All language subtitles for Wonders of Life 4of5 Size Matters_Subtitles01.ENG

PROFESSOR BRIAN COX:
Our world is covered in giants.

The largest things that ever lived
on this planet weren't the dinosaurs.

They're not even blue whales.
They're trees.

These are Mountain Ash.

They're the largest
flowering plant in the world.

They grow about a metre a year.

And these trees are 60, 70,
even 80 metres high.

But to get this big,

you need to face some very
significant physical challenges.

(MAJESTİC IİNSTRUMENTAL MUSTIC)

These giants can live to well over
300 years old.

But they don't keep growing forever.

There are limits to how big
each tree can get.

As with all living things,

the structure,
form and function of these trees

has been shaped by the process
of evolution through natural selection.

But evolution doesn't have a free hand.

İt is constrained by the universal
laws of physics.

Fach tree has to support its mass

against the downward force
of Farth's gravity.

At the same time,

the trees rely on the strength
of the interactions between molecules

to raise a column of water
from the ground

up to the leaves in the canopy.

And it's these fundamental properties
of nature that act together

to limit the maximum herght of a tree,

which, theoretically, lies somewhere
in the region of 130 metres.

With its forests and mountains,

oöceans and deserts,

I've come to Australia to explore
the scale of life's sizes.

I want to see how the laws of physics
goövern the lives of all living things.

From the very biggest
to the very smallest.

The size of life on Earth
spans from the tallest tree

over 100 metres tall and
with a mass of over 1,000 tons,

to the smallest bacterium cell

with a length less than
a millionth of a millimetre

and a mass less than a million
millionths of a gram.

That spans over 22 orders
of magnitude in mass.

I want to see how size influences
the natural world.

How do the physical forces of nature

dictate the lives
of the big and the small?

Do organisms face different challenges
at different scales?

And do we all experience
the world differentiy based on our size?

The size you are profoundly influences
the way that you live your life.

It selects for the properties
of the natural world

that most affect you.

So, I suppose that, whilst we all live
on the same planet,

we occupy different worlds.

I'm heading out to the Neptune Islands,
west of Adelaide in South Australia

in search of one of nature's
largest killing machines.

Ihese beasts are feared
around the world,

a fear not helped by
Hollywood film makers.

I'm here to swim with
great white sharks.

How big... How wide can they open
their jaws?

-MAN: About three feet wide.
-About three feet.

MAN: They can swallow a man whole.

Yeah, so about three... (CHUCKLES)

Three foot wide, can swallow
a man whole. (CHUCKLES)

The skipper has a special permit
to use bait to lure the sharks in.

The crew ready the cages.

COX: Last time I dived, it was
in the Marina in Brighton.

I did see afish.

İt was about that big.

From that to the largest
marine predator.

(CLEARS THROAT)

As the sharks start to circle,

it's time to get in.

COX: Here he is. Here he comes.

Just look at that.

He's just checking us out.

Oh, he's turning straight for us.

Look at those teeth.

Graceful, elegant thing,

shaped by natural selection.

Brilliant at what it does,

which is to eat things.

(COX LAUGHING)

Oh, I never thought
you could be that close to one of those.

Great whites
are highly-evolved predators.

Around two-thirds of their brain
is dedicated to their sense of smell.

They can detect as little
as one part per billion blood

in this water.

The tiniest speck of blood
will attract the shark.

These fish can grow to a huge size

but still move
With incredible speed and ağility.

They've been sculpted by evolution,

acting within the bounds
of the physical properties of water.

Well, he's about five metres long.

He weighs about a ton.

And he's probably the most
efficient predator on EFarth.

When he's attacking,

he can accelerate up to over
20 miles an hour.

And they can launch themselves
straight out of the water.

There he is. There he is.

Whoa! Whoa!

Oh, oh!

Ithink I need to remove my hands.

That was one of the most

awe-inspiring sights I've ever seen.

The great white, just straight in front
of you, with its mouth open.

