The pouched mammals, or marsupials, differ from the highest group of mammals, placentals, in a striking way. The young are born at a very early stage in their development; in the opossum from only 8 to 13 days after fertilization has occurred. They then crawl to a pouch on the abdomen of the female where they spend the early part of their lives, being suckled on milk from the mammary glands.
The kangaroo is probably the best known marsupial. Others include the Wombat, Koala, Native Cat, Tasmanian devil, Wallaby and the Banded Anteater. Not all marsupials have a pouch or marsupium as well-developed as that of the Kangaroo. Some merely have 2 flaps of skin while others, for example the Woolly opossum, have no pouch. In this case the youngsters hang on to the nipples of the female as she moves around. Eastern Barred BandicootPeramales gunnii
In the placental mammals (e.g. rabbit) the developing embryo obtains its nourishment from the mother by way of a placenta, a specialised organ formed by the union of the womb lining and certain embryonic tissues. Such a device permits a long period of development and consequently the young are well developed at birth. In marsupials, however, the young are not nourished by a placenta, except in the Bandicoot, but even here the placenta is relatively simple in structure. As embryos, the only nourishment they have is the store of yolky material in the egg and a supply of "milk" from the womb (uterus) lining. The yolk is quickly exhausted and consequently the young are born at an immature stage. They have to reach the pouch where, clinging to the nipples with their mouths, they can obtain the nourishment required for their firther development.
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At birth, the offspring of the Virginia opossum are no larger than a honey bee. The female does not pick up the newborns and place them in her pouch. They find their own way there, a quite remarkable feat for such immature creatures. Associated with this, the forelimbs and their nervous supply are well developed at birth. The hind limbs are relatively undeveloped at this stage. The mother may lick the fur on her abdomen to assist the youngster's journey to the pouch.
The length of the time that the young spend in the pouch varies from about 7 weeks in the Marsupial Cats to 4 months in the Rat kangaroo. Young of the Virginia opossum are nursed for nearly 2 months and they do not become independent of the mother for at least 3 months. Generally, a large number of young are produced. Ten is an average litter for the Virginia opossum.
The IUCN assessments show that many marsupials have declined in abundance and distribution and provide a gloomy prognosis for the future. Why have marsupials fared so poorly? In all regions, large areas of native vegetation have been fragmented and removed for agriculture and original faunas have been transformed by the introduction of exotic species. In the tropical regions of South America, large tracts of forest have been logged for timber or cleared for production of cattle or of cash crops such as coffee, bananas, soy and sugar cane. Sharp increases of human populations in the Amazon basin are now being accompanied by increased frequncy of fire. As fragmentation proceeds, the remaining forest becomes more susceptible to drought and climate changes. Marsupials are llikely to be at more immeditae risk in the dry tropical forests where fragmentation effects are acute and several species have small populations and reduced ranges.1