Marsupial mating systems are not well known. Some species may be monogamous, but most are probably polygy-nous or promiscuous. The marsupials are a group defined by their reproductive biology. Female marsupials give birth to live young, but at a very early stage after a very short gestation (for example 21 days in the common wombat, 17 days in the brushtail possum, 32 days in the eastern gray kangaroo). Female marsupials have two uteri and two vaginas. Young developing in one or both uteri are born through a third opening, which only develops when the female is due to give birth for the first time. In most marsupials this birth canal is a temporary structure that seals over after the birth of every litter, but in certain diprotodonts (the kangaroos and the honey possum) it becomes permanent.
Newborn young make their own way to the mother's teats, which are usually located within a pouch (the "marsupium" for which the group is named). The young latch onto a teat and continue their development sustained by milk that changes in composition to suit their needs as they grow. The diprotodonts have some of the best developed pouches—designed to carry the young securely while the mother hops, ands burrows her way through daily life. The number of teats varies between species and gives a rough guide to maximum litter size—a female cannot rear more young than she has teats. Diprotodont litters are quite small—one or two is normal for most possums, whereas single young are the norm for kangaroos, rat kangaroos, wombats, and the koala. The mountain pygmy possum may give birth to as many as eight young, but only the four strongest will find a teat and survive.
Most mothers continue to suckle to their young when they have outgrown the pouch. Possums and wombats carry larger youngsters on their back. By accompanying their mother as she forages, young animals learn by imitation what foods are good to eat. Young kangaroos that have left the pouch may
remain close to their mother and continue to suckle for many weeks, reaching into the pouch to drink from the same teat they used as a newborn. By this stage there is often a new baby in the pouch attached to a different teat, in which case the mother produces a different kind of milk for each of her two offspring.
In many diprotodonts, including the honey possum and several species of kangaroo, the interval between one young vacating the pouch and the birth of a replacement can be as little as one day. This is thanks to a remarkable phenomenon known as embryonic diapause. Because gestation is so short, it does not interrupt the normal estrus cycle, and a female can come into season very soon after her first young is born. The new mother may mate and conceive during this "post-par-tum" estrous, but as long as the pouch is occupied by a suckling youngster the second embryo or litter does not develop beyond a very early stage. Instead it remains in a state of suspended animation, ready to be reactivated should the first young be lost or when it is almost ready to leave the pouch. The process is controlled by the same hormones that regulate milk production.
With the exception of the honey possum, individuals of which rarely live to see their first birthday, diprotodonts are quite long-lived animals. Possums, cuscuses, ringtails, gliders, and small macropods have a life expectancy between six and 14 years, koalas often live into their late teens, and wombats and large kangaroos well into their twenties. Even the diminutive feathertails and pygmy possums can live seven and 10 years respectively.
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