With the boat moored up
away from shark-infested waters,

I want to explore why it's in our oceans

that we find the biggest
animals on Farth.

From giant sharks to blue whales,

the largest animals that have
ever lived, have lived in the sea.

The reason why is down to physics.

This is a container full of salt water
and I'm going to weigh it.

Now you see?
That says 25 kilograms there.

That's actually its mass.

Its weight is the force that the Earth
is exerting on it due to gravity,

which is 25 times about 10,

which is 250 kilograms
metres per second sguared.

That might sound pedantic
but it's gonna be important in a minute.

See, what happens if I lower
this salt water into the ocean?

Its weight has effectively disappeared.

It's effectively zero.

Now, of course gravity is still
acting on this thing, so

by the strictest sense of the word

it still has the same weight
as it did up here.

But Mr Archimedes told us

that there's another force
that's come into play.

There's a force proportional
to the weight of water

that's been displaced by this thing.

And because this thing is essentially
the same density as sea water,

it's made of sea water,

then that force is egual and opposite
to the force of gravity.

And so they cancel.

So, it's effectively weightless.

And that is extremely important indeed
for the animals that live in the ocean.

The cellis of all living things are
predominantly made up of salty water.

So, in the ocean, weight is
essentially unimportant.

(INDISTINCTI CONVERSATION)

Because of Archimedes' principle,

the supportive nature of water

releases organisms from
the constraints of Farth's gravity,

allowing the evolution
of marine leviathans.

But this comes at a cost.

Water is 800 times denser than air.

And so, whilst it provides supporkt,

it reguires a huge amount of effort
to move through it.

Not only does the shark have
to push the water out of the way,

it also has to overcome drag forces

created by the frictional contact
with the water itself.

The solution for the shark
lies in its shape.

If you look at him, that great white,

he's got a distinctive streamline shape.

His maximum width

is around a third of the way
down his body.

And that width itself should be around
a guarter of the length.

That ratio is set by the...

The necessity for something that big

to be able to swim,

effectively and guickly
through this medium.

This shape reduces
drag forces to a minimum

and optimises the way water flows
around the shark's body.

It is the result of evolution,

shaped by the laws of physics.

AHh! Whoa! Oh-ho!

(LAUGHING)

Ah, gone again.
That was straight out of /aws.

That streamline shape of the shark

is something that you see
echoed throughout nature.

I mean, think of a whale,
or a dolphin or a tuna.

They're all that same
torpedo-like shape.

And that's because they're contending
with problems that arise from the same

laws of physics and convergent evolution

has driven them to the same solution.

For life in the sea,

the evolution of giants
is constrained directliy

by the physical properties of water.

But out of the ocean,

life now has to contend with
the full force of Farth's gravity.

And it's this force of nature

that dominates the lives
of giants on land.

This is the hot, dry outback
north of Broken Hill in New South Wales.

I'm here to explore how gravity,

a force whose strength is governed
by the mass of our whole planet,

moulds, shapes and, ultimately, limits
the size of life on land.

(TRIBAL MUSIC PLAYING)

I've come to track down
öone of Australia's most iconic animals.

The red kangaroo.

Red kangaroos are Australia's
largest native land mammakl,

one of 50 species of macropods,

so-called on account
of their large feet.

(WHISPERS) There. There, there.

You see, very close there.

The kangaroos are the most remarkable
of mammals because they hop.

There's no record, even in the fossil
record, of any other large animal

that does that.

But it makes them
very fast and efficient.

When Joseph Banks, who's
one of my scientific heroes,

first arrived here with Captain Cook

on the £Zimdeavourin 1770,

he wrote, "that they move so fast
over the rocky, rough ground,

"where they're found, even my
greyhound couldn't catch them."

What was he doing with a greyhound?

Kangaroos are herbivorous.

They scratch out a living
feeding on grasses.

(FLIES BUZZING)

While foraging, they move
in an ungainly fashion,

using their large muscular tail
like a fifth leg.

