THUNDER
Fresh water.
Some animals can't breed without it.
But correctly predicting its arrival
can be the difference between success...
..and disaster.
A giant African bullfrog.
He's the size of a dinner plate,
and he weighs half a kilo.
He's been buried for the last six months.
Now he needs to find a mate...
..and he doesn't have long.
The newly-arrived rains have created pools that trigger
this year's mating jamboree.
He's not the first to the party.
And what is more, it will be all over within the next 24 hours.
This pool is already crowded with males,
like a downtown bar on a Friday night.
The females will soon arrive, ready to lay their eggs.
So this is where a male needs to be.
But the battle-scarred faces are a warning.
This pond is not for the faint-hearted.
The males occupy the centre of the pond and fight to hold
a place there.
Their calls will attract females, but they will have to get
to the centre if they're to meet the strongest males.
So if a male is going to mate, that's where he needs to be.
Which, for a latecomer, means running the gauntlet.
As any prize-fighter knows,
dodging the hits is all part
of winning the bout.
And by doing what frogs do best,
he reaches the centre unscathed.
But his victory may not last long.
New contenders are arriving.
And this is one of those they've all
been waiting for - a female.
She wants to get to the centre too.
After all, that's where she will meet the strongest males.
Swimming there underwater is her best chance
of reaching them.
If a male is to mate, he must stay in the centre.
And he won't be able to do that without a fight.
These battles are no-holds-barred
and could even be fatal.
The latecomer accepts defeat.
Not everyone can win in the mating game.
So it's his rival who holds the centre of the pond
and gets the females.
The pool will soon dry up.
So for this year, at least,
he's missed his chance.
Patagonia.
On the high plateau,
the spring thaw reveals
a land of lakes.
The water here comes not from rains,
but from melting snow and ice.
One of the rarest birds in the world travels long distances to get
here and breed.
A hooded grebe, one of just 700 that still survive.
This is a female, so she's extremely important if this species
is to survive.
She doesn't fight to claim a mate.
She dances.
The most experienced males are the best dancers
because they've done all this before.
So her challenge is to mate with one of them.
To do that,
she will have to prove that she too can dance.
Just getting noticed would be a start.
Couples are pairing off all around,
but her calls go unanswered.
SHE CALLS
Two is company, and as perhaps she should have known,
three is a crowd.
A solitary male must be a better possibility...
..but he's not keen either.
The lakes will freeze again before long,
so an invitation to dance is a very welcome sign.
THEY CALL
But a rival for his affections brings it to a swift end.
And just as she was getting started.
It's not going well.
And if she can't have him...
Perhaps finding a gift for him will help.
If she can find a partner, they will nest among the milfoil
plants when its flowers appear on the water surface.
Presenting strands of it is part of the courtship ritual.
She just needs to find someone to give it to.
His bending over backwards is perhaps a sign
that he is not interested.
But who's this?
FLAMENCO MUSIC PLAYS
Could he be the dance partner that she's been looking for?
The timing of her moves is crucial.
His precision shows that he's danced this routine before,
and more importantly, that he has already been a parent.
It's looking good.
Finally, having completed the performance,
she seals their bond
with a strand of milfoil...
..a symbol of their commitment to the work ahead.
And one of hope for the survival of this most endangered of birds.
THEY CALL
Many species migrate to lakes
at a specific time in order to breed.
And for some, doing so comes at great cost.
This female sockeye salmon started from far out in the Pacific Ocean
and then continued up one of Alaska's great rivers
for a further 160 miles in order to reach Iliamna Lake.
Her arrival coincides with that of others.
Lots of them.
Every summer, over three and one half million sockeye come
here. It's the largest salmon run in the world.
When they entered fresh water, they stopped feeding,
so the bodies of the females now have room for developing eggs.
But their energy is dwindling, and they have yet to reach
the spawning grounds that lie upstream beyond the lake.
But now they face more immediate threats.
While they try desperately to evade the grizzly bears
that attack them from the lakeshore,
another threat comes from deep water.
Iliamna Lake is home to freshwater seals.
Despite these onslaughts,
great numbers of the migrants survive
and swim on upstream, undeterred.
