Monotremes belong to the class of mammals in the animal kingdom. They are however different from mammals in that they do not give birth to live young ones. Instead they lay eggs. Monotremes possess all other characteristics of mammals especially the unique feature of feeding the young ones through milk produced through the mammary glands.
The name monotreme come the fact that these animals have only one opening for three functions namely urination, defecation and reproduction. Marsupials, which are also mammals, have only single opening but placental mammals have three different openings.

Unlike other mammals, monotremes lay egg. The egg is retained inside the mother for some time wherein the egg is nourished with nutrients. They also lactate but do not posses nipples to feed the young. They take care of the young one for relatively longer duration.

The most popularly known monotremes are the ant-eaters and the platypus. They have a longer lifespan but lower reproduction rate. Probably that is the reason why they spend longer duration of time taking care of the young ones. The metabolic rate is lower though they are endothermic.

Monotremes existed in larger numbers in the past. Today, however the number is lesser and there are only five species of monotremes. Their habitation is restricted to Australia and New Guinea. Fossils of monotremes discovered suggest that they evolved about 150 million years ago though all of the earlier variants are extinct today.

They have undergone evolutionary changes. The monotremes of today have many changes from the fossils that have been discovered. The fossils itself suggest that the number of monotremes were sparse. One of the characteristics that have changed with time is the ability to maintain constant temperature in different climatic conditions. Even the low metabolic rate and a lower body temperature is an evolutionary adaptation.