All language subtitles for Ultimate Swarms BBC EN Sub_Subtitles01.ENG

Swarms are one of the greatest
spectacles on earth.

At times horrifying,
and also a thing of wonder.

As part of the swarm,

the smallest of creatures can
become a force of nature.

I'm George McGavin, a zoologist
and explorer

and I'll be travelling the globe
to get right into the heart

of some of
the world's most impressive swarms.

HE LAUGHS

I want to show you that far from
being the ultimate nightmare,

swarms are one of nature's most
ingenious solutions.

Ow!

Swarms are extremely powerful.

By joining together,

even the simplest of creatures can
achieve the impossible.

And by understanding how
swarms work,

we're gaining some fascinating
insight into our own lives.

My journey begins in North America,
in southern California.

I am hitting the highway in pursuit
of a swarm most sane people

would go out of their way to avoid.

At some point,

this little insect's sting might
have totally ruined your day.

Whoo, damn!

But in extreme cases,
this is a swarm that can kill.

I don't think you want to be outside
right now. There is a swarm of bees.

The honeybee.

A terrifying swarm,

but one that worldwide is worth
a staggering $180 billion a year.

Without them, over a third of all
the food we eat wouldn't exist.

And nowhere is the bee's pollinating
handiwork more crucial than here.

California.

More fruit and vegetables
are produced in this state

than anywhere else in America.

And bees are so important
to the process that every year,

farmers actually import almost
2 million hive-loads of them.

This is the perfect place to
understand the secret of the swarm

that takes group intelligence
to a completely new level.

To understand how it works,

I need to get right to the start
of the swarm, to the moment a queen

gathers a loyal team of workers
around her and goes house hunting.

Now, in this temporary
swarming state,

they are really non-aggressive

because they're not protecting
anything,

they're not protecting young,
or honey -

they are simply
protecting or shielding the queen

that's in the heart of their swarm
until a new home is found,

and they are so relaxed
non-aggressive

that I can actually put my hand
right into the swarm here

and just jiggle off a little
handful of bees.

Now, there's no way
I could do this with a normal hive.

It's only possible when they're
in this temporary swarming state.

Now, if I can persuade the worker
bees that I'm a queen bee,

they should swarm around me.

It's only going to be possible
because the swarm sticks together

by following chemical
signals called pheromones.

Looks have nothing to do with it.

It's an attraction so powerful
that the bees should be prepared

to ignore the obvious
and accept me as their queen.

To put it to the test,

honey bee expert Norman Gary is
giving me a queen bee makeover.

I'm going to put droplets
of a pheromone mix on your clothing.

That pheromone is the odour
that the queen bee emits

that makes the other workers swarm
around it.

These are all females, did you know
that? The females are now loving you.

Yeah.

Have fun, George! Hah!

'There's no backing out now.

'My only defence is a bit of insect
repellent on my face.'

I'm gonna start low, George,

because they tend to migrate up,
so here we go.

There you go.

'At this stage, it's hard to tell

'whether the bees are going
to stick around.

'And if I'm honest, right now,
I'm not sure I want them to.'

This is not funny, because...

It's in my eye, it's in my eye!
Argh!

Just relax.

I am relaxed.
I'm so chilled, I can't tell you.

It looks like the workers
have been fooled.

I'm really beginning to get
a sense of what it must feel like to

be the queen bee in the centre
of a massive swarm of bees.

And being part of the swarm is
starting to feel even weirder

than it looks. I can now feel
the weight of the bees on my chest.

There's a fair few of them
and collectively,

they're beginning to feel quite
heavy, like wearing a wet shirt.

The other thing that's quite
obvious is the heat.

Because I'm covered
in a layer of active bees

that are vibrating their wings,
they are generating

quite a bit of heat and that's being
transmitted to me through my shirt.

I can really feel like I'm wearing
a sort of woollen jumper.

'It's incredibly unsettling.

'I'm only safe from attack
because this swarm is in

'this temporary state,
but that needs to change.

'The bees face a critical
decision on where to set up

'their permanent home.

'It's vital they get it right.

'Get it wrong,
and the swarm will die.

