Saturday, May 26, 2007

Not All Marsupials Have Pouches, So What Do They Have In Common?

Among the mammals discovered by the first explorers of Australia and America were curious creatures the females of which possessed a pouch of loose skin on the abdomen where they sheltered and suckled their young. Zoologists in the 17th-century classified these animals as Marsupialia, from the Latin word for purse.

Of the 270 marsupial species in the world today, around two-thirds are found in Australasia; the rest live in Central and South America, apart from two species in North America. We now know that not all female marsupials have deep, bag-like, forward-opening pouches, like the kangaroo. Such pouches are found only in climbing and leaping species. In burrowing marsupials such as bandicoots and wombats, the abdominal pouch opens backwards, to avoid filling with soil. In mouse opossums, there are merely two narrow flaps of skin, and a few marsupials, such as the numbat (banded anteater), are completely pouchless.

Rather than the possession of a pouch, what defines an animal as a marsupial is the incomplete state in which its young are born. In mammals like ourselves, babies develop for a relatively long period inside the womb. This extended development is made possible by the placenta, an organ through which the mother's blood passes nutrients to the foetus.

Marsupials have no placenta and babies spend a very short time in the womb - in the case of some opossums, only 12 days. At birth, marsupial babies are virtually embryos, with only rudimentary limbs, nor fur, and closed eyes and ears. A kangaroo weighing 32 kg (70 lb) bears a baby only about 10 mm (1/2 in) long, while over a dozen newborn opossums would fit into a teaspoon.

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