0:00:04 > 0:00:10Swarms are one of the greatest spectacles on earth.
0:00:10 > 0:00:15At times horrifying, and also a thing of wonder.
0:00:18 > 0:00:19As part of the swarm,
0:00:19 > 0:00:23the smallest of creatures can become a force of nature.
0:00:23 > 0:00:26I'm George McGavin, a zoologist and explorer
0:00:26 > 0:00:30and I'll be travelling the globe to get right into the heart
0:00:30 > 0:00:33of some of the world's most impressive swarms.
0:00:33 > 0:00:35HE LAUGHS
0:00:38 > 0:00:43I want to show you that far from being the ultimate nightmare,
0:00:43 > 0:00:47swarms are one of nature's most ingenious solutions.
0:00:47 > 0:00:48Ow!
0:00:59 > 0:01:01Swarms are extremely powerful.
0:01:07 > 0:01:08By joining together,
0:01:08 > 0:01:12even the simplest of creatures can achieve the impossible.
0:01:14 > 0:01:19And by understanding how swarms work,
0:01:19 > 0:01:23we're gaining some fascinating insight into our own lives.
0:01:32 > 0:01:37My journey begins in North America, in southern California.
0:01:45 > 0:01:49I am hitting the highway in pursuit of a swarm most sane people
0:01:49 > 0:01:51would go out of their way to avoid.
0:01:54 > 0:01:55At some point,
0:01:55 > 0:02:00this little insect's sting might have totally ruined your day.
0:02:00 > 0:02:03Whoo, damn!
0:02:03 > 0:02:06But in extreme cases, this is a swarm that can kill.
0:02:06 > 0:02:10I don't think you want to be outside right now. There is a swarm of bees.
0:02:12 > 0:02:13The honeybee.
0:02:19 > 0:02:21A terrifying swarm,
0:02:21 > 0:02:26but one that worldwide is worth a staggering 180 billion a year.
0:02:29 > 0:02:33Without them, over a third of all the food we eat wouldn't exist.
0:02:35 > 0:02:41And nowhere is the bee's pollinating handiwork more crucial than here.
0:02:41 > 0:02:42California.
0:02:46 > 0:02:50More fruit and vegetables are produced in this state
0:02:50 > 0:02:52than anywhere else in America.
0:02:52 > 0:02:56And bees are so important to the process that every year,
0:02:56 > 0:03:01farmers actually import almost 2 million hive-loads of them.
0:03:04 > 0:03:09This is the perfect place to understand the secret of the swarm
0:03:09 > 0:03:13that takes group intelligence to a completely new level.
0:03:19 > 0:03:21To understand how it works,
0:03:21 > 0:03:25I need to get right to the start of the swarm, to the moment a queen
0:03:25 > 0:03:30gathers a loyal team of workers around her and goes house hunting.
0:03:30 > 0:03:33Now, in this temporary swarming state,
0:03:33 > 0:03:35they are really non-aggressive
0:03:35 > 0:03:37because they're not protecting anything,
0:03:37 > 0:03:38they're not protecting young, or honey -
0:03:38 > 0:03:41they are simply protecting or shielding the queen
0:03:41 > 0:03:45that's in the heart of their swarm until a new home is found,
0:03:45 > 0:03:48and they are so relaxed non-aggressive
0:03:48 > 0:03:51that I can actually put my hand right into the swarm here
0:03:51 > 0:03:56and just jiggle off a little handful of bees.
0:03:57 > 0:04:00Now, there's no way I could do this with a normal hive.
0:04:00 > 0:04:04It's only possible when they're in this temporary swarming state.
0:04:04 > 0:04:08Now, if I can persuade the worker bees that I'm a queen bee,
0:04:08 > 0:04:10they should swarm around me.
0:04:12 > 0:04:16It's only going to be possible because the swarm sticks together
0:04:16 > 0:04:20by following chemical signals called pheromones.
0:04:20 > 0:04:23Looks have nothing to do with it.
0:04:23 > 0:04:28It's an attraction so powerful that the bees should be prepared
0:04:28 > 0:04:31to ignore the obvious and accept me as their queen.
0:04:31 > 0:04:33To put it to the test,
0:04:33 > 0:04:38honey bee expert Norman Gary is giving me a queen bee makeover.
0:04:38 > 0:04:44I'm going to put droplets of a pheromone mix on your clothing.
0:04:44 > 0:04:48That pheromone is the odour that the queen bee emits
0:04:48 > 0:04:51that makes the other workers swarm around it.
0:04:51 > 0:04:56These are all females, did you know that? The females are now loving you.
0:04:56 > 0:04:57Yeah.
0:04:57 > 0:05:00- Have fun, George!- Hah!
0:05:01 > 0:05:04'There's no backing out now.
0:05:04 > 0:05:08'My only defence is a bit of insect repellent on my face.'
0:05:08 > 0:05:10I'm gonna start low, George,
0:05:10 > 0:05:13because they tend to migrate up, so here we go.
0:05:14 > 0:05:16There you go.
0:05:20 > 0:05:23'At this stage, it's hard to tell
0:05:23 > 0:05:25'whether the bees are going to stick around.
0:05:25 > 0:05:29'And if I'm honest, right now, I'm not sure I want them to.'
0:05:29 > 0:05:33This is not funny, because...
0:05:36 > 0:05:39It's in my eye, it's in my eye! Argh!
0:05:42 > 0:05:44Just relax.
0:05:44 > 0:05:48I am relaxed. I'm so chilled, I can't tell you.
0:05:51 > 0:05:53It looks like the workers have been fooled.
0:05:53 > 0:05:57I'm really beginning to get a sense of what it must feel like to
0:05:57 > 0:06:03be the queen bee in the centre of a massive swarm of bees.
0:06:03 > 0:06:07And being part of the swarm is starting to feel even weirder
0:06:07 > 0:06:12than it looks. I can now feel the weight of the bees on my chest.
0:06:12 > 0:06:15There's a fair few of them and collectively,
0:06:15 > 0:06:20they're beginning to feel quite heavy, like wearing a wet shirt.
0:06:20 > 0:06:22The other thing that's quite obvious is the heat.
0:06:22 > 0:06:25Because I'm covered in a layer of active bees
0:06:25 > 0:06:28that are vibrating their wings, they are generating
0:06:28 > 0:06:32quite a bit of heat and that's being transmitted to me through my shirt.
0:06:32 > 0:06:35I can really feel like I'm wearing a sort of woollen jumper.
0:06:38 > 0:06:41'It's incredibly unsettling.
0:06:41 > 0:06:45'I'm only safe from attack because this swarm is in
0:06:45 > 0:06:49'this temporary state, but that needs to change.
0:06:49 > 0:06:53'The bees face a critical decision on where to set up
0:06:53 > 0:06:55'their permanent home.
0:06:55 > 0:06:58'It's vital they get it right.
0:06:58 > 0:07:02'Get it wrong, and the swarm will die.
0:07:02 > 0:07:08'So how do 40,000 individuals reach a unanimous decision?'
0:07:08 > 0:07:11Although the queen is central to the formation of the swarm,
0:07:11 > 0:07:14it's not her that takes any part in the decision
0:07:14 > 0:07:17about where the swarm will end up, where they'll have their new home.
