Cetaceans are the only completely aquatic mammals and are among the most spectacular of all vertebrates. Seafarers have appreciated the beauty and grace of cetaceans for centuries. Whale hunting (whaling) began as early as 1000 C.E. Although some cultures consumed whale meat and utilized the meat and by-products as dog and cattle feed, whales historically have been hunted for their baleen ("whale bone") and oil derived from their blubber. Small cetaceans, particularly dolphins, have also been hunted, but their biggest threat comes from being incidentally caught in tuna fishing seines. Although the pressures of exploitation are ever present, international agreements protect some cetaceans, and some populations that were overhunted are increasing. Modifications of seines and equipment have greatly reduced the number of dolphins accidentally killed by fishermen.
Cetaceans are remarkable swimmers and divers. Their bodily form, skeleton, physiology, and behavior are modified for feeding, communicating, locomoting, and reproducing within salt and fresh waters. All are intelligent and exhibit complex social behavior; many can echolocate. Some species of whales are the largest animals that have ever existed, exceeding the biggest dinosaurs in body size and weight.
As unlikely as it seems, modern cetaceans are more closely related to living cattle, antelopes, and bison than to any other group of modern mammals. Cetaceans evolved from primitive terrestrial artiodactyls and represent a secondary adaptation to a completely aquatic existence. Fossil evidence of the most primitive whales come from Eocene rocks in India and Pakistan, sediments deposited in the eastern Tethys Sea nearly 50 million years ago. These
early cetaceans were amphibious, toothed predators, spending time on both land and in the water. Recent paleontological discoveries indicate that some supported their bodies on land with hoofed digits on the hands and on the soles of the feet; they probably moved like a modern sea lion. In the water, the animals propelled themselves by paddling webbed hands and feet and by body undulations.
Living cetaceans (Order Cetacea) are grouped into the suborder Odontoceti (67 species, thirty-five genera, and nine families of toothed whales, porpoises, and dolphins) and suborder Mysticeti (eleven species, six genera, and four families of huge whales with plates of baleen instead of teeth). All cetaceans have a nearly hairless, fusiform (torpedo-shaped) body that lacks sweat and sebaceous glands. Just beneath the skin is a thick, insulating, fibrous layer composed of fat and oil (blubber). Hind limbs, external ears, and ear muscles are absent. Nostrils (blowhole) open at the highest point of the head: odontocetes have a single blowhole, mysticetes a double blowhole. The blowhole connects directly with the lungs (which prevents milk from entering lungs of suckling calves), and it is closed during submergence. The water spout from a blowhole is condensation of water vapor entering the cooler air from the warmer lungs, not water ejected from the lungs. The bones are spongy and saturated with oil.
Cetaceans are the fastest of all marine animals. Powerful vertical movements of the tail raise and lower the body and propel the cetacean forward. The dorsal fin and flippers are used to steer. Most dolphins are shallow divers (to 30 m), but the sperm whale descends to more than 1,000 m.
Cetaceans must breathe atmospheric air, but they are adapted to alternate between normal breathing (eupnea)and long periods of not breathing (apnea). Most of the smaller dolphins and porpoises can hold their breath for up to five minutes; some whales (the sperm whale, for example) can remain submerged for an hour or longer. Various physiological adaptations are associated with cessation of breathing during prolonged diving and submergence. Before diving, the animal expels most of the air from its lungs. Cetaceans have twice as many erythrocytes (red blood cells that transport oxygen to tissues) per volume of blood as do terrestrial mammals, and up to nine times as much myoglobin (a molecule in muscle that stores oxygen and releases it to the tissues). The oxygen from these sources accounts for up to 90 percent of the supply utilized during diving. Heart rate decreases to half of that at the surface, thus decreasing oxygen use. Vascular networks shunt the peripheral blood supply to the brain and decrease the supply to the muscles; the oxygen debt sustained in muscular tissue is repaid when the animal surfaces to breathe again.
Most cetacean species are gregarious and have long parental care and maturation. A single calf is born after a gestation ranging from ten to seventeen months. Born underwater, the calf is pushed to the surface by its mother for the first breath.
Odontocetes (dolphins, porpoises, belugas, narwhals, sperm whales, and beaked whales) occur in all oceans, seas connected to oceans, and some rivers and lakes in North and South America, Asia, and Africa. All have conical teeth, and most species eat fish and squid; sperm whales take giant squid, large sharks, and fish; killer whales prey on fish, seals, porpoises, and small baleen whales. All have acute echolocation ability, used for communication, orientation, and detecting and stunning prey.
Mysticetes (right whales, rorquals, gray whales, and pygmy right whales) inhabit all oceans and are filter feeders, engulfing huge concentrations of zooplankton (minute crustaceans and other tiny animals). Instead of teeth, these cetaceans have long, thin plates of baleen (modified mucous membrane) that act as sieves; the plates are suspended from the palate at right angles to the long axis of the head. The animals swim with their mouths open through swarms of zooplankton. When the mouth is closed, the baleen acts as a strainer, trapping the zooplankton inside but allowing the water to pass through. The largest baleen whale and largest known animal (living or extinct) is the blue whale (Bal-aenoptera musculus). The longest blue whale ever measured was 34 m, and the heaviest was 190,000 kg. Many species of baleen whales migrate long distances. Gray whales (Eschrichtius robustus), for example, feed in the North Pacific during the summer and migrate 10,000 to 22,000 km to winter along the Korean coast in the western Pacific, and the coast of Baja California and Sonora in the eastern Pacific.
—Mary Ellen Holden
See also: Artiodactyls; Mammalia; Oceans; Plankton; Rivers and Streams
Chadwick, Douglas H. 2001. "Evolution of Whales." National Geographic 200, no. 5: 64-77; Gingerich, Philip D., et al. 2001. "Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan." Science 293: 2239-2242; Mead, James G., and Robert L. Brownell, Jr. 1993. "Order Cetacea." In Mammal Species of the World, 2d ed., edited by Don E. Wilson and DeeAnn M. Reeder, pp. 349-364. Washington, DC: Smithsonian Institution Press; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vol. 2. Baltimore: Johns Hopkins University Press; Rice, Dale W. 1984. "Cetaceans."
In Orders and Families of Recent Mammals of the World, edited by Sydney Anderson and J. Knox Jones, Jr., pp. 447-490. New York: John Wiley and Sons.
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