NARRATOR:
Tornados of bat wings...
Multitudes of
silver fish...
Animal armies
on the march...
These are nature's
greatest gatherings --
swarms of epic
proportions.
Who controls these mobs?
How are a million minds
able to act as one?
Flocks and schools,
herds and hives --
this is what happens
when animals come together.
They achieve what
no individual ever could.
This is nature's
collective genius...
rising in
the gathering swarms.
NARRATOR: Of all the mysterious
animal gatherings in the world,
a hidden grove
in Mexico
shelters the most
beautiful.
Their bodies decorate the trees
like leaves.
They are monarch butterflies --
up to half a billion
of them.
As the morning sun
warms their wings, they stir.
They've traveled all the way
from North America to be here,
escaping the cold of winter.
Some came from
as far as Alaska --
an incredible 3,000 miles --
to arrive at a place that none
had been to before.
How they know the way
and find this hidden spot
is still a mystery.
Even here, 10,000 feet
above sea level,
the nights are cold.
They cluster together
to conserve heat
until touched by the sun.
Then, they cascade
like autumn leaves...
In this still glade,
the silence is broken by
the sound of their wings.
[Wings fluttering]
They gather
at mountain streams.
Water is their only sustenance
for the five months
they spend here.
They then take to the air
to attract a mate.
In spring, they will fly north
to recolonize
North America.
Over the summer, three
generations will live and die.
It's up to a fourth generation
to find the way back
to this secluded grove
for the winter.
And every fall, they do.
Not all gatherings
are quite so predictable.
This colorful congregation
in the Australian outback
is a rare sight, indeed.
It's made up of thousands upon
thousands of budgerigars --
a type of parakeet.
They may be the world's
most popular pet birds,
but in the wild,
they live far from
human company.
Usually they travel in
small flocks,
seeking the grass seed that
makes up their staple diet.
But when food is plentiful,
they appear at waterholes
in spectacular numbers.
Wave upon wave descend
to drink in turn.
Each bird alights
for just a few seconds
before another
takes its place.
But wherever birds gather,
so do their predators.
The black falcon --
a formidable hunter.
But the budgies'
sheer numbers
are also their secret weapon.
It looks like the falcon
has all he can eat.
But as the flock splits,
he doesn't know where to turn.
And while he's distracted,
more budgies sneak a drink.
Some act as lookouts,
watching the drinkers' backs.
The falcon tries again,
but he's quickly spotted.
He isolates a target,
but it just drops
like a stone.
He tries again,
but the same thing happens.
Working together,
the budgies survive.
By settling for a moment,
each gets a drink
while the rest run rings
around the falcon.
In the chaos, even a single bird
presents quite a challenge.
The black falcon is outwitted by
the power of the flock,
and it's not
the only benefit
of massing together.
When birds assemble,
something
extraordinary happens.
They act like a super-organism,
traveling with
a single purpose.
The knowledge of each bird
adds together
and helps the flock come to
a consensus.
It finds food and makes choices
far better than
any individual.
This is one of the largest
flocks of budgies ever seen.
Together, a million birds
appear to
think and react
with one mind.
The budgies are living proof
that when finding food
or facing predators,
it's better to be
part of a crowd.
Fish schools work on
the same
collective principal.
Sardines traveling up
the coast of South Africa
are part of the greatest
fish migration on the planet.
Their synchrony relies on
a pressure sense
that runs along
their bodies.
This "lateral line"
detects the movements of
their nearest neighbor
and allows them
to move as one.
It also detects predators.
And it's not just sharks that
make a meal of the sardines.
A superpod of common dolphins
is also on their trail.
They're joined by
an attack squadron
of cape gannets.
A swarm of predators is drawn in
by the swarming prey.
But the sardines
resist the attacks
by staying deep and keeping
the school together.
En masse, it's difficult to take
this super-shoal by surprise.
Yet attacks
from below and above
begin to break down the
sardines' collective defenses.
They fragment the shoal
into smaller targets.
In mini-shoals the predators
begin to have the upper hand...
until one arrives
to take the lot.
The Bryde whale.
Twenty tons of hungry blubber.
As soon as they can,
the splinter groups seek refuge
in the master shoal.
They regroup, and despite
the strength of the as*ault,
still find
there is safety in numbers.
To breed in the coldest place
on earth,
emperor penguins must also
become part of a crowd.
Their chicks are born
in the depths of
the Antarctic winter.
Their only protection
from the cold
is their parent's brood pouch.
But as winter storms set in,
this isn't enough.
As temperatures reach 40 below,
even the adults'
only chance of survival
is to huddle together.
They start to waddle
with a single purpose.
All converge on
the same central point
until a huddle
begins to form.
