'Mating.'
You've got about a mil there already.
'For every species on Earth, it's vital for the survival of the fittest.
But the way some creatures make babies is quite extraordinary.'
- I see it, I see it.
Yes!
- There it is.
'And there are few mammals with a stranger reproductive strategy than marsupials.'
Look at the little feet.
'With their alien pouches...' It's amazing, I'm holding what looks like a little fetus.
'..their strange love songs... (Grunting) It's an easy pick-up joint.
'..bizarre sexual anatomy...' This tube is a third vagina.
'..and aggressive fights for mates.'
They've got this henched stature, this huge chest.
I've never seen mating like this before.
'We're in Australia's Outback... ..to investigate Sex in the Wild.'
Sex in the Wild is made possible 'I'm Joy Reidenberg.
As a comparative anatomist, I'm fascinated by the strange sex lives of the weirdest mammals on Earth...marsupials.
Veterinarian Mark Evans and I have come to Australia to explore how these animals reproduce and their unique breeding behavior.
Isolated for millions of years in one of the toughest environments on the planet, hostage to fires, freak floods and droughts... ..marsupials have evolved remarkable breeding strategies to survive.
No other mammal is born at such an early stage of development.
The tiny newborn crawls for its life, up into a pouch to suckle and grow.
While Mark's travelling to Australia's Outback to explore the sex lives of the largest marsupials of all, male Red Kangaroos, I'm on my way to the arid South West, to visit a kangaroo sanctuary to get up close to some females.
This mob of Western Grey Kangaroos is being cared for at Uralla by owner, Mandy English.
Female kangaroos have a smaller home-range than males and when Mandy releases them into the wild, the forests and grasslands of this 150-acre haven become home.
Because these semi-wild kangaroos can tolerate humans close to them, I have an extremely rare opportunity to get close as they mate, give birth and raise their young.'
These are some of your rescues right here?
- That's Bella.
- Hi, Bella.
And this is Phoenix, and he's probably six years old now.
'There's one female I'm especially keen to meet, a three-year-old called Alice who was rescued by Mandy.'
Alice's mum was hit by a car.
The accident killed her mum so we went and picked her up and brought her home and we hand-reared her and now she has been released.
So, she's out in the wild but comes back to visit?
- She comes back to visit, yes.
- Oh, nice.
Nice to meet you.
'Excitingly, Mandy thinks Alice might be pregnant.'
We saw her being mated not that long ago, so we're hoping very soon, she might have a little joey.
- Like, in the next few days?
- I hope so, yes.
Wow, that would be great.
Yes, she hasn't had a baby before, so it will be her first joey.
(JOY) That would be awesome.
'Kangaroos give birth to their young after a gestation period of around 30 days.
Alice might not look pregnant but she could be just days away from giving birth.
To find out, veterinarian Christina Leslie carries out an ultrasound scan.'
Just trying to get the best view, good girl, good girl.
Good girl.
Is there a fetus there or not?
Alright, sweetheart.
- There's our little joey.
- Right there!
I see it, yes!
Okay.
Wow!
See this little image here, this little white blob?
You can see the fluid around it, there's our baby, 1.2 by 0.7 cm.
- What would that weigh?
- Maybe one gram.
How soon do you think this baby's due?
Within the next three days, four days, maybe.
- Oh, that's very exciting.
- It is, isn't it?
If you can look really carefully, can you just see that little fluttering in the center there?
- It's the joey's heartbeat, isn't that fantastic?
- Yes!
'In Australia's unpredictable environment, long pregnancies are risky.
So kangaroos have evolved to give birth after just four to five weeks.
Their tiny, immature young face an epic test of strength.
They have to climb up to their mother's pouch to grow.
Only the strongest survive.'
I think that's the way to do a pregnancy, don't you?
I'd much rather a 30-day pregnancy than a nine-month pregnancy.
Delivering a little jelly bean.
How easy would that be?
And do all the rest of the development where you can see what's going on.
You guys got it right, I have to say, you got it right.
What a great system, huh?
Yeah, good girl, you've been such a good girl.
'In order to see how this miniscule baby even came about, we're going to have to explore the strange world of kangaroo and marsupial sex.'
Marsupials have evolved an unusual strategy: keep the pregnancy short and nurse the premature babies in pouches.
This means mothers can produce babies quickly when times are good... or discard them when times are tough.
