CETACEAN

any of several species of aquatic mammals, including the whales, porpoises, and dolphins, that make up the order Cetacea. Cetaceans are found in oceans worldwide, as well as in some tropical lakes and rivers. The order Cetacea is divided into three suborders: the Archaeoceti, a group of toothed cetaceans that are now extinct; the Odontoceti, about 70 species of modern toothed cetaceans, including sperm whales, killer whales, narwhals, beluga, dolphins, and porpoises; and the Mysticeti, 13 species of baleen, or whalebone, whales. The two types of modern cetaceans are mainly distinguished by the shapes of their heads and the nature of their teeth. Most odontocetes have simple teeth of uniform shape numbering from 2 to 300; the mysticetes have no teeth and feed instead by straining small organisms from the water through baleen plates, which are long, bristly structures of horn anchored to the upper jaw. Cetaceans originated in the early Tertiary or Late Cretaceous (about 70 million years ago), when they diverged from a group of land-dwelling, carnivorous mammals. The specific land-dwelling ancestors of the cetaceans are uncertain, however. There is also uncertainty as to whether the modern toothed cetaceans and baleen whales evolved from the same archaeocete relative or separately from different land-dwelling mammals. Cetaceans range in length from about 1.3 m (4.3 feet) in the smaller porpoises to about 30 m (100 feet) in the blue whale. Weights range from about 45 kg (100 pounds) to 136,000 kg (150 tons). Modern cetaceans have tapered bodies, no external hind limbs (although vestigial limb structures are found internally), and tails ending in a horizontal blade of two lobes, or flukes. Vertical tail movements produce forward propulsion, while the paddle-shaped flippers balance and steer the cetacean. Among odontocetes the teeth are used for seizing prey (fish and squid), which is not chewed before it is swallowed. This necessitates a specialized, multichambered stomach suitable for prolonged digestion of the unchewed food. Baleen whales eat plankton and small crustaceans and fishes that they strain from the water in two ways: by swimming forward with the mouth open slightly, keeping a continuous stream of food-filled water moving through the baleen plates, or by gulping mouthfuls of water and expelling them through the baleen. Baleen whales have a reduced sense of smell, and odontocetes have none. Hearing, on the other hand, is exceptionally well developed in all cetaceans. Porpoises, in particular, are capable of hearing sounds pitched 10 times higher than the human limit. All cetaceans use a variety of sounds to communicate socially, and odontocetes produce high-pitched clicks whose echoes allow them to discriminate objects and to navigate. Because cetaceans are mammals, they must come to the water's surface to breathe through blowholes located on top of their heads. Odontocetes have single blowholes, while baleen whales have pairs. After inhaling, the cetacean holds its breath while it dives beneath the surface. Smaller cetaceans can hold their breath for several minutes, but the larger species may stay underwater for an hour or longer. Sperm whales are known to have dived to depths of 1,100 m (3,600 feet) or more. Baleen whales are usually solitary except during the breeding season, but odontocetes spend most of their lives in organized schools ranging in number from a few animals to 1,000 or more. Odontocete schools can be complex in structure, with subgroups divided by family, age, and sex. These subgroups often swim, dive, and feed independently, except when the school is frightened or traveling rapidly. Play is common among odontocetes but rare in the baleen whales. Play takes various forms: sexually related prodding and rubbing of the genitals, balancing floating objects on the fins or flukes, and swimming and diving in formation or riding waves. Cetaceans frequently assist other cetaceans that are in trouble (known as epimeletic behaviour). Both baleens and odontocetes will stand by or support a wounded or sick animal. Females assist each other in giving birth, and mothers frequently shield their young. All cetaceans are seasonal breeders, with impregnation taking place during the period from spring to fall. The gestation period is 11 to 12 months, except in the sperm whale, in whom it is 16 months. Birth takes place underwater, after