But when they want to,

these large marsupials can cover ground

at considerable speeds.

To take a leap,

kangaroos have to work against
the downward pull of Farth's gravity.

This takes a lot of energy.

As animals go faster,

they tend to use more energy.

Not so with the kangaroos.

As the roos go faster,

their energy consumption
actually decreases.

It then stays constant,

even at sustalined speeds
of up to 40 kilometres per hour.

This incredible efficiency for such
a large animal

comes directliy from
the kangaroo's anatomy.

Kangaroos move so efficiently because

they have an ingenious
energy storage mechanism.

See, when something hits the ground
after falling from some height,

then it has energy
that it needs to dissipate.

If you're a rock,

that energy İs dissipated as sound

and a little bit of heat.

But if you're a tennis ball,

then some of that energy İs reused.
Because a tennis ball is elastic,

it can deform, spring back

and use some of that energy to throw
itself back into the air again.

Well, a kangaroo is very similar.

It has very elastic tendons in its legs,

particularly its Achilles tendon

and also tendons in its tail.

And they store energy

and then they release it,
supplementing the power of the muscles,

to bounce the kangaroo through the air.

Now, an adult kangaroo is 85, 90 kilos,

which is heavler than me.

And, when it's going at full speed,

it can Jump around nine metres.

That's the distance from me to that car.

The evolution of the ability to hop

gives kangaroos a cheap and efficient
Way to get around.

But not everything
can move like a kangaroo.

The red kangaroo is the largest animal
in the world

that moves in this unigue way,

you know, hopping across the landscape
at high speed.

And there are reasons, why
there aren't, you know, giant

hopping elephants or dinosaurs
and they're not really biological.

It's not down to the details
of evolution

by natural selection
or environmental pressures.

The larger an animal gets,
the more severe the restrictions

on its body shape and its movement.

To understand why this is the case,

I want to explore what happens
to the mass of a body

when that body increases in size.

Take a look at this block.
Let's say it has width one,

length one and height one.

Then its volume is one,

multiplied by one, multiplied by one,

which is one cubic things,
whatever the measurement İs.

Now its mass İs proportional
to the volume,

so we could say that the mass
of this block is one unit as well.

Let's say that we're gonna double
the size of this thing.

In the sense that we want to double
its width,

double its length,

double its height.

Then its volume is two,
multiplied by two, multiplied by two

eguals eight cubic things.

Its volume is increased
by a factor of eight,

and so İts mass İs increased
by a factor of eight as well.

So although I've only
doubled the size of the blocks,

I've increased the total mass by eight.

As things get bigger,

the mass of the body goes up by the cube

of the increase in size.

Because of this scaling relationship,

the larger you get,
the greater the effect.

As things get bigger,

the huge increase in mass
has a significant impact

on the way large animals
support themselves against gravity

and how they move about.

No matter how energy efficient

and advantageous it is
to hop like a kangaroo,

as you get bigger, it's just not
physically possible.

Going supersize on land

comes with tremendous
constraints attached.

This is the left femur, the thigh bone,

of an extinct animal called
the Diprotodon,

which is the largest known
marsuplal ever to have existed.

This would have stood as tall as me.

It would have been four metres long,

weighed between
two and two and a half tons,

so the size of a rhino.

Now, it's known that it was all
over Australia.

Iİt was the big herbivore.

And it got progressively bigger

over the 25 million years
that we have fossils for it.

Then, around 50,000 years ago,

coincidentally when humans
arrived in Australia,

the Diprotodon became extinct.

The Diprotodon is thought to have looked
like a giant wombat.

And being marsupials,

the females would have carried their
sheep-sized offspring

in a huge pouch.

To support their considerable bulk,

the Diprotodon's skeleton
had to be very strong.

This imposed significant constraints

on the shape and size of its bones.

This is the femur of the closest
living relative of the Diprotodon.

It's a wombat, which is an animal
around the size of a small dog

and you see that, superficially,
the bones are very similar.

Let me take a few measurements.