While I stay at Uralla, hoping to see actual mating and possibly a rarely-seen marsupial birth, Mark is almost 2,000 miles away, at Fowler's Gap Research Station in New South Wales.'
(MARK) 'I'm hoping to find out how large male kangaroos compete for dominance and the chance to mate, in the wild.
And it doesn't get wilder than this.
Here, where temperatures hit around 120 degrees Fahrenheit, kangaroo ecologist Dr. Adam Munn is uncovering how males beat the heat and find mates.'
There he is.
He's there.
Oh, he's beautiful.
He's a really stunning guy.
'This is a Boomer a dominant male kangaroo.
As the largest male around, he has first pick of the females.
But girls are a precious resource in this vast wilderness and rival males search far and wide to find them.
To maintain his status, he will have to fight.'
Do you see most of the fighting in the more juvenile males or in the real kind of adults, the super kings, if you like?
The serious fights that you see for dominance of an area and therefore the right to mate with all those females, they will be full-on fights between very large males.
'Boomers use a range of scare tactics to put rivals off.
Standing tall, they wave their huge biceps at their opponent.
If this doesn't work, it's time to fight.
Males kick-box their huge legs into their rival's stomach.
Although fights look harsh, winning is more about strategy than brute force.'
The interesting thing is, it's not the kicking that is important, it's really the ability to grab and hold the other animal and twist it down to the ground that determines who wins.
'We're heading to an isolated watering hole.
During the day, kangaroos sit out the heat.
But in the cool evenings, they come from miles around to drink.
For dominant males, having all the females in one spot is the perfect chance to find a mate.'
Look at that sunset!
'Kangaroos have few natural enemies but the arrival of humans around 50,000 years ago, followed by dingoes, means that wild kangaroos are extremely wary.
Because they see very well in the dark, Adam is using a thermal camera, which can pick up body heat, plus an infra-red camera, so they can't see us but we can see them.
Using this new technique, he hopes to reveal how males find mates in the dark.'
(ADAM) Just panning, looking for males.
Here he comes, look on here, look on here.
- Are you definitely sure he's a male?
- No, that's a boy.
Looks like a big male.
He's interested in that girl.
She's interested in him.
This is cool, this is really cool.
They're checking each other out.
One of the really neat tricks of those dominant males is that they can tell when a female's pretty much ready to mate by checking that urine.
He's sniffing her now.
It's an easy pick-up joint.
It is the dodgy disco, when they come for water and he uses that opportunity to pick out his next mate.
But he can't follow them all, so while he's chasing one female, potentially there could be another one that another male has a chance with.
Yeah, that's right, that's what they call sneaky copulators that get in there and sire young with females because that dominant male can't be everywhere at once.
I know the feeling.
(JOY)'At Uralla, I'm watching kangaroo mating playing out right in front of me in broad daylight.
Western Grey females reach sexual maturity just after their first birthday and three-year-old Reeba is in heat.
Head of the mob, Phoenix, isn't letting her out of his sight.
This six-year-old, semi-wild male has fathered over 20 joeys.'
These Western Grey kangaroos only come into season once or twice a year and when they're receptive, they are only receptive for five to seven days.
'This limited mating period means that Phoenix has to guard against rivals to ensure he fertilizes Reeba's egg.'
This big male's the dominant male and so he has the right to be the one that mates the girls that are in heat.
Let's see if we can get a little closer.
He's so much bigger than her, he's about twice her size.
He's grabbing her tail, look at that.
Every time he sniffs her to make sure she's still in heat, she runs ahead.
She's really playing hard to get.
'But Phoenix has competitors, smaller, lower-ranking males.
While his back is turned, five-year-old Zonty makes his move.'
Uh-oh, we have competition now, here comes another male.
He knows that she's in heat, he's going to try and get access too.
'Zonty chases Reeba.
He tries to mount her.
But Phoenix spots him.
And returns to reclaim his prize.
Zonty tries again.
And again.'
He's panting, she's panting, their noses are flaring, they're drooling, and once she's tired, that's when he'll finally get access.
(MARK) 'Unlike kangaroos that live in the open, male Koalas face a very different problem finding mates.
Hidden by dense foliage, they need to broadcast their location.
In the Brisbane Valley, leading expert in koala ecology and mating, Dr. Bill Ellis, explains how males use their voices to find females.'
They will go right to the very tops of these trees.
- Really?
