Australia.
A vast country of breathtaking natural beauty.
Teeming with extraordinary and iconic animals.
Among them, eastern grey kangaroos
Found in the south-east in their millions.
For the males, known as bucks,
Their drive to perpetuate their species
Means a life full of risk...
..And reward.
From joey to buck...
..It's a battle for survival.
As they roam the land,
Climb up the social ladder...
..And fight for their mating rights...
..In a bid to secure their legacy.
This is the secret life of the kangaroo.
(kookaburra laughing)
(birdsong)
In each young kangaroo buck's life,
The moment comes when he has to face a challenge.
Fighting and defeating a bigger, higher-ranking male
Could ensure access to better grazing...
..And the chance to mate.
But the stakes are high.
(hissing)
A serious injury could be fatal.
Sniffing is the prelude to confrontation.
The kangaroo on the left is justin.
He's been named by researchers studying the behavior
Of a large population of eastern grey kangaroos
At sundown national park,
On the border of queensland and new south wales.
His challenger on the right is a new arrival to the area,
A wandering male trying to assert his dominance.
The intruding male instigates the challenge.
He coughs, which can be a sign of submission.
But then he provokes justin and paws at him to goad him.
Justin can either back down or retaliate.
In an instant, both kangaroos
Are up on the balls of their feet,
Standing as tall as they can,
Each hoping to intimidate his opponent.
For a moment, the intruding male appears to back down.
But after a brief pause...
..He renews the challenge.
Like a boxer in a ring, he circles around his opponent.
Perhaps trying to outmaneuver him.
Kangaroos usually attack head-on.
He circles again.
Justin times his flying kick perfectly.
It's a decisive blow.
Justin swings his body to cement his victory
In a display of strength.
Then he sprays urine to assert his dominance.
Meanwhile, two younger males
Playfully spar behind the older males.
Imitating the moves they'll need when they're older.
The intruder has been defeated.
His hand is dangling, clearly injured.
If it doesn't heal quickly,
He'll find it hard to feed
And his boxing days will be over.
Fighting is a potentially defining moment
In these young bucks' lives.
It's a form of ritualized behavior
Which separates them from the females.
While female kangaroos live in established home ranges,
Clustering together in a largely peaceful social group known as a mob...
..Life for males is far more hazardous.
Many roam the countryside in search of mates.
This necessitates being exposed to risk
To secure the future of their genes.
And, above all, males need to assert themselves.
For all bucks, a large part of life is about body image.
Large males like tim,
Who only visit sundown during breeding season,
Like to impress with their big muscles and strong stature.
Both to attract potential mates
And to scare off rivals.
Giant is an alpha male
Who regularly visits a mob of kangaroos
Which lives and grazes on a golf course
In anglesea, victoria.
Researchers named him with good reason.
His large and muscular arms are a sign of great strength.
Kangaroo males never stop growing...
..And studies have found that in some kangaroo species
Muscle size is directly related to mating success.
Forelimb measurements show that whereas
Female muscle growth is proportional to body size,
Male upper-body musculature
Becomes disproportionate to the rest of their bodies
As they mature...
..Resulting in beefed-up arms and chests.
A fully-grown male can weigh 190 pounds...
..And can stand seven-and-a-half feet tall
With an added three feet of tail.
But while larger muscles may lead to more opportunity for passing on his genes,
The extra bulk may also come at a cost
For a large male like giant.
While there's plenty of grass here on the golf course,
Maintaining a larger body weight can be much harder
When food and water are scarce...
..A common occurrence during scorching australian summers.
As well as on the golf course,
Wandering males are a common sight
In the seaside town of anglesea.
Particularly in the woodland near the beach.
And even urban parks.
Males will travel through on their way to find fresh mobs
And new grazing.
These large, wandering males may have better reproductive success
Roaming around during breeding season
As it allows them access to more females.
The greatest distance a male eastern grey has been recorded covering
Is 155 miles over a nine-month period.
(birdsong)
Kangaroos are well-adapted
For a life both out on the pasture and on the move.
Whenever they're out from the protective cover of woodland,
They're potentially vulnerable.
But they can escape quickly by hopping.
Adult eastern greys have been clocked hopping
At a top speed of 39 miles per hour.