'So how do 40,000 individuals reach
a unanimous decision?'

Although the queen is central
to the formation of the swarm,

it's not her that takes
any part in the decision

about where the swarm will end up,
where they'll have their new home.

But in order to show you
how that works,

I'm going
to have to move this swarm off me.

Wonderful. Perfect, perfect.

With the swarm now airborne,

they don't have long to make
a decision

and to see how they do
it, we've set up an experiment.

I've given the bees
a choice of three new hives,

only one of which is suitable.

Hive A has a small enough
entrance to keep out any predators,

but it's not big
enough for the swarm to grow.

Hive B is larger, so there's
room for the bees to expand,

but the entrance is too big to
protect them.

It all rests on hive C.

The one they should really
go for is this one over here.

It's the right size,
it's got room for growth,

the entrance hole is also
the right size,

so this, if I was a bee,
this is the one I'd be going for.

The question is,
will the bees choose the right one?

The first thing the swarm does is
send out the scouts.

It's their job to search for a new
home, inspect the premises,

and report back.

They're looking for a nice, roomy
hive, safe from predators

and south-facing, to make
the most of the early morning sun.

And after a thorough investigation
of all three hives,

it's time to head back to base.

What's really amazing about all this
is the way that the scout bees

communicate that vital information
back to the swarm.

It's all about the waggle dance.

With their best dance moves,

the scouts tell other bees which
direction to go.

The length of the waggling
gives them a distance.

One second of waggle equals half a
mile of flying. But that's not all.

The energy that they perform
that dance

indicates the quality of the house.
Look at this one here.

She is really going for it. This is
a really high-energy waggle dance.

Lots of enthusiasm.
She's doing it over and over again.

And that will tell
the rest of the swarm

that what she's found could be
the perfect home.

We used to think that the decision

was all down to the bees'
waggle dance.

When enough bees dance for the same
hive, a decision was made.

But recently, scientists realised
there was something much

more interesting
happening on the dance floor.

Some of the dancing bees were
on the receiving end

of a sneaky
head-butt, a signal to stop dancing.

So it's actually a combination
of enthusiasm and bullying

that helps the bees agree
and prevents a decision deadlock.

So what does this swarm
make of the three hives?

Jeez! They're everywhere.

Well, those 40,000 bees have now
taken to the air,

absolutely full of...the noise
is incredible

and they're all heading to that
hive over there

and so far, not one sting.

Ooh! Ow!

Something just got me!
Sorry, correction - one sting!

And while two of the hives
are still empty,

hive C is proving popular and
the bees are moving in en masse.

They've made the decision
collectively

and they've picked this hive. This
was the one I thought they'd pick.

It's absolutely right,
is the right size.

It's the perfect angle,
the perfect everything.

And it's their group
intelligence,

the ability to use the power
of the swarm to make the right

decision that makes honeybees
the ultimate team...

Because it's a fact that hundreds
of individuals

make a better decision together
than a single expert.

And that's something both animal
swarms and humans have in common.

It's a bizarre phenomenon first
noticed back in 1906

at a county show much like this one.

Ha ha!

Scientists were amazed by what
happens during a simple competition

asking people to guess the weight
of a cow.

When they analysed the results,
not only was the average

of everyone's guesses more accurate
than the winning guess,

it was also a much better guess than
that of the experts who took part.

So today, we're going to put
it to the test

with the help of Zinny here.

We asked a selection of people
at the Royal Bath Show

to guess Zinny's weight.

How accurate will they be as a group
and can they beat one of the pros?

Right, well, the results are in

and all I've got to do now is to add
up all the estimates

from the crowd, get a total,

divide that by the number of people
who guessed...

The answer we get is 588 kilograms.

Now, a livestock expert guessed 584,
so they're very close.

But who's closest?

It's time for the cow to reveal all.

Haven't got her back legs in yet!
Haven't got her back legs in.

595 kilograms. Incredible!

That's just seven kilograms higher
than the group's estimate.

The crowd got it just about spot-on,
and they beat the experts.

It seems many brains
really are better than one,

and that's due to something
known as The Wisdom of the Crowd.