- It's quite remarkable.
'This is a koala's love song.'
(Grunting) 'The loud call broadcasts his location in the trees.
To make this extraordinary bellow, koalas have a unique sound-producing organ.
Using an extra pair of vocal folds, they create a call 20 times deeper than you'd expect for an animal its size.'
What do we know about what that bellow is communicating?
It's a really honest signal about just how big the male is that makes the bellows.
This animal should be the size of a lion, you know, based on how deep and how low those frequencies were.
The more bellowing we hear in the environment, the more females seem to be moving.
And so the females hear the bellow and they go looking for that male.
'But it's not just females deciphering the male's bellow.
Other males listen in.
(Bellows) If the bellow signals that he's huge and will beat them in a contest for females, they'll steer clear.'
(JOY) 'For male kangaroos, finding mates is a much more exhausting challenge.
At Uralla, it looks like my chance to see the way kangaroos actually have sex is going to happen.
Phoenix's chase has gone on for over three hours but finally, Reeba decides to accept him as her mate.'
Oh, he is so close, he's got her hips, and he's going to try to push her back over the penis.
He's leaning back, tilting his pelvis up and he's balanced in a tripod right now, with his hind legs and tail, so he got to pull her into position.
Notice where his front paws are, they're actually hooked in between her thighs, on each side.
He's actually yanking her hips back onto his pelvic area so that he can push her vagina over his penis.
'Male kangaroo anatomy is fascinating.
The penis is shaped like an 'S' and is hidden inside the body below the testicles.
When aroused, the engorged penis is pushed downwards and forwards, moving it towards the female.
Phoenix maneuvers around Reeba's thick tail, using the curve in his penis to aim it upwards.
He uses the heat-sensitive tip to feel around for her genital opening, before guiding it in.
Unbelievably, Zonty continues to try his luck.'
It's amazing, he has to chase off another male, while he's trying to mate.
I've never seen mating like this before.
It's really amazing to see.
Reeba, however, has had enough.
Oh, look at this, she managed to get herself off his penis but he's still not letting her go because he's trying to get it back into the same spot so he can do this again.
Look at that, she is trying to jump away from him.
'Phoenix has ejaculated multiple times over the past half an hour, shooting 100 million sperm inside her each time.'
Now it looks like he's just resting but he's actually doing something very smart, he's waiting, with his penis in the female, for another opportunity to have another ejaculation.
'To ensure that his sperm fertilizes her egg, Phoenix's semen forms into a plug.
Shortly after he ejaculates, it coagulates into a rubbery mass that temporarily blocks up the base of Reeba's reproductive tract.'
It's a very smart solution to make sure that you are the father.
Because your semen is on the inside and there's a nice barrier to keep everybody else's semen out.
'Phoenix finally sets his exhausted mate free.
His duty done, he takes a break.'
(MARK) 'On Australia's East coast, koalas have adapted superbly to the challenging environment by evolving the ability to eat toxic eucalyptus leaves.
But relying solely on such a specific food source has left them highly vulnerable to deforestation and disease.
At the forefront of efforts to save them is world-leading reproductive zoologist, Dr. Steve Johnston.
At Dreamworld, near Brisbane, Steve is helping to boost koala numbers.
To do this, he's collecting koala sperm using an artificial vagina.'
It's a perfect size, isn't it?
A perfect size for a koala, yes.
And what you're trying to do, you're trying to mimic the vagina of the female so when the male's penis hits that, he will ejaculate in there.
'Steve fills the fake vagina with warm water and blows it up to get a good fit.
He'll deploy this at the right moment during mating when he collects sperm from resident male Bailey.'
Hello, Bailey.
How are you doing?
'It's quite a skill.'
He's a big old boy.
'Bailey's sperm will be used in Artificial Insemination to help increase genetic diversity amongst captive koalas.
He's usually kept away from the ladies, but today, he's in luck.'
Yeah, I'm just letting the girls know that he's here.
'Bailey will only perform if he finds a fertile female.
And he's only too happy to check out what's on offer.'
(Bellowing) That vocalization from a female could be a "I might be interested".
That bellowing behavior certainly is an indicator that she's interested.
'To avoid distraction, he's taken to a separate room to mate.
Having sex up a tree is an art.
Male koalas stick to a strict pre-mating protocol... they sniff, urinate and, to hold on, they bite the female's neck...hard.'