0:07:17 > 0:07:20But in order to show you how that works,
0:07:20 > 0:07:23I'm going to have to move this swarm off me.
0:07:27 > 0:07:31Wonderful. Perfect, perfect.
0:07:32 > 0:07:35With the swarm now airborne,
0:07:35 > 0:07:38they don't have long to make a decision
0:07:38 > 0:07:42and to see how they do it, we've set up an experiment.
0:07:42 > 0:07:46I've given the bees a choice of three new hives,
0:07:46 > 0:07:48only one of which is suitable.
0:07:48 > 0:07:53Hive A has a small enough entrance to keep out any predators,
0:07:53 > 0:07:57but it's not big enough for the swarm to grow.
0:07:57 > 0:08:01Hive B is larger, so there's room for the bees to expand,
0:08:01 > 0:08:04but the entrance is too big to protect them.
0:08:04 > 0:08:07It all rests on hive C.
0:08:07 > 0:08:11The one they should really go for is this one over here.
0:08:11 > 0:08:14It's the right size, it's got room for growth,
0:08:14 > 0:08:16the entrance hole is also the right size,
0:08:16 > 0:08:22so this, if I was a bee, this is the one I'd be going for.
0:08:22 > 0:08:26The question is, will the bees choose the right one?
0:08:26 > 0:08:32The first thing the swarm does is send out the scouts.
0:08:32 > 0:08:36It's their job to search for a new home, inspect the premises,
0:08:36 > 0:08:40and report back.
0:08:40 > 0:08:44They're looking for a nice, roomy hive, safe from predators
0:08:44 > 0:08:50and south-facing, to make the most of the early morning sun.
0:08:50 > 0:08:54And after a thorough investigation of all three hives,
0:08:54 > 0:08:57it's time to head back to base.
0:08:59 > 0:09:03What's really amazing about all this is the way that the scout bees
0:09:03 > 0:09:07communicate that vital information back to the swarm.
0:09:13 > 0:09:15It's all about the waggle dance.
0:09:22 > 0:09:24With their best dance moves,
0:09:24 > 0:09:27the scouts tell other bees which direction to go.
0:09:27 > 0:09:30The length of the waggling gives them a distance.
0:09:30 > 0:09:36One second of waggle equals half a mile of flying. But that's not all.
0:09:36 > 0:09:39The energy that they perform that dance
0:09:39 > 0:09:43indicates the quality of the house. Look at this one here.
0:09:43 > 0:09:47She is really going for it. This is a really high-energy waggle dance.
0:09:47 > 0:09:50Lots of enthusiasm. She's doing it over and over again.
0:09:50 > 0:09:53And that will tell the rest of the swarm
0:09:53 > 0:09:58that what she's found could be the perfect home.
0:09:58 > 0:10:00We used to think that the decision
0:10:00 > 0:10:03was all down to the bees' waggle dance.
0:10:03 > 0:10:09When enough bees dance for the same hive, a decision was made.
0:10:09 > 0:10:12But recently, scientists realised there was something much
0:10:12 > 0:10:15more interesting happening on the dance floor.
0:10:19 > 0:10:23Some of the dancing bees were on the receiving end
0:10:23 > 0:10:27of a sneaky head-butt, a signal to stop dancing.
0:10:27 > 0:10:31So it's actually a combination of enthusiasm and bullying
0:10:31 > 0:10:37that helps the bees agree and prevents a decision deadlock.
0:10:37 > 0:10:40So what does this swarm make of the three hives?
0:10:42 > 0:10:44Jeez! They're everywhere.
0:10:44 > 0:10:49Well, those 40,000 bees have now taken to the air,
0:10:49 > 0:10:52absolutely full of...the noise is incredible
0:10:52 > 0:10:56and they're all heading to that hive over there
0:10:56 > 0:10:57and so far, not one sting.
0:10:57 > 0:10:59Ooh! Ow!
0:10:59 > 0:11:04Something just got me! Sorry, correction - one sting!
0:11:04 > 0:11:07And while two of the hives are still empty,
0:11:07 > 0:11:13hive C is proving popular and the bees are moving in en masse.
0:11:13 > 0:11:16They've made the decision collectively
0:11:16 > 0:11:19and they've picked this hive. This was the one I thought they'd pick.
0:11:19 > 0:11:22It's absolutely right, is the right size.
0:11:22 > 0:11:24It's the perfect angle, the perfect everything.
0:11:27 > 0:11:29And it's their group intelligence,
0:11:29 > 0:11:33the ability to use the power of the swarm to make the right
0:11:33 > 0:11:37decision that makes honeybees the ultimate team...
0:11:40 > 0:11:44Because it's a fact that hundreds of individuals
0:11:44 > 0:11:48make a better decision together than a single expert.
0:11:49 > 0:11:54And that's something both animal swarms and humans have in common.
0:11:59 > 0:12:04It's a bizarre phenomenon first noticed back in 1906
0:12:04 > 0:12:07at a county show much like this one.
0:12:08 > 0:12:10Ha ha!
0:12:15 > 0:12:21Scientists were amazed by what happens during a simple competition
0:12:21 > 0:12:24asking people to guess the weight of a cow.
0:12:24 > 0:12:27When they analysed the results, not only was the average
0:12:27 > 0:12:30of everyone's guesses more accurate than the winning guess,
0:12:30 > 0:12:35it was also a much better guess than that of the experts who took part.
0:12:35 > 0:12:38So today, we're going to put it to the test
0:12:38 > 0:12:39with the help of Zinny here.
0:12:43 > 0:12:46We asked a selection of people at the Royal Bath Show
0:12:46 > 0:12:48to guess Zinny's weight.
0:12:48 > 0:12:53How accurate will they be as a group and can they beat one of the pros?
0:12:53 > 0:12:55Right, well, the results are in
0:12:55 > 0:13:00and all I've got to do now is to add up all the estimates
0:13:00 > 0:13:04from the crowd, get a total,
0:13:04 > 0:13:08divide that by the number of people who guessed...
0:13:09 > 0:13:12The answer we get is 588 kilograms.
0:13:12 > 0:13:18Now, a livestock expert guessed 584, so they're very close.
0:13:20 > 0:13:21But who's closest?
0:13:24 > 0:13:27It's time for the cow to reveal all.
0:13:27 > 0:13:30Haven't got her back legs in yet! Haven't got her back legs in.
0:13:34 > 0:13:38595 kilograms. Incredible!
0:13:38 > 0:13:43That's just seven kilograms higher than the group's estimate.
0:13:43 > 0:13:47The crowd got it just about spot-on, and they beat the experts.
0:13:48 > 0:13:52It seems many brains really are better than one,
0:13:52 > 0:13:57and that's due to something known as The Wisdom of the Crowd.
0:13:57 > 0:14:00Our highest guess was 1,400 kg,
0:14:00 > 0:14:03and our lowest 200 kg.
0:14:03 > 0:14:06So, both way off,
0:14:06 > 0:14:09but when a crowd works together the odd crazy guess
0:14:09 > 0:14:11doesn't really matter.
0:14:13 > 0:14:16And that works for nature, too.
0:14:16 > 0:14:18One animal might make a bad decision,
0:14:18 > 0:14:22but unless others have the same reaction, the swarm isn't fooled.