Our highest guess was 1,400 kg,

and our lowest 200 kg.

So, both way off,

but when a crowd works together
the odd crazy guess

doesn't really matter.

And that works for nature, too.

One animal might make a bad
decision,

but unless others have the same
reaction, the swarm isn't fooled.

That's why taking the option to ask
the audience in a game show

is such a smart move.

They get it right about 90%
of the time,

compared with a 65% success rate
from the experts.

It's no wonder swarming has helped
some creatures become

the ultimate decision-makers.

But not all swarms
are about boosting brainpower.

Some are about working together
to solve a problem.

The next stop on my journey
is Christmas Islands,

a tiny volcanic outcrop
in the Indian Ocean.

This dramatic, otherworldly
landscape

is the setting for an extraordinary
swarm on the move.

And right now is the best time
to see it in action.

Oh, my God, that is so...

The island is about to go
into lockdown.

Every year as the monsoon rains
arrive,

60 million red crabs emerge
out of the trees.

A swarm facing the ultimate
challenge.

These land crabs live in a forest.

But like their coastal ancestors,

they're still completely dependent
on the sea to breed.

And between the forest
and the beach

lies a gruelling six-mile treck.

This one is a female.

You can see underneath...

Gosh, she's really skittish.

If I hold her very carefully
you can see underneath

that brown mass at the end of her
abdomen under her tail,

is something like 100,000 eggs.

The reason that the females are
a little more difficult to handle,

on their mind just now
is to get these eggs...

Ow! ..into the sea as fast
as they can.

So that's what she has to do now,
so I'm just going to let her...

Oh, if I can get...

..get on her way.

She knows exactly which way to go.

The Christmas Islands crabs follow
the same well-trodden route

for generations.

But the arrival of humans
on the island

has given them a few extra obstacles
to deal with.

But nothing can get in the way
of their epic journey.

In just over a week, the crabs
will cover up to six miles.

For a creature of this size that
spends most of its life underground,

that's like running several
back-to-back marathons

with no training.

So how do they do it?

Scientists have recently discovered

it's all thanks
to a special internal sugar reserve.

Powered by a massive sugar rush
once a year,

these laidback forest creatures

suddenly turn into long-distance
athletes.

Oh, my goodness! Look at this!
This is...

This is unbelievable.

There's just crabs as far
as the eye can see.

Every inch of this rock is covered.

Amazingly, after days of walking
and dodging obstacles,

tens of millions of crabs have
survived and made it to the beach.

Just in time for their next big
challenge.

Tonight's the night.

They've got to shed their eggs
on a pre-dawn high tide,

when the difference between high
and low tide is at the smallest,

because if they fall in the sea,
they drown, unbelievably.

Being land crabs,
they can't survive in the sea.

But they've got to get their eggs
in the sea.

They all have just one tiny
window of opportunity.

And according to my calculations,

that should be in about
six hours' time.

So, I'm taking my place with
the crabs and waiting for high tide.

It's 3am, and all around me
female crabs have just started

frantically scrambling
to the edge of the surf.

Well, this is it.

The annual mass spawning
has started,

and it is just one of the most
incredible things I've ever seen.

It's easy to forget how treacherous
this moment is for the crabs.

Many of them will get
washed away and drowned.

But it's a risk they have to take.

The crabs are actually spawning
all around me.

Every time they spawn, they put
their claws up

and they shake themselves,

and as they do that
they shed 100,000 eggs each.

So many trillions of eggs have been
shed into the Indian Ocean

that it's turned into soup.

That is just full of now
freshly-hatched crab eggs.

The minute they hit the seawater,
the eggs hatch into tiny larvae.

But now they're at the mercy
of tides and currents.

Most will end up as fish food or get
swept away into deeper waters,

never to be seen again.

Which is why this swarm is so vital.

To ensure the survival of just
a few crabs,

nature has to throw
a lot of zeros at the problem.

Over the next three days,
something like three trillion

individual crab larvae will be
released into the Indian Ocean.

But despite the incredible numbers,
the baby crabs will only make it

back to shore every six
or seven years.

And when they do, the scenes
are spectacular.

