What group of mammals includes the platypus and echidnas?

What group of mammals includes the platypus and echidnas? The answer is monotremes. Monotremes are mammals in the order Monotremata and are the only living mammals that reproduce by laying eggs rather than giving birth to live young.

Only five monotreme species survive today: one platypus species and four echidna species. They are native to Australia and New Guinea. Despite their egg-laying reproduction, monotremes share a defining mammalian trait: females produce milk to nourish their young. Unlike most mammals, however, they lack nipples; milk is released through specialized skin areas and licked from the fur.

The name “monotreme” comes from Greek roots meaning “single opening,” referring to the cloaca, a shared outlet for the digestive, urinary and reproductive systems. The platypus is semi-aquatic, while echidnas are mainly terrestrial and use their long tongues to feed on ants and termites.

They are sometimes mistakenly grouped with reptiles because they lay eggs, but they are mammals because they have hair, produce milk and possess other mammalian anatomical features. Their unusual biology preserves an ancient branch of mammal evolution while remaining highly specialized.