0:14:24 > 0:14:27That's why taking the option to ask the audience in a game show
0:14:27 > 0:14:29is such a smart move.
0:14:29 > 0:14:32They get it right about 90% of the time,
0:14:32 > 0:14:37compared with a 65% success rate from the experts.
0:14:38 > 0:14:41It's no wonder swarming has helped some creatures become
0:14:41 > 0:14:43the ultimate decision-makers.
0:14:45 > 0:14:49But not all swarms are about boosting brainpower.
0:14:49 > 0:14:53Some are about working together to solve a problem.
0:14:57 > 0:15:01The next stop on my journey is Christmas Islands,
0:15:01 > 0:15:04a tiny volcanic outcrop in the Indian Ocean.
0:15:08 > 0:15:11This dramatic, otherworldly landscape
0:15:11 > 0:15:15is the setting for an extraordinary swarm on the move.
0:15:17 > 0:15:21And right now is the best time to see it in action.
0:15:24 > 0:15:27Oh, my God, that is so...
0:15:28 > 0:15:31The island is about to go into lockdown.
0:15:35 > 0:15:37Every year as the monsoon rains arrive,
0:15:37 > 0:15:4260 million red crabs emerge out of the trees.
0:15:49 > 0:15:52A swarm facing the ultimate challenge.
0:16:00 > 0:16:03These land crabs live in a forest.
0:16:04 > 0:16:06But like their coastal ancestors,
0:16:06 > 0:16:10they're still completely dependent on the sea to breed.
0:16:13 > 0:16:16And between the forest and the beach
0:16:16 > 0:16:19lies a gruelling six-mile treck.
0:16:23 > 0:16:26This one is a female.
0:16:26 > 0:16:28You can see underneath...
0:16:30 > 0:16:32Gosh, she's really skittish.
0:16:32 > 0:16:34If I hold her very carefully you can see underneath
0:16:34 > 0:16:37that brown mass at the end of her abdomen under her tail,
0:16:37 > 0:16:40is something like 100,000 eggs.
0:16:40 > 0:16:45The reason that the females are a little more difficult to handle,
0:16:45 > 0:16:48they're much more flighty, is because all they've got
0:16:48 > 0:16:51on their mind just now is to get these eggs...
0:16:51 > 0:16:55Ow! ..into the sea as fast as they can.
0:16:55 > 0:16:58So that's what she has to do now, so I'm just going to let her...
0:16:58 > 0:17:00Oh, if I can get...
0:17:00 > 0:17:02..get on her way.
0:17:02 > 0:17:04She knows exactly which way to go.
0:17:06 > 0:17:09The Christmas Islands crabs follow the same well-trodden route
0:17:09 > 0:17:11for generations.
0:17:17 > 0:17:19But the arrival of humans on the island
0:17:19 > 0:17:22has given them a few extra obstacles to deal with.
0:17:28 > 0:17:32But nothing can get in the way of their epic journey.
0:17:37 > 0:17:41In just over a week, the crabs will cover up to six miles.
0:17:43 > 0:17:47For a creature of this size that spends most of its life underground,
0:17:47 > 0:17:50that's like running several back-to-back marathons
0:17:50 > 0:17:52with no training.
0:17:56 > 0:17:58So how do they do it?
0:18:04 > 0:18:07Scientists have recently discovered
0:18:07 > 0:18:10it's all thanks to a special internal sugar reserve.
0:18:20 > 0:18:23Powered by a massive sugar rush once a year,
0:18:23 > 0:18:25these laidback forest creatures
0:18:25 > 0:18:28suddenly turn into long-distance athletes.
0:18:34 > 0:18:38Oh, my goodness! Look at this! This is...
0:18:40 > 0:18:43This is unbelievable.
0:18:43 > 0:18:45There's just crabs as far as the eye can see.
0:18:45 > 0:18:48Every inch of this rock is covered.
0:18:50 > 0:18:55Amazingly, after days of walking and dodging obstacles,
0:18:55 > 0:18:59tens of millions of crabs have survived and made it to the beach.
0:19:01 > 0:19:04Just in time for their next big challenge.
0:19:04 > 0:19:05Tonight's the night.
0:19:05 > 0:19:09They've got to shed their eggs on a pre-dawn high tide,
0:19:09 > 0:19:12when the difference between high and low tide is at the smallest,
0:19:12 > 0:19:16because if they fall in the sea, they drown, unbelievably.
0:19:16 > 0:19:18Being land crabs, they can't survive in the sea.
0:19:18 > 0:19:20But they've got to get their eggs in the sea.
0:19:23 > 0:19:26They all have just one tiny window of opportunity.
0:19:32 > 0:19:34And according to my calculations,
0:19:34 > 0:19:37that should be in about six hours' time.
0:19:39 > 0:19:43So, I'm taking my place with the crabs and waiting for high tide.
0:19:56 > 0:20:00It's 3am, and all around me female crabs have just started
0:20:00 > 0:20:03frantically scrambling to the edge of the surf.
0:20:11 > 0:20:12Well, this is it.
0:20:12 > 0:20:14The annual mass spawning has started,
0:20:14 > 0:20:18and it is just one of the most incredible things I've ever seen.
0:20:19 > 0:20:24It's easy to forget how treacherous this moment is for the crabs.
0:20:24 > 0:20:27Many of them will get washed away and drowned.
0:20:30 > 0:20:33But it's a risk they have to take.
0:20:33 > 0:20:36The crabs are actually spawning all around me.
0:20:36 > 0:20:38Every time they spawn, they put their claws up
0:20:38 > 0:20:40and they shake themselves,
0:20:40 > 0:20:45and as they do that they shed 100,000 eggs each.
0:20:48 > 0:20:52So many trillions of eggs have been shed into the Indian Ocean
0:20:52 > 0:20:54that it's turned into soup.
0:20:56 > 0:21:02That is just full of now freshly-hatched crab eggs.
0:21:05 > 0:21:09The minute they hit the seawater, the eggs hatch into tiny larvae.
0:21:10 > 0:21:13But now they're at the mercy of tides and currents.
0:21:15 > 0:21:19Most will end up as fish food or get swept away into deeper waters,
0:21:19 > 0:21:21never to be seen again.
0:21:22 > 0:21:26Which is why this swarm is so vital.
0:21:26 > 0:21:29To ensure the survival of just a few crabs,
0:21:29 > 0:21:33nature has to throw a lot of zeros at the problem.
0:21:33 > 0:21:36Over the next three days, something like three trillion
0:21:36 > 0:21:42individual crab larvae will be released into the Indian Ocean.
0:21:45 > 0:21:49But despite the incredible numbers, the baby crabs will only make it
0:21:49 > 0:21:53back to shore every six or seven years.
0:21:54 > 0:21:57And when they do, the scenes are spectacular.
0:22:01 > 0:22:04A super-swarm of tiny crabs defies the odds
0:22:04 > 0:22:07and climbs back out of the ocean.
0:22:09 > 0:22:12For a land crab trapped in a forest,
0:22:12 > 0:22:15this swarm has the ultimate survival strategy.
0:22:18 > 0:22:19And across the globe,
0:22:19 > 0:22:24there are similarly amazing sights as other swarms set off on the move.
