There are three kinds of mammals; placental mammals, marsupials, and monotremes. Monotremes are very small in number while the rest of the mammals are in large numbers. The placental mammals are especially dominant and they are existing on earth for over 100 million years. They have over 5000 species. The marsupials, on the other hand, have only about 300 species, and are found in Australia and America. The Virginia Oppossum is the only species of marsupial that inhabits North America. Australia, a continent, was part of Gondwana, the continent that included Africa, Madagascar, New Zealand, Antarctica, and South America. After Gondwana was split up, Australia was separated and has remained so till today. Marsupials moved to Australia prior to the division and have been living separately from placental mammals. Both placental mammals and marsupials are covered in fur or hair. They are warm-blooded tetrapods. The examples of marsupials are the koala, kangaroo, opossums, the Tasmanian Devil, wallabies, pademelons, quokkas, marsupial mice, bandicoot, wombats, the marsupial mole, numbat, bilby, and musky rat-kangaroo. The examples of placental mammals are humans, whales, elephants, otters, horses, rodents, cows, deer, horses, cats, lions, tigers, dogs and many others. Placental mammals give birth to well-developed and nourished babies, after keeping them in the womb for a certain period, where they are nurtured and sustained through the mother's placenta. The placenta is an organ, which attaches the embryo of a placental mammal to the mother's blood, which provides the embryo with nutrients for growth. A marsupial possesses a special pouch to carry their offspring or young. Marsupials give birth earlier that placental mammals, but to an undeveloped offspring. They do not possess placenta. The yolk and uterine secretions in the uterus provide nutrition for the embryo during its early development. When they are born, their eyes, ears and rear limbs are very poorly developed. They climb up to their mother's pouches so their front legs are more well-developed than the rest of their bodies at the time of birth. In the pouch, the offspring attach themselves to the nipples and feed on their mother's milk and continue developing. After they mature, they then move out of the pouch. According to the Animal Diversity Web, some marsupial species are born with pouches, and others develop a pouch at the start of the reproduction process. The basal body temperature (BBT) of marsupials is about 35°C and that of placental mammals is about 40°C. The body temperature of an animal decides the metabolic rate within the animal. The standard metabolic rate (SMR) of marsupials is around 30% less than that of placentals. The total energy expenditures during reproduction in marsupials is also lower than placental mammals. Tooth form differs substantially among species of marsupials. However, a simple characteristic to determine the members of the group is that the number of incisors in the upper jaw varies from the number in the lower. This number is same in almost all placental mammals. In placental mammals the dental formula is 3/3 1/1 4/4 3/3, that makes 44 teeth and marsupial mammals is 5/4 1/1 3/3 4/4, that makes 50 teeth. The numbers are of pairs of incisors, canines, premolars, and molars in the upper and lower jaws respectively. Another difference is that placental mammals usually have two sets of teeth. One set grows in young animals and another set of adult teeth replace full sets of baby teeth. In marsupials, only certain teeth can be replaced by the set of adult teeth instead of the whole set. Marsupials like kangaroos and wallabies hop with their long back legs. Ironically, hopping at low speeds needs more energy than that needed for hopping at high speeds. Other marsupials move by running, climbing, or waddling with four limbs. Placentals mammals usually walk and run with four limbs. The marsupials are less intelligent than placental mammals due to the size, structure and shape of the brain. In placentals, the brain case is large and broad. In marsupials, the brain case is small and narrow, which encloses a smaller and simpler brain compared to that of placental mammals. Corpus callosum, the part of the brain that joins the two halves of the cerebral cortex, is not present. Due to the small size of the brain, marsupials also have restricted vocal ability. They are not totally silent, but few of them can let out loud sounds of excitement. In placental mammals, vision, olfaction, hearing and touch are the ways, which enable to perceive or sense an object. Communication can happen in various forms as well. Some species of marsupials communicate with acoustic signals, especially at the time of mating or territorial encounters. There are also many species that have color patterns which may be used to convey information. Another important difference between the marsupial and placental mammals is the time of gestation or the time for which the offspring remains inside the uterus. Marsupials have a short gestation period, which is only about six weeks. Most of the development takes place after its birth. Placental mammals have a longer gestation period compared to marsupials. The period is about nine months in humans. Most of the development takes place inside the uterus during pregnancy. The most important difference between placentals and marsupials is their reproductive systems. The sex organs of male marsupials and placentals do not differ much. However, in case of females, it is very different. Female marsupials possesses a pair of reproductive structures, which include two vaginae, two cervices, two uteri and two fallopian tubes. On the contrary, placental female mammals have only have one vagina, cervix, and uterus. The only similarity is that they both have two fallopian tubes. Marsupials' vaginae lie on both sides of the ureter, and each one of them have similar functionality. In male marsupials, most of them with the exception of macropods (the marsupial family, which includes kangaroos, wallabies, tree-kangaroos, pademelons, and several others) have a bifurcated penis, separated into two columns. The penis is posterior to the scrotum. In placental male mammals, the penis is not bifurcated and the penis is anterior to the scrotum unlike marsupials. ▸ Marsupials are found in large numbers in Australia where placental mammals are absent. ▸ The smallest marsupial is the long-tailed planigale and the largest marsupial is the red kangaroo. ▸ The only species of marsupial that lives in North America is the Virginia opossum. ▸ The smallest mammal is Etruscan shrew and the largest mammal is blue whale.