The length of the Diprotodon femur is

around 75 centimetres.

The length of the wombat femur

is arocund 15 centimetres.

So this is about five times the length
of the wombat femur.

But now look at
the cross-sectional area.

Assuming the bones are rougğhly circular
in cross section,

we can calculate their area using pi

multiplied with the radius sguared.

İt turns out that, although
the Diprotodon femur

is around five times longer,

it has a cross-sectional area
40 times that of the wombat femur.

A bone's strength depends directiy
on its cross-sectional area.

The Diprotodon needed thick leg bones

braced in a robust skeleton

Just to provide enough strength
to support the giant's colossal weight.

As animals get more massive,
the effect of gravity

plays an increasingly
restrictive role in their lives.

The shape and form of their body
is forced to change.

If you look across the scale
of Australian vertebrate life,

you see a dramatic difference
in bone thickness.

This is a line of femur bones
of animals of different sizes.

We start with the smallest,
one of the smallest marsupials

in Australia, the marsupial mouse
or the Antechinus.

Then the next one.
İt's an animal known as Potaroo.

Again, it's a marsupial around
about the size of a rabbit.

Then we have the Tasmanian devil,
a wombat, the dingo.

Then the largest marsupial in Australia
today, the red kangaroo.

This is the femur of the Diprotodon.

And then here the femur
of a Rhoetosaurus,

which was a sauropod dinosaur

17 metres long
and weighing around 20 tons.

And so you see, as animals get larger,

from the smallest marsupial mouse
all the way up to a dinosaur,

the cross-sectional area of their bones
increases enormously

Just to support that increased mass.

Being big and bulky,

giants are more restricted

as to the shape of their body
and how they get about.

That's why red kangaroos
are the largest animals

that can move in the way that they do.

At a much greater size,
their bones would be very heavy,

have a greater risk of fracture

and they reguire far too much
energy to move at high speeds.

İt's ultimately the strength
of Farth's gravity

that limits the size
and the manoeuvrability

of land-based giants.

But for the bulk of life on land,

gravity is not the defining
force of nature.

At small scales,

living things seem to bend
the laws of physics,

which is, of course, not possible.

The world of the small is often hidden
from our view,

but there are ways to draw out
these tiny creatures.

Ihis is the domain of the insects.

These animals can clearly do things
I can't do

and appear to have super powers.

They can walk up walls,
Jjump many times their own height

and can lift many times
their own weight.

There are over 900,000 known species
of insects on the planet.

That's over 7596 of all animal species.

Some biologists think,
that there may be an order of magnitude

more yet to be discovered.
That would be 10 million species.

And they are very small
so you can fit a lot of them

on planet Earth at any one time.

In fact, it's estimated there are
over 10 billion billion

individual insects alive today.

Of all the insect groups,

it's the beetles, or Coleoptera,

that have the greatest
number of species.

The biologist, J. B. S. Haldane said,

"that if one could conclude

"as to the nature of the creator
from the study of creation,

"then it would appear that God
has an inordinate fondness

"for stars and beetles."

With so much variation in colour,
form and function,

beetles have fascinated
naturalists for centurles.

Fach species is wonderfully adapted

to their own unigue niche.

Well, this is the beginnings of
biology as a science that you see here.

İt's this desire to collect
and classify,

which then, over time, becomes
a desire to

explain and understand.

I'm going to take a picture.

Here in the suburbs of Brisbane,

every February, there is
an invasion of beetles.

The ruüles göverning their lives
play out very differentiy to ours.

This is the rhinoceros beetle,
named for obvious reasons.

But actually it's only the males

that have the distinctive
horns on their heads.

The beetles spend much of their
lives underground as larvae

but then emerge en masse as adults
to find a mate and breed.

Much of this time,

the males spend fighting over females.

(BEETLE HISSES)

See that,

distinctive (CHUCKLES) posture
that he's adopting there.

That's because, I think, he's seeing
his reflection in the camera lens

and so he rears up.

