Most mammals give birth to live young. However, a small and unique group defies this common expectation by reproducing through egg-laying. These creatures are known as monotremes.
The Unique Mammals That Lay Eggs
The mammals that lay eggs are known as monotremes, belonging to the order Monotremata. There are only five living species of monotremes: the platypus and four types of echidnas. The platypus (Ornithorhynchus anatinus) is a semi-aquatic mammal recognized by its distinctive duck-like bill, beaver-like tail, and webbed feet. This animal is found in freshwater systems across eastern Australia, including Queensland, New South Wales, Victoria, and Tasmania.
Echidnas, often called spiny anteaters, are terrestrial monotremes covered in stiff spines, resembling hedgehogs or porcupines. The four species include the short-beaked echidna (Tachyglossus aculeatus) and three species of long-beaked echidnas: the Eastern, Western, and Sir David’s long-beaked echidnas (Zaglossus bartoni, Zaglossus bruijni, and Zaglossus attenboroughi). Short-beaked echidnas are widely distributed across almost all Australian environments, from snowy mountains to deserts, as long as ants and termites are available. Long-beaked echidnas primarily inhabit high-altitude forests in New Guinea.
Why They Are Still Mammals
Despite their egg-laying reproduction, monotremes possess fundamental characteristics that classify them as mammals. Like all other mammals, they have hair covering their bodies, which provides insulation. Monotremes are also warm-blooded, though their metabolic rate and body temperature are typically lower than most other mammals.
A defining mammalian trait is the production of milk to nourish their young, and monotremes share this characteristic. However, unlike placental mammals and marsupials that have nipples, female monotremes secrete milk through specialized pores or ducts onto patches of skin on their abdomen. The young then lap up the milk directly from the mother’s fur or skin. Monotremes also possess a single bone in their lower jaw and three middle ear bones, features shared with other mammals.
Egg-Laying and Early Development
Female monotremes lay eggs. Platypuses typically lay one to three small, leathery eggs. These eggs are similar in texture to reptile eggs, rather than the hard, calcareous shells of bird eggs. The female platypus constructs a nursery burrow along a riverbank, incubating the eggs by curling her body around them. The incubation period for platypus eggs lasts about 10 days.
Echidnas usually lay a single, soft-shelled, leathery egg into a temporary pouch on their belly. Gestation for echidnas is approximately three weeks, and the egg hatches inside the pouch after about 10 to 11 days. The newly hatched young, called “puggles,” are tiny and underdeveloped. These puggles remain in the mother’s pouch (for echidnas) or burrow (for platypuses) and feed on the secreted milk. They continue to develop before emerging and becoming independent, which can take several months.