There are far more salmon
than the predators can eat.
At last, they reach the shallow streams.
So far on their journey, there has been safety in numbers.
But now, at the spawning ground,
it's every fish for itself.
A female starts digging a nest
and quickly attracts the attention of a male.
Quivering his fins is a male's version of a pick-up line.
She has a mate for now, but with more fish
continuing to arrive,
room for nests is running out.
Rival females try to steal her mate, her nest or both.
But she quickly sees them off.
Nests now cover the stream bed.
She releases her eggs...
..he fertilises them...
..and their work is done.
But she doesn't start on a return journey.
This was always a one-way trip.
None of the salmon, male or female,
has eaten since leaving the ocean...
..and now their exhausted bodies begin to collapse.
Pacific salmon only breed once
in their lives, and having done so,
they die.
They seemed inexhaustible while being driven
by the urge to reproduce.
These have won the mating game.
But it cost them their lives to do so.
Salmon from the Pacific migrate
in their search to find a safe place
to lay their eggs.
But when suitable sites for nests are particularly rare, competition
can get intense.
These are the Appalachian Mountains in the eastern United States.
Here, in just a very few rivers, live monsters
with a truly ancient ancestry.
Some call them snot otters...
..others mud devils.
Their most common name, however, is hellbender.
But common they are not.
Hellbenders are very rare indeed.
This male is very well camouflaged, and he rarely moves.
So in practice, he's almost invisible.
Hellbenders are a kind of giant salamander.
He may grow up to 60 centimetres long.
They're usually very shy,
but during the few weeks each year
when summer turns to autumn and the river starts to flow
more strongly, the hellbenders start to move around.
He's looking for somewhere to nest -
a rock, but not just any rock.
This particular slab he knows well.
He's chosen it before.
Such rare, heavier stones don't get easily washed away.
The space underneath it is well hidden and the current flowing
swiftly through it will bring the oxygen needed
for developing eggs.
The females are already on the move,
battling their way upstream towards him.
He just needs to hold on to his den until they get there.
But every year other males arrive to challenge him.
They come at him from all sides.
They're even lining up to try and take over his den.
But as he fights, the strong current carries him away from the den
and another rival sees a chance.
Back in the nick of time.
Exhausting.
But this is the time when he has to secure a mate.
With temperatures dropping, the turbulent winter water becomes
rich in oxygen needed by developing eggs.
These days, increasing erosion of the land
is blocking hellbender dens with sediment.
So now this one, more than ever, is worth fighting for.
He's been forced out of his home.
But after all he's been through, he's not about to concede.
His rival gives up.
He is still the boss.
And now, at last, a welcome visitor.
He greets her with what is known to hellbender watchers
as a courtship kiss.
Her belly is full of eggs
and she slips in beside him.
Then strangely, after days of brawling,
rival males are also allowed to enter.
Perhaps he's now too exhausted to fight
or has some strategy we don't know about.
Other females then visit the den and add their eggs
to the communal clutch, which he will protect.
He has become a baby-sitter for the whole hellbender community.
In the freshwater world, it's often the males who look after the young.
But even when these duties are left to one parent,
the mating game may still demand carefully timed coordination.
THUNDER RUMBLES
In South America, water levels across the Amazon basin rise
and fall with the seasonal rains.
If this male splash tetra is going to mate successfully,
he needs the water level to rise.
It's low at the moment,
but every drop of rain brings it closer to his requirements.
As the rain begins to ease, he tries to find out if he's high enough yet.
A few well-aimed splashes help him to estimate the distance.
He's looking for a place
where a female can lay her eggs away from predators.
But to assess the situation properly,
he does something truly extraordinary.
The water he splashes on to the leaf
momentarily delays his fall back into the river.
Now he turns his attention to a female.
Because what comes next needs the two of them.
If they're to mate successfully,
they must hit the leaf simultaneously.
After she's laid her eggs, he spreads his jaw and fins
to help him cling on just long enough to fertilise them.
This extraordinary way of laying keeps the eggs safe
from underwater predators.
But for him, it creates one final challenge.
Out of the water, the eggs are in danger of drying out.
