whale aquatic mammal of the order Cetacea, found in all oceans of the world. Members of this order vary greatly in size and include the largest animals that have ever lived. Cetaceans never leave the water, even to give birth. Although their ancestry has been much debated, DNA studies and skeletal evidence from extinct early whales indicate that whales evolved from the ancestors of artiodactyls, a group that includes hippopotamuses, cows, pigs, and deer.
Characteristics and Behavior
Like other mammals, whales breathe air, are warm-blooded, and produce milk to feed their young. Their adaptations for aquatic life include a streamlined form, nearly hairless skin, and an insulating layer of blubber, which can be as thick as 28 in. (70 cm) in some Arctic species. The forelimbs of whales are modified into flippers, and the hind legs are reduced to internal vestiges. Many species possess
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