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The Body Form
The body form of whales has been shaped by the special demands of a marine lifestyle. They are highly streamlined, a trait enhanced by the disappearance of certain organs, like genitalia and ears, from an external to an internally cloaked position. External hair has been greatly reduced in whales, which enhances their sleekness. Heavy muscles in the caudal peduncle, or tail-stock, power the up-and-down movement of the large, laterally flattened tail flukes to generate strong forward movement. The dorsal fin present in most species helps stabilize the whale's movement and may play some role in thermoregulation; it is small in most whales and absent in a few, though the male orca, actually an oversized dolphin, sports a six-foot-tall dorsal fin.
The Blowhole
All whales still need to breathe air, and thus are tied to the surface. They do this via one or two blowholes situated at the top of the head. This placement not only makes sense to access an oxygen reservoir always above the animal, but also allows for seamless breathing during a whale's swift, undulating locomotion. The spouts of water vapor and spray accompanying a whale's surface respiration can be useful tools for long-distance cetacean identification, as the shapes and angle of the spout differ between species because of variation in blowhole placement and shape.
Diving Adaptations
Cetaceans in general show significant adaptations for diving significant distances underwater, which is a physiologically stressful activity on account of the crushing pressure of surrounding water at depth. Among the great whales, the sperm whale is best known as a diver: It may plunge more than a mile beneath the surface in pursuit of deep-water prey like squid. In their blood, whales have rich reserves of hemoglobin, which transports oxygen, and they have a high tolerance for carbon dioxide. During a dive, the whale's metabolism slows markedly, while blood flow services only the brain, heart and other essential organs. Structures like the lungs and alveoli collapse to reduce the pressure differential between the whale's internal anatomy and the ambient water.
Comparisons
Some of the whale's aquatic adaptations are mirrored in other marine organisms. Open-ocean fish tend to show a similar streamlined form, though their tail fins are vertically compressed and their swimming motion typically a side-to-side one. Another group of marine mammals, the pinnipeds, which include seals, sea lions and walruses, are also capable of deep dives, and exhibit some of the same physiological adaptations for it. Indeed, most whales are surpassed in diving capabilities by certain earless seals like the elephant seal.