0:22:26 > 0:22:29Individually, each animal has no idea which way they're heading.
0:22:31 > 0:22:35But as a group, somehow they all move in the same direction.
0:22:37 > 0:22:41But how do you get tens of millions of individuals
0:22:41 > 0:22:43to work together as a team?
0:22:45 > 0:22:48Well, strangely, it's all thanks to having nobody in charge.
0:22:51 > 0:22:53It might sound like a recipe for disaster,
0:22:53 > 0:22:56but if you look at a colony of leafcutter ants,
0:22:56 > 0:22:58everyone is doing their own thing.
0:23:01 > 0:23:06No single ant is in charge of organisation, not even the queen.
0:23:09 > 0:23:11But with every ant ignoring the bigger picture
0:23:11 > 0:23:13and focusing on the one job,
0:23:13 > 0:23:17the process actually becomes highly efficient.
0:23:20 > 0:23:23So, by thinking like ants, we're now changing the way
0:23:23 > 0:23:27we look at some of our own logistics.
0:23:28 > 0:23:32Every day, millions of us travel through the world's transport hubs.
0:23:34 > 0:23:37Getting from A to B by the most efficient route is vital
0:23:37 > 0:23:40to keeping things running smoothly.
0:23:41 > 0:23:46Something the ants do really well. So, how would they run an airport?
0:23:47 > 0:23:51By designing software capable of thinking like a swarm,
0:23:51 > 0:23:52we've been finding out.
0:23:54 > 0:23:57An American airline tried to solve a long-running debate -
0:23:57 > 0:24:02was it faster to board a plane by giving passengers allocated seating
0:24:02 > 0:24:05or by allowing them to pick their own seats?
0:24:05 > 0:24:06Surprisingly,
0:24:06 > 0:24:11when the computer programme used digital ants to fill the plane,
0:24:11 > 0:24:14it showed that sometimes, letting people choose their own seats
0:24:14 > 0:24:17is quicker than giving them seat numbers.
0:24:20 > 0:24:23Because when there's no top dog to make decisions for us,
0:24:23 > 0:24:26like the ants, we all have to think for ourselves,
0:24:26 > 0:24:29and it doesn't result in the chaos you'd expect.
0:24:34 > 0:24:37And new technology is taking this even further,
0:24:37 > 0:24:40with robots that use swarm intelligence.
0:24:43 > 0:24:47Just like insects, these robots all do their own thing.
0:24:52 > 0:24:56By reacting to each other, they can combine to do things
0:24:56 > 0:24:58that would be impossible on their own.
0:25:01 > 0:25:04The hope is that one day these robots could be sent out
0:25:04 > 0:25:07into some of our most dangerous locations.
0:25:07 > 0:25:10Artificial swarms working together on the front line,
0:25:10 > 0:25:13or on search and rescue missions,
0:25:13 > 0:25:16replacing humans and potentially saving lives.
0:25:18 > 0:25:21But while some swarms work together day after day,
0:25:23 > 0:25:26other swarms exist for only a few hours.
0:25:29 > 0:25:31Coordination is critical.
0:25:31 > 0:25:34Get it wrong, and your whole life has been wasted.
0:25:36 > 0:25:40This is a swarm that's seen on just a few nights of the year.
0:25:42 > 0:25:45- Oh, my God! - It's a blackout.
0:25:45 > 0:25:46GIRL SCREAMS
0:25:49 > 0:25:52Yes. They are alive.
0:25:55 > 0:25:57To be in with a chance of finding this swarm,
0:25:57 > 0:26:00I'm travelling to the States, to Wisconsin.
0:26:05 > 0:26:09I'm chasing a massive swarm that appears from nowhere,
0:26:09 > 0:26:13so this part of my journey will be a race against time.
0:26:15 > 0:26:17To get a sense of the challenge ahead,
0:26:17 > 0:26:20I've come to the local weather centre in La Crosse,
0:26:20 > 0:26:25where they've been keeping a close eye on the swarm's past movements.
0:26:25 > 0:26:29That is just incredible! It's like an explosion.
0:26:29 > 0:26:31Right, this was just after sundown.
0:26:31 > 0:26:35Some biological target is coming out of the river
0:26:35 > 0:26:38and then being carried by the wind away from the Mississippi River.
0:26:38 > 0:26:41If I could see that, if that happened again,
0:26:41 > 0:26:43or even anything remotely like that...
0:26:43 > 0:26:46I would be very, very happy.
0:26:48 > 0:26:53So, what swarm could be so intense that it shows up on weather radars?
0:26:59 > 0:27:02Well, for just a few days every summer, amazingly, millions,
0:27:02 > 0:27:08some say trillions of mayflies take to the sky to form dense clouds.
0:27:10 > 0:27:14It's a blizzard of insects thick enough to stop traffic.
0:27:16 > 0:27:20But despite the massive size of this swarm, it's unpredictable,
0:27:20 > 0:27:22so finding it won't be easy.
0:27:25 > 0:27:28Time for me to take to the airways.
0:27:28 > 0:27:31Well, I'm in the right place and it's more or less the right time.
0:27:31 > 0:27:33But if I'm going to see a really big swarm,
0:27:33 > 0:27:37I've got an idea of how I can enlist some local help.
0:27:38 > 0:27:43Downtown La Crosse. It's now 14.10. WIZM.
0:27:45 > 0:27:47I'm George McGavin and we're from the BBC.
0:27:47 > 0:27:50RADIO DJ: Swarms of mayflies have attracted the attention
0:27:50 > 0:27:54of famed entomologist and TV host George McGavin.
0:27:54 > 0:27:57For a lot of people around here it's going to be surprising
0:27:57 > 0:27:59to hear that you want to be in the swarm. When we see
0:27:59 > 0:28:01those swarms we want to get in our car, get inside
0:28:01 > 0:28:03- and get away from it. - Why? It's fantastic.
0:28:03 > 0:28:07For an insect to be able to halt a train or a car, you know,
0:28:07 > 0:28:09that's pretty special.
0:28:09 > 0:28:11That's something I really want to see.
0:28:14 > 0:28:15We just need mayflies, lots of them.
0:28:15 > 0:28:19- And they can call any time, day or night?- Absolutely. We'll be there.
0:28:19 > 0:28:21We'll be hot on the heels of the swarm.
0:28:30 > 0:28:34I just hope we get the phone calls. I just hope we get folks ringing.
0:28:34 > 0:28:36We've done what we can,
0:28:36 > 0:28:40but it's impossible to predict where the first sighting will come from.
0:28:41 > 0:28:43One thing we do know is,
0:28:43 > 0:28:47they all start off in the same place - the Mississippi River.
0:28:49 > 0:28:53Mayflies spend most of their lives hidden underwater.
0:28:54 > 0:28:55So by looking in the river,
0:28:55 > 0:28:59I should be able to get right to the source of this swarm.
0:29:00 > 0:29:03So now, hopefully, in here
0:29:03 > 0:29:06are one or two or more mayfly nymphs.
0:29:09 > 0:29:13In a good year, a grab of that size might contain 15 nymphs,
0:29:13 > 0:29:15and if you think of that, it's a very small area
0:29:15 > 0:29:17and if you multiply it by this vast area,
0:29:17 > 0:29:22that's the sort of volumes of animals that we're hunting for.