A super-swarm of tiny crabs
defies the odds

and climbs back out of the ocean.

For a land crab
trapped in a forest,

this swarm has the ultimate
survival strategy.

And across the globe,

there are similarly amazing sights
as other swarms set off on the move.

Individually, each animal has no
idea which way they're heading.

But as a group, somehow they all
move in the same direction.

But how do you get tens of millions
of individuals

to work together as a team?

Well, strangely, it's all thanks
to having nobody in charge.

It might sound like a recipe
for disaster,

but if you look at a colony
of leafcutter ants,

everyone is doing their own thing.

No single ant is in charge
of organisation, not even the queen.

But with every ant ignoring
the bigger picture

and focusing on the one job,

the process actually becomes
highly efficient.

So, by thinking like ants,
we're now changing the way

we look at some of our
own logistics.

Every day, millions of us travel
through the world's transport hubs.

Getting from A to B by the most
efficient route is vital

to keeping things running smoothly.

Something the ants do really well.
So, how would they run an airport?

By designing software capable
of thinking like a swarm,

we've been finding out.

An American airline tried to solve
a long-running debate -

was it faster to board a plane by
giving passengers allocated seating

or by allowing them to pick
their own seats?

Surprisingly,

when the computer programme used
digital ants to fill the plane,

it showed that sometimes, letting
people choose their own seats

is quicker than giving them
seat numbers.

Because when there's no top dog
to make decisions for us,

like the ants, we all have to think
for ourselves,

and it doesn't result in the chaos
you'd expect.

And new technology is taking this
even further,

with robots that use
swarm intelligence.

Just like insects,
these robots all do their own thing.

By reacting to each other,
they can combine to do things

that would be impossible
on their own.

The hope is that one day
these robots could be sent out

into some of our most
dangerous locations.

Artificial swarms working together
on the front line,

or on search and rescue missions,

replacing humans and potentially
saving lives.

But while some swarms work
together day after day,

other swarms exist for only
a few hours.

Coordination is critical.

Get it wrong,
and your whole life has been wasted.

This is a swarm that's seen
on just a few nights of the year.

Oh, my God!
It's a blackout.

GIRL SCREAMS

Yes. They are alive.

To be in with a chance of finding
this swarm,

I'm travelling to the States,
to Wisconsin.

I'm chasing a massive swarm
that appears from nowhere,

so this part of my journey
will be a race against time.

To get a sense of the challenge
ahead,

I've come to the local weather
centre in La Crosse,

where they've been keeping a close
eye on the swarm's past movements.

That is just incredible!
It's like an explosion.

Right, this was just after sundown.

Some biological target is coming
out of the river

and then being carried by the wind
away from the Mississippi River.

If I could see that,
if that happened again,

or even anything
remotely like that...

I would be very, very happy.

So, what swarm could be so intense
that it shows up on weather radars?

Well, for just a few days every
summer, amazingly, millions,

some say trillions of mayflies take
to the sky to form dense clouds.

It's a blizzard of insects thick
enough to stop traffic.

But despite the massive size of this
swarm, it's unpredictable,

so finding it won't be easy.

Time for me to take to the airways.

Well, I'm in the right place and
it's more or less the right time.

But if I'm going to see a really
big swarm,

I've got an idea of how
I can enlist some local help.

Downtown La Crosse.
It's now 14.10. WIZM.

I'm George McGavin
and we're from the BBC.

RADIO DJ: Swarms of mayflies have
attracted the attention

of famed entomologist
and TV host George McGavin.

For a lot of people around here
it's going to be surprising

to hear that you want to be
in the swarm. When we see

those swarms we want to get
in our car, get inside

and get away from it.
Why? It's fantastic.

For an insect to be able to halt
a train or a car, you know,

that's pretty special.

That's something
I really want to see.

We just need mayflies, lots of them.

And they can call any time, day or
night? Absolutely. We'll be there.

We'll be hot on the heels
of the swarm.

I just hope we get the phone calls.
I just hope we get folks ringing.

We've done what we can,

but it's impossible to predict where
the first sighting will come from.

One thing we do know is,

they all start off in the same
place - the Mississippi River.

