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Marsupials’ reproductive systems differ markedly from those of placental mammals. During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas. Female marsupials have two lateral vaginas, which lead to separate uteri, but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent. As with all marsupials, the female kangaroo has 3 vaginas and uteruses (uteri). The two outermost vaginas are used for sperm transportation to the uteruses. Babies are born through the middle one.
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 time of extreme draught and starvation the female kangaroo can practice birth control by putting the babies growing in the uteruses “on hold”, stopping their future development until the 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 are unique to kangaroo, populations can increase rapidly when food is plentiful.
Kangaroo females get pregnant in the regular way. They shed an egg from their ovary and it drifts down the fallopian tube where, if it meets up with sperm, the egg is fertilized and then embeds itself in the wall of its mother’s uterus. BUT, here’s the big difference between us regular mammals and marsupial mammals, no placental connection is formed. As soon as the marsupial egg has consumed its own yolk to stay alive and develop (just like a bird egg), it has to be born. So the whole pregnancy is only about 28 days long!
At the end of the pregnancy the expectant mother takes up a sitting position and grooms her pouch. The baby emerges from an opening at the base of her tail called the cloaca. The infant is very tiny, only about the size of a lima bean. It’s pink and largely undeveloped except for its two front arms that are crucial for its climb up its mother’s abdomen to the pouch. The baby, which is little more than a foetus, makes this climb completely unaided and guided only by instinct. Once inside the pouch the baby finds one of its mother’s four nipples and takes the end of one in its mouth. The baby doesn’t have the muscles to suck at this stage. Instead, the nipple swells inside the baby’s mouth so that it can’t disengage and milk is secreted very slowly into its mouth. Later, once the baby’s jaw is more developed, it will be able to disengage and suck at will.
BABY 1: firmly attached to the teat, the mother kangaroo can basically wipe her paws on it. Red kangaroo joeys spend about 235 days in the pouch before popping out, and grey kangaroos joeys stay pouched for a better part of year. Starts to explore outside the mother’s pouch. As the joey experiments with the new plant foods full of carbohydrates, it suckles less.
This lets mother’s baby know that its ready for BABY 2, at which point it allows another fertilised egg to begin cooking. After just 33 days of development, BABY 2 makes the journey to the pouch and starts to suckle. Amazingly, the presence of BABY 2 does not mean that BABY 1 gets the boot. Instead, the momarroo’s mammary glands start churning out 2 different kinds of milk- carbohydrate rich meals for the neonate and fat rich meals for the yearling.
About 2 weeks after BABY 2 made the pilgrimage from cloaca to pouch, the female kangaroo got busy with a male kangaroo and she now has a fertilised egg waiting in her womb. And it will stay there, real chill like, until BABY 2 starts vacationing outside pouch. The mother kangaroo’s body is now supporting 3 joeys, all at completely different stages of development.
1. The Life of Mammals by David Attenborough, Inside Nature’s Giants by David Dugan, Aardvarks to Zebras by Melissa S. Tulin.
2. Dawson, Terence J. (1995). Kangaroos: Biology of the Largest Marsupials. Cornell University Press/Comstock Publishing. ISBN 0-8014-8262-3. Zoo and Wild Animal Medicine by Murray Fowler, 2nd edition.
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