0:29:22 > 0:29:23Oh, here, we got one.
0:29:25 > 0:29:26So, that's what we're after.
0:29:26 > 0:29:27This is a mayfly nymph,
0:29:27 > 0:29:31and they're perfectly adapted for existing in silt.
0:29:31 > 0:29:34They're very streamlined. They've got two tusks
0:29:34 > 0:29:36at the front of the head for excavating through the mud.
0:29:36 > 0:29:39They've got these really strong front legs, as well,
0:29:39 > 0:29:42which have little sort of prongs on them.
0:29:42 > 0:29:44So they are the perfect silt inhabitants,
0:29:44 > 0:29:48and they stay there for perhaps a year, up to two years...
0:29:49 > 0:29:53Until all conditions are right to trigger a mass emergence.
0:29:55 > 0:29:58It's thought water temperature has a lot to do with it,
0:29:58 > 0:30:01but lots of other factors like weather and food supply
0:30:01 > 0:30:03also play a role,
0:30:03 > 0:30:09making it frustratingly difficult to predict what's going on down there.
0:30:09 > 0:30:11Unless you're a mayfly, that is.
0:30:17 > 0:30:21With just 24 hours to emerge, find a mate, breed
0:30:21 > 0:30:23and get back to the river,
0:30:23 > 0:30:26it's a race against time for survival.
0:30:27 > 0:30:28Miss it by just a few hours,
0:30:28 > 0:30:32and you'll miss your one and only chance to find a mate.
0:30:36 > 0:30:38And I'm racing against the clock, too.
0:30:40 > 0:30:44Time to get back out on the road and keep looking.
0:30:46 > 0:30:50A swarm of mayflies could form anywhere along the river.
0:30:51 > 0:30:53But towns and roads are a good place to start looking
0:30:53 > 0:30:57because there's one thing they just can't seem to resist.
0:30:58 > 0:30:59Lights.
0:31:01 > 0:31:02The brighter, the better.
0:31:06 > 0:31:10So our shiny new modern lights play havoc with them,
0:31:10 > 0:31:13and bring chaos to our towns.
0:31:16 > 0:31:20It could be that the lights are a giant mayfly singles bar.
0:31:22 > 0:31:24With just hours to breed,
0:31:24 > 0:31:28there's not much time to fly about and look for a mate.
0:31:29 > 0:31:31So, by all heading to the nearest bright light,
0:31:31 > 0:31:34you improve your chances of finding a partner.
0:31:37 > 0:31:39And just as I was about to give up hope
0:31:39 > 0:31:42of getting to see this incredible event in action,
0:31:42 > 0:31:44there's some promising news.
0:31:46 > 0:31:51I've just had a phone call from a boat yard up here.
0:31:51 > 0:31:54This looks like the marina,
0:31:54 > 0:31:56so I hope I'm in the right place.
0:31:57 > 0:31:59Oh! Oh-ho!
0:32:00 > 0:32:03Mayflies, flying. Look, there's one on the window.
0:32:03 > 0:32:05Whoa, this is more like it.
0:32:05 > 0:32:08Whoa! Look at this!
0:32:08 > 0:32:11'We've caught the very beginnings of a swarm.'
0:32:13 > 0:32:16These guys will have left the river late last night last night,
0:32:16 > 0:32:18looking for a landing spot
0:32:18 > 0:32:21where they can go through one final transformation.
0:32:23 > 0:32:24This is incredible.
0:32:24 > 0:32:29This is the emergence of an adult mayfly from the sub-adult,
0:32:29 > 0:32:34and this is a process that has been going on for 300 million years.
0:32:36 > 0:32:37This is incredible to watch.
0:32:38 > 0:32:41Now, it's still got to get the end of its abdomen out,
0:32:41 > 0:32:45and it's got to draw out those long tails.
0:32:46 > 0:32:50And this exact process is being repeated
0:32:50 > 0:32:52hundreds of millions of times.
0:33:04 > 0:33:05In a few hours' time,
0:33:05 > 0:33:09all the new adults will rise into the air in a huge swarm.
0:33:12 > 0:33:16The mayfly have spent their entire lives in the mud,
0:33:16 > 0:33:19waiting for this brief and all-important moment.
0:33:23 > 0:33:26By evening, the sky fills with millions of them,
0:33:26 > 0:33:29in an intense breeding frenzy.
0:33:32 > 0:33:36Being part of the swarm is a once in a lifetime event for the mayfly.
0:33:37 > 0:33:42But other insects have to deal with a whole life in amongst the swarm.
0:33:42 > 0:33:46But then working together efficiently, is vital.
0:33:50 > 0:33:54Ant colonies are completely dependent on keeping the supplies
0:33:54 > 0:33:55coming in thick and fast.
0:33:58 > 0:34:00With millions of hungry mouths to feed,
0:34:00 > 0:34:02any delay could be a disaster.
0:34:05 > 0:34:09So, they avoid a crisis by following a surprising highway code.
0:34:11 > 0:34:16To go as fast as possible, the ants have learned to slow down.
0:34:18 > 0:34:21That means no boy racers and no overtaking.
0:34:24 > 0:34:27And what works for the ants also works for us.
0:34:29 > 0:34:33Twice a day, our roads have to cope with millions of people
0:34:33 > 0:34:36all trying to be at the right place at the right time.
0:34:38 > 0:34:42Designing our transport networks to cope is a huge challenge.
0:34:44 > 0:34:46So, now, our road planners
0:34:46 > 0:34:49are starting to pick up a few tips from the ants.
0:34:56 > 0:35:00By restricting each and every car to around 50 miles an hour,
0:35:00 > 0:35:02actually makes the journey faster.
0:35:03 > 0:35:05And although it might not seem like it,
0:35:05 > 0:35:07reducing the number of lanes
0:35:07 > 0:35:11and banning overtaking makes things even more efficient.
0:35:14 > 0:35:17And now several car companies are taking this a step further
0:35:17 > 0:35:20by looking into technology that keeps cars
0:35:20 > 0:35:23close behind each other and at a constant speed...
0:35:24 > 0:35:26..just like the travelling ants.
0:35:28 > 0:35:30That is just weird.
0:35:30 > 0:35:34These super-smart cars have sensors that read their speed
0:35:34 > 0:35:37and distance from the vehicle in front.
0:35:37 > 0:35:39I'm not touching the brake at all.
0:35:39 > 0:35:43All you have to do is hold the wheel and, if that car in front stops,
0:35:43 > 0:35:46the brakes are applied automatically...
0:35:46 > 0:35:47bringing me to a halt.
0:35:50 > 0:35:53I have to say, it really makes you very nervous.
0:35:53 > 0:35:56It's a very unsettling feeling
0:35:56 > 0:35:59not to be in control of the car, as it were.
0:36:00 > 0:36:02Stopping here - oh.
0:36:02 > 0:36:05Oh. Oh.
0:36:05 > 0:36:07- Oh. - HE LAUGHS
0:36:07 > 0:36:09I didn't touch the brake.
0:36:11 > 0:36:13Oh, my goodness.