Mayflies spend most of their lives
hidden underwater.

So by looking in the river,

I should be able to get right
to the source of this swarm.

So now, hopefully, in here

are one or two or more
mayfly nymphs.

In a good year, a grab of that size
might contain 15 nymphs,

and if you think of that,
it's a very small area

and if you multiply it
by this vast area,

that's the sort of volumes
of animals that we're hunting for.

Oh, here, we got one.

So, that's what we're after.

This is a mayfly nymph,

and they're perfectly adapted
for existing in silt.

They're very streamlined.
They've got two tusks

at the front of the head
for excavating through the mud.

They've got these really strong
front legs, as well,

which have little
sort of prongs on them.

So they are the perfect silt
inhabitants,

and they stay there for perhaps
a year, up to two years...

Until all conditions are right
to trigger a mass emergence.

It's thought water temperature
has a lot to do with it,

but lots of other factors like
weather and food supply

also play a role,

making it frustratingly difficult to
predict what's going on down there.

Unless you're a mayfly, that is.

With just 24 hours to emerge,
find a mate, breed

and get back to the river,

it's a race against time
for survival.

Miss it by just a few hours,

and you'll miss your one
and only chance to find a mate.

And I'm racing against
the clock, too.

Time to get back out on the road
and keep looking.

A swarm of mayflies could form
anywhere along the river.

But towns and roads are a good place
to start looking

because there's one thing
they just can't seem to resist.

Lights.

The brighter, the better.

So our shiny new modern lights
play havoc with them,

and bring chaos to our towns.

It could be that the lights
are a giant mayfly singles bar.

With just hours to breed,

there's not much time to fly about
and look for a mate.

So, by all heading to
the nearest bright light,

you improve your chances
of finding a partner.

And just as I was about to give up
hope

of getting to see this
incredible event in action,

there's some promising news.

I've just had a phone
call from a boat yard up here.

This looks like the marina,

so I hope I'm in the right place.

Oh! Oh-ho!

Mayflies, flying.
Look, there's one on the window.

Whoa, this is more like it.

Whoa! Look at this!

'We've caught the very beginnings
of a swarm.'

These guys will have left the river
late last night last night,

looking for a landing spot

where they can go through one
final transformation.

This is incredible.

This is the emergence of an adult
mayfly from the sub-adult,

and this is a process that has been
going on for 300 million years.

This is incredible to watch.

Now, it's still got to get
the end of its abdomen out,

and it's got to draw out
those long tails.

And this exact process
is being repeated

hundreds of millions of times.

In a few hours' time,

all the new adults will
rise into the air in a huge swarm.

The mayfly have spent their entire
lives in the mud,

waiting for this brief
and all-important moment.

By evening, the sky fills
with millions of them,

in an intense breeding frenzy.

Being part of the swarm is a once
in a lifetime event for the mayfly.

But other insects have to deal with
a whole life in amongst the swarm.

But then working together
efficiently, is vital.

Ant colonies are completely
dependent on keeping the supplies

coming in thick and fast.

With millions of hungry
mouths to feed,

any delay could be a disaster.

So, they avoid a crisis by following
a surprising highway code.

To go as fast as possible,
the ants have learned to slow down.

That means no boy racers
and no overtaking.

And what works for the ants
also works for us.

Twice a day, our roads have to cope
with millions of people

all trying to be at the right
place at the right time.

Designing our transport networks
to cope is a huge challenge.

So, now, our road planners

are starting to pick up
a few tips from the ants.

By restricting each and every car
to around 50 miles an hour,

actually makes the journey faster.

And although it might not
seem like it,

reducing the number of lanes

and banning overtaking makes things
even more efficient.

And now several car companies are
taking this a step further

by looking into technology
that keeps cars

close behind each other
and at a constant speed...

..just like the travelling ants.

That is just weird.

These super-smart cars have sensors
that read their speed

and distance from the vehicle
in front.

I'm not touching the brake at all.

All you have to do is hold the wheel
and, if that car in front stops,

the brakes are applied
automatically...

bringing me to a halt.

I have to say,
it really makes you very nervous.

It's a very unsettling feeling

not to be in control of the car,
as it were.

