Growing up, I always thought that joeys — baby kangaroos residing in the pouches of their mothers — were born fully developed, then placed in the pouch for ease of transport like a marsupial BabyBjörn. When I got older, I thought that joeys existed in their mothers’ pouches for the entirety of their development — from eggs all the way through the time that they are old enough to emerge. It turns out that both of these assumptions are entirely wrong, and that kangaroo gestation and development is far more interesting than I had ever believed.
Kangaroo pregnancy begins in the way that most of us are familiar with. An egg is released from an ovary and travels through the fallopian tube, and if it becomes fertilized, will implant on the uterine wall. However, unlike humans, kangaroo fetuses do not obtain nutrition through a placenta. Instead, developing kangaroos are nourished by a yolk sac, which is able to provide nutrition for only about twenty-eight days. At this point, the kangaroo must be born.
When a kangaroo is ready to give birth after less than a month of gestation, she sits down and her baby emerges from her cloaca, a sort of all-purpose orifice that is used for sex, birth, and excretion. The lima bean-sized infant then climbs up the mother and into her pouch, driven purely by instinct. Once inside the pouch, this new joey will drink from one of its mother’s four nipples; although its muscles are too weak at this point for it to suckle, the mother’s nipple will expand in the joey’s mouth and release milk to facilitate feeding. A joey will typically remain in its mother’s pouch for about nine months, after which it will begin to venture out (although it must still be fed by its mother until it reaches eighteen months of age).
But it gets even more interesting. Within days of giving birth to her joey, a mother kangaroo will become pregnant again. However, we encounter a dilemma because a new joey cannot occupy a pouch until the older one has begun to leave it. Therefore, the mother will enter a stage called embryonic diapause, in which development is suspended once the fetus reaches about one hundred cells. Once the joey leaves the pouch, fetal development resumes. Therefore, female kangaroos of reproductive age are almost always pregnant. As an even more extreme example of this, the related swamp wallaby actually becomes pregnant in one uterus several days before giving birth from another, meaning that these mothers actually experience overlap between consecutive pregnancies. This adaptation of perennial pregnancy allows kangaroos and other marsupials to delay fetal development and birth during times of drought or famine, giving their offspring the best chance for survival.
I have one more aspect of kangaroo gestation to wow you with, and I’ve saved the best for last. If a joey leaves its mother’s pouch at nine months and its sibling is born about twenty-eight days later, both young kangaroos must be fed by their mother at the same time since joeys are nourished this way for an additional nine months after vacating the pouch. However, a newborn joey and an older joey have different nutritional needs — the older a joey gets, the more protein and fat and the fewer carbohydrates it requires. Therefore, the mother is able to simultaneously produce two different types of milk to meet the needs of both of her offspring.
Sometimes the truth is stranger than even our whackiest childhood ideas, so do with this information what you will. I personally think it makes great party conversation, and believe it or not, I actually do go to parties sometimes.