Original subtitles

Narrator: How do a million high-speed bats avoid crashing into each other?

It's an absolute wonder to behold.

Narrator: What did this dolphin want with this diver?

I was blown away that it just swam right up to me.

Narrator: And how is one bug a secret mathematician?

It's always some weird number.

Narrator: Nature is awe-inspiring,

But sometimes it just doesn't make sense.

Man: I have never seen anything like this.

Our team of experts investigates the weirdest animal behavior...

That's amazing.

...And the most unexpected events...

What is causing that?

...Ever caught on camera.

My god!

These are...

-- Captions by vitac -- www.Vitac.Com

Captions paid for by discovery communications

In downtown austin, texas,

Crowds gather to watch a breathtaking nightly display.

This is one of the greatest natural phenomena

You can see anywhere on the planet.

The mexican free-tail bats come out from under the bridge

In austin, texas,

And fly out in these massive clouds

To go eat insects across the countryside.

Narrator: Many bat colonies number in the thousands,

But crammed under congress avenue bridge

Are over 1.5 million.

This is the biggest urban bat colony in the world.

Narrator: That means in summer, bats outnumber people in this city.

They're almost like a plume of smoke.

It's an absolute wonder to behold.

What's amazing about this spectacle

Is these millions of bats emerge,

Yet there's no congestion.

They don't even seem to collide with one another.

Narrator: So, how do they manage to fly out at the same time

Without flying into each other?

So, one possible explanation

Is that they're using their echolocation

To see where everyone is and avoid bumping into them.

Cooke: Well, we've all heard of echolocation.

It's the incredible system

Whereby bats can navigate in the dark.

They send out high-frequency sounds,

And then they listen for their reflection back.

Narrator: Is it possible the texas bridge bats are using echolocation

To minimize the number of deadly mid-air collisions?

It's really hard to imagine, with that many bats,

Millions of bats, all whirling around,

That they simply have this perfect system

Where the echolocation means they avoid one another.

You're shouting and listening to your echoes,

But everybody around you is also shouting as loud as they can.

So, how do you tell your echo from that bat's or that bat's

Or the one coming right at you?

So, something else is going on here.

Narrator: If echolocation is not the answer, what is?

Is there a clue in the way that british daubentons bats fly?

Scientists in the u.K.

Discovered that bats will choose a wing man, a leader to follow.

Narrator: Thermal imaging cameras revealed that these bats

Tend to forage in pairs

And seem to obey a simple set of traffic rules.

It goes left, you go left.

If it goes right, you go right.

And as long as everybody cooperates,

That should keep everyone safe.

Narrator: Could the bats in texas also be playing a giant game

Of follow the leader to avoid catastrophic collisions?

The problem with that theory, though,

Is that these bats are all changing position all the time,

And it's really not clear that

There's any particular leader in these clouds.

Narrator: So, how do they fly out from under this bridge

In their millions without getting injured?

Burke: Bat biologists are so intrigued by this behavior

That they film bats with ultra high-speed cameras

That slow the action right down.

♪

Narrator: When scientists analyzed the footage,

They were amazed to discover

That, actually, thousands of collisions do occur,

Many more than expected.

Yet incredibly, the bats carry on flying

Without missing a wing beat.

So it seems bats really do go bump in the night.

The question now is how are they able to recover

From mid-air collisions so quickly?

Could the answer lie in the unique structure of their wings?

Boerma: They're mammals just like we are,

And so their anatomy is really similar to ours, too.

And so their wings are made from an arm and a hand.

I can look at my arm and my hand,

And I can see the same joints in my body

That a bat has in their wings.

Not only that, but they actually have muscles

Embedded within the wing membrane

That can control the stiffness of the wing.

That affects the aerodynamics.

Narrator: A bat's flexible wings can change shape during flight

To a much greater extent than those of insects or birds.

And this may give them superior aerial agility.

The beauty of bat wings is that they can handle a bump.

They can deform.

They're made out of a flexible membrane,

So it's not like when two airplanes hit each other,

And they both crash to the ground.

If bats smack wings a little, they're okay.

Narrator: And that's why it's not raining bats every night

In austin, texas.

♪

January 2013.

Kona, hawaii.

Dive instructor keller laros

Is leading a busy nighttime scuba dive at garden eel cove.

There are 50 snorkelers and divers in the water

At this manta ray hotspot.

Many have underwater cameras

To film the wildlife they encounter.

We're sitting, and we're watching the manta rays

As they swoop back and forth

And feed on the plankton attracted to our light.

All of a sudden, I heard a really loud,

Piercing sonic burst.

[ animal squeaking ]

It sounded like a dolphin, and as I turned,

A bottlenose dolphin was right behind me.

[ dolphin squeaking ]

It was all alone.

Normally, they travel in pairs or threes or sixes.

