Original subtitles

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.