Whales: Adapting to swimming life
The major differences between the whales and the early mammals are all attributable to adaptations for a swimming life. The forelimbs have become paddles. The rear limbs have been lost altogether, though there are a few small bones buried deep in the whale's body to prove that the whale ancestors really did, at one time, have back legs. Fur, that hallmark of mammals, functions as an insulator due to air being trapped between hairs and is therefore of little use to a creature that never comes onto dry land. Consequently whales have lost that too, though there are a few bristles on the snout to demonstrate that they once had a coat. Insulation, however, is still needed and whales have developed blubber, a thick layer of fat beneath the skin that prevents their body heat from escaping even in the coldest sea. The mammals' dependency on air for breathing must be a considered a real handicap in water, but the whale has minimized that problem by breathing more efficiently than most land livers. Man only clears about 15% of the air in his lungs with a normal breath. The whale, in one of its roaring, spouting exhalations, gets rid of about 90% of its spent air. As a result it only has to take air in at extended intervals. It also has in its muscles a particularly high concentration of a substance called myoglobin that enables it to store oxygen. This form of oxygen storage allows the fin back whale, to reach depths of 500 metres and swim for forty minutes without surfacing for air.