Narrator: A bottlenose dolphin is a rare sight

At this shallow dive location.

At night, they usually head out to deeper waters to hunt.

But then, as keller watches the unexpected guest,

He spots something ominous.

I notice that there's a fishing line hanging out of his mouth

And was wrapped around his pectoral fin.

It was obviously in distress, and I sensed the urgency.

I'd seen fishing lines seriously cut and injure manta rays

And see them actually lose fins from fishing line.

And it's that important that you get it off.

Narrator: Using his dive knife, he starts to cut away the fishing line.

Dolphins are known for playing with humans,

But this animal could be in a lot of pain,

And most injured wild animals will avoid human contact.

So, what is this dolphin doing?

Is it possible that the dolphin panicked after it approached

Keller, and now it's scared stiff?

When an animal is caught and handled by a human

And then an animal is unable to get away.

Sort of almost goes into a -- almost like a trance-like state.

Sort of safety shutdown, I guess.

Essentially, the stress hormones pumping through their body

Are so overwhelming that, instead of trying to fight back,

They just freeze, and they go motionless.

Narrator: But keller's dolphin encounter is different.

The dolphin actually chooses to approach the diver.

It's not being approached itself.

Laros: I looked up, and I said,

"oh, you've got a fishing line, come here."

And I literally gestured like that.

And then dolphin just swam straight up to me.

You know, when you say to your dog,

"come here," you expect the dog to come.

But when I said, "come here" to the dolphin,

I was blown away that it just swam right up to me.

That animal is completely

And utterly happy in the presence of keller.

Narrator: As he works on the dolphin's fin, it stays close by,

But then, without warning,

And before the fishing line is removed, it leaves.

If the dolphin really has approached keller for help,

Why would it swim away mid-rescue?

Narrator: In kona, hawaii, a dolphin tangled in fishing line

Approaches diver keller laros for help.

Mid-rescue, it suddenly leaves and swims to the surface,

But why?

This dolphin clearly needed to surface for air, which it did.

But as soon as it had enough air,

It went straight back down.

Narrator: It picks keller out again from the group of 50 divers

And swims right up to him.

Yeah, there I am in a wetsuit at night, you know, with a mask,

And the dolphin came back and found me again.

Narrator: But how did it single keller out from such a large group?

Could it be that the dolphin actually recognized him?

Well, there are many examples of dive sites

And even boat safaris where the same individual dolphins

Will appear in response to a boat

Or a diver being present in the water.

It's possible that this what was going on,

That this dolphin actually knew the diver.

Narrator: But did the diver know the dolphin?

Well, it turns out that keller

Is more of a manta ray specialist.

I can tell manta rays apart by the spots on their abdomen.

It's like a fingerprint.

But dolphins?

They all pretty much look alike to me.

Narrator: As far as he was concerned,

This was just any old dolphin that came by to say hi.

That is, until he got back home and reviewed the footage

When he realized something astonishing.

Narrator: On a routine night dive off kona, hawaii,

Scuba instructor keller laros encounters a dolphin

In need of his help.

It approaches, and keller works to cut away

A tangled fishing line.

But why did the dolphin single out keller in particular?

I noticed that it had a white scar,

Kind of a rectangular-shaped scar behind its left eye.

And I realized, that makes this dolphin look different

Than all the rest.

The previous summer, keller had taken his son

On a snorkeling trip with school friends,

And they'd taken some underwater photos.

So, when my son's senior yearbook came out,

We were looking at the pictures.

And one of the pictures he'd used was him

Snorkeling with this dolphin.

It was the same dolphin.

White scar behind its eye.

So, I'd actually met this dolphin the previous summer.

Narrator: Whether it not it remembered him,

The dolphin certainly does seem to have known

That keller could help it out.

Laros: I think the first sign of trouble

Was that the dolphin squeaked really, really loud.

Very sharp, focused sonar blast.

[ dolphin squeaks ]

It was almost like a call for help.

It came up and sort of leaned into me.

It was so intent and so focused

On getting this fishing line removed.

So, it was no accident.

This dolphin was clearly trying to make sure

That the diver understood what needed to be done.

Baker: Somewhere along the line, that dolphin has worked out

That keller is the solution to its problem.

That's the incredible thing.

Narrator: But how did it figure out that humans were the answer?

Or that keller and the other divers were safe to approach?

It seems to me what's happening here

Is some form of habituation.

Oh, my god.

Narrator: Habituation is when wild animals repeatedly interact with humans

And learn not to be fearful of them.

It's not always a good thing.

It can put them at risk, but in this unique case,

It may have saved the dolphin's life.

Laros: I think that dolphin came to find humans

Because it knew that humans would be in a position

Where we could help it,

And I was just lucky enough that I recognized the dolphin's need.

As I cut the line off, I really think the dolphin felt relief,

And it took off.

