Britannica Sources used in this answer

Why are whales classified as mammals rather than fishes despite their similar body shapes?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

Whales are classified as mammals rather than fishes because they possess defining mammalian characteristics that distinguish them from the class of fishes, despite their similar aquatic body shapes. This external resemblance is a result of convergent evolution, where unrelated species independently evolve similar traits as adaptations to the same environment—in this case, life in the water.1

Whales are categorized as mammals because they share the following biological traits:
* Respiration: Unlike fishes, which primarily use gills to extract dissolved oxygen from water, whales breathe air using lungs and must surface regularly to do so.23
* Reproduction and Development: Whales are viviparous, meaning they give birth to live young rather than laying eggs. Furthermore, they nourish their offspring with milk produced by mammary glands.452
* Physiology: Whales are warm-blooded and possess hair, which is a unique mammalian feature (though often restricted to the fetal stage or isolated whiskers in adult whales).42

These fundamental anatomical and physiological differences confirm their classification within the class Mammalia.42