Stopping here - oh.

Oh. Oh.

Oh.
HE LAUGHS

I didn't touch the brake.

Oh, my goodness.

These cars are so clever that even

when the car in front brakes
unexpectedly,

the car has everything under
control.

Ah!

Oh!

Oh!

Ah!

Let me just get my breath back now.

HE SIGHS
That was unbelievable.

I'm not sure I'm quite
ready for a car with swarm powers

but it's a genius idea.

And with almost 40 million
vehicles on UK roads,

technology like this could be
the answer to keeping traffic

flowing with ant-like precision.

But while some swarms are helping us
overcome problems,

others are just adding to them.

In America, there's an unseen swarm
that has silently invaded

and is slowly taking over.

In the last 40 years,
it's spread into 23 states

and yet remains largely
hidden from view.

The American government has spend
over $150m trying to control it.

It's a swarm that
I have never experienced before

and one that is not without
considerable risk.

For my chance to meet this swarm,

I'm heading to Bath, Illinois,
in the American Midwest.

And it seems I'm not the only one

here to meet this highly
destructive swarm.

There was a couple of guys that got
black eyes that got hit in the face

with no protection, and they're
walking around here today.

They got a big, old shiner.

That looks painful.
It was.

This one was the first hit yesterday
and one on this eye.

And you're back for more.
Back for more. This is dangerous.

Yeah. We've been coming eight years.

This is the first time we've ever
got hurt, though.

People say they eat them
but, as nasty as they look,

I ain't touching them.

Just what have I let myself in for?

To find out, the Illinois Natural
History Team is going to get me

face-to-face with the problem.

Today, we're going to use
a technique called electrofishing,

and we're going to put anywhere from
4,500 to 5,000 volts into the water.

Well,
this is the calm before the swarm.

I have no idea what's going to
happen.

Wow!

This is crazy!

Argh!

That is unbelievable.

Oh!

This is what I'm here for,
the Asian silver carp,

a fish with a unique panic reflex,
causing it to leap out of the water.

There are literally
thousands of these carp.

Arg!

This is absolute insanity.

Whether it's a predator,
a boat or our electric current,

a reflex response spreads
through the swarm,

setting off a chain
reaction of leaping fish.

It's a vital way of measuring

just how many
fish are lurking down there.

At the moment, the best guess
is 2,500 carp per mile of river

but the numbers are still growing.

And as carp numbers are going up,

the amount of other
wildlife is going down.

I've never seen
so much fish in my life.

Oh!

This is a swarm out of control
and ecologically dangerous.

Getting rid of it isn't going to be
easy, but the battle is on.

The big concern is that the carp
are only 50 miles away

from the great lakes of America.

If they continue to spread
in that direction,

they could devastate
a $4bn fishing industry.

The US government have been
spending $150m in the last two years

just to control this species.

Everybody all right?

I got one on the backside.

Unbelievable.

So far, they've tried everything
from poison to electric barriers,

but nothing seems to be able
to stop the carp.

So, the locals have decided to take
matters into their own hands.

This fishing tournament
is their chance to fight back.

We're now in the middle of this
fishing chaos.

There's about 50 boats here. There's
some more boats coming towards us.

Whoa! That hit him right on the
head, that.

Carnage!

Oh, jeez!

HE LAUGHS

I'm hanging on.

But why have the carp got
out of control?

Ooh!

They eat algae and because they eat
so much of it,

they are basically eating all
of the food

that the fish here would normally
eat.

The carp can eat up to
a third of their body weight a day.

Their ferocious appetite has
pretty much emptied

the river of food for anything else.

This swarm is taking over.

And the trouble with all this
is, it's a purely man-made problem.

These fish were
introduced 40 years ago to clean

the algae off commercial catfish
ponds.

When they escaped into nearby
rivers,

the carp found themselves
in the river full of food

and nothing to stand in their way.

With each carp able to produce up to
two million eggs a year,

it didn't take
long for the fish to become a swarm.

Something that doesn't happen
back in their native home, Asia,

where predators
keep their numbers in check.

They've just exploded.
This is a swarm of epic proportions.