We see that dolphin to this day, and it's doing well.

Narrator: This distressed dolphin knew it was in trouble

And sought out humans for help.

Fortunately, keller was on hand to make sure

This mystery had a happy ending.

♪

June 2013, east coast, america.

In nine different states, from new jersey to virginia,

Millions of cicadas emerge all at the same time.

Nelson: Imagine one acre.

That's maybe a large plot of land for a house.

We're talking about 1.5 million cicadas an acre.

Narrator: These cicadas were laid and hatched 17 years ago.

They've been biding their time,

Developing underground ever since.

They're known as periodical cicadas,

And for a very good reason.

Riskin: The mystery around them

Is why they only come out every 13 or 17 years.

Not every 10, not every 5.

It's always some weird number.

Narrator: What's the reason for their strangely timed appearance?

Is there a clue in the actions of other swarming insects?

So, one possibility is that the cicadas

Are just moving around in search of food.

Nelson: Desert locusts, from the biblical plague,

They form these huge populations of individuals

Which can destroy crops.

These come up in their billions,

And when they do, they form these vast clouds.

Narrator: Hungry locusts must travel up to a mile a day

To find enough food to support the swarm.

But adult cicadas barely feed, drinking only a little tree sap,

Causing minimal damage to plants.

Burke: They emerge en masse,

But they seem to stay put and die in one place.

So, something else is going on.

Narrator: If they're not looking for food,

Why else might these cicadas have suddenly turned up?

Riskin: Another possibility is that they're all coming out

At the same time for safety in numbers.

They don't want to be eaten, so if there's zillions of them,

The predators will be overwhelmed,

And so individual cicadas are more likely to survive.

The theory is you're simply flooding the market.

♪

So, if there is more animals in the market,

Then the native predators can tackle --

You simply cannot eat all those animals in one go.

Then some, by definition, get away and succeed.

Narrator: If this mass emergence is all about safety,

Wouldn't a less risky strategy be to just stay put underground?

There's another reason why animals might emerge en masse,

And that's to mate.

Narrator: So, is this huge gathering a periodical cicada mating frenzy?

So, over a period of just a couple weeks,

All these adult cicadas push up through the soil.

They've been waiting for the cue,

And they all come out of the soil together.

These are mating machines.

They don't really feed as adults.

It's all about sex.

♪

Narrator: Cicadas come out en masse to maximize their chances.

Like speed dating.

The more potential partners there are,

The likely they are to be lucky in love.

Nelson: So maybe that's what we're seeing here.

A timed event

Where they're all coming out of the ground to mate.

Riskin: The problem with that theory, though,

Is that it doesn't explain why they would only come out

In such weird numbers of years.

Nelson: And I think the solution to this is found

When you actually look at those numbers.

13 and 17 are very unique numbers.

They're prime numbers.

What that means is the only thing that divides into them

Is itself and one.

Riskin: And you would think that that is

Just something you learn in math class,

And it has no bearing at all on the natural world.

But it turns out that prime numbers

Are very important for cicadas.

Narrator: Across the eastern usa,

Periodical cicadas emerge every 13 or 17 years.

They all come out at the same time

So they can mate en masse, but why the strange timings?

It turns out their long, prime number population cycles

Help the cicadas to avoid as many predators as possible.

Think of it from a predator's perspective.

They also have population cycles,

With their numbers rising and falling,

Depending on how much food there is to go round.

If their boom in population matched that of the cicadas,

They could almost wipe them out.

Imagine a predator with a two- or three-year life cycle.

If the cicadas came out, say, every 6 or 12 years,

The peak in predator numbers would coincide

With the cicadas every time.

But 13 and 17?

Nothing divides into that.

If you have a predator that comes out every other year,

It's never going to match the cycle of the cicadas.

Narrator: There are no predator life cycles

That synch up with the prime number boom years

Of the periodical cicada.

As a strategy for the cicada, it works really, really well

'cause it is near impossible to time your own life cycle,

If you're a predator, to hit those mass emergences.

It's an amazing strategy.

It's a bizarre strategy, and I believe a unique one.

But the question is how do they know

When 13 years or up or 17 years are up?

How do they know when to emerge?

Narrator: Are cicadas secret mathematicians?

Potentially, because somehow,

They're tracking the passage of time.

Nelson: Well, what we think happens is that the cicadas

Are actually keeping track of the sugar

Flowing through the tree.

The sugar only flows through the tree

When the leaves are out.

Narrator: Track the sugars, and the cicadas can track the seasons.

Now, as every spring goes by,

They can taste when the sugars spike as the trees go into leaf,

And it's this way that they're able to monitor

Each passing year, each passing spring.

Narrator: The cycles of tree sap probably affect how cicadas develop.

In readiness to emerge en masse every 13 or 17 years.

♪