What is "monotreme"?

Monotreme refers to a group of mammals that lay eggs instead of giving birth to live young. They are unique in that they have a number of reptilian features, including the fact that they lay eggs and have a cloaca, a single opening for excreting waste, mating, and laying eggs. Monotremes are found only in Australia and New Guinea and include the platypus and echidnas. They are also distinct in that they lack teeth as adults and produce milk to feed their young through specialized glands in their skin. Monotremes are considered to be one of the most primitive groups of mammals, and their unusual characteristics have fascinated scientists for centuries.