Look at the size of that one.

That's got to be 30 pounds.

If that hit you, it would
break your jaw.

PEOPLE SHOUT

And although this may all look
a bit unconventional,

this event does play a small
part in a serious attempt

to control these fish.

By the end of the event,

over two tons of carp will have been
cleared from the river.

ALL CHEER

And that's in addition to
the hundreds of tons caught

every
year through commercial fishing,

some of which is actually now
exported back as food

to the carp's original home, Asia.

But all this is just keeping
things in check.

It's not enough to actually defeat
the swarm and stop it spreading...

Because a dangerous
combination of unlimited food,

lots of space and freedom
from predators allows some animals

to swarm to biblical proportions.

And when swarms get to this scale,
they also have a dark side.

It doesn't take much to throw
hundreds

of carp into mass confusion.

In seconds,
fear passes from fish to fish...

even though most of them
won't know what they're afraid of.

Because in a swarm,
individuals are so closely connected

that they share moods and reactions.

In the event of danger, a quick,
united response like this

can mean the difference
between life or death.

But, occasionally,
the system goes wrong.

An event sends a contagious
emotion through the swarm

and causes complete chaos.

And, unfortunately, that's something
you see in humans, too.

CHEERING

When thousands of strangers come
together for a shared experience,

it can create a powerful connection.

When everything's going well,
it can be an incredible experience.

But when the mood changes,
things can get very nasty.

It takes just one event
to cause a crowd

to stampede like wildebeest...

out of control
and with dangerous consequences.

SIREN BLARES

Anger spreads person to person
till a peaceful demonstration

becomes a full-blown mob.

People stop thinking as individuals
and begin to follow the herd.

Emotions are high
and behaviour starts to change.

Within the group, people start
losing their inhibitions.

They do things they'd never consider
doing if they were on their own.

And because the group makes
individuals feel powerful

and less accountable,

normally responsible people start
acting completely out of character.

So, finding yourself in a crowd

can play a huge role in how you
behave as an individual.

SIREN BLARES

Now, thankfully, most human
gatherings pass without incident.

But there is a swarm where big
crowds are nearly always fatal.

A plague of locusts is never
a welcome sight for farmers.

But when the numbers explode,
it's also bad news for the swarm.

At peak populations, the locusts
suddenly start craving protein...

..and the meat closest to hand is
the insect ahead.

To avoid being eaten
alive by their nearest neighbours,

the locusts need to keep moving.

This cannibalistic swarm is not just
tearing through the crops,

it's also at risk of devouring
itself.

But overcrowding doesn't always
work against the swarm.

The animal I'm off to see next
has turned

overcrowding into its greatest
asset.

This is a creature found in numbers
so high,

it brings people
out into the streets.

To join them, I'm heading south.

I'm in Texas, home to the largest
gathering

of mammals anywhere on the planet.

Every summer,
as night falls in Austin, Texas,

a seemingly endless swarm
fills the air.

Wow! Look at this. They're just like
flooding out of the bridge right now.

Wow! Look back into the distance,
guys.

It almost looks like a plume
of smoke.

The residents are outnumbered.

Over a million creatures
are swarming the skies.

This bridge is the biggest
urban roost

for Mexican free-tailed bats
anywhere in the world.

Oh, look at this. Look at this.

This has become a real spectacle.

There's just
hundreds of thousands of bats

and they're streaming
out from under the bridge.

This is a colony of biblical
proportions.

So what do the bright lights of
the city have to offer these bats?

Like many riverside cities,

there's a glut of insects
in the summer months.

But what really makes Austin
a bat hotspot is its architecture.

When Congress Bridge
was rebuilt in the '80s,

the new structure was full of deep,
narrow openings.

It was the ideal bat home
and they moved in en masse,

but this convenient city pad
isn't without problems.

Space is limited and the bats
are an easy target

for birds of prey
and other predators.

So, as impressive as this is,

a three-hour drive away, there's
a bat swarm ten times bigger.

And a swarm of this size can achieve
the seemingly impossible.

But getting a good view of it
isn't going to be easy.

This area of Texas is
riddled with caves,

making it the perfect
habitat for bats.

