Into the Air Secrets of Bones

'Bones.

'They offer structure...

'support...

'and strength.

'But they have a much bigger story to tell.

'Vertebrates may look very different on the outside...

'..but one crucial thing unites them all...

'..the skeleton.

'I'm Ben Garrod,

'an evolutionary biologist with a very unusual passion.'

This is unbelievable!

There are too many skeletons for me to look at all at once!

'As a child, I was fascinated by bones.

'Now, skeletons have become my life.

'And I put them together for museums and universities all over the world.

'I'm going to explore the natural world...

'..from the inside out...'

'..to see how the skeleton has enabled animals to move...

'..hunt...

'and even sense the world.'

I will take you on a very personal journey to discover how this

one bony blueprint has shaped such massive

diversity across the animal kingdom

and how it has come to dominate life on planet Earth.

'This time, we're going to uncover how bones...'

Oh, wow!

That's absolutely amazing.

'..have enabled animals to do the most remarkable thing of all.

'Take to the air.

'I'm going to reveal the secrets of bones.'

Pretty much every group of animals, from fish...

to frogs...

..and mammals...

to snakes have had a go at getting airborne.

But only a few have dramatically changed their skeleton

and truly mastered powered flight.

The ultimate flyers have to be the birds.

Their bones have adapted not only for a life up in the sky...

..but also down on the ground.

And even under the water.

How did the skeleton enable birds to become so successful?

First up, the evolution of wings.

Surprisingly, the blueprint for all vertebrate wings can be seen

in the primate skeleton.

Like this gorilla.

And you and me.

These five digits are known as the pentadactyl limb,

and first appeared in land animals over 300 million years ago.

Basically, it was from five fingers like ours

that winged flight has evolved independently three times.

And to see how these bones first helped animals take to the skies,

I'm going back to the time of the dinosaurs.

The first vertebrates to become true flyers with fully-formed wings

and sustained flight were a type of flying reptile -

the pterosaurs.

This is a fossil cast of Pterodactylus antiquus,

a young pterosaur, about as big as a starling.

The smallest, though, were only the size of sparrows.

But the biggest pterosaurs, they were massive.

They had a wingspan of over 10m.

They were the largest flying animals to have ever lived.

Their wings would fill this room.

Pterosaurs dominated the skies for 150 million years

and had wings modified from the original five-fingered blueprint.

This is clear when compared alongside a human hand.

The first three digits adapted as grasping claws,

and the fifth digit was lost.

But the fourth digit grew really long,

as a support for the wing membrane.

And, in some species, could be several metres in length.

The word 'pterodactyl' comes from Greek origins,

and actually means 'wing finger.'

When pterosaurs were wiped out around 65 million years ago,

other flying animals flourished.

Including bats.

They developed a completely different method

of taking to the air.

They evolved a second way of flying...

..once again, based on the pentadactyl limb.

Unlike the pterosaurs, in the bats, only their first digit,

or thumb, became hooked for grasping.

The other four fingers grew extremely long, giving them

superb control over the shape of their wings in flight.

Now, on this fruit bat here,

also known as one of the megabats - sounds cool, doesn't it? -

you can see some amazing skeletal adaptations.

The first, and most obvious, is up here.

It's this wonderful forelimb,

the wing.

You can see the very long bones here,

but they end in these four

very, very elongated digits.

And these serve to open up as much

skin and soft tissue as possible,

allowing for these very broad, strong wings.

The bones are also very flexible, which helps cope with

the extreme forces acting on the skeleton during flight.

Basically, the whole skeleton works together to become

as aerodynamic and as lightweight as possible.

I'll be honest - bats amaze me because, to me,

they're fat, little, hairy mammals, that manage to stay up in the air

and they do it very well.

But more impressive to me is the fact that

there are approximately 1,000 species of bats across the globe.

And this accounts for nearly a quarter

of all mammal species on Earth.

The success of bats can largely be attributed to their flying prowess.

And that is mainly down to their skeleton.

