There are up to three kangaroo babies in this tableau: the "young-at-foot" hopping out in front, another joey hidden inside its mother's pouch, and a third--a tiny embryo--lurking in the womb. Female kangaroos, you see, are permanently pregnant, but they can halt development of embryos through a strategy called diapause.
Although joeys can be born after less than a month of gestation, they spend nearly eight more months suckling on a teat in their mother's pouch. The female red kangaroo can mate and become pregnant again during this time, but the new embryo will grow no larger than a clump of 70 to 100 cells. When the nursing joey reaches 200 days old, development of the embryo restarts; a day before birth, the mother kicks the older joey of out her pouch to make space for its younger sibling.
But the new young-at-foot isn't quite gone yet. It sticks around and continues nursing up to one year of age. Female kangaroos' teats produce different types of milk tailored to the different nutritional needs of a joey in the pouch and at-foot.