Look at that. He's...
He's trying to scare himself off.

(LAUGHING)

(BEETLE HISSES)

You can also hear that hissing sound.
That's him contracting his abdomen,

which again is a...İs a

defensive posture that he adopts
to scare other males.

Gram for gram, these insects are among
the strongest animals alive.

And I can demonstrate that by just
getting hold of the top of his head.

Doesn't hurt him at all. But watch

what he is able to do.

Look at that.

So he's hanging onto this branch, which
is many times his own bodyweight.

Absolutely no distress at all.

As things get smaller,

it's a rule of nature that
they inevitabiy get stronger.

The reason is guite simple.

Small things have relatively
large muscles

compared to their tiny body mass
and this makes them very powerful.

The beetles also appear
to have a cavalier attitude

to the effects of gravity.

They fight almost like sumo wrestlers.

Their aim is to throw each other
off the branch.

If they should fall,

they just bounce and walk off.

Ifl fell a similar distance
relative to my size, I'd break.

So, why does size make
such a difference?

Time for a bit of fundamental physics.

ALI things fall at the same rate
under gravity.

That's because
they're following geodesics

through curved space-time,
but that's not important.

The important thing for biology

is that, although everything
falls at the same rate,

it doesn't meet the same fate
when it hits the ground.

A grape bounces.

A melon

doesn't bounce.

Now the reasons
for that are guite complex, actually.

First of all, the grape
has a larger surface area

in relation to its volume,
and therefore its mass, than the melon.

And so, although in a vacuum

if you took away the air,

they would both fall at the same rate.

Actually, in reality the grape falls
a bit slower than the melon.

Also, the melon is more massive

and so İit has more kinetic energy
when it hits the ground.

Remember from physics class?

Kinetic energy is a half mv sguared.

So if you reduce m,
you reduce the eneragy.

The upshot of that is
that the melon has a lot more energy

when it hits the ground.
It has to dissipate it in some way

and it dissipates it by exploding.

The influence of Farth's gravity
on your life

becomes progressively diminished
the smaller you get.

For life at the small scale,

a second fundamental force of nature
starts to dominate.

And it's this that explains many
of those apparent superpowers.

For me, the force of gravity
is the thing that defines my existence.

İt's the... It is the force
that I really feel the effects of.

But there are other forces at work.

For example, if I lick my finger
and wet it,

then I can pick up a piece of paper,
I can holdit up

against the downward pull of gravity.

That's because the force
of electromagnetism is important.

In fact, it's the cohesive forces
between water molecules

and the molecules that make up my finger
and the molecules that make up the paper

that are dominating
this particular situation.

And that's why this piece of paper
doesn't fall to the floor.

Now, many insects can use
a similar effect.

Take a common fly, for example.

Their feet have specially enlarged pads
onto which they secrete a sticky fluid.

And that allows them to adhere
to rather slippery surfaces,

like the glass of this jam Jjar.

It allows them to do things that,
for me, would be absolutely impossible.

And it's all down to
the relative influence

of the different forces of nature
on the animal.

So the capacity to walk up walls

and fall from a great height
Without breaking,

plus super strength,

are not superpowers at all.

They're just abilities gained naturaliy

by animals that are small
and lightweight.

But this is just the beginning of my
Journey into the world of the small.

Down at the very small scale,

it becomes possible to live within
the lives of other individuals.

Worlds within worlds.

But just how small can animals get?

This macadamia nut plantation,
an hour outside of Brisbane,

is home to one of the very smallest
members of the animal kingdom.

These are species of micro hymenoptera

known as trichogramma.

They're basically very small wasps,
and when I say small,

I mean small.

Can you see that?

They're like specks of dust.

They're less than
half a millimetre long.

But each one of those is a wasp.

İt's got compound eyes,
it's got six legs, it's got wings.

They've even got a little stripe
on their abdomen.

And they're very precisely adapted
to a specific evolutionary niche.

The trichogramma wasps may be small,
but they're very useful.

