Mammal Classification
Cetaceans
Recent molecular and paleontological studies suggest that whales, porpoises and dolphins share a common ancestor with hippopotamuses, giraffes and other artiodactyls. The most likely ancestral group is the Mesonychidae, which included a wolflike animal, Mesonyx, that ran on feet equipped with five small hoofs rather than claws. Subsequent cetacean ancestors took to the water, possibly around the time when India was still separate from Asia (see Mammal Migration).
All cetaceans are streamlined in shape, insulated with blubber rather than hair, and equipped with a circulatory system highly adaptive to the temperature changes of the ocean. A combination of foreshortened forelimbs and unusually long digits creates rigid paddles for steering. In addition, the problem of water pressure — a formidable obstacle to deep-sea diving — has been solved by several adaptations, including the ability to make use of oxygen stored in the muscles and the ability to collapse the lungs rapidly.
Cetaceans either possess baleen plates or they have teeth. Baleen whales use their plates of keratin to filter small invertebrates from seawater. The 120-ton blue whale has more than 300 baleen plates in each side of its upper jaw — surprisingly, about the same number as its much smaller relative, the minke whale.
Toothed cetaceans pursue fish and other marine animals for food, rather than swimming through concentrations of plankton as do baleen whales. Having to hunt for food has caused them to develop superior swimming techniques. Porpoises can swim up to 28 miles (45 km) per hour. Similar speeds have been observed in bottlenose dolphins, which pursue fish onto mud flats, sliding right out of the water to seize their prey.
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