As soon as the male establishes a neck bite, he'll swing his penis out and start to attempt to find the female's reproductive opening.
'Steve quickly introduces his sperm collector and hopes Bailey doesn't notice!'
Okay, so I'm just going to...
He's actually thrusting into the artificial vagina.
- Okay.
- There he goes.
There's his last two thrusts.
What I'm going to do now, is I'm just going to attempt to just manipulate him a little bit to try to direct the seminal fluid into the collection vial.
- To be able to get gravity on your side.
- Exactly.
And there it is, you've got some.
The sample will come out over a couple of seconds.
You've got about a mil there already.
Probably closer to two mil, I reckon we'll get.
Yeah, there he goes, there's his thrusting.
He just did a second neck bite so we can separate them off.
That is perfect.
Perfect, absolutely perfect.
Bless her, she is amazing.
'Steve's research suggests that koalas have an ingenious solution to sex in the tree tops.
Unlike male kangaroos, who ejaculate sperm first and then create a plug to hold it in, koalas overcome gravity by making the plug first, and then depositing the sperm.
During sex, a koala releases two different types of semen.
He shoots the first, sperm-free, fraction into the female's vagina, which quickly coagulates into a thick, mucus plug.
After 40 thrusts, he pushes his penis through this plug and his final two thrusts deposit semen on the far side.
This time, it's full of sperm.
But the strategically placed plug prevents it slipping out.'
It's a very neat solution to having sex up a tree.
Correct.
It's very el basico and, you know, some of the best things in science are.
'But collecting semen is only the start.
Koalas have a very specific mating ritual and Steve's worked out how to replicate it exactly to trick this female into conceiving.'
I'm going to support your head, sweetheart.
'If he gets it right, he'll pass on Bailey's healthy genes and produce a valuable new baby.
Using a glass rod shaped like a koala penis, Steve simulates sex.
Koalas only release an egg after the physical act of sex.'
Seven, eight, nine...
I'm gonna go up to 40, and two final thrusts.
'Amazingly, koala sex is always around 42 thrusts.'
- She's been very good, you're a very good koala.
- A very clever girl.
'To ovulate, studies suggest koala females also need semen because it contains an 'ovulating factor'.
This, as well as 42 penis thrusts, triggers her ovaries to release an egg.'
If you can hold that nice and steady... Just release the pressure off a little at the top.
You got it, beautiful.
Oops, sorry, mate!
I got a little bit of sperm on you.
You just sprayed me with sperm?
That's nice, Steve!
Well, we're friends, aren't we?
So, we'll just um... She is a very good girl, isn't she?
She is amazingly quiet.
- Incredible, aren't you?
- So there we go, she's done.
You're done, sweetheart.
You can take her home, back to her little enclosure.
She can be ready to produce a baby in 35 days, we hope.
'Perhaps the most interesting adaptation of sexual anatomy in Australia belongs, not to a marsupial, but to another extremely successful mammal... ..the echidna.
During courtship, echidna males often pursue a fertile female in a long 'mating train', with the dominant male in the lead.
When he gets his girl, there's a prickly problem - spines.
In this rare footage of echidna sex, the male digs a hole in the ground under the female's spines and approaches her from the side - with a penis that's a quarter of his body length.
The penis is not just large, its anatomy is unique.
It has four heads and research suggests that two enlarge and line up with the female's branched reproductive tract.
Sperm travels up a main urethra, which is thought to split further into multiple, tiny channels that pool semen in cuplike tips.
He presses them against the female's vaginal openings, alternating the heads each time he mates.
The intriguing sex-lives of these mammals are also being explored by Steve Johnston.
The star of his pioneering research at The University of Queensland lives at Currumbin Wildlife Sanctuary.'
Come and see your friend.
- Have you got him?
- Yeah, I've got him.
Hey, buster.
Gonna show these people what you can do?
He sort of masturbates on demand for us.
So, what are you doing with your hand?
I'm just giving him something to rub against.
He typically will mate in a C-shaped position, so this is sort of mimicking his natural mating position.
- He's getting an erection now.
- Yeah.
Oh, my God, that is weird!
Four heads on it!
- Four heads to start with.
- Unbelievable.
Just watch what happens.
As the penis becomes erect, two of them, on the left-hand side, are shutting down, and the two on the right-hand side are becoming larger.
So, does he urinate through the penis as well or not?
The penis is only used for emitting semen, and he urinates at the base of his penis.