Flexible wings allow them to catch highly acrobatic prey.

They can turn 180 degrees in less than half a wingspan.

Bats may be brilliant flyers,

but birds are the true masters of the sky...

..with almost ten times more species inhabiting practically

every habitat on the planet.

So, what's special about their wings?

They developed an entirely independent, third way of flying.

And, again, it all began with five fingers.

Rather than the elongated digits found in bats and pterosaurs,

some bird bones fused together.

Others disappeared completely.

This gave rigidity to the wing

and provided a platform for feathers to generate lift.

But it's not all about wings.

Birds had to make other significant changes

within their skeletons to become such successful flyers.

In order to overcome gravity,

it's important to become both lightweight and strong.

This is exactly what birds have done.

You can see on this pheasant how many of their bones

have fused together for strength.

The first of which is this area here, where they've fused

a load of their vertebrae and their pelvis into one big superstructure.

And you can very clearly see the edge of the wing

where you've got, not only a loss of some of the digits,

but also a fusion of several bones into one, yet again.

There's another skeletal adaptation which I love.

It's these little processes you get between the ribs,

linking one rib to the next, to the next.

This all serves to stiffen the whole ribcage,

again, making it really, really strong.

Now, you're a bird, you can fly, you've got these big wings,

you've got these massive muscles,

you need somewhere to attach these things to.

And what birds have evolved

and developed is this wonderful structure here, as well.

This is the breastbone, or the keel.

This big, flattened projection you can see here serves to anchor

all of these big muscle attachments

which, ultimately, allows the bird to fly.

One other crucial adaptation has helped birds take to the air.

And, this time, the secret is INSIDE their skeleton.

Their bones have evolved to be as light as possible.

Here, we've got a wonderful image from a scanning electron microscope

from within a bird's bone.

And you can see this whole network of rigid, internal strut-like

supports which actually prevent the bones from buckling during flight.

Compare this to a human bone,

and you can see the difference instantly.

It's much thicker, it's very dense, there's lots of marrow.

Ultimately, it's incredibly heavy.

It's the last thing you want when trying to fly.

To see how all these adaptations to the skeleton have come together,

I'm going to look at an extraordinary bird

that we see everyday -

and is often taken for granted.

The humble pigeon.

It's hugely successful.

There are over ten million pigeons in the UK alone.

And there are thought to be around 260 million in the world.

There's one particular survival technique that has allowed

the pigeon to thrive.

Specialist bird handler Lloyd Buck is going to reveal the secret

with a very short and simple flight.

-Ready?

-Yeah.

There's a little pea there.

-I'll try and get out of the way.

-OK.

By filming Smudge in slow motion,

we can see how pigeons have an explosive takeoff,

able to fly vertically, upwards, for more than 20m.

This exceptional flying ability is down to their complex physiology.

So, when they do their vertical takeoff, what's going on?

Well, it's amazing to watch, isn't it?

And when you see it slowed down,

you get more of an idea what's happening.

And you see, she's putting all her energy into one purpose,

-to clear the ground and go up as fast as she possibly can.

-Yeah.

This slow, you can see how Smudge first bends her knees.

Then pushes off the ground,

whilst flinging her wings above her head.

This powerful jump allows her to clear the ground enough

to make a complete downward stroke

without her wings touching the floor.

She can accelerate from 0 to 60mph in less than two seconds.

To see how pigeons are such skilled flyers,

we need to take a closer look at their bones.

I thought it'd be nice to have a real good

comparison between the live bird and my sort of bird.

The first thing that sticks out is this massive keel.

Quite big for a bird that size.

I've never seen a pigeon's skeleton before today.

I'm only used to seeing live ones, like Smudge here.

When I first looked at it, I thought, "That's not quite right, is it?"

But, actually, when you look at Smudge, look...

Right down there, you can really see it.

And there are so many adaptations that just points to this

being an absolute powerhouse of muscle in flight.

