The embryo is nourished during its brief gestation by a fluid secreted by the mother's uterus. The young are born in a very undeveloped state; at birth the great gray kangaroo is about 1 in. (2.5 cm) long and the opossum about 1 1⁄2 in. (3.8 cm) long. Immediately after birth the young crawl to the mother's nipples and remain attached to them while continuing their development. As they are still too helpless to suckle, milk is squirted into them by the periodic contraction of muscles over the mother's mammary glands.
In nearly all marsupials the female's nipples are covered by a pouch, or marsupium, formed by a fold of abdominal skin. Even after the suckling stage the young return at times to the pouch for shelter and transportation. In many species the young are carried on the mother's back after the suckling stage. In addition to having a less efficient reproductive system than the placental mammals, marsupials are of generally lower intelligence.
Marsupials were once widespread over the earth, but were displaced in most regions as the more successful placental mammals evolved. The Australian region, which has been isolated from contact with other regions since the Cretaceous period, had almost no native placental mammals, and the marsupials were able to continue their evolution there without competition. They underwent an adaptive radiation in Australia comparable to that of placental mammals in the rest of the world, evolving many forms that superficially resemble various placental mammals and fill the same ecological niches. Thus, there are animals known as Tasmanian wolves (see thylacine ), marsupial moles, marsupial mice, and native cats (see dasyure ), which live very much like the correspondingly named placental mammals and, in many cases, are strikingly similar in appearance. See also bandicoot , numbat , phalanger , Tasmanian devil , wombat .
See H. Tyndale-Biscoe, Life of Marsupials (1973); A. K. Lee and A. Cockburn, Evolutionary Ecology of Marsupials (1985).
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