Size Matters Wonders of Life

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, 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.

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.

It is constrained by the universal laws of physics.

Each tree has to support its mass

against the downward force of Earth'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 height of a tree,

which theoretically lies somewhere in the region of 130 metres.

With its forests and mountains...

Oceans and deserts...

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

I want to see how the laws of physics

govern 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 tonnes,

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.

And 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 differently, based on our size?

The size you are

profoundly influences the way that you live your life.

It selects from 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.

These beasts are feared around the world,

a fear not helped by Hollywood filmmakers.

I'm here to swim with great white sharks.

ENGINE STARTS UP

-How big... How wide can they open their jaw?

-Three foot wide.

-About three feet.

-They can swallow a man whole.

-Yes.

So about three...

Three foot wide, can swallow a man whole.

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

The crew ready the cages.

The last time I dived was in the marina in Brighton.

I did see a fish.

It was about that big.

From that to the largest marine predator.

CLEARS HIS THROAT

As the sharks start to circle, it's time to get in.

There he is. There he comes.

Just look at that. He's just checking us out.

Well, 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.

HE LAUGHS

Well, I never would've 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 million 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 agility.

They've been sculpted by evolution,

acting within the bounds of the physical properties of water.

Now, he's about five metres long.

He weighs about a ton.

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

When he's attacking,

he can accelerate up to over 20 miles an hour.

They can launch themselves straight out of the water.

There he is! There he is.

Whoa!

Whoa!

I felt the need to remove my hands.

That was one of the most awe-inspiring sights I've ever seen.

A great white, just straight in front of me 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 Earth.

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 saltwater

and I'm going to weigh it.

You see, that says 25 kilograms there.

That's actually its mass.

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

which is 25 times about ten,

which is 250 kilogram metres per second squared.

That might sound pedantic, but it's going to be important in a minute.

See what happens if I lower this saltwater 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 has essentially the same density as seawater,

because it's made of seawater,

then that force is equal 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 cells of all living things are predominantly made up of salty water

so in the ocean, weight is essentially unimportant.

Because of Archimedes' principle, the supportive nature of water

releases organisms from the constraints of Earth'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 support,

it requires 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 that distinctive streamlined shape.

His maximum width is about a third of the way down his body,

and that width itself should be around a quarter of the length.

That ratio is set by the necessity for something that big

to be able to swim effectively and quickly 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.

Whoa!

HE LAUGHS

That's cunning! That was straight out of Jaws!

That streamlined shape of a shark

is something that you see echoed throughout nature.

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

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

directly by the physical properties of water.

But out of the ocean,

life now has to content with the full force of Earth'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.

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

..the red kangaroo.

Red kangaroos are Australia's largest native land mammal,

one of 50 species of macropods,

so-called on account of their large feet.

-(WHISPERS)

-There! There.

There's two 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 Endeavour in 1770,

he wrote that "They move so fast

"over the rocky, rough ground where they're found,

"even my greyhound couldn't catch them."

I mean, what was he doing with a greyhound?

Kangaroos are herbivorous

and scratch out a living feeding on grasses.

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 Earth'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 sustained speeds of up to 40 kilometres per hour.

This incredibly efficiency for such a large animal

comes directly from the kangaroos' 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 is dissipated as sound and a little bit of heat

but if you're a tennis ball...

..then some of that energy is 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 the 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 heavier 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 move around.

But not everything can move like a kangaroo.

The red kangaroo is the largest animal in the world

that moves in this unique way,

hopping across the landscape at high speed,

and there are reasons why there aren't 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 movements.

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 is.

Now, its mass is 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 going to 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,

equals eight cubic things.

Its volume has increased by a factor of eight,

and so its mass has 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 a 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 a Diprotodon,

which is the largest known marsupial 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,

and it's known that it was all over Australia,

it was the big herbivore,

and it got progressively bigger

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

and 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 skeleton had to be very strong.

This imposed significant constraints on the shape and size of its bones.

This is the fever 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.

But let me take a few measurements.

The length of the Diprotodon femur

is...what, around 75 cm.

The length of the wombat femur is around 15 cm,

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

But now look at the cross-sectional area.

Assuming the bones are roughly circular in cross-section,

we can calculate their area using pi multiplied by the radius squared.

It 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 directly 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 is an animal known as the Potoroo.

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

Then we have the Tasmanian Devil,

a wombat,

a dingo,

then the largest marsupial in Austria today,

the red kangaroo.

And 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'd require far too much energy to move at high speeds.

It's ultimately the strength of Earth'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.

This is the domain of the insects.

These animals can clearly do things I can't do

and appear to have superpowers.

They can walk up walls,

jump 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 75% of all animal species.

Some biologists think that

there may be an order of magnitude more yet to be discovered.

That would be ten million species,

and they're 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 ten 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 JBS Haldane said that

if one could conclude as to the nature of the Creator

from a 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 centuries.

Each species is wonderfully adapted to their own unique niche.

This is the beginnings of biology as a science that you see here,

it's this desire to collect and classify,

which then, over time, becomes the desire to explain and understand.

I'm going to take a picture.

