Upon surfacing after a dive, seals engage in what is often termed “recovery breathing.” This involves a series of rapid, deep breaths designed to quickly resupply their oxygen stores and efficiently eliminate carbon dioxide that has accumulated in their system during the breath-hold. This efficient gas exchange allows them to prepare for subsequent dives with minimal surface time.
On land or ice, where seals spend time resting, molting, or raising their young, their breathing is rhythmic and consistent, much like terrestrial mammals. They rely on their lungs to take in oxygen from the atmosphere. Their respiratory system is adapted to handle both the rapid, intense breathing required post-dive and the more relaxed, steady respiration needed during periods of rest. The act of breathing is largely under their voluntary control, enabling them to consciously initiate breath-holding for dives and resume breathing when they return to the surface.