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, cattle, swine, deer, and chevrotains; DNA evidence suggests that whales are most closely related to hippopotamuses.
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