Antarctic Weddell seals are also excellent divers, capable of diving to depths of 600 meters and staying for up to an hour. What's the secret to this deep-diving and long-duration ability? Scientists conducted research in a laboratory setting, simulating the diving conditions, and discovered that the Weddell seal's physiological functions undergo significant changes during descent to adapt to the demands of the dive.
Seals that are good at diving
Upon diving, the Weddell seal's heart rate immediately drops from 55 beats per minute to 15 beats per minute, and its cardiac blood flow decreases from 40 liters per minute to 6 liters per minute. Most other organs receive only 5-10% of their normal blood volume, but its blood pressure remains normal at 160 mmHg.
During descent, blood sugar levels drop significantly and continue to decline for the first 5-10 minutes after surfacing, but cardiac function increases dramatically during this time. Because respiration is impossible during descent, the body's oxygen reserves are quickly depleted, and glucose metabolism relies solely on anaerobic glycolysis to produce lactic acid. Consequently, blood lactic acid concentrations are very high, reaching three times the normal level. Upon surfacing, lactic acid concentrations decrease rapidly. The energy required for descent is supplied by lactic acid.
The lungs metabolize slowly and can absorb lactic acid, which is different from normal times when seals' lungs metabolize glucose.
The brain's metabolism during descent is not yet fully understood. If it also utilizes lactic acid, then the brain's glucose metabolism would need to be eight times higher than usual to meet its needs. If not, it means that the brain can meet its energy requirements even without oxygen. In reality, after 70 minutes of descent, the brain only consumes 33% of its total glucose.
The strange thing is, where does so much lactic acid come from? The brain and lungs couldn't possibly produce lactic acid; it must come from other parts of the seal's body. Experiments show that lactic acid comes from muscles and skin, because these areas have very low blood flow, only 15%. Due to the lack of blood and oxygen, these organs can only perform anaerobic metabolism, producing lactic acid. However, anaerobic metabolism produces very little energy, which explains why those organs with such low blood flow consume so much glucose to produce lactic acid.
The Weddell seal brain has extremely low oxygen consumption, which is highly advantageous for diving. Weddell seal blood contains 1000 milligrams of oxygen, and the brain uses only 3-4% of that oxygen in 70 minutes, while the human brain uses 90% in the same time. The Weddell seal heart uses 14% of its oxygen in 70 minutes, compared to 57% in the human heart. Considering only the oxygen consumption of the Weddell seal brain and heart, it has the potential to extend diving time.
In contrast to humans, Weddell seals have remarkably small heads, only about 5% the size of a human brain. So it's not without reason that such a large creature possesses a head less than 1/1000th of its body weight; this may be one of the secrets to the Weddell seal's adaptation to deep-sea and long-distance diving.
