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,
they're much more flighty, is because all they've got
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!
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.