Kangaroos are the largest marsupials in the world. Red kangaroos can grow to over six feet tall and weigh up to 200 pounds. Marsupials are mammals whose young are undeveloped at birth and are carried in a pouch, where they remain until fully developed. This pouch has everything the baby kangaroo, called a joey, needs to finish developing: food and shelter.
Like all marsupials, kangaroos give birth to young that are very undeveloped. A baby kangaroo, or joey, is about one-inch long at birth, isn't developed much beyond the embryo stage and looks like a worm. The joey travels from the birth canal to the pouch by instinct. Once inside, the joey attaches to one of four teats and remains attached without letting go for 120 to 130 days.
Because kangaroos can mate as soon as a joey is born, a female can have three offspring at a time: an embryo whose development is suspended, another embryo developing in the pouch and a joey old enough to live outside the pouch but still nursing. Inside the pouch are four teats that produce two different kinds of milk for the different stages of a joey's development. Two of the teats produce a high-carbohydrate milk for the older joey; the other two produce a high-fat milk for the newborn.
Going In and Out
When a joey enters its mother's pouch, it goes in head first and then rolls on its back until it is facing out. A female kangaroo can control whether or not the joey stays in or out of the pouch with muscle contractions. When alarmed, she can contract her muscles to keep the pouch closed and the joey inside. She can relax her muscles to allow the joey to slip out.
A kangaroo’s pouch opens forward, unlike a koala's pouch, which opens backward. The pouch, called a marsupium, is actually a fold of abdominal skin that covers and protects the mother's mammary glands.
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The joey stays in the pouch for five to nine months, depending on the type of kangaroo. For the next two or three months, the joey spends less and less time in the pouch, returning to nurse or for transportation. A joey continues to nurse until it is 12 to 18 months old.