That piece of anatomy is extraordinary.
Yeah.
And also, the physiology of the blood supply and the neural system to control that type of erection must be quite phenomenal.
- He's quite a lad.
- Yeah, he's focused.
'A semen sample from this echidna reveals sperm that swim in bundles!
Up to 100 sperm cement their heads together into a bundle which powers up the female's vagina.
The larger the bundles, the faster they move.
Marsupials also have very unusual sperm.
It can be hooked, like wombat and koala sperm, or, in the case of the honey possum, super-long.
At over a third of a millimeter, it's almost visible to the naked eye.
These crawl like snakes up to the female's egg.
Or sperm can double up and swim in pairs, like the South American, Grey Short-tailed opossums.
After racing up the female's oviduct, they break apart.
Kangaroos have to keep their sperm cool in the heat of the Outback.
Strangely, unlike most mammals, marsupial testicles lie in front of the penis.
When they're hopping or fighting, they're up, out of the way.
But when they need to cool them off, they relax their cremaster muscle and lower their scrotum away from the body.
Or, if it's really hot, they add a bit of moisture.
(JOY) 'At Uralla, Alice just 24 hours away from the birth of her first baby seeks shade from the midday sun.
Soon, she will need to find food for two.
But for now, it's time to rest.
To understand what really makes a marsupial unique, I need to explore the female's reproductive tract.
Unlike all other mammals, female kangaroos have not one, but three vaginas.
This crazy layout is hard to explain without seeing it and the best way to see it is to dissect one.
Sadly, kangaroos are hit by cars all too frequently in the Outback and I found this female Red Kangaroo on the road a few hours ago.
I brought her in to check out what really makes a female marsupial different from other mammals.
And to do this, I'm going to remove her reproductive tract.'
We're looking at the female reproductive tract of a kangaroo.
This is where the penis is inserted.
And right here is where sperm would then enter.
The sperm then does something absolutely amazing that doesn't happen in regular mammals, it only happens in marsupials.
It has two tubes that it can travel up.
It goes from here into a right vagina and a left vagina.
Sperm then continues up through here and then there are two uteri it can go into.
Humans have only one uterus but the kangaroo has two.
The sperm then passes through that into this, which is the oviduct, and that's where the egg, which is released from these ovaries, travels down.
The egg is fertilized in the oviduct and then it travels down into the uterus, implants in the uterus, stays there for 30 days, develops into an embryo, then, as a fetus, at 30 days, it is released and it actually travels down through something amazing.
It doesn't go through the same tubes the sperm came up, it goes through a third tube right in the middle here.
This center tube right here is the birth canal, and it is the third vagina.
You might wonder, "Why does this animal have three vaginas?
Why don't they just fuse into one?"
Well, they can't fuse into one because there is actually a tube separating them.
'These tubes are the pipes that take urine from the kidneys, down to the bladder.
Called ureters, they pass between the vaginas on either side, keeping them apart.
This prevents the vaginas fusing into one, larger tube.
Only a tiny fetus can slip down the central birth canal.
So the fetus will actually travel down that center tube and then it's actually born.
'To cope with their unpredictable environment, most female kangaroos have another, very special feature.
They can produce young continuously.
To do this, they store an undeveloped embryo in their uterus.
If one joey dies, the stored embryo develops and takes its place.
It's now 30 days since Alice conceived and the time has come to prepare herself for motherhood.
The skin inside her pouch secretes a protective layer of wax that dries into a dark scale.
But, as birth approaches, she removes it.'
She's cleaning her pouch right now, trying to get rid of all the brown waxy substance that's accumulated in there.
And that's in preparation for the baby.
'She holds her pouch open with her forepaws and sticks her head right inside, licking the scale away until it is soft and moist.'
And that behavior begins about one to two days before she gives birth.
But when she starts doing that more intensely, it means that the birth is imminent.
'But Alice isn't in control of her actions.
Pouch cleaning is actually triggered by the presence of her unborn fetus.
As birth approaches, it produces increasing levels of prostaglandin, a hormone-like substance, which stimulates Alice to clean.'
Usually about one to two hours before the baby comes, she'll start cleaning non-stop.
And that's what we're waiting for right now.
'It's early evening.
Alice's baby is moments from being born.
She settles into the birth position, tilting her pelvis forward to help her joey's climb.
And then her baby is born.
