Seals are marine mammals adapted to aquatic environments. They spend a significant portion of their lives underwater, navigating the ocean’s depths. Found across diverse habitats, from icy polar regions to temperate coastlines, seals rely on their exceptional diving capabilities for survival. Their ability to remain submerged for extended periods is due to unique biological features that allow them to thrive where air-breathing creatures typically cannot.
Breath-Holding Records and Averages
The duration a seal can hold its breath varies considerably among species, depending on factors like age, activity level, and individual condition. Elephant seals are among the most impressive divers, known to stay underwater for up to two hours. Southern elephant seals commonly dive for 20-30 minutes, reaching depths of 400 to 800 meters, though their maximum recorded dive can exceed two hours. Weddell seals also exhibit extraordinary breath-holding capabilities, with recorded dives lasting up to 96 minutes and reaching depths over 900 meters.
Other seal species also demonstrate notable breath-holding abilities, though typically for shorter durations. Harbor seals can hold their breath for up to 40 minutes, with average dives lasting three to seven minutes. Grey seals are capable of submerging for over an hour, often spending four to eight minutes underwater during typical dives. These variations highlight how different species have adapted their diving performance to suit their ecological niches and hunting strategies.
Physiological Adaptations for Diving
Seals possess physiological adaptations that enable their prolonged underwater excursions. A primary adaptation involves efficient oxygen storage. Seals have a higher blood volume compared to terrestrial mammals of similar size, with elephant seals’ blood volume constituting up to 20% of their body weight. This large blood volume is rich in hemoglobin, the protein responsible for oxygen transport, with some species like elephant seals possessing 50% more hemoglobin than land mammals.
Beyond blood, seals store substantial oxygen in their muscles through high concentrations of myoglobin, an oxygen-binding protein. Myoglobin levels in seal muscles can be 10 to 15 times higher than in humans, creating an internal oxygen reserve. To manage oxygen consumption during dives, seals employ a diving response that includes a reduction in heart rate, known as bradycardia. Their heart rate can slow from 75-120 beats per minute to as low as 4-15 beats per minute, conserving oxygen. This reduced heart rate is coupled with peripheral vasoconstriction, restricting blood flow to non-essential organs and shunting it to the brain, heart, and lungs.
Seals also tolerate lower oxygen levels (hypoxemia) and higher carbon dioxide levels in their blood and tissues. Elephant seals, for example, routinely tolerate extreme hypoxemia, nearly depleting their blood oxygen stores. The spleen in seals, especially in deep-diving species like Weddell seals, is large and contracts during a dive to release oxygen-rich red blood cells.
Why Seals Dive So Deep and Long
Seals dive deeply and for extended periods primarily to access food resources unavailable at shallower depths. Many prey species, such as fish, squid, and crustaceans, reside in deeper waters or in deep scattering layers. By diving extensively, seals exploit these abundant food sources, ensuring they meet their considerable energy requirements. This foraging strategy is fundamental to their survival, with some seals spending 80-100% of their day continuously diving for small prey.
Another reason for prolonged and deep dives is predator avoidance. Large marine predators like killer whales and sharks often hunt near the surface. By diving to greater depths, seals escape these threats. Elephant seals, for instance, take short “power naps” during deep dives, capitalizing on the depths’ safety. These dives also facilitate efficient travel and migration, allowing seals to move between feeding grounds or breeding sites with less energetic expenditure than surface travel.