Seals are marine mammals that cannot breathe underwater. They must regularly return to the surface for air. Despite this, seals exhibit remarkable adaptations, allowing them to spend extended periods beneath the ocean’s surface. They can hold their breath and withstand the immense pressures of deep dives.
Why Seals Need Air
Seals are mammals. Like all mammals, they possess lungs and rely on atmospheric oxygen for respiration. This process involves inhaling oxygen from the air and expelling carbon dioxide as a waste product. Unlike fish, seals do not have gills to extract oxygen from water. This physiological difference necessitates their return to the surface to breathe.
Masterful Underwater Adaptations
Seals have evolved a suite of physiological and anatomical adaptations for impressive underwater capabilities. One adaptation is bradycardia, where their heart rate significantly slows during a dive, reducing oxygen consumption. A harbor seal’s heart rate, for instance, can drop from 75-120 beats per minute to as low as four to six beats per minute when diving. They also exhibit peripheral vasoconstriction, a mechanism that redirects blood flow away from extremities and non-essential organs to prioritize the brain, heart, and muscles, ensuring these vital areas receive sufficient oxygen.
Seals possess increased oxygen storage capacity compared to land mammals. Their blood contains higher concentrations of hemoglobin, and their muscles have elevated levels of myoglobin, both proteins that bind and store oxygen. This allows them to carry more oxygen within their bodies. Before a deep dive, some seals, like harbor seals, exhale to reduce the air in their lungs, storing oxygen primarily in their blood and muscle tissues rather than relying on lung volume.
Another key adaptation involves their respiratory system, specifically collapsible lungs and flexible rib cages. These features prevent nitrogen absorption and “the bends” by allowing their lungs to collapse under pressure, pushing air into non-absorptive airways. Furthermore, seals demonstrate a remarkable tolerance to higher levels of lactic acid and carbon dioxide, which are metabolic byproducts, allowing their muscles to continue functioning even with reduced oxygen availability. Seals may also sense blood-oxygen levels, helping them regulate dive durations more efficiently.
Diving Feats: How Long and How Deep
Seals’ extraordinary adaptations enable impressive diving feats. While typical dives for harbor seals last around three minutes, they can stay submerged for up to 30 minutes. The ringed seal can hold its breath for approximately 45 minutes.
Elephant seals are efficient divers, capable of holding their breath for up to two hours. These animals can descend to depths exceeding 1,700 meters (5,577 feet). Deep and prolonged dives are primarily undertaken for hunting prey, avoiding predators, traveling, or resting. Their ability to spend significant time underwater is a direct result of unique physiological adaptations, enabling them to exploit food sources unavailable to less specialized marine life.