Which mammal lays eggs?

Only five living species of mammal lay eggs: the duck-billed platypus and four species of echidna, all found in Australia and New Guinea. These animals, called monotremes, represent an ancient branch of the mammal family tree that split off before live birth evolved in other mammals. Despite laying eggs, monotremes are true mammals because they produce milk to feed their young — though they lack nipples and instead secrete milk through patches on their skin. The platypus is further remarkable for having a bill that detects electrical signals from prey and, in males, venomous spurs on the hind legs.