0:36:13 > 0:36:15These cars are so clever that even
0:36:15 > 0:36:18when the car in front brakes unexpectedly,
0:36:18 > 0:36:21the car has everything under control.
0:36:21 > 0:36:22Ah!
0:36:22 > 0:36:24Oh!
0:36:25 > 0:36:27Oh!
0:36:27 > 0:36:29Oh!
0:36:29 > 0:36:30Ah!
0:36:30 > 0:36:32Let me just get my breath back now.
0:36:36 > 0:36:38- HE SIGHS - That was unbelievable.
0:36:40 > 0:36:44I'm not sure I'm quite ready for a car with swarm powers
0:36:44 > 0:36:46but it's a genius idea.
0:36:47 > 0:36:51And with almost 40 million vehicles on UK roads,
0:36:51 > 0:36:55technology like this could be the answer to keeping traffic
0:36:55 > 0:36:57flowing with ant-like precision.
0:37:04 > 0:37:08But while some swarms are helping us overcome problems,
0:37:08 > 0:37:11others are just adding to them.
0:37:11 > 0:37:15In America, there's an unseen swarm that has silently invaded
0:37:15 > 0:37:17and is slowly taking over.
0:37:21 > 0:37:24In the last 40 years, it's spread into 23 states
0:37:24 > 0:37:26and yet remains largely hidden from view.
0:37:28 > 0:37:33The American government has spend over 150m trying to control it.
0:37:34 > 0:37:38It's a swarm that I have never experienced before
0:37:38 > 0:37:41and one that is not without considerable risk.
0:37:46 > 0:37:48For my chance to meet this swarm,
0:37:48 > 0:37:52I'm heading to Bath, Illinois, in the American Midwest.
0:37:55 > 0:37:57And it seems I'm not the only one
0:37:57 > 0:38:00here to meet this highly destructive swarm.
0:38:01 > 0:38:04There was a couple of guys that got black eyes that got hit in the face
0:38:04 > 0:38:07with no protection, and they're walking around here today.
0:38:07 > 0:38:08They got a big, old shiner.
0:38:08 > 0:38:11- That looks painful. - It was.
0:38:11 > 0:38:14This one was the first hit yesterday and one on this eye.
0:38:14 > 0:38:17- And you're back for more. - Back for more.- This is dangerous.
0:38:17 > 0:38:19Yeah. We've been coming eight years.
0:38:19 > 0:38:22This is the first time we've ever got hurt, though.
0:38:23 > 0:38:25People say they eat them but, as nasty as they look,
0:38:25 > 0:38:26I ain't touching them.
0:38:28 > 0:38:30Just what have I let myself in for?
0:38:36 > 0:38:40To find out, the Illinois Natural History Team is going to get me
0:38:40 > 0:38:43face-to-face with the problem.
0:38:43 > 0:38:46Today, we're going to use a technique called electrofishing,
0:38:46 > 0:38:50and we're going to put anywhere from 4,500 to 5,000 volts into the water.
0:38:58 > 0:39:00Well, this is the calm before the swarm.
0:39:00 > 0:39:02I have no idea what's going to happen.
0:39:10 > 0:39:13Wow!
0:39:13 > 0:39:14This is crazy!
0:39:17 > 0:39:19Argh!
0:39:19 > 0:39:21That is unbelievable.
0:39:21 > 0:39:23Oh!
0:39:23 > 0:39:26This is what I'm here for, the Asian silver carp,
0:39:26 > 0:39:31a fish with a unique panic reflex, causing it to leap out of the water.
0:39:32 > 0:39:35There are literally thousands of these carp.
0:39:35 > 0:39:36Arg!
0:39:36 > 0:39:38This is absolute insanity.
0:39:41 > 0:39:46Whether it's a predator, a boat or our electric current,
0:39:46 > 0:39:49a reflex response spreads through the swarm,
0:39:49 > 0:39:52setting off a chain reaction of leaping fish.
0:39:55 > 0:39:57It's a vital way of measuring
0:39:57 > 0:39:59just how many fish are lurking down there.
0:40:01 > 0:40:06At the moment, the best guess is 2,500 carp per mile of river
0:40:06 > 0:40:08but the numbers are still growing.
0:40:10 > 0:40:12And as carp numbers are going up,
0:40:12 > 0:40:14the amount of other wildlife is going down.
0:40:17 > 0:40:20I've never seen so much fish in my life.
0:40:20 > 0:40:23Oh!
0:40:23 > 0:40:27This is a swarm out of control and ecologically dangerous.
0:40:30 > 0:40:33Getting rid of it isn't going to be easy, but the battle is on.
0:40:35 > 0:40:38The big concern is that the carp are only 50 miles away
0:40:38 > 0:40:41from the great lakes of America.
0:40:42 > 0:40:44If they continue to spread in that direction,
0:40:44 > 0:40:47they could devastate a 4bn fishing industry.
0:40:49 > 0:40:54The US government have been spending 150m in the last two years
0:40:54 > 0:40:56just to control this species.
0:40:56 > 0:40:57Everybody all right?
0:40:57 > 0:40:59I got one on the backside.
0:41:01 > 0:41:03Unbelievable.
0:41:07 > 0:41:12So far, they've tried everything from poison to electric barriers,
0:41:12 > 0:41:14but nothing seems to be able to stop the carp.
0:41:20 > 0:41:23So, the locals have decided to take matters into their own hands.
0:41:28 > 0:41:31This fishing tournament is their chance to fight back.
0:41:31 > 0:41:34We're now in the middle of this fishing chaos.
0:41:34 > 0:41:38There's about 50 boats here. There's some more boats coming towards us.
0:41:40 > 0:41:43Whoa! That hit him right on the head, that.
0:41:44 > 0:41:47Carnage!
0:41:47 > 0:41:48Oh, jeez!
0:41:49 > 0:41:50HE LAUGHS
0:41:50 > 0:41:52I'm hanging on.
0:41:52 > 0:41:55But why have the carp got out of control?
0:41:57 > 0:41:58Ooh!
0:42:00 > 0:42:04They eat algae and because they eat so much of it,
0:42:04 > 0:42:06they are basically eating all of the food
0:42:06 > 0:42:08that the fish here would normally eat.
0:42:11 > 0:42:14The carp can eat up to a third of their body weight a day.
0:42:16 > 0:42:18Their ferocious appetite has pretty much emptied
0:42:18 > 0:42:20the river of food for anything else.
0:42:24 > 0:42:27This swarm is taking over.
0:42:27 > 0:42:31And the trouble with all this is, it's a purely man-made problem.
0:42:31 > 0:42:33These fish were introduced 40 years ago to clean
0:42:33 > 0:42:36the algae off commercial catfish ponds.
0:42:39 > 0:42:41When they escaped into nearby rivers,
0:42:41 > 0:42:44the carp found themselves in the river full of food
0:42:44 > 0:42:46and nothing to stand in their way.
0:42:48 > 0:42:52With each carp able to produce up to two million eggs a year,
0:42:52 > 0:42:55it didn't take long for the fish to become a swarm.
0:42:57 > 0:43:00Something that doesn't happen back in their native home, Asia,
0:43:00 > 0:43:03where predators keep their numbers in check.
0:43:04 > 0:43:09They've just exploded. This is a swarm of epic proportions.
