The Wonder of Milk Mammary glands, the source of milk, are unique to mammals, as is milk itself, though pigeons produce something similar in their crops for their young.
Mammary glands are what give our class of animals their name.
We are the mammals or the mammary gland possessing animals. Though all mammals have mammary glands and produce milk, not all mammals have teats. The exceptions are the two monotremes the Echidna and the Platypus. In monotremes the milk is secreted onto the surface of the skin like sweat and licked off the body hairs by the young. In later mammals the mammary glands became more complicated with the possession of special ducts which channel the milk into the teats.