Sea lions are marine mammals, and like all mammals, they rely on breathing air for survival. This means they cannot breathe underwater, but they possess a highly specialized physiology that allows for extended periods of breath-holding, known as apnea. Their success in the ocean is directly tied to their ability to manage and conserve oxygen stores while submerged for minutes at a time. These adaptations enable them to hunt, travel, and rest beneath the surface.
The Mechanics of Breath-Holding
Unlike some other marine mammals, sea lions often exhale a significant portion of the air in their lungs before submerging. This partial exhalation reduces buoyancy, making it easier for the animal to descend and remain at depth without expending extra energy.
Once underwater, the sea lion’s respiratory system automatically seals itself off against the surrounding water pressure. Their nostrils are naturally structured to remain closed, only opening with muscle effort when the animal breathes at the surface. As the sea lion descends, the pressure causes the lungs to collapse, which stops gas exchange. This collapse prevents nitrogen gas from being forced into the bloodstream and tissues, mitigating the risk of decompression sickness.
Specialized Oxygen Storage Systems
The ability to sustain a dive relies on oxygen reserves stored throughout the body, not in the lungs. Sea lions have evolved to maximize their oxygen storage capacity. A major component of this strategy is a significantly higher overall blood volume, which acts as a reservoir for oxygen-carrying cells.
This large blood volume is coupled with a high concentration of hemoglobin within the red blood cells, increasing the blood’s oxygen-binding capacity. The remaining oxygen is stored in the muscle tissue, where the protein myoglobin is found in high concentrations. Myoglobin is structurally similar to hemoglobin but binds and stores oxygen directly within the muscle fibers.
The myoglobin in diving mammals is chemically unique, featuring a positive electrical charge that prevents the protein molecules from clumping together. This allows sea lions to pack a dense, highly concentrated myoglobin store into their muscles. This muscular oxygen reserve is important because it allows working muscles to draw oxygen directly, reducing their reliance on the circulating blood supply during a dive.
Activating the Mammalian Dive Reflex
The oxygen stored in the blood and muscle is conserved through the mammalian dive reflex. This reflex is triggered by breath-holding and facial submersion in water. The most noticeable effect is an immediate reduction in heart rate, a phenomenon called bradycardia.
For a California sea lion, the heart rate may slow from a resting rate of around 95 beats per minute to as low as 20 beats per minute during a dive. This reduction significantly lowers the rate at which oxygen is consumed by the heart muscle. Simultaneously, the reflex initiates peripheral vasoconstriction, which is the constriction of blood vessels in the extremities and non-essential organs like the digestive tract and skin.
This shunting mechanism redirects blood flow away from tissues that can tolerate low oxygen levels. The circulating oxygen is prioritized and delivered almost exclusively to the most oxygen-sensitive organs: the brain and the heart. This redistribution of blood ensures that the animal’s cognitive functions and central circulation are maintained for the duration of the dive. The sea lion can also exhibit an anticipatory slowing of the heart rate even before entering the water, suggesting a level of cognitive control over this reflex.
Maximum Dive Performance and Recovery
The efficiency of their oxygen conservation strategy determines the limits of a sea lion’s dive performance. While most foraging dives for a California sea lion are relatively short, lasting about 1.5 to 3 minutes, they are capable of greater feats. Extreme dives can push the duration to 10 minutes and the depth to nearly 274 meters, though some species, like the New Zealand sea lion, have been recorded diving deeper than 500 meters.
When a sea lion remains submerged past its aerobic dive limit, the body must switch to anaerobic metabolism. This process allows the muscles to continue generating energy without oxygen, but it results in a rapid buildup of lactic acid. The accumulation of lactic acid is isolated within the muscles due to vasoconstriction, preventing it from entering the main bloodstream during the dive.
Upon surfacing, the sea lion must recover the oxygen deficit and clear the accumulated lactic acid from its muscles. The heart rate increases, and the peripheral blood vessels dilate, allowing the lactic acid to flush into the bloodstream where it can be metabolized. Deep or prolonged dives require a proportionally longer recovery period at the surface to replenish oxygen stores and eliminate metabolic byproducts before the animal is ready to dive again.