looks like nothing but a built-in baby sling.

But if you peeked inside,

you'd see it's far more complex than a simple pocket.

It has to be, because the joey inside

is not your average baby.

An adult male red kangaroo can stand over

1 1/2 meters tall and weigh 90 kilograms.

That's larger than a grown man.

But the newborns start out the size of jelly beans.

They're blind, deaf, and hairless to boot.

After all, they only spend 33 days

inside their mom before birth.

That's like a human having a baby

when she's two months pregnant.

So the underdeveloped roo isn't ready to face

the harsh Australian wilderness.

That's where the pouch comes in.

It's a pocket of skin that acts like a second womb,

giving the joey a safe, cozy environment to grow.

And, like a pregnant belly,

the pouch can stretch to fit the baby as it gets bigger.

It's lined with powerful,

but flexible, muscles and ligaments.

To keep the joey safe, mom can tighten those muscles

to shut the pouch flush against her body,

just like pulling a drawstring bag closed.

And it will need the extra space,

because over the course of eight months,

that bean-sized baby will reach the size

of a large house cat.

That's thousands of times its birth weight.

That rapid growth is thanks to the pouch's four nipples,

which spout milk that contains germ-fighting antibodies

to keep the little roo from getting sick.

But that's just the start.

You see, the nutrient levels change

to meet the baby's needs as it ages.

For example, sulfur, a major building block of hair,

peaks around three months in.

That's the same time the baby starts growing fur.

The best part?

Mom can produce multiple types of milk at the same time,

each squirting from its own nipple.

So she can suckle two babies

in different age groups simultaneously.

Another special feature about the kangaroo pouch

is that it's lined with sweat glands

that release antimicrobial substances,

which help protect the baby roos

from harmful viruses, bacteria, and parasites.

But there's one more way that pouch's design

keeps the joey safe.

It's totally hairless, and that skin-to-skin contact

keeps the baby warm and cozy.

Basically, it's the ultimate nursery.

But nothing lasts forever.

Eventually, the joey will need to leave the pouch.

At about 5 months old, it pokes its head out.

And a month later, it takes its first tentative

steps into the world.

There, it will explore for a few short seconds

before high-tailing it home.

But as it gets older and bolder, it stays out longer,

until eight months in:

It's ready to leave the nest, well, the pouch, for good.