Each penguin must find
a place to tuck in.
But by speeding up the action,
it's clear that something
more organized
is going on.
Soon, the huddle
numbers thousands,
and still more keep coming.
While those on the outside
take the brunt of the storm,
those on the inside
take tiny steps
that move the huddle
in waves.
The densely packed penguins
continue to shift and rotate
from the center.
By keeping constantly
on the move,
no penguin is left
permanently out in the cold.
In fact, everyone gets a turn
in the warm center.
The huddle is
so efficient,
penguins in
the inner core
often overheat.
Then, the colony breaks down.
The coldest penguins
that were on the outside
form a new huddle.
As others join,
the cold penguins
become warm and snug
once more.
Step by step,
huddle after huddle,
the penguins
weather the storm.
Emperors are the only creatures
on Earth
to breed in
the Antarctic winter.
But they need the warm embrace
of the crowd
to make this strategy work.
Throughout nature,
coming together to breed
is a popular idea.
The Mississippi River
plays host to
one of the most spectacular
of these gatherings.
On a single day,
thousands upon thousands
of mayflies
emerge from the river.
They've lived for a year
as aquatic larvae,
molting from one stage
to the next,
preparing to take
their first flight.
Within a day or two,
they undergo their final
metamorphosis,
into adults
ready to breed.
They soon spread
their new wings,
this time
in a mating flight.
By emerging together,
they overwhelm any predators --
there are simply
too many to eat.
They lay their eggs
back in the river,
an insurance against
the millions of mayflies
that get lost
along the way.
Some years,
the swarms can be so dense
they slow traffic
to a crawl.
At their peak,
they can cover
four and half thousand
square miles --
and even show up on radar.
But it's easy for them
to get lost.
They navigate by flying at
a set angle to the moon --
and more and more,
this ancient beacon
is as good as useless.
Artificial "moons"
are everywhere,
and these beckoning lights
lure the mayflies
straight into town.
They mistake the reflections on
the road for the water surface
and descend to lay their eggs.
The road is soon covered by
countless confused insects
in futile attempts
to reproduce.
Yet an estimated
two and a half thousand times
the world's human population.
By an extraordinary coincidence,
the invasion often coincides
with the Fourth of July,
adding a bizarre contribution
to the festivities.
WOMAN: This is the most
ridiculous thing!
MAN: Sounds like bubble wrap
when you're walking.
NARRATOR: Amid the celebrations,
billions upon billions
of mayflies
enjoy the last day
of their lives.
Mayflies aren't
the only creatures
that reproduce in
great congregations.
On the Pacific coast of Mexico
and California,
from February to September,
at the highest tides
of the month,
there's another
bizarre invasion.
These are gulf grunion --
fish with a seemingly
suicidal desire
to breed out of water.
For just a few minutes,
the entire shoreline
becomes an orgy
of mating fish.
It starts as the females
burrow backwards,
laying their eggs
in the exposed sand.
Their plan is to deposit them
as far from the reach of
aquatic predators as they can.
Each wave carries in more fish,
all with the same purpose
in mind.
Among the writhing masses,
the males must find a female
and fertilize her eggs
before they are buried
for good.
He wraps his chosen partner
in a slippery embrace
until the couple
are washed back out to sea.
It will be two weeks
before the tide reaches
this height again.
The eggs will then hatch,
and the fry return to
the ocean.
The grunion may have outwitted
the egg predators of the sea,
but the adults are in danger
from fish eating birds.
Yet by taking this risk,
far more of their eggs survive
than if the fish had laid them
in the water.
Some animals
use even stranger techniques
in a bid to
avoid predators.
Once every two decades,
trees across
the eastern United States
are subjected to what seems like
an alien invasion.
These are periodic cicadas.
For the past 17 years,
these nymphs have lived
deep underground.
Now, under the cover
of darkness,
they all emerge
together.
For all that time,
they sucked the sap
from the roots of trees
and counted the passing years.
Now, millions,
maybe billions, of cicadas
join the secret invasion.
They make their way
to the nearest tree
and start to climb.
Soon, the nymphs break out of
their old bodies --
a miracle that can take
as little as
ten minutes.
But before their crumpled wings
can fly,
they must be pumped
full of blood --
a marvel of
insect hydraulics.
Many are eaten,
but by emerging all together,
cicadas swamp predators
with more food
than they can eat.
As this happens just once
every 17 years,
their predators can't time
their own breeding cycles
to benefit.
The cicadas' deafening
mating calls
come from a vibrating drum
hidden inside
the male's abdomen.
[Cicadas droning loudly]
The female homes in
on the ear-piercing sound.
[Droning]
This is the only moment
in 17 years
that the cicadas spend
any time together.
