Marsupials

Marsupials belong to the order Marsupalia, one of three subclasses of mammals (Metatheria). Marsupials are named for the marsupium, which means pouch in Latin; most female marsupials carry their young in pouches. The order Marsupalia includes eight families, 75 genera, and 250 species. Marsupials are divided into two groups based on the number and shape of the incisor teeth. One group has numerous small incisors (the Polyprotodontia) and includes the carnivorous and insectivorous marsupial mice and American opossum. The second group has a few, large incisors (Diprotodontia) and includes the herbivorous marsupials such as kangaroos and wallabies. The majority of species of marsupials, such as kangaroos, wallabies, koalas, bandicoots, wombats, and Tasmanian devils inhabit the Australasian region (Australia, New Guinea, and the surrounding islands) to the east of Lombok in Indonesia, which marks the boundary between the Australian and Asian fauna. Approximately one-third of the species, most of which are opossums, are native to the Americas. Marsupials live underground (i.e. marsupial moles), on land (kangaroos), in trees (tree kangaroos and koalas), and in water (yapok), and inhabit rainforests, deserts, and temperate regions. Many species are nocturnal, while others are active by day. Marsupials may be herbivorous, carnivorous, insectivorous, or a combination of the three. Marsupials range in size from mouse-size to as large as adult humans.

All marsupials are born partially developed, and are small, blind, hairless; they have well-developed front legs with sharp claws and poorly developed hind legs. Immediately after birth, marsupial embryos crawl out of the birth canal using their front claws into the marsupium (pouch) where they attach themselves to a milk-secreting teat (nipple) for nourishment. The young marsupials are so helpless that they cannot suck milk right away. Contractions of muscles around the teat periodically squirt milk into the mouths of the attached embryos. The marsupial pouch helps keep the young attached to a teat. Newborn marsupials, born to species without pouches, stay attached by holding on to their mothers with their claws, and are aided by a swollen teat, which fills the baby's mouth, and makes it difficult for the young to detach. Female marsupials carry their young everywhere they go. When the young can no longer fit in the pouch or become too large for the mother to carry around, they detach and begin to live independently.

The oldest known fossils of marsupials date from the upper Cretaceous period (65–100 million years ago). Marsupials were once a dominant group with a wide distribution, and in the past were well represented in South America. The opossums of the Americas are extremely adaptable and some species have increased in number. The marsupial fauna of Australasia remained intact due to the isolation of this area from the rest of the world for millions of years, but many species are currently on the endangered list. Encroaching agriculture, urban sprawl, and the introduction of placental mammals have put some species of marsupial in danger of extinction. A century ago, the skins of large kangaroos were in great demand. Kangaroo hides have been used for leather and their meat used for human consumption, and for pet foods. Some species of kangaroo considered by farmers to be pests have been slaughtered in great numbers. Today, conservation groups, wild animal refuges and sanctuaries, and cooperation from ranchers and farmers are helping to keep the current populations of marsupials protected.