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Why are whales, dolphins, and porpoises mammals?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Whales, dolphins, and porpoises are classified as mammals because they possess the defining biological characteristics of the class Mammalia, despite their aquatic lifestyle.1

Key mammalian features found in cetaceans include:

Respiration: Unlike fish, which extract oxygen from water using gills, cetaceans breathe air through lungs.1
Reproduction and Nursing: They are viviparous, meaning they give birth to live young rather than laying eggs. Mothers produce nutrient-rich milk to nourish their calves through specialized mammary glands.213
Physical Traits: While many adult cetaceans appear hairless, they possess hair during their fetal stage. Additionally, their internal anatomy—such as the presence of a diaphragm and specific skeletal structures—aligns with mammalian biology rather than that of fish.214

Although their body forms are adapted for swimming, these animals are descendants of terrestrial, four-legged mammals that transitioned to an entirely aquatic existence over millions of years.14