0:43:10 > 0:43:12Look at the size of that one.
0:43:12 > 0:43:14That's got to be 30 pounds.
0:43:16 > 0:43:18If that hit you, it would break your jaw.
0:43:20 > 0:43:24PEOPLE SHOUT
0:43:32 > 0:43:35And although this may all look a bit unconventional,
0:43:35 > 0:43:39this event does play a small part in a serious attempt
0:43:39 > 0:43:40to control these fish.
0:43:46 > 0:43:47By the end of the event,
0:43:47 > 0:43:50over two tons of carp will have been cleared from the river.
0:43:52 > 0:43:54ALL CHEER
0:43:54 > 0:43:57And that's in addition to the hundreds of tons caught
0:43:57 > 0:44:00every year through commercial fishing,
0:44:00 > 0:44:03some of which is actually now exported back as food
0:44:03 > 0:44:04to the carp's original home, Asia.
0:44:07 > 0:44:10But all this is just keeping things in check.
0:44:10 > 0:44:14It's not enough to actually defeat the swarm and stop it spreading...
0:44:17 > 0:44:20Because a dangerous combination of unlimited food,
0:44:20 > 0:44:25lots of space and freedom from predators allows some animals
0:44:25 > 0:44:28to swarm to biblical proportions.
0:44:30 > 0:44:34And when swarms get to this scale, they also have a dark side.
0:44:37 > 0:44:39It doesn't take much to throw hundreds
0:44:39 > 0:44:41of carp into mass confusion.
0:44:43 > 0:44:47In seconds, fear passes from fish to fish...
0:44:47 > 0:44:50even though most of them won't know what they're afraid of.
0:44:54 > 0:44:58Because in a swarm, individuals are so closely connected
0:44:58 > 0:45:00that they share moods and reactions.
0:45:10 > 0:45:14In the event of danger, a quick, united response like this
0:45:14 > 0:45:16can mean the difference between life or death.
0:45:19 > 0:45:22But, occasionally, the system goes wrong.
0:45:31 > 0:45:34An event sends a contagious emotion through the swarm
0:45:34 > 0:45:35and causes complete chaos.
0:45:43 > 0:45:47And, unfortunately, that's something you see in humans, too.
0:45:49 > 0:45:51CHEERING
0:45:51 > 0:45:55When thousands of strangers come together for a shared experience,
0:45:55 > 0:45:58it can create a powerful connection.
0:46:01 > 0:46:04When everything's going well, it can be an incredible experience.
0:46:04 > 0:46:08But when the mood changes, things can get very nasty.
0:46:10 > 0:46:12It takes just one event to cause a crowd
0:46:12 > 0:46:14to stampede like wildebeest...
0:46:14 > 0:46:17out of control and with dangerous consequences.
0:46:17 > 0:46:20SIREN BLARES
0:46:20 > 0:46:24Anger spreads person to person till a peaceful demonstration
0:46:24 > 0:46:26becomes a full-blown mob.
0:46:27 > 0:46:30People stop thinking as individuals and begin to follow the herd.
0:46:31 > 0:46:36Emotions are high and behaviour starts to change.
0:46:36 > 0:46:40Within the group, people start losing their inhibitions.
0:46:40 > 0:46:44They do things they'd never consider doing if they were on their own.
0:46:45 > 0:46:48And because the group makes individuals feel powerful
0:46:48 > 0:46:50and less accountable,
0:46:50 > 0:46:54normally responsible people start acting completely out of character.
0:46:57 > 0:46:58So, finding yourself in a crowd
0:46:58 > 0:47:01can play a huge role in how you behave as an individual.
0:47:01 > 0:47:02SIREN BLARES
0:47:05 > 0:47:08Now, thankfully, most human gatherings pass without incident.
0:47:10 > 0:47:14But there is a swarm where big crowds are nearly always fatal.
0:47:18 > 0:47:22A plague of locusts is never a welcome sight for farmers.
0:47:24 > 0:47:28But when the numbers explode, it's also bad news for the swarm.
0:47:31 > 0:47:35At peak populations, the locusts suddenly start craving protein...
0:47:36 > 0:47:40..and the meat closest to hand is the insect ahead.
0:47:41 > 0:47:44To avoid being eaten alive by their nearest neighbours,
0:47:44 > 0:47:46the locusts need to keep moving.
0:47:48 > 0:47:51This cannibalistic swarm is not just tearing through the crops,
0:47:51 > 0:47:54it's also at risk of devouring itself.
0:47:56 > 0:48:00But overcrowding doesn't always work against the swarm.
0:48:00 > 0:48:03The animal I'm off to see next has turned
0:48:03 > 0:48:05overcrowding into its greatest asset.
0:48:11 > 0:48:14This is a creature found in numbers so high,
0:48:14 > 0:48:16it brings people out into the streets.
0:48:18 > 0:48:20To join them, I'm heading south.
0:48:30 > 0:48:32I'm in Texas, home to the largest gathering
0:48:32 > 0:48:34of mammals anywhere on the planet.
0:48:36 > 0:48:39Every summer, as night falls in Austin, Texas,
0:48:39 > 0:48:42a seemingly endless swarm fills the air.
0:48:45 > 0:48:49Wow! Look at this. They're just like flooding out of the bridge right now.
0:48:55 > 0:48:57Wow! Look back into the distance, guys.
0:48:57 > 0:48:59It almost looks like a plume of smoke.
0:48:59 > 0:49:02The residents are outnumbered.
0:49:02 > 0:49:05Over a million creatures are swarming the skies.
0:49:15 > 0:49:17This bridge is the biggest urban roost
0:49:17 > 0:49:21for Mexican free-tailed bats anywhere in the world.
0:49:23 > 0:49:25Oh, look at this. Look at this.
0:49:25 > 0:49:26This has become a real spectacle.
0:49:30 > 0:49:32There's just hundreds of thousands of bats
0:49:32 > 0:49:35and they're streaming out from under the bridge.
0:49:35 > 0:49:37This is a colony of biblical proportions.
0:49:41 > 0:49:46So what do the bright lights of the city have to offer these bats?
0:49:46 > 0:49:47Like many riverside cities,
0:49:47 > 0:49:50there's a glut of insects in the summer months.
0:49:52 > 0:49:56But what really makes Austin a bat hotspot is its architecture.
0:49:58 > 0:50:01When Congress Bridge was rebuilt in the '80s,
0:50:01 > 0:50:04the new structure was full of deep, narrow openings.
0:50:06 > 0:50:11It was the ideal bat home and they moved in en masse,
0:50:11 > 0:50:14but this convenient city pad isn't without problems.
0:50:19 > 0:50:21Space is limited and the bats are an easy target
0:50:21 > 0:50:25for birds of prey and other predators.
0:50:53 > 0:50:56So, as impressive as this is,
0:50:56 > 0:51:01a three-hour drive away, there's a bat swarm ten times bigger.
0:51:06 > 0:51:10And a swarm of this size can achieve the seemingly impossible.
0:51:15 > 0:51:19But getting a good view of it isn't going to be easy.
0:51:22 > 0:51:24This area of Texas is riddled with caves,
0:51:24 > 0:51:26making it the perfect habitat for bats.
