Marsupials are animals that give live birth to their young after a brief gestation and that young spends the next part of babyhood in the mom's pouch. (A few marsupials, however, do not have a pouch.) Most marsupial babies permanently attach to a nipple until they develop to a more mature stage. At that time they detach and nurse at will. Marsupials normally have a lower body temperature and metabolic rate and slightly different digestive system than placental mammals.

Their metabolic rate is normally 2/3 the rate of a placental mammal of the same size. When the babies are born after a brief gestation, they are not able to regulate their own body temperature and depend on the warmth from the mother's pouch. If they are detached from the nipple and removed from the pouch before they can regulate their own temperature, they will perish. Normally this regulation begins about 1/2 way through their life in the pouch.