monotrèmes

Monotremes are a small and ancient order of mammals that, unlike all other mammals, lay eggs instead of giving birth to live young. This unique reproductive strategy, along with several other distinct characteristics, sets them apart from marsupials and placental mammals. There are only five living species of monotremes, all found in Australia and New Guinea. These include the platypus and four species of echidnas, commonly known as spiny anteaters.

The most striking difference between monotremes and other mammals is their oviparous nature. After fertilization, the female monotreme lays a single, leathery egg, which she incubates until hatching. Another notable feature is their cloaca, a single posterior opening that serves as the outlet for the digestive, urinary, and reproductive tracts, a trait shared with birds and reptiles. Monotremes also possess a lower body temperature than most other mammals, and they lack nipples; milk is secreted through pores in the skin and licked by the young.

Despite their reptilian-like egg-laying, monotremes are unequivocally mammals, possessing fur, producing milk, and being warm-blooded, though their thermoregulation is less precise than in other mammals. Their evolutionary lineage is thought to have diverged from the main mammalian line very early on, making them living fossils that provide invaluable insights into mammalian evolution. The platypus is perhaps the most bizarre, with its duck-like bill, webbed feet, and venomous spur on the hind leg of the male. Echidnas, on the other hand, are covered in spines and have long, sticky tongues for feeding on ants and termites.