Which mammal group includes the only living species that lays eggs?

The mammal group that includes the only living egg-laying mammals is the monotremes. The group contains the platypus and four living echidna species, all native to Australia or New Guinea.

Monotremes are mammals because they have hair, are warm-blooded, and produce milk for their young. Their reproduction is unusual: females lay eggs rather than giving birth to live young. After hatching, the offspring drink milk released through skin openings because monotremes do not have nipples. Young echidnas are often called puggles.

The name monotreme comes from Greek roots meaning “single opening,” referring to the cloaca used for urinary, digestive, and reproductive functions. The other major living mammal lineages, marsupials and placentals, do not lay eggs. A common mistake is to call monotremes reptiles or birds; their egg-laying is an ancient retained trait, but their anatomy and biology clearly place them among mammals.