0:51:28 > 0:51:32The caves offer them protection from the elements and from aerial attack.
0:51:34 > 0:51:36But to find the real reason
0:51:36 > 0:51:39ten million bats flock to this particular cave...
0:51:40 > 0:51:43I need to get much closer to the swarm.
0:51:44 > 0:51:48This isn't going to be a pleasant journey.
0:51:48 > 0:51:51I'm now entering one of the most overcrowded
0:51:51 > 0:51:52and toxic places on Earth.
0:51:54 > 0:51:56So, I've come prepared.
0:52:00 > 0:52:02Wow. The smell.
0:52:02 > 0:52:06Ammonia is actually quite intense.
0:52:08 > 0:52:11The caves are piled high with bat droppings, guano,
0:52:11 > 0:52:15releasing dangerously high levels of poisonous gases.
0:52:18 > 0:52:22Levels of ammonia are so high that it bleaches the bats' fur.
0:52:23 > 0:52:28But, amazingly, the bats themselves aren't harmed.
0:52:28 > 0:52:29By slowing down their metabolism,
0:52:29 > 0:52:32their bodies are able to neutralise the toxic gas.
0:52:39 > 0:52:42This pile of guano is absolutely enormous -
0:52:42 > 0:52:45it must be metres thick.
0:52:45 > 0:52:49That roof is absolutely jam-packed with bats and look at them.
0:52:50 > 0:52:52It's every crevice.
0:52:52 > 0:52:54There's a big group of them right here.
0:52:57 > 0:53:01The levels of ammonia are now potentially fatal
0:53:01 > 0:53:05and, without these masks, we'd be unconscious in a matter of minutes.
0:53:06 > 0:53:10Very little can survive these lethal conditions...
0:53:10 > 0:53:12apart from the bats, that is.
0:53:14 > 0:53:17If you're wondering what a million bats look like, it looks like this.
0:53:17 > 0:53:21This is one of the most incredible sights I think I've ever seen.
0:53:27 > 0:53:29This is a giant bat creche.
0:53:31 > 0:53:34Every year, ten million pregnant female bats
0:53:34 > 0:53:37fly to these caves to give birth.
0:53:38 > 0:53:43It's more or less predator-free down here, thanks to the deadly fumes,
0:53:43 > 0:53:46so it's a safe place for mothers to leave their young
0:53:46 > 0:53:47while they go off and hunt.
0:53:48 > 0:53:51The really amazing thing about these bats is,
0:53:51 > 0:53:55the adult females roost in a different part of the cave.
0:53:55 > 0:53:57These are just the young and so to feed them,
0:53:57 > 0:54:01they have to actually find their own individual flock
0:54:01 > 0:54:03in amongst this lot.
0:54:03 > 0:54:05Just imagine you having to find your baby
0:54:05 > 0:54:08in among a million other close-packed babies.
0:54:08 > 0:54:09It's just unbelievable.
0:54:13 > 0:54:15But there's something even smarter
0:54:15 > 0:54:17about how these baby bats are organised.
0:54:20 > 0:54:24They're crammed in at over 4,000 bats per square metre,
0:54:25 > 0:54:28and it's this incredible density of animals that holds the key
0:54:28 > 0:54:30to their success.
0:54:30 > 0:54:33I've brought a thermal imaging camera, which will show us
0:54:33 > 0:54:36which parts of the cave are hot.
0:54:36 > 0:54:39So, if I pan the thermal image camera across the roof,
0:54:39 > 0:54:41you can see it's not very hot.
0:54:41 > 0:54:44And then when I get to the roof - look at that.
0:54:44 > 0:54:48Red is hot and white is even hotter.
0:54:48 > 0:54:53These young bats are in a roost that is probably 20 degrees hotter
0:54:53 > 0:54:54than the rest of the cave,
0:54:54 > 0:54:58which means that they're able to grow at a phenomenal rate,
0:54:58 > 0:55:00making it the perfect incubator.
0:55:01 > 0:55:04The whole secret to this animal's success
0:55:04 > 0:55:07is the fact that it is part of a swarm.
0:55:07 > 0:55:09On their own, they wouldn't survive.
0:55:11 > 0:55:15Not only have the bats created a hostile environment
0:55:15 > 0:55:17that few others can survive in,
0:55:17 > 0:55:19their real genius is that,
0:55:19 > 0:55:22by filling it with millions of individuals,
0:55:22 > 0:55:25they've even managed to change conditions in the cave
0:55:25 > 0:55:26to suit the swarm.
0:55:27 > 0:55:30Well, this may be the perfect environment for rearing bats,
0:55:30 > 0:55:34but it's definitely not the perfect environment for humans.
0:55:34 > 0:55:38If I stay here much longer it could be fatal, so I'm off.
0:55:42 > 0:55:46By now, the bats have started their nightly exodus to feed.
0:55:54 > 0:55:57And for me, the only way back out of the cave is right through
0:55:57 > 0:55:59the eye of this gigantic bat swarm.
0:56:02 > 0:56:06There's literally tens, if not hundreds of thousands of them.
0:56:06 > 0:56:10I've never seen so many bats in one place in my life.
0:56:10 > 0:56:15This is the start of a ten million-strong swarm
0:56:15 > 0:56:19that will fly around and out of this cave for the next three hours.
0:56:28 > 0:56:32Surprisingly, these Mexican free-tailed bats are not very agile.
0:56:32 > 0:56:35They're not particularly manoeuvrable bats.
0:56:38 > 0:56:40Being tucked down in these caves
0:56:40 > 0:56:43means the bats have to commute long distances for food,
0:56:43 > 0:56:46so these are the speedy cross-country fliers
0:56:46 > 0:56:47of the bat world.
0:56:50 > 0:56:53Having to deal with some obstacles is a bit more of a challenge.
0:56:53 > 0:56:55Argh! Argh!
0:56:55 > 0:56:58Oh! Oh!
0:57:01 > 0:57:02This getting quite intense now.
0:57:02 > 0:57:04Argh!
0:57:04 > 0:57:06That was on my beard, that one.
0:57:10 > 0:57:13I'm being absolutely bombarded by bats.
0:57:17 > 0:57:21This would be some people's worst nightmare
0:57:21 > 0:57:25but, for me, to be inside in the eye of this swarm of bats
0:57:25 > 0:57:27is just the most thrilling experience.
0:57:36 > 0:57:39These bats are the ultimate opportunists.
0:57:39 > 0:57:40By swarming together,
0:57:40 > 0:57:44they're able to make their home where most others would fail...
0:57:46 > 0:57:50..proving there are huge benefits to life in the swarm.
0:57:54 > 0:57:56Swarms are a way for animals
0:57:56 > 0:57:59to become significantly more successful.
0:58:00 > 0:58:04In working together, swarms become cleverer, stronger,
0:58:04 > 0:58:07and individuals are much more likely to survive.
0:58:10 > 0:58:14Understanding swarms is now also shedding light on how humans behave.
0:58:16 > 0:58:20With urban populations expected to double in the next 40 years...
0:58:21 > 0:58:25..we're all going to have to get used to living amongst the crowd.
0:58:28 > 0:58:32Unlocking the secrets of the swarm could become critical.
0:58:59 > 0:59:03Subtitles By Red Bee Media Ltd