Animals Bearing Under-Developed Young
Marsupials are mammals. They are classed in a field of their own because they differ markedly from many mammals. There are over 250 species of mammals. Over 140 of the marsupial species are found only in Australia.
Kangaroos, wallabies, opossums, wombats, gliders, koalas, numbats, bilbies, possums, woylies and bandicoots are all marsupials. The largest is the red kangaroo and the smallest the flat-headed marsupial mouse.
Like mammals, marsupials are covered in fur and give birth to live young which are suckled by the female. Female marsupials do not have a placenta to the womb and are unable to nourish the embryo in that way. Instead, after a very short gestation period, the young are born in a very underdeveloped state. The hind limbs are only partially formed but the forelegs are quite strong and clawed. Once out of the birth canal, they claw their way through the mother's fur until they find a teat. In most cases, the teat is located in a protective pouch but in some species there is no pouch. They continue their development, reaching a transition period where they spend time outside the pouch (or off the nipple) until they are ready to fend for themselves. They may remain in close proximity to the mother or family group for some time.
Pouches differ in their size and characteristics. A kangaroo's pouch opens at the top, the koala's opens to the rear as do the pouches of animals which dig with their front feet. This prevents the pouch filling with dirt as they dig. Muscles around the pouch opening can be tightened to prevent the joey falling out. In some, the nipple expands to hold the tiny baby securely.
Most marsupial babies are called 'joeys' although a young echidna is called a puggle. The correct term for the pouch is marsupium. The kangaroo can have up to four teats. One may have a newborn attached to it while the previous year's young suckles from the outside from a different teat. Each teat supplies milk of a different composition to meet the different needs of the young. So much for a 'primitive' species!
Another feature which one would think was hardly 'primitive' if the ability of many marsupials to practise embryonic diapause. Once a female is impregnated, she can then delay development of the embryo until conditions are optimal for a successful pregnancy. Should her current offspring die, the development of the embryo can be brought on immediately. Some animals can even choose the sex of their next baby.
The young are born blind and hairless. They instinctively claw their way to a teat. The mother may lick her fur to provide a clear path to the pouch or teat but otherwise does not help the joey. The time spent in the pouch depends on the species. Kangaroos begin to venture out to play at around 4 months and by 8 months it is generally too big to fit in. At this point it continues to suckle from outside the pouch. Opossums stay in the pouch for around two months then ride on the mother's back, clinging on with claws and tail.
Many marsupial species are now extinct. Many have been hunted mercilessly for their fur or for meat. Loss of habitat, land clearing, introduction of new predators, competition for food and water both from introduced pests and domestic stock, have all taken their toll on the marsupials of the world.