... (144 m.y. ago). The 250 or so species of the metatheria, or marsupials, are distinguished by their short gestation periods, tiny, feebly developed offspring, and (in most species) a protective pouch or marsupium. This pouch lies over the mammary glands of the female and the young crawl into it immediately after birth to feed and complete development. Eutheria, or placental mammals, comprises some 3800 or so species. They are all born at a more advanced state of development than marsupials: some are even able to run or swim within minutes of birth and most are weaned sooner than marsupials. The differences between the reproductive patterns of Eutheria and Metatheria reflect different evolutionary responses to the stresses of the environment (Lillegraven 1974). Eutherian young are produced with a higher potential for survival, but at greater cost to the mother. The cost of producing young for metatherian mothers is much less and ...

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