Mammals are a class of vertebrate animals defined by common traits such as being warm-blooded, having hair or fur, and producing milk to nourish their young through mammary glands. While these characteristics are universal across the class Mammalia, the method of reproduction varies dramatically among different groups. This difference in how they bring their young into the world serves as the basis for classifying all living mammals into three distinct groups.
Monotremes
Monotremes, belonging to the order Monotremata, are considered the most ancient and primitive of the three mammal groups. Their defining feature is that they are the only mammals that reproduce by laying eggs, a process known as oviparity. The eggs have a leathery shell, similar to those of reptiles, and are retained inside the mother’s body before being laid.
The five surviving species are the platypus and four species of echidnas, all found exclusively in Australia and New Guinea. Female monotremes lack nipples, instead secreting milk from modified sweat glands onto a patch of skin, which the young lap up. The platypus lays its eggs in a burrow, while the female echidna develops a temporary pouch to hold and incubate the single egg.
Marsupials
Marsupials are characterized by a reproductive strategy involving an extremely short internal gestation period, followed by extensive development outside the womb. The young are born in a highly undeveloped, almost embryonic state. The gestation period is typically brief, often lasting only a few weeks.
Upon birth, the tiny, blind newborn must instinctively crawl up the mother’s fur and into the specialized pouch, called the marsupium. Once inside, the young attaches itself firmly to a nipple. The pouch acts as a second womb, where the young marsupial, or joey, remains for many months, nursing and completing its development. Familiar examples include kangaroos, koalas, opossums, and wallabies.
Placentals
Placental mammals, also known as Eutherians, are the most numerous and diverse group, representing over 90% of all mammal species. Their defining characteristic is a prolonged gestation period where the fetus develops entirely inside the mother’s uterus. This extended internal development is made possible by the placenta, a complex organ formed from both maternal and embryonic tissues.
The placenta serves as an interface, exchanging oxygen and nutrients from the mother to the developing fetus and removing waste products. This system allows the offspring to be born in a relatively advanced and mature state, increasing its chances of survival. Placental mammals include virtually all familiar mammals, from bats and whales to rodents and primates.