Cetaceans

Human contact with cetaceans—whales, dolphins, and porpoises—has a rich history, beginning with some of our earliest civilizations. Although ancient people believed cetaceans were fish, they are actually aquatic mammals, which means they bear live young, produce milk to feed their offspring, and have hair (albeit just a few sensory hairs). The Greek philosopher Aristotle (384-322 B.C.) was the first to record this fact; in his Historia Animalium, Aristotle noted that whales and dolphins breathe air through a blowhole, and therefore have lungs; and that instead of laying eggs like fishes, they deliver their offspring fully developed.

Modern biologists believe that life first appeared in the sea; from these marine beginnings, land-dwelling organisms such as mammals gradually evolved. Cetaceans have returned to the marine environment after an ancestral period on land. As evidence of their terrestrial pedigree, consider that a whale fetus possesses four limb buds, a pelvis, tail, and forelimbs with five fingers like any land mammal. Adult whales and dolphins have the streamlined, fish-like appearance befitting their watery existence, but they have maintained and modified key terrestrial features (e.g., a much-reduced pelvic girdle in the tail, and forelimbs now used as flippers for swimming). A blowhole atop the head (one in dolphins, two in whales) replaces the nostrils, and thus the passageways for food and air are completely separate, as opposed to the usual terrestrial condition, in which food and air partly share a common tube. Other anatomical changes in cetaceans include a reduced neck, sensory modifications, and the addition of a thick layer of blubber to insulate against the cold of the ocean depths and to provide extra energy stores.

The order Cetacea is divided into three suborders. The Archaeoceti are a group of extinct cetaceans with elongated bodies, and are known only from fossils that are still being discovered and described. The living cetaceans are the Mysticeti or baleen whales, ten species restricted to the ocean, and the Odontoceti or toothed whales, whose many species (including dolphins and porpoises) are found in diverse habitats ranging from deep oceans to freshwater rivers great distances from the sea. Cetologists (scientists who study cetaceans) still disagree about how many toothed whales may be distinguished, but at least 68 species are recognized.