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Lungs
The lungs of marine mammals are proportionately smaller than those of humans, but these animals use oxygen far more efficiently. Marine mammals possess increased amounts of myoglobin in their blood, which makes for improved oxygen retention. Dolphins and whales quickly inhale oxygen through a blowhole as they surface and are also able to exchange 90 percent of the air they have inhaled, each time they exhale. The muscles of marine mammals can function without oxygen during periods when they hold their breath. Sperm whales are capable of diving to depths that exceed a mile and are able to tolerate remarkable atmospheric pressure. Bottlenose dolphins can remain submerged for up to 10 minutes and reach depths of 1770 feet.
Streamlined Bodies
Whales, dolphins, seals and sea otters possess a streamlined body form for effective movement in water. Most marine mammals lack fur, and their smooth skin creates less drag. The appendages of these marine mammals are modified to further reduce drag and for maximal propulsion. Seals and sea lions breed and give birth on land, and their flippers also are adapted for locomotion across rocks and ground. Sea lions make more use of their forelimbs on land than seals do; a seal moves principally by undulating its body. Polar bears use their large paws to pull themselves through the water.
Thermoregulation
Marine mammals rely on thick fur or blubber as insulation against the cold water. Blubber contains fats that store energy and do not dissolve in water, while the thick fur of polar bears and sea otters traps warm air against their skin and prevents it from becoming wet. Marine mammals possess a complex circulatory system, through which they can control blood flow to their extremities. During this process, termed vasodilation, cold blood that is returning to the body core from the flippers and fins is warmed by receiving heat from the arteries going to the extremities.
Communication and Camouflage
Whales and dolphins communicate under water using sounds and echolocation. This highly sophisticated process also allows these animals to locate prey, which they would otherwise not be able to see due to poor underwater visibility. Dolphins and whales use counter-shading for camouflage. The dark backs of many of these animals blend into the dark water when viewed from above, while their light ventral area blends in with the sunlit ocean surface when viewed from below.