The caves offer them protection from
the elements and from aerial attack.

But to find the real reason

ten million bats flock to this
particular cave...

I need to get much
closer to the swarm.

This isn't going to be
a pleasant journey.

I'm now entering
one of the most overcrowded

and toxic places on Earth.

So, I've come prepared.

Wow. The smell.

Ammonia is actually quite intense.

The caves are piled high with bat
droppings, guano,

releasing dangerously high
levels of poisonous gases.

Levels of ammonia are so high
that it bleaches the bats' fur.

But, amazingly,
the bats themselves aren't harmed.

By slowing down their metabolism,

their bodies are able to
neutralise the toxic gas.

This pile of guano is
absolutely enormous -

it must be metres thick.

That roof is absolutely jam-packed
with bats and look at them.

It's every crevice.

There's a big group of them
right here.

The levels of ammonia are now
potentially fatal

and, without these masks, we'd be
unconscious in a matter of minutes.

Very little can survive these
lethal conditions...

apart from the bats, that is.

If you're wondering what a million
bats look like, it looks like this.

This is one of the most incredible
sights I think I've ever seen.

This is a giant bat creche.

Every year, ten million pregnant
female bats

fly to these caves to give birth.

It's more or less predator-free down
here, thanks to the deadly fumes,

so it's a safe place for mothers
to leave their young

while they go off and hunt.

The really amazing thing about these
bats is,

the adult females roost
in a different part of the cave.

These are just the young
and so to feed them,

they have to actually find their own
individual flock

in amongst this lot.

Just imagine you having to find
your baby

in among a million other
close-packed babies.

It's just unbelievable.

But there's something even
smarter

about how these baby
bats are organised.

They're crammed in at over 4,000
bats per square metre,

and it's this incredible density
of animals that holds the key

to their success.

I've brought a thermal imaging
camera, which will show us

which parts of the cave are hot.

So, if I pan the thermal image
camera across the roof,

you can see it's not very hot.

And then when I get
to the roof - look at that.

Red is hot and white is even hotter.

These young bats are in a roost
that is probably 20 degrees hotter

than the rest of the cave,

which means that they're able to
grow at a phenomenal rate,

making it the perfect incubator.

The whole secret to this
animal's success

is the fact that it is part
of a swarm.

On their own, they wouldn't survive.

Not only have the bats created
a hostile environment

that few others can survive in,

their real genius is that,

by filling it with millions
of individuals,

they've even managed to change
conditions in the cave

to suit the swarm.

Well, this may be the perfect
environment for rearing bats,

but it's definitely not the perfect
environment for humans.

If I stay here much longer
it could be fatal, so I'm off.

By now, the bats have started
their nightly exodus to feed.

And for me, the only way back
out of the cave is right through

the eye of this gigantic bat swarm.

There's literally tens, if not
hundreds of thousands of them.

I've never seen so many bats in one
place in my life.

This is the start of
a ten million-strong swarm

that will fly around and out of this
cave for the next three hours.

Surprisingly, these Mexican
free-tailed bats are not very agile.

They're not particularly
manoeuvrable bats.

Being tucked down in these caves

means the bats have to commute
long distances for food,

so these are the speedy
cross-country fliers

of the bat world.

Having to deal with some obstacles
is a bit more of a challenge.

Argh! Argh!

Oh! Oh!

This getting quite intense now.

Argh!

That was on my beard, that one.

I'm being absolutely
bombarded by bats.

This would be some people's
worst nightmare

but, for me, to be inside in the eye
of this swarm of bats

is just
the most thrilling experience.

These bats are the ultimate
opportunists.

By swarming together,

they're able to make their home
where most others would fail...

..proving there are huge benefits
to life in the swarm.

Swarms are a way for animals

to become significantly
more successful.

In working together,
swarms become cleverer, stronger,

and individuals are much more
likely to survive.

Understanding swarms is now also
shedding light on how humans behave.

With urban populations expected to
double in the next 40 years...

..we're all going to have to get
used to living amongst the crowd.

Unlocking the secrets of the swarm
could become critical.

Subtitles By Red Bee Media Ltd