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Macropod Reproduction and the Development of Joey in the Pouch

Macropod (kangaroo and wallaby) reproduction is truly fascinating.  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 it's mother's uterus.  BUT, and 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 it's 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 fetus, 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.
But the amazing stuff doesn't stop there.  Once mum has given birth, she will mate again and become pregnant only this second baby won't be born after 28 days like the first one.  Instead, the second baby develops until it is a bundle of around 100 cells and then stops growing.  It just sits there and waits for the tenant in the pouch to vacate.  This ability to suspend a pregnancy is called embyonic diapause and almost all kangaroos and wallabies are capable of it.  The advantage to this is that mum can replace a joey very quickly if she loses one.
The following pictures are sequenced pictures of a joey developing in the pouch.  Thank you to Rex Paperd and his ultra-tame wallaroo for providing them.

Newborn joey.  Day 1.

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Day 3 in the pouch.

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Day 10.

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6 weeks.

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60 days.

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90 Days.

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5 months.

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6 months

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7 months

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8 months.  Joey getting in and out.

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You can see how the question "How old is the joey?" does not have a simple answer.  To say that a baby that has just come out of the pouch is eight months old is misleading to most people because they don't understand the process.  An eight month old placental mammal like a dog or a cat would be approaching sexual maturity at that age.  But an eight month old joey (counting from it's birth) is only the equivalent of a newborn placental.  To avoid this confusion I usually answer "She's been out of the pouch for a few weeks."