Kangaroos can achieve these speeds
Due to special tendons in their ankles.
Up to 50 per cent of the energy used for the first hop
Is reused in the next hop by the extensor tendons
Which act like elastic bands.
To make it even easier, when a kangaroo is hopping,
He doesn't need to make any effort to breathe.
The kangaroo's diaphragm muscle, which controls breathing,
Is pulled down by the action of hopping up...
..And is pushed in the other direction when he lands.
This means each breath is perfectly in time with his hopping.
And it's far easier to breathe when moving
Than standing still.
This energy-efficient way of traveling
Means that males can cover vast distances
In search of mobs and mates with relative ease.
But being on the move means that males constantly face
More dangers than females.
As well as having to deal with heat and possible dehydration,
They risk being hit by cars...
..Shot by farmers... And caught in wire fences.
But it's not just full-grown bucks who face peril.
For kangaroo males, a more hazardous life begins in the pouch,
The pocket where marsupial mothers raise their young,
Known as joeys.
Ensuring the survival of their joeys can be a challenge for kangaroo mums.
And it's especially difficult
Getting male joeys through to the point of independence
When they leave the pouch at around nine months.
Over 50 per cent of male joeys die before they're weaned,
A far higher proportion than of female joeys.
Researchers are uncertain why male joeys are more vulnerable.
One theory is that their bodies expend more energy
Putting on muscle and growing larger bones,
Rather than developing fat reserves
That help them survive in leaner times.
Researchers have also found that raising male joeys
Can deplete a kangaroo mother.
Studies have shown that mums with male joeys
Have to eat more grass...
Which puts them under pressure,
Especially when food supplies are scarce.
So when there's food around, they need to eat faster.
Females are very unlikely to successfully raise joeys two years running
If the first is a boy.
If a mum has a girl first,
She has an 86 per cent chance of her next baby surviving.
Whereas if her first is a boy,
Her declining body condition reduces that success rate
To just 50 per cent.
During the years between weaning from their mothers...
..And their first serious efforts at fighting and mating...
..Young males will need to feed and feed.
When they're born, kangaroo joeys are tiny...
..Less than one inch in size...
..And weigh less than a tenth of an ounce.
By the time they leave the pouch,
They will weigh around 13 pounds.
And to become sexually mature and start mating,
They'll need to put on bulk.
The growing kangaroos need to maximize the nutrition they can get
From their low-energy grass diet.
And their ability to regurgitate their food when it's semi-digested
Helps them do this.
Kangaroos have a digestive system
Very similar to other grazers -
Like cows and sheep.
This process, known as chewing the cud,
Combined with special bacteria in their guts
Which ferments the grass,
Turns grazing into an energy-efficient way of feeding.
This enables kangaroos to survive
Over areas of inhospitable habitat in australia.
One reason why the population of eastern greys
Stands at a healthy 16 million.
Joeys stay with their mothers until they're fully weaned
At around 18 months.
They won't be sexually-mature adults
Until they're around four to five years old.
But, like any male teenagers,
There's plenty to learn about life as a young buck.
How to approach females.
How to look cool.
And how to fight.
For the dominant males in waiting,
Play-boxing is part of essential preparation
For their fight to the top.
In sundown national park,
Four-year-old doodle is a typical teenage male.
While he's not big enough to mate yet,
He's working hard on his image...
..And trying to look macho.
He needs to assert his masculinity
By standing as tall as he can,
With one forearm out
And the other scratching his chest or his back.
A key part of looking cool
Comes down to this nonchalant attention seeking.
While he still lives in the same national park as his mother,
Doodle has moved further away from her home range since weaning.
Female kangaroos typically stay within the same areas as their mothers
And males are more likely to disperse.
It's thought that this may be an evolutionary strategy
Which reduces the chances of inbreeding.
Doodle still lives close to his half-brother.
They share a dad.
They were born two months apart
And have migrated to the same area together.
Once they're fully grown, they can start to travel greater distances
And take off from sundown...
Along with other larger males.
But while they're still here, like any teenage boys,
They'll practice their moves on the females as much as they can.
(birdsong)
The adolescent bucks' interactions with females
Are rather like teenage crushes.
Doodle might be too young to mate,
But that doesn't stop him from trying to woo an older female.
His efforts mostly consist of following the female around...
