What order of mammals includes whales, dolphins, and porpoises? The answer is Cetacea. Cetacea is the scientific group containing all living whales, dolphins and porpoises, whose ancestors evolved from land-dwelling mammals into fully aquatic animals.
Modern cetaceans are divided into two major lineages. Baleen whales, or Mysticeti, include blue, humpback and bowhead whales and filter food with plates of baleen. Toothed whales, or Odontoceti, include dolphins, porpoises, sperm whales, belugas and narwhals; many use echolocation to locate prey and navigate underwater.
Taxonomy can make this answer look more complicated. In modern classifications, Cetacea is often treated as an infraorder within the larger order Artiodactyla, or Cetartiodactyla, alongside even-toed ungulates. Older textbooks and many quiz references describe Cetacea as an order, so “Cetacea” remains the standard canonical answer to this question.
Cetaceans breathe air through blowholes, propel themselves with horizontal tail flukes and have forelimbs modified into flippers. Genetic and fossil evidence places them closest among living relatives to hippopotamuses, despite their dramatically different body forms.