A baby red kangaroo (Macropus rufus) is about the size of a jelly bean. Born after about 34 days of gestation, it's less than an inch long — or 100,000 times smaller than its adult height (roughly 4 feet). This newborn kangaroo, called a joey, isn't quite ready for prime time, however. Unlike most mammals, joeys are born while they're still embryos, which means they lack sight, hearing, and hair. They spend the next six months in their mother's pouch, or marsupium, where they suckle from a teat and continue to develop before finally taking their first steps into the world. If the word "marsupium" sounds familiar, it's probably because that's where the term "marsupial" comes from. Marsupials are a mammalian class that includes kangaroos, wombats, koalas, possums, and more — about 330 species altogether.
Kangaroos are some of the supermoms of the animal kingdom. Not only do they have a special pouch for their babies, but they can create two distinct types of milk to care for both the developing embryo and the more mature joey. They can even suspend their ability to conceive during times of drought, and then regain that ability when conditions are more favorable. With their remarkable adaptability, it's no wonder kangaroos outnumber Australians nearly two to one.
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