If you look at the bones in the upper arm, the humerus,

they're really short. Having that short, stubby, stocky little bone

really allows that power, again.

Even that one bone says this is a very strong bird.

-Incredible.

-Brilliant.

You can see these massive legs. They just keep going up.

-And they're really strong, aren't they?

-Yeah.

-You can tell that this bird is one hell of a flyer.

-Yeah.

Their stocky and flexible legs, big, muscular keel

and short, manoeuvrable wing bones allow them to perform

powerful vertical take-offs.

But why do they need this skill?

It's as much being a feral pigeon.

They spend a lot of time feeding on the ground.

So, if a ground predator or an aerial predator comes in to try

and kill them, they need to be able to get away as quickly as possible.

-And the vertical take-off is a brilliant method.

-Yeah.

They can out-climb a peregrine, if they need to.

A peregrine has no chance of matching them for climbing speed.

And also, they can just keep going over distance,

getting close to 100km an hour.

-They could do up to 800km in a day.

-In one day?

-At that speed.

-That's phenomenal, isn't it? It really is.

PIGEON WARBLES

HE WHISTLES

The pigeon is the ultimate all-rounder.

But the basic bird skeleton has adapted in other species

for extremely specialised forms of flight.

WINGS SCUD RAPIDLY

For speed.

Manoeuvrability.

And long-haul travel.

At more than 3.5m,

the albatross has the longest wingspan of any bird.

They require a good run-up

to allow enough air to move over their wings to generate lift.

Once airborne, they rarely need to flap their wings,

using a soaring technique

to glide on wind currents for thousands of kilometres.

How can they undertake such epic journeys?

The wing bones are very, very long and very, very straight.

This wing allows the animal to soar and glide,

in much the same way that an aeroplane's wing would.

On top of that, there is another very specific adaptation,

and that's a very large tendon that sits in the shoulder area

and travels all the way down the humerus, up and over the elbow.

This allows the wing to be locked into place.

This ability to effectively lock their wings during gliding

allows them to fly effortlessly,

conserving valuable energy.

We can see from this X-ray image

that part of their tendon has also become hard and bony.

Known as a spreader bone, it offers stability and support to their wings

during long periods of flight, and reduces muscle fatigue.

This bird can glide like almost no other.

And it can travel for 15,000km,

from the moment they take off to the moment they return to the ground.

As one of the heaviest flying birds,

the albatross needs a colossal wingspan to cope.

But some big birds have tiny wings.

I'm on my way to the Royal Veterinary College near London

to understand why their skeletons have specialised in this way.

-Hello! Be careful here. It's really muddy as you go in.

-OK.

Professor John Hutchinson is an expert in animal locomotion.

It's safe.

Hello!

They're pretty mellow.

Cheeky animals, too!

The peck won't hurt you, it's the kick you've got to worry about.

But they won't use it unless they're threatened.

Why do you have a field full of emus?

Yeah, yeah, so emus are just really cool birds.

Although they look kind of dinosaur-like, and they don't fly,

so that seems primitive, actually, they're specialised,

they're advanced, for a bird. Because most birds fly.

They've lost their flight and become an extreme runner,

a real athlete on land.

Oh-ho, looky!

John and his team have been studying how the emu's anatomy

is adapted for running and not flight.

-Being this close, you can really see, there's almost no wing.

-Yep.

-Where is it?

-Well, it's just a little nubbin,

dangling down in front of the knee here.

Really small, really fragile.

I happen to have a wing of an emu.

-Check that out, isn't that cool?

-So tiny, yeah.

There's the humerus, forearm bones, radius and ulna,

and there's the wrist and the hand.

Very, very short hand. And the claw.

They don't get much bigger than that.

It really shows you close-up just how small,

and just that massive reduction they've had.

It makes sense to lose flight

when it's no longer favoured by natural selection.

Because flight is energetically expensive.

Yeah, I guess so. It's almost like a trade-off.

They've lost the ability to fly,

but they're compensated by having massive legs.

Exactly. A bird can't be both a super-fast runner and a great flyer.