Alice's tiny newborn's umbilical cord snaps after just a tenth of the climb from the birth canal.
Weighing just one gram, it uses its front limbs to haul itself up her fur.
It's astonishing to watch this perilous ascent towards safety.
The young disappears over the rim of the pouch in just three minutes.
This extreme close-up footage reveals the baby's Herculean task.
The lungs are barely formed.
It's thought oxygen is absorbed through the skin to fuel this climb up to the warmth and security of the pouch.
It's the morning after Alice gave birth and I'm dying to see her baby.'
Do you think she's gonna let us look into the pouch?
I doubt it.
Let's see what's going on.
Can we take a look?
Let's see.
Oh, that is awesome!
So tiny, wow!
I can see the big arms just moving around in there, even though the head is latched on to that nipple.
That's its new umbilical cord, really.
It's only 31 days old, now.
At that stage, if it were a human, it'd still be looking a lot like a fish.
Yeah, okay.
There's some little movements of the chest that I can see.
And the nostrils, they're open.
In a human embryo at this stage, the lungs haven't developed.
So, to see something that's actually breathing through open nostrils, that's pretty incredible.
It's really precocious in its development.
I saw claws on the end of the paws.
So we've got focus and energy spent only on the front end of the animal to get the mouth ready for nursing and for those front legs to be ready for crawling.
But then, it's gonna really focus on the hind end.
And now you have these giant, iconic kangaroo legs that start developing and enormous tail for balance.
That's an advantage to having a marsupial with a pouch as opposed to a uterus to look in.
This is a lot easier to look in and see what's going on.
'Although a pouch is such a recognizable feature of all marsupials, we rarely have the chance to see one up close.
The female kangaroo that sadly died on the road, however, gives me the chance to show you what makes this piece of anatomy so extraordinary.'
When we look at a marsupial pouch, this is iconic for kangaroos.
You imagine a kangaroo with a pouch with a joey hanging out, that is classic.
Take a look inside the pouch.
When I pull it wide open like that, you can see, some parts of it are pretty thin.
If you look inside, you can see light shining through some areas of the pouch where it's particularly thin.
'Kangaroo mothers control the size of their pouch opening, using strong muscles that line it.'
It's very stretchy, very roomy.
The skin is pretty stretchy in this area.
I can really expand it over here.
It's a little bit thicker right on the edge here.
This is where a muscle helps pull it in tight and close the opening of the pouch to help keep the joey, the baby, inside this area.
'The entrance to the pouch has a semi-circle of muscles on each side.
When these muscles relax, the pouch opens.
But if the mother is alarmed, she contracts these muscles, pulling the pouch tight against her body.'
- G'day, Joy!
- Good morning, David.
'For veterinarian David Schultz, a pioneering breeding scientist, the unique marsupial pouch might be the opening he needs to help save a smaller, hairier cousin of the kangaroo, the critically endangered, Southern Brush-tailed Rock Wallaby.
At the University of Adelaide's Waite Campus, I'm going to help him boost these wallaby's numbers.
First, we need to take a joey from the pouch of a rare wallaby mother and give it to a foster mother from a different, more common, wallaby species.
David suspects one of his rare females has a joey and we need to check.
But singling her out is no easy task.'
This is our expert catching team.
I see you've got nets ready at the go.
'The team is going to open their night-time shelter.
There's a mix of captive wallabies inside and it's my job to stop them all running out at once while the team bags the right one.'
I have every confidence that you'll be able to block this totally.
Are you saying my butt's big?
(Laughter) Clever lady.
- (Thudding) - Wow!
Hey!
No, it's black.
'It's a Black-footed Rock wallaby, not the Brush-tailed female we're after.'
Alright, let her go.
- They can really jump!
- Oh, yeah.
- Got her!
- Well done, Jon.
'It's a rough job, but when we transfer her precious joey to a surrogate mother, it will actually speed up this wallaby's reproductive cycle, thanks to a remarkable evolutionary adaptation.
While one joey develops in the pouch, another embryo is held in reserve in the mother's uterus, waiting to come to life.
When the joey leaves the pouch, suckling is reduced, and triggers this second embryo to grow and develop.
In around 30 days, a new joey will be born.
Called embryonic diapause, it means almost all wallaby and kangaroo mothers can nourish three separate youngsters at once: an older joey that has left the pouch, a young one developing inside it and an embryo still waiting to be born.
