🔬 Mammal

Mammals are a class of animals whose females feed their young on milk. The milk is produced by special mammary glands, and these give the class its scientific name — Mammalia, from the Latin mamma, 'breast'. The name was introduced by the Swedish naturalist Carl Linnaeus in 1758.

Besides mammary glands, mammals have fur or hair and three tiny auditory ossicles in the middle ear. The Mammal Diversity Database (MDD) of the American Society of Mammalogists, version 2.5 (July 2026), lists 6,904 species: 6,791 living and 113 that have gone extinct since about 1500; they belong to 27 orders and 169 families. In 2005 only 5,416 species had been described: since then scientists have been discovering new species and splitting old ones. Mammals include the cow, the goat, the cat, the whale, the bat and ourselves.

Almost all mammals give birth to live young. The exception is the monotremes: the platypus and echidnas lay eggs, but still feed their young on milk afterwards.

All mammals produce milk, yet only a few are milked. According to FAO, almost all the world's milk comes from cows, buffaloes, goats, sheep and camels, and less often from yaks, mares, reindeer and donkeys. Milk varies greatly between species: its water content ranges from 83 % in yaks to 91 % in donkeys, while the fat, protein and sugar are exactly what that species' young need. These few animals are the source of all the world's dairy products — from koumiss to cheese.