..And grazing alongside her.
Sometimes he tries grazing in front of her...
..Hoping to get noticed.
It's not the most sophisticated approach.
And then...
..He clucks.
Clucking is the noise males make
To express their interest in females.
(clucking)
It's the kangaroo equivalent of the pick-up line.
But his efforts are falling on deaf ears.
The female has a small joey in her pouch.
This means that she won't be ready to mate again for a few months.
Eastern grey females are seasonal breeders...
..With peak births occurring between December and March.
But they're polyestrous,
Meaning that they can have more than one breeding cycle in a year.
While a joey is in the pouch for nine months,
A female may return to estrus
When the joey is around five months old.
If the female conceives at this point,
She's able to do something extraordinary.
She can keep that embryo in waiting
Until her current joey is ready to vacate the pouch.
She can also conceive again at different times of the year
If her current joey doesn't survive.
So young males like doodle work on the basis of persistence.
Keeping up his moves on the female could mean that one day...
..Doodle could be in the right place at the right time
And get a chance to mate.
Pestering females is a constant part of a buck's life at any age.
Justin, who previously won the fight with another male,
Is trying to woo a female resting in the shade.
He makes his interest clear
By clucking and posturing in front of her.
But she's doing her best to ignore him.
Her trembling reveals that she doesn't want him near her.
Justin's scratching is a sign
That he's not being too forceful.
She sniffs the scent glands in his chest.
But that doesn't seem to pique her interest.
But he doesn't give up
And follows her when she takes off.
Her lack of interest could be a sign
That she's currently not in estrus.
For the males, the key to successful mating
Is knowing when a female is more likely to be receptive to their advances.
So male kangaroos will check females regularly
For any signs of reproductive readiness.
Doodle sniffs a female's urine
As well as her cloaca...
The opening used for both reproduction and excretion.
He can detect hormones that will tell him
Whether she'll soon be ready.
As females get closer to ovulating,
Males like justin become increasingly persistent.
But if he's not high enough ranking, or still too young,
Then this is mostly as far as he gets.
Some males have to resort to sneaky tactics
To ensure that they get the chance to mate.
While it's predominantly the larger males
Who reserve the mating rights...
..Researchers at sundown have identified another class
Of lower-ranking males who do sometimes get the girls.
These males have been dubbed 'homeboys'.
Smaller, less assertive males
Who are less likely to be strong enough
For the vagaries of life on the road.
So they stick around with sundown's regular mobs.
And when the opportunity arises -
Usually outside the main breeding season -
They're there to take it.
Dna tests have revealed a surprising number of joeys
Are fathered by these more opportunistic lower-ranking males.
It's September and early summer in australia.
It's now that the main breeding season gets underway
And wandering males seize their chances.
Tim joins a mob of kangaroos in a small park...
..On his way back to sundown.
He identifies a female and spends several days
Grazing near her.
He detects that she's close to estrus
And wherever she goes, he follows.
He waits until the timing's right.
Tail stroking is part of the ritual.
And then he gets what he's been waiting for.
Mating can be a protracted process...
With regular breaks in the action.
But even after that, tim's work still won't be done.
Although he'll play no part in parenting,
He needs to ensure that he successfully reproduces.
So he'll spend time guarding his mate from rival males.
Then he'll be on the move again.
It's not long before tim arrives at sundown...
..Along with a number of other larger males,
Including jimmy,
Who've made their way back here for breeding season.
Now the competition between them all begins in earnest.
(hissing)
The posturing, preening and picking fights escalate...
..To establish who is top buck and who gets to mate.
For the most part, males know their rough position in the pecking order.
And the hierarchy is often kept in place with long, penetrating stares.
(coughs)
A submissive cough can be enough
To prevent an escalation in hostilities.
If that fails, males need to stand up to show how big they are.
Males of all ages are jostling for position.
For the teenagers at sundown, this is still an important time of year.
Rivalries are established.
And they get to fight with their peers...
..Determining the pecking order for the seasons to come.
(hiss)
At four years old,
Doodle may not yet be able to take on the top bucks...
..But he's getting himself into plenty of contests.
This is a typical teenage boy fight.
There's less decisive kicking...
..And more grappling and arm waving.
Doodle leans his head right back to protect his eyes
From his opponent's sharp claws.
