The three types of mammals classified on the basis of how their young develop are the following: monotremes, marsupials, and placental mammals. Quite very distinct from other mammals, monotremes can lay eggs rather than give birth to their young. Only three types of mammals can be classified as monotremes. These are the platypus and two species of spiny anteaters known as echidnas. Monotremes exist only in Australia and New Guinea, where they lay leathery-shelled eggs that they incubate in a special pouch. Although monotremes lack mammary glands, they secrete milk to feed their young through ducts located at the mother’s abdomen. Unlike monotremes and other mammals, most marsupials develop their young through a protective pouch during post-birth development.
This is because most marsupials can only develop embryos in the mother’s uterus for only a short period of time. The young marsupials are born very immature, and they are placed in a pouch where the young can reach the mother’s nipple for milk. Examples of marsupials are kangaroos, Virginia opossums, koalas, wombats, and Tasmanian devils. Lastly, placental mammals are the most common type because they have placentas that are more complex compared to marsupials. Placental mammals have the ability to retain their young in the uterus for a longer period of time until the young can undergo full embryonic development before being born. Placental mammals include bats, moles, monkeys, elephants, rodents, whales, and even human beings.