She then uses her egg-laying
tube as a saw,
laying 30 eggs
in each incision.
When they hatch
they'll burrow into the soil.
Job done, the cicadas drop dead
in their millions.
But deep underground,
their offspring are thriving,
and will take their turn
in 17 years' time.
Not all bizarre gatherings
are of nature's design.
In the 1970s silver carp were
accidently introduced
into the Mississippi
after escaping from
a fish farm.
Originally from Asia, t
hey now outnumber local fish
by ten to one.
Their expl*sive leaping
is startling.
Some clear 10 feet
in a single jump.
Their behavior is triggered
by the motors of
passing boats.
The fish mistake pressure waves
from the outboards
for the movement of predators,
and literally
jump with fright.
With the river
filled to bursting,
when one fish leaps,
it scares its neighbor,
creating a chain reaction.
Here, anglers don't even need
a rod or line --
the carp do the job
for them.
But with some carp weighing
collisions with people
can be dangerous.
Having colonized the Mississippi
and many of its tributaries,
they are now poised to invade
the Great Lakes.
And this is by no means
the first swarm
created by human hands.
In the skies of North Africa,
desert locusts are on the move.
They thrived when we first
started to grow crops,
and they've been plaguing us
ever since.
The view from inside the swarm
shows just what a formidable
invading force they have become.
A single swarm can cover
over 400 square miles
and contain
As they fly, they synchronize
their wingbeats.
This reduces turbulence,
making their flying
more efficient.
Like fighter pilots,
they fly in perfect formation.
They even have crash detectors
to help them avoid
mid-air collisions.
They react six times faster
than a human pilot can,
so even in crowded airspace,
split second maneuvers
get them out of trouble.
In their search for food,
they can cover 150 miles
in a single day.
And when they reach
agricultural crops,
their populations explode.
They consume their own
body weight in food every day,
and when there's nothing left
to eat, they move on.
Each day, a large swarm
can devour 200,000 tons --
enough to feed
half a billion people.
No other swarm
has had such an impact on us.
But some animal gatherings
are impressive
for different reasons.
There are caves across
the Southwest
that are home to
the largest concentrations
of mammals on the planet.
Here, 40 million
free-tailed bats gather together
in one colossal
maternity ward.
These females have traveled
from Central America
to give birth here,
and the cave is like
a giant crèche where everyone
shares the childcare.
At dusk, adults and young
take to the air.
They swirl around the entrance
before leaving their roosts
for the night.
The young learn from
their mothers.
Among the millions,
she recognizes her own offspring
by its call,
and they stay together
as they fly.
They stream towards
the best sources of food,
information they gleaned from
their last night's outing.
The first bats out
are ahead of the crowd,
but being first has
its disadvantages.
Red-tail hawks
know they're coming...
...and catch them
with a flick of the claw.
Peregrines join the attack,
but these birds only hunt while
there's still light
to see by.
As darkness falls,
the bats stream out,
unmolested.
And with such
incredible numbers,
the whole exodus
can last two hours.
They all follow
the same pattern,
swirling around the entrance
in a bat tornado.
At normal speed you get a sense
of how difficult
this must be.
But bats react many times
faster than we do.
Collisions are rare.
The vortex always spins
in the same direction,
and at the center,
information is exchanged.
Together, the bats decide
where they should forage
for the night.
Such collective decision-making
is most developed among
social insects, such as bees.
Bees create swarms when they are
looking for a new home.
And while house hunting,
they can turn up
just about anywhere.
A motorcycle makes a good
temporary bivouac --
there are plenty of nooks
and crannies to provide shelter.
The first bees to land
release a pheromone
that draws
the others in.
But as the sun beats down
and temperatures soar,
they need to find
a more permanent home.
The workers fan their wings
to cool the colony down,
but in the heat of the day,
it isn't enough.
The overheated bees
send a chemical signal
to scout bees,
who set off to find
alternative accommodations.
A scout must find a cavity
large enough to contain
the whole hive.
She inspects the property,
pacing the floor and measuring,
just like a human surveyor.
Once the scout is happy,
she leaves
to let the swarm know.
The returning scout
uses a "waggle dance"
to give instructions as to where
the new home can be found.
The straight part of the dance
shows the direction
the bees must fly.
Its duration tells them
how far they have to go.
A second's worth of dancing
equals half a mile of travel.
When the dancing bee leaves,
other scouts follow.
With her guidance,
this advance party makes
a beeline to the exact spot.
Now, it's their turn
to check it out.
They, too, pace out the cavity,
confirming the initial
measurements.
Then they hold
a committee meeting,
and by some unknown process,
arrive at a consensus.
The whole scouting party lets
the swarm know the good news
by rushing through
with vibrating wings
in what's known as
a "buzz run."
