The kangaroo reaches sexual maturity at around the ages of 16 months for females and 24 months for males. They have no fixed breeding period, but they mate more often when food is plentiful than during periods when food is scarce.
As with all marsupials, the female kangaroo has three vaginas and two uteruses (uteri). The two outermost vaginas are used for sperm transportation to the two uteruses. Babies are born through the middle one. (See photo). By contrast, female placental mammals have only one uterus and one vagina.
With this unusual reproductive system a female kangaroo can be in a continuous state of pregnancy, with a fertilised egg in one uterus waiting to be released, a baby growing in the second uterus, one in her pouch and another hopping outside but coming to its mother for milk. Another unique feature of these animals is that during times of extreme drought and starvation the female kangaroo can practice birth control by putting the babies growing in her uteruses "on hold", stopping their future development until conditions improve. This is called embryonic diapause. When the mother's pouch becomes free the next baby will be born and move into the pouch and the fertilised egg "on hold" in a uterus will start developing into a new foetus. Because of this multiple-offspring strategy and other adaptabilities unique to the kangaroo, populations can increase rapidly when food is plentiful.
The male kangaroo has a two-pronged penis located behind its scrotum. (Most animals have the penis located in front). This two-pronged structure enable the male to inseminate the dual vaginas of the female kangaroo. When flaccid, the penis is withdrawn into the animal's body. Another adaptation for Australia's harsh environment is that the male kangaroo's body shuts down sperm production during period of severe drought in order to conserve energy.
The kangaroo egg, which is about 0.12 mm in diameter, descends from the female's ovary into an uterus where it is fertilized. Once fertilised the eggs is encased in a very thin shell similar to that of birds and reptiles. This shell is just a few microns thick and disintegrates when the egg reaches the third phase of gestation. A remnant from the evolutionary past this unusual characteristic is common amongst marsupial mammals. The gestation period for a kangaroo is approximately 30 days and varies amongst the different types of kangaroos.
As the time approaches for the young kangaroo to be born the female kangaroo cleans out its pouch by sticking her head into her pouch licking the inside of it clean. It then takes up a "birthing position" by sitting on its back with its tail between its legs and the hind legs extended straight forward. It also leans the trunk of its body forward. It then licks its birth canal opening possibly to stimulate the birth.
The young kangaroo, no larger than a jelly-bean (2 cm), and weighing less than one gram, soon emerges from the birth canal. It is born blind, hairless, with stumpy forelimb and hardly any trace of its hind legs. Even though it is still so underdeveloped the young newborn has an excellent sense of direction, knowing which way is up and down, and also an acute sense of smell. Using its little forelimbs in a swimming motion, the young joey crawls laborious up its mother's fur to the pouch. This journey takes it about three minutes. The joey's journey is made entirely by itself. The mother does not assist it in any way. Once inside its mother's pouch the joey quickly attaches itself firmly to one of four nipples in the pouch.
Once it has attached itself to its mother's nipple the young joey will stay hidden for up to six and a half months. Then it will start to tentatively pop its head out of its mother's pouch and observe the world around it. About two weeks later it will have gained enough confidence to venture out of the pouch and hop about close to its mother. However if frightened it will immediately jump back into the safety of the pouch. By the time it is about 8 months old the joey no longer uses its mother's pouch.
The female kangaroo can produce two types of milk depending on the joey it is feeding. The milk produced in the nipple on which an embryonic joey is attached will be different from the milk produced to feed a joey which has already left the pouch and only comes back to be weaned.
Because the young joey attaches itself so firmly onto its mother's nipple and is very difficult to pull away from it, early European explorers thought that baby kangaroos just miraculously grew off the nipple in the mother's pouch. This is because they could see no apparent opening inside the pouch the from which the joey could have emerged. They didn't know of the little joey's perilous journey from the birthing canal to the pouch.