Both bucks keep hopping, learning to balance with their tails.
Testosterone levels are raging.
And young upstarts are brawling all over the place.
Fights are more common among the lower ranks
Who don't have high status to defend.
But at this time of year,
The larger males have one thing on their minds.
Mating rights are hard-won and not given up lightly.
All-out battle poses risks,
But if they have to fight, the big males come well-armed.
As well as their muscular forearms,
Their long, sharp claws are a real threat to eyes and ears.
The muscular tail acts as a fifth limb during warfare.
The bucks can balance on their tails to produce a powerful kick.
While the sharp claws on the tips of their toes
Can do damage.
An even greater threat comes from the force of the kick itself,
Which can break bones and injure organs.
Luckily, male kangaroos have thicker skin on their stomachs.
This acts as a dermal shield,
Which offers some protection from the force of a running kick.
Justin tries his luck with another female.
But just as he's getting interested,
Alpha male tim arrives on the scene.
All justin can do is defer to the higher-ranking male.
And tim turns his attention to the female.
Tim rubs his scent on the nearest bush.
This is a way both of showing his supremacy over other males...
..And leaving a message for other females.
He also does this by urinating,
Though he catches the unfortunate female
In the face.
Tim continues to prowl the meadow, asserting his dominance.
Males who are nervous, or don't want to fight,
Sometimes tremble in the presence of an alpha like tim.
He'll expect a male below him to give a submissive cough.
When he doesn't get it, tim walks on his tiptoes
To intimidate the other kangaroo.
(hisses)
That does the trick.
The subordinate coughs as he backs away.
As they graze, tim continues to growl at the nervous male.
(birdsong)
(squawking)
Younger males will take any chance they can
To raise their status.
Teenager doodle is chasing a smaller rival...
..When they startle an older, larger male.
Doodle mistakes his reaction for a sign of submission
And lunges at the higher-ranking buck.
Initially it appears that doodle's boldness has paid off.
But the bigger male quickly regains his composure
And stands tall to show his superior size
With a big scratch.
Doodle submits and peace is restored.
It's been a few weeks since justin first fought and beat the visiting male.
His rival suffered an injury to his wrist.
But with his arm now healed,
The intruder returns for round two.
Brief, frantic periods of fighting
Are often broken with a pause to posture and assess the opponent.
The conflict resumes again.
It's not always easy to declare a clear winner in a kangaroo fight.
But this time justin doesn't manage to see off the intruder...
..So he won't gain status from this fracas.
Unlike justin, who is still fighting his way up the ranks,
A larger alpha like tim will rarely need to fight.
When he challenges a smaller kangaroo over a female,
The subordinate will jump away quickly.
But when tim comes up against a proper rival,
Jimmy, who could threaten his access to females,
He knows he needs to challenge him.
Tim's persistent posturing and scratching
Right where jimmy's feeding can't go unchallenged.
And tim gets what he's been asking for.
Within seconds, tim and jimmy are squaring up.
It's over quickly, but it's decisive.
Jimmy rubs his scent on the ground to assert his dominance.
Then declares himself the winner with a high stand.
Tim's lowered head implies that he agrees.
He picked on the wrong opponent this time.
Having declared himself the winner over tim,
Jimmy does what only a high-ranking male can do.
He assumes mating rights.
The female he selects has a joey at foot,
But by smelling her urine
Jimmy determines that she's in estrus
And therefore capable of conceiving again.
After sniffing and tail-groping...
..Jimmy gets the opportunity to pass on his genes
To the next generation.
At first, the female seems willing.
Then she breaks away.
Jimmy is reluctant to end the mating,
But the female defiantly stands her ground.
The pair mate again.
But once more, she wants to get away.
After finally escaping jimmy's grip,
The female resumes grazing.
Jimmy guards her closely.
After successfully mating,
He won't be letting rivals anywhere near her.
For those males who aren't getting a chance to mate,
Cementing their position in the pecking order remains the top priority.
Undeterred by his last fight,
Justin continues to battle his way up the ranks.
Moving from one fight to the next.
If he survives this season unscathed,
He may have what it takes to one day be top buck in sundown.
Meanwhile, doodle is still trying his luck with the females.
Maybe next year.
♪♪