The mammals that lay eggs are called monotremes. There are only five existing species of monotremes, all found in Australia and New Guinea. These include:
- The platypus (duck-billed)
- Four species of echidnas (also known as spiny anteaters): the short-beaked echidna, eastern long-beaked echidna, western long-beaked echidna, and Sir David's long-beaked echidna
Monotremes are unique among mammals because they lay eggs instead of giving birth to live young, but they still nurse their hatchlings with milk. The eggs have a leathery exterior, and after hatching, the young ones (called puggles in echidnas and newborns in platypus) depend heavily on their mother's milk for nourishment. These mammals represent an ancient evolutionary branch that retains some reptilian traits while being true mammals.