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Mammalian Characteristics
The mammalian class is composed of warm-blooded vertebrate animals. They are characterized by the presence of body hair to insulate and control body temperature and mammary glands, possessed by the females, which produce milk to nourish their developing young. Some additional common characteristics include high metabolism, three middle ear bones and a single lower jaw bone.
Monotremes
Monotremes, native to Australia and New Guinea, are the only extant order of prototherians, the smallest subclass of the mammalian species. This class is not very diverse; only five species exist, which include the duck-billed platypus and four species of echidnas, or spiny ant eaters. They do not give live births and are considered primitive due to their egg-laying, reptilian characteristic. The gestation period of their young occurs outside of the body, for 10 days, in leathery eggs. Monotremes possess mammary glands but lack nipples. Therefore, once their young hatch, they are nourished by licking milk secreted by the mammary glands into the mother's fur.
Marsupials
Marsupials (also known as metatherians) are a more diverse mammalian infraclass than the monotreme. They are native to Australia, New Zealand and Central and South America. Commonly called pouched mammals, this infraclass includes species of kangaroos, koalas and wombats. Marsupial prenatal development begins inside the womb with nourishment from the mother through a placenta. However, the gestation period is short which results in the birth of a very tiny premature embryo. After birth, development continues inside the external pouch known as the marsupium. The newly born offspring will climb from the birth canal to the mother's nipple and latch on to feed on milk produced by the mother. This development stage can range from weeks to months depending on the species.
Eutherians
Eutherians, also known as placental mammals, are a large portion of the mammalian class. They are the most diverse in size, habitat and appearance. Similar to marsupials, prenatal development begins in the womb with nourishment through a placenta. However, their placental nourishment is much more complex, enabling a longer gestation period. The eutherian offspring are born fully developed with the ability to perform most biological functions independently. After birth, continued nourishment is provided through milk produced by the mother.