They're natural parasites

of an insect pest species called
the nut borer moth

which attacks the macadamia nuts.

The micro wasps lay their eggs
inside the eggs of the moths,

killing the developing moth larvae.

So, what you're seeing here
is the surface of a macadamia nut

and here is a small cluster
of moth eggs.

And there you see the wasp
is walking over the eggs.

They're almost pacing out the size
to see whether the eggs are suitable

for their eggs to be laid inside.

And if we're lucky...

There you go, you see that?

That...

There we go.

The wasps emerge just nine days
later as full-grown adults.

At this scale, they live
in a very sticky world

dominated by strong,
intermolecular forces.

To them, even the air is a thick fluid
through which they essentialliy swim

using paddle-like wings.

Incredibiy, these tiny animals
can move about across several trees

seeking out the moth eggs.

But what I find more remarkable
is that they do all this

operating with very restricted
brain power.

One of the limiting factors that
determines the minimum size of insects

is the volume of their
central nervous system.

In other words, the processing power
you can fit inside their bodies.

And these little wasps
are pretty much at the limit.

They have less than 10,000 neurons
in their whole nervous system.

To put that into perspective, most tiny
insects have 100 times that many.

But that's still enough to allow them
to exhibit guite complex behaviour.

These micro wasps exist
at almost the minimum possible size

for multi-cellular animals.

But the scale of life on our planet
gets much, much smaller.

The wasps are giants

compared to life
at the very limit of size on Farth.

The smallest organisms on our planet

are also our oldest
and most abundant type of life forms.

These weird rocky blobs

in the shallows of Lake Clifton,
Just south of Perth,

are made by bacteria.

These mounds are called thrombolites
on account of their clotted structure

and they are built up over centuries

by colonies of microscopic
bacterial cells.

Now, although these colonies are rare,

by most definitions, bacteria are
the dominant form oflife on our planet.

On every surface, across every
landscape, you find bacteria.

And, in fact, numerically speaking,

there are more bacteria living
on and inside my body

than there are human cells.

Bacteria come in many shapes and forms.

And they're not actually
animals or plants,

instead sitting in their own,
unigğue taxonomic kingdom.

Compared to the cells we're made of,

bacteria are structurally much simpler
and far, far smaller.

Bacteria are typically arocund
İwo microns in size.

That's two millionths of a metre,
which is very hard to picture.

But it means that you could fit

around half a million of them
on the head ofa pin.

Or to look at İit another way,
if I took a single bacterium

and scaled it up to the size
of this coin,

then I would be 25 kilometres high.

Bacterial-type organisms were
the first life on Farth

and they've dominated
our planet ever since.

Excluding viruses, which by most
definitions are not alive,

bacteria are the smallest
free-living life forms we know or.

But what ultimately puts the limit
on the smallest size of life?

Single-cell life needs to be big enough

to accommodate all the molecular
machinery of life.

And that size ultimately depends
on the basic laws of physics.

It depends on the size of molecules,
which depends on the size of atoms,

which depends on fundamental
properties of the universe,

like the strength of the force
of electromagnetism

and the mass of an electron.

And when you do those calculations,
you find out that the minimum size

of a free-living organism
should be around 200 nanometres,

which is about
200 billionths of a metre.

And that should be universal.

It shouldn't only apply
to life on Earth,

but it should apply to any carbon-based
life anywhere in the universe,

because it depends on fundamental
properties of the universe.

From the smallest bacterium
to the largest tree,

it's your size that determines how
the laws of physics gövern your life.

Gravity imposes itself on the large

and the electromagnetic force
rules the world of the small.

But the conseguences of scale
for life on Farth

extend beyond dictating
the relationship you have

With the world around you.

Your size also influences how energy
itself flows through your body.

These are southern bent-wing bats,

öne of the rarest bat species
in Australia.

Every evening, they emerge
in their thousands from this cave

in order to feed.

When fully grown,

these bats are just
five and a half centimetres long

and weigh around 18 grams.