Because this wallaby has an embryo on standby, when David moves her joey to a foster mother, it will trigger its development and birth.
By doing this, David can speed up the breeding process and rapidly increase their numbers.
But you can never guarantee there's one in there.'
Okay, this is crunch time.
Oh, yes, the kid's still here.
Oh, I see it!
Look at the little feet!
- Oh, my goodness.
- What I'm going to do now... is I'm going to just ease the little guy out.
So you're pulling the teat out of its mouth.
I see.
I've worked my way out there.
It's so amazing, I'm holding what looks like a little fetus.
If it were a human, it would be still in the embryonic phase.
Yes, yes.
It seems like it's so much more developed, it's incredibly well co-ordinated.
'At Adelaide Zoo, this baby's foster mother is waiting.
She's at the right point of lactation to accept a new joey but we have to be quick.'
Okay, this is the treatment room, Joy.
'And now, the critical moment.
To survive and grow, our joey needs to latch onto the foster mother's teat.'
(DAVID) So, we pour him into the pouch.
So at this stage, are you just warming it up?
We're just warming him up.
I'm actually not allowing both sides of the pouch to go on top of him because it might warm him up too quickly.
That would just cause a massive shock to the system.
So I'm just going to hold the fur up.
I've got the teat now, right-oh.
'This is a nerve-racking moment.
I can sense that even David is on edge.'
I'm trying to pick it up correctly with my forceps so when I push it into the mouth, the whole lot goes in.
'At last.
Success.'
Okay, that's good.
Right-oh, it's all out.
'Although taking a joey from its mother may seem cruel, the foster mother will accept the infant as her own and its birth mother will quickly develop another joey.
At Uralla, Alice's new baby is in her pouch.
All kangaroo mothers nourish their babies with milk and a joey embarks on an astonishing period of growth.
It weighs less than a gram but in just ten months, it will grow 4,000 times heavier.
As it develops, its tiny bud-like hind legs catch up with the front limbs in size and then, they overtake.
By five months, they outgrow the pouch all together.
And, at six months, this joey takes its first faltering steps in the outside world.
(MARK) 'In the Outback, Adam's working out how joeys keep up with their mums.
First up, hopping lessons.'
How long does it take them to be able to find their feet, as it were, when they're born?
It must be completely weird coming out of the pouch then suddenly realizing, "My God, look at these stilts!
What's going on here?"
When they come out for the first time, they really don't know what power they have.
You see some joeys suddenly be two feet in the air, looking around, going, "How did I get here?"
But within a few days, they really learn to control the power in those little springs.
What really interests me is how the youngsters manage to keep up with Mum when she's travelling at 40km/h.
The joeys are much smaller than the mother, and you can see they do more hops over the same distance, compared with the mother, and because they're smaller, that energetic cost of locomotion must be higher.
But the benefit that the joey has is, it can support that because it's getting really rich milk from the mother.
'In this arid outback, the need to produce milk for their young forces mothers and their joeys to hop for miles to find water.
Here, in the harshest landscape of all, they sometimes congregate around a leaking water pipe.
Because they're so timid, we have to wait downwind and hope the scorching weather drives them in for a drink.'
You've got a mother, with a young joey, barreling in here and then stopped.
Why the hesitation?
It's a good sign of a good mother.
So, she's really cautious coming in to water.
She's very careful, she's got a joey with her.
(MARK) Hold on, who's this one?
There's another one.
This is another female.
Yeah, she's pretty big.
She might be the older sibling.
So, we've got Mum, a previous joey, the most current joey.
And she looks as if there's something in her pouch too.
Yeah, there's a young one in the pouch too.
You can see how saggy it is.
There's that weight at the bottom of it.
Amazing.
Look at these two now, they look as if they're kissing, but are transferring saliva.
Yeah, yeah, yeah, so the joey goes up and she'll use that, that's good water.
'Red Kangaroo joeys are weaned at the end of their first year.
The joey still pesters her for milk but she increasingly ignores it and finally prevents it from suckling at all.'
But look at what's happening now.
We've got the older one, she's suckling the big one.
I'd have thought she'd be weaned by now, she's a big animal.
Why isn't she pushing her away, just saying, "Hello, it's not for you"?
Yeah, she's pinching the milk that should be going to that young at foot.
A sneaky drinker?
- I've never seen that before.
- What never?
No, never.
