Monotreme

Monotremes are a group of mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals. They are one of the three main groups of living mammals, along with marsupials and placentals. The monotremes are represented by only five species: the platypus and four species of echidnas (or spiny anteaters). They are the only surviving members of the order Monotremata.

Characteristics

Monotremes are distinguished by a number of unique characteristics. They lay eggs rather than giving birth to live young, have no teats, and have a single opening for the digestive, reproductive, and urinary tracts, known as the cloaca.

Egg-laying

Monotremes are the only group of mammals that lay eggs. The eggs are small, leathery and similar to those of reptiles. The female incubates them by curling around them.

No teats

Monotremes have no teats. Instead, milk is secreted from mammary glands on the mother's belly and the young lap it up.

Cloaca

Monotremes have a single opening, or cloaca, for the digestive, reproductive, and urinary tracts. This is a characteristic they share with birds and reptiles.

Species

There are only five living species of monotremes: the platypus and four species of echidnas.

Platypus

The platypus is a unique mammal that is native to Australia. It has a duck-like bill, webbed feet, and a beaver-like tail. It is also one of the few venomous mammals.

Echidnas

There are four species of echidna: the short-beaked echidna and three species of long-beaked echidna. Echidnas are covered in spines and have a long tongue that they use to catch ants and termites.

Evolution

Monotremes are among the most primitive of mammals, with fossils dating back to the Cretaceous period. They are thought to have split from the mammalian lineage before the evolution of marsupials and placental mammals.

See also

• Mammal
• Marsupial
• Placental mammal
• Egg-laying mammals

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