Because of their size,

they face a constant struggle
to stay alive.

Now, we're using a thermal camera here

to look at the bats and you
can see that they appear as

streaks across the sky.
They appear as brightly as me.

That's because they're roughly
the same temperature as me.

They're known as endotherms.

They're animals that maintain
their body temperature.

And that takes a lot of effort.

I mean, these bats have
to eat something like

three guarters of their own
body weight every night

and a lot of that energy goes
into maintaining their temperature.

As with all living things,

the bats eat to provide energy
to power their metabolism.

Although, like us, they have a high body
temperature when they are active,

keeping warm is a considerable
challenge on account of their size.

The bats lose heat mostly
through the surface of their bodies.

But because of simple laws
göverning the relationship

between the surface area of a body

and its volume, being small
creates a problem.

(BATS SOUEFAKING)

So let's look at our blocks again,

but this time
the surface area to volume.

Here's a big thing.
İt's made of eight blocks

so its volume is eight units.

And its surface area is two by two
on each side, so that's four,

multiplied by the six faces is 24.

So, the surface area to volume ratio
is 24 to eight, which is three to one.

Now look at a smaller thing.
This is one block.

So its volume is one unit.

The surface area İs one by one by one,

sixtimes, so İt's six.

So this has a surface area
to volume ratio of sixto öne.

So, as you go from big to small,

your surface area
to volume ratio increases.

Small animals, like bats,
have a huge surface area

compared to their volume.

As a result, they naturally lose heat
at a very high rate.

To help offset the cost of losing
so much energy in the form of heat,

the bats are forced to maintain
a high rate of metabolism.

They breathe rapidiy,
their little heart races

and they have to eat a huge amount.

So a bat's size

clearly affects the speed
at which it lives its life.

Right across the natural world,

the size you are has a profound effect
on your metabolic rate,

or your speed of life.

For Australia's small marsupial mouse,

even at rest, his heart is racing away.

But the fox-sized Tasmanian devil,

he ticks along at a much slower rate.

Then there's me, living life
at a languid 60 beats a minute.

(HEART BEATING)

Looking beyond heart rate,

your size influences the amount
of energy you need to consume

and the rate at which
Yyou need to consume İt.

Bigger bodies have more celils to feed.

So you might expect that the total
amount of energy needed

goes up at the same rate
as any increase in size.

But that's not what happens.

If you plot

the amount of energy an animal uses
against its mass

for a huge range of sizes,

from animals as small as flies
and even smaller,

all the way up to whales,

then you do get a straight line.

But the slope is less than one.

So that implies that, gram for gram,

large animals use less energy
than small animals.

This relationship
between metabolism and size

significantly affects the amount of food

larger animals have to consume
to stay alive.

Now, if my metabolic rate
scaled one to one

with that of a mouse, then

I would need to eat about four kilograms
of food a day.

In my language, that's around
67,000 kilojoules of energy,

which more colloguially
is 16,000 calories.

That is eight times

the amount that I take in
on average, on a daily basis.

Fach of the celis in my body
reguires less energy

than the egulvalent celis
in a smaller-sized mammal.

The reason why this should be
so is not fully understood.

It's also not clear
whether this rule of nature

gives an advantage to big things

or is actually a constraint
placed on larger animals.

Take the relationship between an
animal's surface area and its volume.

Big animals have a much
smaller surface area

to volume ratio than small animals.

And that means that their rate
of heat loss is much smaller.

And that means that there's
an opportunity there for large animals.

They don't have to eat
as much food to stay warm

and therefore they can afford
a lower metabolic rate.

Now this helps explain the lives
of large, warm-blooded endotherms

like birds and mammals,

but doesn't hold so well
for large ectotherms,

life's cold-blooded giants.

Now there's another theory that says
that it wasn't really

an evolutionary opportunity
that large animals took

to lower their metabolic rate.
It was forced on them.

İt was a constraint, if you like.

