Born tiny, they finish growing latched to a teat in a pouch
Marsupials deliver young after a short pregnancy—often twelve to thirty-three days—then the pink, blind newborn crawls to a teat in a pouch or fur fold. Gestation is brief; lactation does the heavy developmental work placentals pack into the womb. The Greek root mársippos simply means pouch.
Marsupialia spans Australasia, Wallacea, and the Americas: kangaroos, koalas, opossums, Tasmanian devils, wombats, and more. Roughly seventy percent of about 334 living species occupy the Australian region; thirty percent live in the Americas, with thirteen Central American species and only the Virginia opossum ranging north of Mexico. They split from placentals some 125–160 million years ago. Sizes run from gram-scale planigales to the extinct ton-scale Diprotodon; living red kangaroos reach about 1.8 metres and ninety kilograms.
Anatomy diverges sharply from eutherians: many females carry a permanent pouch; epipubic bones project from the pelvis; most lack a corpus callosum bridging brain hemispheres. Teeth often number forty to fifty. Females have two uteri and two vaginas plus a median birth canal; many males have a bifid penis forward of the scrotum. Body temperature averages about 35°C versus 37°C in placentals. Pouch openings face forward in hopping kangaroos and backward in many quadrupedal climbers.
A joey may match an eight-to-twelve-week human fetus at birth, from a few grams upward, then stay pouch-bound for months while milk supplies nutrients and immune protection. Early birth limits how long a fetus burdens the mother in hard seasons, at the cost of intense external care. Grasping forelimbs must develop early, which some argue constrained wing- or hoof-like specializations—though exceptions such as the pig-footed bandicoot complicate that story. Convergent lookalikes include the wolfish thylacine and gliding sugar gliders beside flying squirrels. The pouch is not a curiosity; it is a portable nursery evolved for unfinished beginnings.
Source: Marsupial
