The males have a forked penis that is behind the testes, not in front as in other mammals. Gestation ranges from 8 to 40 days. The embryonic young are born very early in their development and must make their own way to the nipple that will nourish them for the rest of their development. The young remain firmly attached to the nipple for several weeks or months until they are mature enough to leave the pouch. Because the embryonic young must climb hand over hand up from the vaginal opening to the pouch, it is unlikely that there would ever be a hooved or winged marsupial.
Social organization is limited to short-term pair bonding.
There are no apparent leaders in a mob. The carnivorous species are much more aggressive, though not as fast as their placental counterparts. Until recently, very diverse marsupials, such as possums, cuscuses, gliders, and koalas, were included in a single family Phalangeridae. This is the oldest known family of marsupials, whose members are primitive and generalized. The big toe is clawless and opposable.