The capillaries,
the supply network to cells,

branches in such a way

that it gets more and more difficult
to get oxygen and nutrients

to cells in a big animal
than in a small animal.

Therefore, those cells must run

at a lower rate.
They must have a lower metabolic rate.

Or it could just be that as
you get bigger, then more of your mass

is taken up by the stuff
that supports you

and support structures like bones are

relatively inert.
They don't use much eneragy.

But whatever the reason,
it's certainly true to say

that the only way that large animals
can exist on planet EFarth

is to operate at a reduced
metabolic rate,

1f this wasn't the case,

the maximum size of a warm-blooded
endotherm like me

or you would be around that of a goat.

And cold-blooded animals,
or ectotherms like dinosaurs,

could only get as big as a pony.

Any bigger, and giants
Would simply overheat.

Now there's öone last conseguence
of all these scaling laws

that I suspect you'll care
about more than anything else.

And it's this:
there's a strong correlation

between the effective cellular
metabolic rate of an animal

and its lifespan.

In other words, as things get bigger
they tend to live longer.

To explore this connection
between size and longevity,

I've left the mainland behind.

For my final destination,

I've come to one of Australia's
remotest outposts.

Named Christmas Island

when it was spotted on
Christmas Day in 1643,

this isolated lump of rock
in the Indian Ocean is a land of crabs.

And in their midst lurks
a giant wonder of the natural world.

This is a Christmas Island robber crab,

the largest land crab
anywhere on the planet.

Now these things can grow to around
50 centimetres in length,

they can weigh over four kilograms

and they are supremely adapted,
as an adult, to life on land.

They can even climb trees.

Över the years, the crabs have become
well adapted to human cohabitation.

These things are called robber crabs

because they have a reputation
for curiosity and for stealing things.

Anything that isn't bolted down.

They'll steal food and cameras,
if they can get half a chance.

(THUNDER RUMBLING)

These giants live on
a diet of seeds and fruit

and, occasionally, other small crabs.

Their large, powerful claws mean
they can also rip open fallen coconuts.

(LAUGHING) Guite a menacing animal,
actually, for a crab.

What's wonderful about these crabs,

is that they live through
a range of scales.

At different times of their lives,

they have a completely different
relationship with the world around them,

simply down to their size.

Throughout their lives, robber crabs
take on many different forms.

They begin their lives as small larvae
swept around by the ocean currents.

As they grow,

some of them get swept up onto
the beaches of Christmas Island,

where they find a shell,
because they are, in fact, hermit crabs.

They live inside the shell for a while,
they continue to grow and, eventually,

as adults, they roam the forest
like this chap here.

So, these crabs, over that lifespan,
inhabit many different worlds.

On land, the adults continue to grow

and now have to support
their weight against gravity.

Compared to the smaller crabs
whizzing around,

these giants move about
much more slowly.

But they also live far longer.

Of all the species of land crab
here on Christmas Island,

the robber crabs
are not only the biggest,

they're also the longest living.

So, this chap here is probabliy
about as old as me.

And he might live to 60, 70,
even 80 years old.

Because of the robber crab's
overall body size,

its individual cells use less energy
and they run at a slower rate

than the celis of their much smaller,
shorter-lived cousins.

The pace of life is slower
for robber crabs.

And it's this that's thought
to allow them to live to a ripe old age.

Your size influences
every aspect of your life.

From the way you were built,

to the way you move

and even how long you live.

Your size dictates how you interact
with the universal laws of nature.

So there's a minimum size

which is set ultimately
by the size of atoms and molecules,

the fundamental building blocks
of the universe.

And there's a maximum size
which, certainly on land,

is set by the size
and the mass of our planet.

Because it's gravity that restricts
the emergence of giants.

But within those constraints,
evolution has conspired

to produce a huge range in size
of animals and plants.

Fach beautifully adapted to exploit
the niches available to them.

Your size influences your
form and construction.

It determines how you
experience the world

and ultimately how long
you have to enjoy İt.