Kangaroos - The 100 Days Project: Day 16 
Teats, Weaning, and that Extraordinary Birth
Photo: Stella Reid, Wildhaven
Leaving the actual mating to a later post, the gestation period for an eastern grey kangaroo is 36 days, give or take a day or two. At that point the embryo is somewhere in the vicinity of 10 millimetres long: the size of a small bean. One can identify the head readily enough, but the hind legs, so definitive to the genus, are so little developed as to be barely identifiable. The arms, on the other hand, are quite another matter. These are already strong enough to enable the tiny kangaroo to drag him/herself up, through the mother’s fur (she lies down to make this a little easier), from the vagina’s entrance (another post: the three vaginas) to the entrance to the pouch, which, in the last stage of what is in effect an extraordinary journey, the tiny roo enters and, having found a teat, attaches him/herself for the next stage of life.
Although it will have ended her/his permanent connection to the teat sometime earlier (a little after the fur emerges), the joey’s head will not emerge from the pouch for half a year (186 days, give or take). By this stage, although not much acquainted with the outside world, he or she is likely, all things going well, the season being a good one, etc., to have made the acquaintance of his her older brother or sister, since an out-of-pouch joey will nevertheless come back to push their head inside the pouch to feed from his/her (now elongated) teat for some months yet (the weaning time is often given as 365 days [all of these times are somewhat shorter for Red Kangaroos]).
The out-of-pouch joey and the in-pouch joey have different dietary needs, and the kangaroo’s teats, under separate hormonal control, can accordingly produce separate formulae.
So, two joeys at the same time!? In fact we should perhaps say three, since kangaroos are capable of producing a further embryo and holding it in suspension (diapause) until the cessation of sucking from the second, in-pouch joey signals that that embryo can be reactivated (diapause can also be brought about by drought and other periods of poor nutrition, etc.).
You can watch an intriguing video of an Eastern Grey helping her embryo toward the pouch here.