Among mammals, the ability to lay eggs is limited to a unique group called monotremes. Monotremes are a primitive group of mammals that includes the platypus and echidnas (also known as spiny anteaters). These fascinating creatures are the only living mammals that lay eggs instead of giving birth to live young. There are only five extant species of monotremes: Platypus (Ornithorhynchus anatinus): Found in Australia, the platypus lays small, leathery eggs and incubates them in a burrow. After hatching, the young are nourished with milk produced by specialized mammary glands. Short-beaked echidna (Tachyglossus aculeatus): Found in Australia and New Guinea, the short-beaked echidna also lays eggs. The female deposits a single leathery egg into a pouch-like structure called a "pouch" located on her abdomen. The egg hatches after about 10 days, and the young echidna continues to develop in the pouch. Western long-beaked echidna (Zaglossus spp.): Found in New Guinea, the western long-beaked echidnas are a group of three species that lay eggs and have similar reproductive processes to the short-beaked echidna. These five species represent the only known mammals that retain the ancestral trait of egg-laying. All other mammalian species give birth to live young through various reproductive methods such as placental birth or marsupial pouches.