Problem solved.
With the female gone, he alone takes the responsibility
for ensuring their eggs will stay wet
until they hatch in a few days' time.
The splash tetra pair had to synchronise their moves,
but in other cases, the mating game requires thousands of individuals
to work in harmony.
In the Great Rift Valley of East Africa,
fractures in the Earth's crust have been filled with fresh water
over millions of years by the annual rains.
But in many of these lakes,
the water doesn't stay fresh for long.
Scalding volcanic waters from deep below ground
carry up salts dissolved from the rocks.
Not much is able to live in these caustic lakes.
But one kind of bird flocks to them.
Lesser flamingos gather here in millions
to feed on the microscopic algae that develop
in these super salty waters.
A male flamingo must be well-fed before he can mate.
And the algae and other microorganisms that flourish
in these briny waters are the only things that lesser flamingos eat.
But these organisms also contain toxins.
The birds do not digest these substances,
but store them in their plumage out of harm's way.
And it's these that turns them pink.
The more well-fed he is, the rosier he becomes
and the more attractive he is to his potential mate.
As the water gets deeper,
the flamingos have to find new ways to collect their food.
He's getting pinker with every mouthful.
There is a female out there for him,
but he can only find her when she and all his friends in the flock
are pink enough to begin their courtship dance.
Each morning, the birds gather around the hot springs to drink
and show off their newly pink feathers.
It's a chance to see who is ready for dancing.
With his plumage in proper order,
he's keen to get the party started and show off to a female.
After weeks of feeding, with the whole flock in full colour,
it seems that everyone is finally in the mood.
Dance fever spreads throughout the flock.
And, at last, the performance begins.
Both males and females are dancing.
Groups merge and co-ordinate their courtship.
Laying their eggs at exactly the same time
maximises their chicks' chance of survival,
and dancing together ensures
that they're all in sync with one another.
If two birds find what they're looking for,
they leave the dancing group.
Exactly how a partner is chosen, we don't fully understand.
But it seems that some flamingos try to select a partner
with the same shade of pink as themselves,
though how they judge that is still a mystery.
Lesser flamingos may have to wait many years
before the water conditions are right to enable them to mate.
But there are lakes where securing a mate
can be a year-round preoccupation.
Lake Tanganyika in East Africa is enormous,
deep and warm.
And it's been this way for millions of years.
Long-term stability allows long-term mating strategies.
This pile of shells, for example, is constantly getting bigger.
It's likely to be several decades old.
It's certainly older than the current ruler,
a male callipterus fish.
He is an underwater architect
and busy collecting empty snail shells to add to his castle.
His pile is home to over 20 females,
who lay their eggs inside the shells.
He does his best to ensure that they only mate with him.
And doing so is a full-time job.
There are eels here
that would happily make a meal of a shell full of eggs.
But not if the king can prevent it.
He stole this castle of shells from a previous owner.
And if he lets his guard down,
someone will certainly take it from him.
Gangs of marauding bachelors are a constant nuisance.
And while the king of the shell castle is busy
proving how tough he is...
..he's being watched by a sneak.
He belongs to the same species, but he's a dwarf.
He won't grow any bigger and he's ready to mate.
He just needs to get near enough to a female.
He is only one 50th of the weight of the king
and if he can reach a shell with a female inside,
he's small enough to slip inside and mate with her.
Busted.
Back to square one.
Every now and then, the king has to tackle someone his own size.
A serious contender is attempting to overthrow him.
Could this be a chance for the dwarf?
He's close.
Once inside with the female, he can fertilise her eggs
without the king ever knowing that he's been there.
He might not be the biggest fish,
but he's definitely got one of the cheekiest strategies in the lake.
These shenanigans continue month after month,
thanks to the lake's extremely stable environment.
But some environments in southern Africa are far from stable.
The grassy plains in Zambia change radically every year.
They flood.
Many are home to lechwe, one of the most aquatic of antelopes.
With hooves that splay out wide and extra-strong hind legs,
this female is able to travel across even deep swamps.
But their watery world is now shrinking.
It hasn't rained in months.
Before long, these swamp meadows on which the lechwe depend will be dry.