This tells the colony
it's time to leave.
Tens of thousands of bees
take to the air
in less than two minutes...
All following the scouts
to their new home.
As the bees settle in,
they release a scent
that attracts
any stragglers.
Communal thinking is the key to
the hive's success...
and they aren't the only insects
to show
the wisdom of the crowd.
As the rains arrive in
the mountains of East Africa,
another social insect
is on the move.
collectively made a decision
to find a new home.
They take their eggs and pupae
with them --
their entire
worldly possessions.
Soldier ants guard the pathways
as the workers carry the load.
A twig blocking the path is like
a tree blocking a main road.
Those first at the scene
call for backup
by releasing
a pheromone.
Soldiers arrive and get to work.
They can lift 50 times
their own body weight.
This takes supreme organization.
They all manage to lift
and push
at the same time.
Obstruction removed,
the traffic soon starts
flowing again.
In defense of the trail,
the ants
take no prisoners.
A soil millipede
is quickly dispatched --
but with their
sensitive antennae,
the soldiers have detected that
it's poisonous.
Nearby ants get the message
and immediately gather
lumps of earth and begin to
bury the problem.
With the millipede
out of the way,
the trail can safely continue...
But more dangers lie ahead.
A praying mantis plucks an ant
from the ranks.
But the stricken ant
sends out a message,
and soon
reinforcements arrive.
The mantis is a deadly predator,
but the ants know exactly what
they are dealing with.
One soldier grabs the mantis's
jaw to immobilize it,
as the rest pile on.
Its fate is sealed by
a clinical decapitation.
All trails lead to the new nest.
It's made entirely from
the bodies of ants.
Linked leg by leg, they form
a living structure.
But they aren't just
physically joined together --
they think together, too.
Deep inside
is the queen.
Her role is to
replenish the colony
by laying around
Yet she is merely
an egg-laying machine.
It's the ants that surround her
that make up
the mind of the swarm.
Although individual ants
aren't intelligent,
the interactions between them
create a different kind of
intelligence --
a super-brain that reflects
the colony's
collective thinking.
Sometimes, even large
gatherings of mammals
can show the kind of
collective intelligence
perfected by
the social insects.
It's called
The Great Migration --
a vast movement of
zebras and wildebeest
across the plains of
East Africa.
Their biggest barrier
is the Mara River,
and here a wrong decision
can easily mean death.
The wildebeest
do everything together,
and usually their
collective thinking pays off.
In contrast, zebras usually
travel in family groups,
and the lead mare
makes the decisions.
Crocodiles regularly put these
different strategies
to the test.
The zebras are wary.
They know that
crocs mean trouble.
But the wildebeest
aren't so canny.
By relying on each other
to warn of danger,
they seem oblivious
to the threat.
This time, the wildebeests'
collective intelligence
failed them...
while the zebras'
individual intelligence
kept them alive.
But when the wildebeest
finally cross the river,
things aren't
so straightforward.
Once one goes, they all do,
those behind
blindly following
those in front.
The zebras deliberately avoid
the apparently mindless masses
and chart their own course
across the river.
With the water full of
vulnerable animals,
the crocs make their move.
But they soon discover that
it's not that easy.
Among the flailing hooves,
the crocs can't single out
a target.
Faced by overwhelming numbers,
the crocs beat a retreat.
Following the crowd has actually
worked for the wildebeest.
But for the zebras, independence
of mind has its own dangers.
Their well-considered plan
didn't account for
the crocodiles.
On their own,
zebras are an easy target.
For the foal and his mother,
it was a close call.
Meanwhile, the wildebeest
have led themselves up
a blind alley --
pinned against a wall
that's impossible to climb.
They flounder, but the mass of
bodies pushing from behind
helps some to start
scaling the wall,
probing it for escape routes.
As the wildebeest
spread along the bank,
they test each section
for a possible way out.
Their numbers increase
the chances
of one of them succeeding.
The zebras make it to their
carefully chosen spot but,
but although
they avoided the crush,
the bank is
too high to climb.
The zebras are still
paying the price
for their independence
of mind.
But by collectively
probing the bank,
the wildebeest finally
find an escape route,
and the rest
are quick to follow.
This time, the wisdom of
the crowd pays off.
The whole herd
funnels up the exit.
It doesn't take
the zebras long
to notice.
But, in the rush for freedom,,
there's one who's been
left behind.
Reunited, the family will rejoin
the great migration
and benefit once more
from the knowledge of the crowd.
Animals in extraordinary numbers
become more than
the sum of their parts.
They create nature's own
special intelligence --
the mysterious wisdom
of many minds,
working as one.
To learn more about what you've seen on this "Nature" program,
visit pbs.org.
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