To have another one come in, an older one still suckling - she's probably using the same teat that that younger one is suckling from.
But the thing is, this mother is letting her do it, so that's a really good indication of a good mother.
She's got the resources to be able to do that.
(MARK) Wow.
What an amazing piece of behavior.
'A kangaroo mother, however, won't always protect her baby.
Part of her survival strategy is infanticide.
If drought sets in and there isn't enough food to produce milk, she'll jettison her joey.
But when conditions are good, her embryo in reserve soon develops and occupies the vacant pouch.'
(JOY) 'At the Uralla sanctuary, Mandy and her volunteers take care of vulnerable orphans.
Without the security and nourishment of their mothers' pouches, Mandy's hungry orphans need round-the-clock care.'
We call this our nursery so we have all our babies from this year in here.
At the moment, we're bottle-feeding 29 joeys.
You know, they're really loving towards each other.
Somebody brings me a joey, for instance, for the first three or four days, it'll cry for its mum.
To me, they remind me a lot of humans, they're just like a baby, yeah.
You see them sucking their thumbs and doing all those things that have a real human quality.
And what's over in the baskets?
This is where we have our smaller joeys, the little babies that are being bottle-fed.
- The blue blanket's moving.
- I think someone's awake in here.
I try to put two in a basket together for company because normally they'd be with their mum.
You can pick up Billy, probably.
Oh, you're so tiny!
He's got a lovely little face, that one.
Hi, Billy.
- He still gets a bottle?
- He still gets five bottles a day.
The teat just goes all the way in his mouth.
There we go, there's breakfast for you.
'Kangaroos and other marsupials have the most complex form of lactation known in any mammal.
In the pouch, joeys receive both nutrients for growth, and immunity from their mother's milk.
As they grow, milk production increases from one to almost 90 milliliters a day.
The composition also changes.
Energy levels rise almost four times to fuel growth and hair-strengthening sulfur levels peak as the fur lengthens.
Kangaroo nipples, unlike those of humans, do different jobs.'
This tiny little nipple, where the newborn attaches, is producing a different kind of milk.
It's called colostrum and it's basically a clear fluid but it's full of antibodies.
Why is that important?
This baby has only been in the uterus for 30 days, and now it's going to finish its development in the pouch.
This is basically the outside world - full of infections, bacteria, all kinds of things that could hurt that baby and so, having colostrum is very important to make sure that that baby has some immunological defense against bacteria.
'Remarkably, kangaroo mothers can produce two different types of milk from different mammary glands within the same pouch - one for a newborn weighing one gram and another for a youngster that's left the pouch at more than four kilograms.
This longer nipple gives a joey room to move around inside the pouch, even though it's still nursing.
Kangaroo nipples even have a handy way of resetting.'
This really long nipple, which the older animal is nursing from, that's going to retract and become a very tiny nipple once that baby leaves the pouch permanently.
When it's weaned and the mother won't let it back in, muscles are going to actually act on that nipple and pull it back and retract it so it becomes tiny again.
(MARK) 'All marsupial mothers nourish their young with milk but koala mothers add a little something else.
Just before emerging from their mother's pouch, babies feed on a substance called pap - a liquefied form of the mother's droppings.
It gives the joeys valuable micro-organisms to prepare their gut for digesting toxic eucalyptus leaves.
Armed with these helpful bacteria, koala babies can survive during droughts on food that few other mammals can eat.'
(JOY) 'It has been a privilege to witness one of the most rarely seen mammal births of all and I've come to say goodbye to Alice.
It is two days after she gave birth and Alice is leaving the safety of Uralla to raise her joey with her mob.
It's extraordinary that any living thing survives here.
But kangaroos thrive.
Their evolutionary adaptations allow them to respond rapidly when conditions are good.
And give birth to tiny young that develop outside their bodies, in pouches.
But if resources dry up, female marsupials can discard their offspring, to increase their own chances of survival.
By controlling their reproduction and adapting their sex lives to one of the most changeable environments on Earth, marsupials have conquered a continent.'
Next time on Sex in the Wild.
We're exploring how marine mammals reproduce underwater.
Mark's in Mexico finding out why gray whales travel halfway around the world to give birth.
While I'm in New Zealand to discover how dolphins perform submarine sex.
2.6 seconds!
Ha ha ha!
And together we'll be looking at how some of the biggest babies on the planet are born in the sea.
Sex in the W