The placental mammals are the largest group of the mammals, it comprises about 4300 species. They are known to be the third subclass of the mammals. The placental mammals are biologically classified into nineteen orders. The largest group of these mammals contains the rodents, such as rats, mice, squirrels, and porcupines. Bats are found in one of the largest group of mammals. The young placental mammals are born at a comparatively advanced period. These mammals include elephants, whales, shrews and armadillos, it also includes pets such as dogs and cats, it includes also the farm and work animals like; cattle, sheep, and horses. The humans are also regarded as the placental mammals.
Some of the placental mammals are found on land, water and in the air, and they differ in their shape and body size. They also differ in their method of surviving, some are carnivores, and some are omnivores while some are herbivores. The differences are also in those that colonized the land, they live in different shelters, for example: human live inside the house while the sheep and cattle in the kraal.
The placental mammals that live on the land are having feet, those who occupied the water are having gills and those who colonized the air are having the wings. The seals, including the sea lions and the walruses can not reproduce in the open oceans, however they spend most of their time in the open oceans, and they depend on the land for reproduction. The largest plant eating mammals that spend the entire lives in water are dugongs and manatees. The bats are the only group of placental mammals to occupy the air.
The jaguar is found to be occupying the land simply because its preys are colonized on the land. ‘The whales, including the huge baleen whales and the dolphins, are well adapted as fast, open ocean predators’. These marine placental mammals occupy areas where there are large amount of food, but where the water temperature is low. These mammals survive the cold conditions in two different ways. Some have very sparse hair and blubber, which helps them to become warm. And others have a double coat of fur, with extremely dense underfur hairs closely packed, and this prevent the skin from being wet.
The majority of the aquatic mammals in the world live in the ocean, nevertheless, some live in the freshwater, for example: river dolphins, and otters adapted to life in fresh water habitats. The rodents are found to have successfully occupied the land, air and the water. They managed to colonize these three habits because they are small in size and this serve as their advantage to evade the extreme conditions.
In conclusion, the placental mammals colonized the air, water and the land very successfully. The humans would not colonize the air because of lack of wings, but they colonized the land. The bats colonized the air. The seals live both aquatic and terrestrial life, they spend most of their time in the water, but they still have to come to land so that they can reproduce.
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3. Msn. Contributor. The Online Encyclopedia [internet], 2006 May 15. Available from, [http://Encarta.msn/text_761561349_1/Mammal/html].