Here in the suburbs of Brisbane,

every February, there's an invasion of beetles.

The rules governing their lives play out very differently 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.

These 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.

See that distinctive 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 trying to scare himself off.

Ha-ha-ha!

INSECT BRISTLES

You also heard that hissing sound.

That's him contract in his abdomen which again is a defensive

posture that he adopts to scare other males.

INSECT HISSES

Gramme for gramme, these insects are among the strongest animals alive.

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

It doesn't hurt him at all, but watch what he is able to do.

Look at that.

So he is hanging on to this branch,

which is many times his own bodyweight.

Absolutely no distress at all.

As things get smaller, it is

a rule of nature that they inevitably get stronger.

The reason is quite 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.

If I fail 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.

All things fall at the same rate under gravity.

That's because they 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.

The reasons for that are quite complex actually.

First of all, the grape has a larger surface area in relation

to its volume and therefore its mass than the melon.

Although, in a vacuum, if you took away the air,

they would both fall at the same rate. Actually, in reality,

the grape falls slower than the melon.

Also, the melon is more massive so it has more kinetic energy

when it hits the ground. Remember physics class.

Kinetic energy is ½ MV squared,

so you reduce M, you reduce the energy.

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 Earth's gravity in 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 a thing that defines my existence.

It's 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, I can pick up a piece

of paper and can hold up against the downward pull of gravity.

That's because the force of electromagnetism is important.

In fact, it is 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.

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

Many insects can use a similar effect.

Take a common fly for example.

Their feet have especially 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 jar.

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

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 supers trength, are not super powers at all.

They're just abilities gained naturally 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 a 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.

It's got compound eyes, six legs and 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.

Theyr'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.

What you're seeing here is the surface of the macadamia nut

and here's 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 a very sticky world,

dominated by strong intermolecular forces.

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

they essentially swim, using paddle-like wings.

Incredibly, 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 their limit.

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

To put it into perspective,

most tiny insects have 100 times that many, but that's still

enough to allow them to exhibit quite complex behaviour.

These micro-wasps exist at almost the minimum possible size

for multicellular 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 earth.

The smallest organisms on our planet are also our oldest

and most abundant type of lifeforms.

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're built up over centuries

by colonies of microscopic bacterial cells.

Although these colonies are rare, by most definitions,

bacteria are THE dominant form of life on our planet.

On every surface across every landscape, you find bacteria.

In fact, numerically speaking, then there are more bacteria

living on and inside my body than there are human cells.

Bacteria come in many shapes and forms

and are not actually animals or plants,

instead sitting in their own unique taxonomic kingdom.

Compared to the cells we're made of,

bacteria are structurally much simpler and far, far smaller.

Bacteria are typically around two 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 of a pin or, to look at it 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.

SPLASH

Bacterial-type organisms were the first life on Earth

and they've dominated our planet ever since.

Excluding viruses, which by most definitions are not alive,

bacteria are the smallest free-living lifeforms we know of.

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 around 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

govern your life. Gravity imposes itself on the large,

and the electromagnetic force rules the world of the small.

But the consequences of scale for life on Earth

extend beyond dictating the relationship

you have with the world around you.

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

BATS SQUEAK FAINTLY

These are southern bent-wing bats...

..one 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 5.5cm long,

and weigh around 18 grams.

Because of their size, they face a constant struggle to stay alive.

BATS SQUEAK, CRICKETS CHIRP

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 -

animals that maintain their body temperature.

And that takes a lot of effort.

These bats have to eat something like

three-quarters 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're 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 governing the relationship

between the surface area of a body and its volume,

being small creates a problem.

BATS SQUEAK

So, let's look at our blocks again,

but this time for surface area to volume.

Here's a big thing -

it'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 3:1.

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

so its volume is one unit.

Its surface area is one by one by one, six times, so it's six.

So, this has a surface area to volume ratio of 6:1.

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 rapidly, 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".

-EXTREMELY FAST HEARTBEAT

-For Australia's small marsupial mouse,

even at rest, his heart is racing away.

-SLOWER HEARTBEAT

-For the fox-sized Tasmanian devil,

he ticks along at a much slower rate.

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

Looking beyond heart rate,

your size influences the amount of energy you need to consume,

and the rate at which you need to consume it.

Bigger bodies have more cells 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 gramme for gramme,

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 colloquially is 16,000 calories.

That is eight times the amount that I take in

on average on a daily basis.

Each of the cells in my body requires less energy

than the equivalent cells 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. It 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 energy.

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

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

is to operate at a reduced metabolic rate.

If 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 one last consequence 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.

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.

Over the years, the crabs have become

well adapted to human co-habitation.

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.

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.

They're really quite 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, and 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 their shell for a while,

they continue to grow, and eventually, as adults,

they roam the forests 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,

robber crabs are not only the biggest,

they're also the longest-living.

So this chap here is probably 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 cells 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,

each beautifully adapted to exploit the niches available to them.

Your size influences your form and constriction.

It determines how you experience the world,

and ultimately, how long you have to enjoy it.

Subtitles by Red Bee Media Ltd