which the mother pushes the calf to the surface for its first breath. Baleen whales nurse their young from 7 to 10 months, and odontocetes nurse even longer. Milk is forced into the mouth of the baby by contractions under the mother's mammary gland. In odontocetes the mother-young relationship lasts for several years, and even a fully grown cetacean may return to its mother in times of stress. Most baleen whales migrate seasonally from feeding to calving grounds, a distance that may be as great as 5,000 km (3,000 miles). Sperm whales also migrate long distances, but other odontocetes' migrations are local in nature. Cetaceans can swim at relatively high speeds, porpoises at 38 km per hour (21 knots, or 24 miles per hour), and whales at 56 km per hour (35 miles per hour). Bottle-nosed dolphins (Tursiops truncatus) any member of the order Cetacea, a group of primarily marine mammals occurring throughout the seas of the world and in certain tropical rivers and lakes. The name whale is often applied by scientists as a general name for the larger species; most of the smaller members of the order are called dolphins (see photograph) or porpoises. The traditional grouping of these mammals in the single order Cetacea is not universally accepted, a disagreement that is reflected in varying systems of classification (see below, Annotated classification and Critical appraisal). Additional reading G.M. Allen, The Whalebone Whales of New England, Mem. Bost. Soc. Nat. Hist., 8:107322 (1916), containing much valuable information on the baleen whales; Harald T. Andersen (ed.), The Biology of Marine Mammals (1969), a modern review of physiology, anatomy, and behaviour of cetaceans; F.C. Fraser, Report on Cetacea Stranded on the British Coasts from 1927 to 1947, 3 vol. (193453), and . . . 1948 to 1966 (1974), information on North Atlantic cetaceans; Handbook of R.H. Burne's Cetacean Dissections (1952), a good source on whale anatomy; Richard J. Harrison and Judith E. King, Marine Mammals, 2nd ed. (1980), a simple guidebook on most aspects of cetaceans; R. Kellogg, The History of WhalesTheir Adaptation to Life in the Water, Q. Rev. Biol., 3: 2976, 174208 (1928); and A Review of the Archaeoceti, Publs. Carnegie Instn. 482 (1936), important papers on the evolution and adaptation of cetaceans; W.N. Kellogg, Porpoises and Sonar (1961), a preliminary synthesis of studies on porpoise echolocation; N.A. Mackintosh and J.F.G. Wheeler, Southern Blue and Fin Whales, Discovery' Rep., 1: 257540 (1929); L. Harrison Matthews, The Humpback Whale, Megaptera nodosa, ibid., 17:792 (1937); and The Sei Whale, Balaenoptera borealis, ibid., 17:183289 (1938), three papers that deal with economically important species; J.R. Norman and F.C. Fraser, Giant Fishes, Whales, and Dolphins, new ed. (1948), an excellent review of the cetacea; K.S. Norris (ed.), Whales, Dolphins, and Porpoises (1966), a readable account of the overall biology of the group; Dale W. Rice and Victor B. Scheffer, A List of the Marine Mammals of the World, Spec. Scient. Rep. U.S. Fish Wildl. Serv. (Fisheries No. 579) (1968), a systematic listing of all modern cetaceans; P.F. Scholander, Experimental Investigations on the Respiratory Function in Diving Mammals and Birds, Hvalrd Skr., 22:1131 (1940); and P.F. Scholander and W.E. Schevill, Counter-Current Vascular Heat Exchange in the Fins of Whales, J. Appl. Physiol., 8:279282 (1955), technical papers that deal with special adaptations of cetaceans; E.J. Slijper, Die Cetaceen (1936, reprinted 1973), and Whales, 2nd ed. (1979; originally published in German, 1958), comprehensive popular books on whales; V.B. Scheffer, The Year of the Whale (1969), a hypothetical narrative of the life history of the sperm whale, with factual information on other species; A.G. Tomilin, Cetacea, vol. 9 of Mammals of the U.S.S.R. and Adjacent Countries (1967; originally published in Russian, 1957); Frederick W. True, Contributions to the Natural History of the Cetaceans: A Review of the Family Delphinidae, Bull. U.S. Natn. Mus. 36 (1889), an early synthesis of cetacea that still has much validity; and Ernest P. Walker et al., Mammals of the World, vol. 2, Cetacea, 2nd ed. (1968), photographs and natural history data of most modern cetaceans. Lyall Watson, Sea Guide to Whales of the World (1981), is a useful reference and field guide to whales, dolphins, and porpoises. Kenneth Stafford Norris