They turn into dusty plains with just a few
small, muddy water holes.
This is the best time for a female to find a mate.
In a few months' time,
when her young are born, things will be easier.
So it's at this point that she starts to think
about getting a partner.
Most of the time, male and female lechwe live quite separate lives.
She is now at her most fertile,
and she starts to leave the safety of the female herd.
Alone, she's more vulnerable,
and the long, dry grass away
from the swamp can only too easily conceal a lion.
She mustn't stay away from the other females for long.
To survive here, you must be fit and strong,
and those are exactly the characteristics
she values in a mate.
The best place to find a male with such qualities is even further
away from the water.
Out here on the dry plains, the males are already
competing for females.
Stabbing the ground with his horns is a male's way
of demonstrating his strength.
His next move is more direct.
She remains unconvinced,
but this performance has caught
the attention of a rival male.
She's here to mate with the strongest male,
and these battles will help her decide which one that is.
But they'd better get a move on.
There are plenty of other males that she could choose.
Having seen off his rival,
he resumes his overtures to her.
Perhaps because they're vulnerable when they get together, their union
is very brief.
Afterwards, he tries to remain close, but she has other ideas.
Her instinct is to seek safety in numbers.
She's got what she came for and she heads back
to the female herd.
She's started her pregnancy at just the right time
to give her offspring, when it arrives, the best start in life.
In Brazil, this wetland, the Pantanal, also dries out every
year, and this is the time when,
like the lechwe, caiman breed.
But if he is to survive long enough to mate, this male will have to find
water of some kind.
The pool he came from has now dried up.
If he doesn't find a new one, he will die.
The land is littered with the remains of those who didn't reach
water in time.
But he is heading for one place
where he knows that water
will remain throughout the dry season.
He's made it.
And he is not the only one to have done so.
So have thousands of others.
And it's not only water that's here.
There are females as well.
Late afternoon is a time to bask.
And although the dwindling pool
is now very crowded, they tolerate
CAIMAN GRUNTS each other...
..for the time being.
But by first light,
the mood has changed.
Surviving the drought is no longer a male's top priority.
If babies are to hatch out when the high water returns and life
is easier, he needs to mate soon.
And if he's to do that, he must claim a patch of this water
as his own...
..because there isn't room for everyone.
All around, males begin to assert their dominance.
The newcomer must stand his ground
if he wants to remain a contender.
But he must also know when to back down.
And there is usually someone
smaller for him to pick on.
By mid-morning, an uneasy calm descends.
The newcomer has secured a patch.
But he still has to convince a female that he's worth mating with.
CAIMAN GRUNTS And he has a plan.
It starts with a grunt.
GRUNTING CONTINUES
The next part of his courtship song, we can't hear, but we can see it.
It is not the muscular movement of his body that make
the water dance,
but the low frequency of his call.
This song contest may be inaudible
to us, but not to female caiman.
It attracts them from far and wide.
Despite his rival's best efforts,
it is the newcomer who seems
to be getting it right,
because he's got company.
The water is so murky that actual
mating is rarely visible...
..but every now and then two caiman appear a little more interested
in one another,
and something is clearly going on below the surface.
Timing is crucial...
THUNDER RUMBLES
..as it is for all animals that come
to breed in those variable and only
too unpredictable of the Earth's environments,
its fresh waters.
In order to tell the story of the breeding migration of red salmon,
the Mating Game team had to choose the best location.
After months of research, assistant producer Olly Scholey struck gold.
So the reason we've come on this shoot really stems back to me
just sitting on Instagram on a Sunday evening
and just scrolling through pictures, and I saw this one photo
of Iliamna Lake that Jason had taken and was just completely sold on
coming here to film red salmon.
There's gin-clear water, and you can just immediately see
in that picture that there's a really fascinating spawning dynamic
going on here.
Jason Ching is a local salmon biologist
and world-renowned wildlife photographer.
He has taken the crew to the exact location where he took his picture.
Cinematographer David Reichert is hoping to film the details
of the salmon courtship as never before.
Filming underwater, one of the big struggles is the visibility,
being able to see a nice, sharp image.
