Pouchbearing

Pouchbearing is a reproductive adaptation in which the female of certain mammals, most notably marsupials, carries and nurtures offspring in a specialized external pouch. The pouch is typically a skinfold or flap of tissue around the mammary region that houses one or more nipples. After a short, relatively underdeveloped birth, the newborn (joey) crawls into the pouch, attaches to a teat, and continues development there. The term is primarily applied to marsupials; monotremes (platypus and echidna) lack pouches, and placental mammals do not use pouches in the same way.

Within marsupials, pouch structure and usage vary. Pouches may open forward or rearward, be relatively large and well-defined or small and vestigial, and may feature a ring of fur or bare skin. The number of teats inside the pouch varies among species, corresponding to the typical litter size. The pouch often provides safety from desiccation and predators while the young undergo rapid postnatal development.

Development and timing: A marsupial birth is usually brief; the underdeveloped joey must reach the pouch, locate a teat, and secure itself. Depending on species, joeys remain attached and grow for weeks to months, being progressively supported by lactation as the mother cycles through reproductive states. Some species will continue to visit or nurse from the pouch for an extended period even after leaving it.

Examples and significance: Known pouch-bearing marsupials include kangaroos, wallabies, koalas, wombats, opossums, and Tasmanian devils. Pouchbearing is a defining feature for many life-history strategies in Metatheria and influences maternal energy allocation, dispersal, and social behavior. The evolution of the pouch is linked to the abrupt transition from embryonic to postnatal life in a relatively short gestation.