Marsupials

Mammals that finish development outside the womb.

Marsupials are mammals whose young are born at a very early stage and complete development attached to a teat, usually inside a pouch. Around 330 species survive, most in Australia and New Guinea, with about 100 in the Americas.

The reproductive difference

A placental mammal nourishes its young through a long-lived placenta and gives birth to a relatively developed offspring. A marsupial has a short gestation — a red kangaroo about 33 days — and the newborn is embryonic: a few centimetres long, blind, hairless, with forelimbs developed just enough to crawl to the pouch.

Neither strategy is more advanced. Long gestation invests heavily in one attempt; the marsupial approach risks little and can be abandoned cheaply in drought, which suits an unpredictable climate.

Convergent evolution

Isolated in Australia, marsupials filled the niches placentals occupy elsewhere and converged on similar forms. Marsupial moles, marsupial mice and the extinct thylacine — strikingly wolf-like and no relation to wolves — are textbook examples that similar environments produce similar solutions.