So when I saw the picture from here, that was really exciting for me.
To be able to work in that kind of water is... You know
you're going to be bringing home really beautiful stuff.
The crew also want to capture the spectacle of this huge salmon run
from the air...
...and this aerial perspective shows them
something completely unexpected.
Just keep it down. Yeah, you've got control.
Hang on, hold up. There's a seal, bottom left.
OK, I'm moving forward.
Freshwater seals, an incredibly rare animal, were predating
upon the salmon as they kind of bunched up in this river mouth.
Oh, he's right on one.
This seal behaviour has never been filmed before, and the crew now
have to overcome the far harder challenge of documenting
it from underwater.
He's away down...
Freshwater seals are extremely shy,
so in order to do this, the crew
need to use remote cameras.
Hopefully, these are going to capture that seal behaviour,
seals attacking the salmon,
and that'll go with the incredible
drone shots that we've already got.
Stop. Yep. Perfect.
Oh, it looks good, mate.
Horizon's good.
So if they come in, if they come in and feed here,
we're going to get some pictures.
Here he comes, here he comes.
He's just sniffing around right now.
So we've just started getting some
little bits of the seals coming in,
but we're not quite in the right place. We haven't been in quite the
right place, but now we've positioned
it right where we think that this seal is going to stop
and wait and then jump up and grab a fish.
Fingers crossed, it works.
Coming in.
OK, he's getting in a good place for this one. Yeah.
After days of catching mere glimpses... That's a good shot.
..the crew finally have their cameras in the perfect position.
Oh, good, good, good, good, good, good, good, good, good! Oh! Oh!
Oh, so close! He missed!
So, there he is. See him coming up there?
He might be about to charge again.
Oh! Got him!
Oh, mate, on both cameras!
It's a great success, but it was
only going to be a matter of time
before the crew meet the salmon's
other predators.
BEAR GRUNTS There's a bear.
Oh. Get ready with the cameras. OK.
It's dangerous to surprise a grizzly,
so Olly lets the bear know
that the crew are close.
Hey, bear! Hey, bear!
I'm watching this bear. Yeah.
Hey, bear. Hey, bear.
That's close enough.
Hey, bear. Hey, bear.
You stay back.
Like, obviously, hindsight is a lovely thing.
We're probably a bit too close to the river and to all the fish.
I think we just need to move back a little bit and just stay
a bit further away.
This is his river, not our river,
so we'll let him catch his lunch.
I mean, it could be amazing because we've got this really
clear water. We could get some really unique shots of bears walking
through salmon and actually able to see the whole bear.
The slight downside to that is either the bear treads on the camera
or grabs it or does anything to it.
It's... Basically the plan is to put quite an expensive camera
underneath the feet of a charging bear.
So...
David positions the remote cameras in deeper water in the hope
of capturing an extremely rare perspective
of a hunting grizzly bear.
Still coming? Yeah. Maybe get out.
OK, got him. Oh, nice!
Yes! OK, we got him.
It works, and they capture a salmon's-eye view of one
of the biggest challenges they face on their breeding migration.
Although the plan for getting the camera close to the bear works
maybe a little bit too well!
We have to look at one upside down now because the bear's completely
HE LAUGHS rolled it!
Is it intact?
Miraculously, the camera survives, and it's now time for David
to finish telling the salmon story
by trying to film them spawning.
So we have found just a magical part of the world.
Just the amount of activity here,
it's just epic, what is happening.
What has really surprised me
about spending so much time
with these salmon is really how
intricate this behaviour is.
There's so much going on with these
fish as they compete
to be really the mating pair.
Yeah, fish... Like, fish sex is, you know... I feel,
HE LAUGHS like, a little awkward...
..watching that for hours, but they don't seem
to care, so I guess it's cool.
With some luck and some ingenuity,
the team capture almost every
challenge that the red salmon must
overcome in order to win
the mating game.
Next time...
The need to breed drives all life,
but the challenges faced
are by no means equal.
Some in the mating game must
overcome overwhelming odds
to have any chance of success,
and the changes WE are creating
are the greatest challenge of all.
PANDA WHINES