It's one or the other. And emus really are at the one extreme

-of being a great runner, not a flyer at all.

-OK.

Show us the wings, come on...

Emus aren't alone when it comes to being superb runners.

Ostriches are the fastest birds in the world,

when it comes to sprinting.

They can reach speeds of over 40mph.

And once again, the secret is in their bones.

These are the leg bones of five different land animals.

Now, they're the femur, which is the bone in the upper thigh.

We've got a camel, a horse, dog,

ostrich and emu.

Now, they all look incredibly similar to each other

and that's because, technically,

they all have the same functional role, which is support

and a lot of weight-bearing at the top of the leg.

But what's weird and quite interesting is that these two,

from the flightless birds here, the ostrich and the emu,

they look really heavy, robust, thick-set.

But they're actually really light.

HOLLOW CLACK

To understand why, I need to saw one open and take a close-up look.

Ha-ha! Oh, wow!

Look at that.

That's absolutely amazing.

Now, this ostrich bone perfectly demonstrates why it's so light.

These big, flightless birds have retained so many

of the characteristics that you'd see in the original flying birds.

These bones genuinely are more air than they are bone.

That same honeycomb structure found in flying birds

is still here in one that lives on land.

But what happens when birds take to the oceans?

Penguins lost flight around 65 million years ago.

About the same time as when the dinosaurs died out.

It's thought they lost this ability

because they no longer had any sort of land predators.

You can see from this little guy,

he's really not too fussed that I'm next to him.

PENGUIN HONKS

HONKING FADES

Exactly!

But it wasn't just the lack of land predators

that led to penguins becoming flightless.

It was also their need to swim.

Penguins can travel at over 20mph.

They need to be fast

to dodge predators like leopard seals

and hunt down their prey.

But, to be this manoeuvrable underwater,

something happened to their bones that then made it impossible to fly.

It's only when you look at their skeletons

and their bones, specifically,

that you can really see the actual story behind what's going on here.

Every single bone is heavier than you'd expect in a bird.

When you've got flying birds, they've got very dynamic

and lightweight bones. And that's perfect for them.

If you live underwater a lot of the time and you hunt underwater,

you need heavy bones.

This allows the skeleton to act as ballast.

If you take a close-up look at a penguin bone under a microscope,

you can see just how dense it is,

compared to that of a flying bird.

If you look at specific areas of the skeleton as well, you can see there

are some perfect adaptations for this hunting, underwater lifestyle.

First of all, if you look at the wings.

Now, they're not very long, but they're very broad

and the leading edge and trailing edge are actually quite sharp.

This allows the penguin

to have a very rigid wing, that you can see here.

And again, this is perfect for slicing through the water.

One of my favourite adaptations in the penguin, though,

are these things. These are massive scapula.

These are the shoulder blades.

They're huge! They're absolutely monstrous!

If you watch a penguin power through the water,

they're constantly paddling and paddling,

and because water offers much more resistance than air,

they really need a lot of power up in the shoulder area,

to really pull themselves through the water.

With their large scapulae, paddle-like wings

and heavy bones, penguins have traded the ability

to fly in the air, to effectively fly underwater.

When you see one shoot past,

they're like a little fat, feathered torpedo!

It's only when you finally see the bubbles

that you kind of remember they're underwater.

We see penguins in an almost comedy light.

And it's wrong, because they're not.

They're predators, and they're good predators.

They live in the Southern Ocean, round Antarctica,

and it's not easy to live down there.

They're tough, tough animals.

'One bird has adapted for life underwater,

'on land,

'and in the sky.

'It really is my ultimate bird skeleton.'

This little bird is such a paradox.

It doesn't look as though it's very good at flying,

and it doesn't look as though it's very good at swimming.

But actually, this wonderful

little interesting bird is both.

'It's the guillemot.'

'Guillemots live in large colonies on coastal cliffs.

'Although they appear quite clumsy when taking off,

'they're surprisingly good flyers,

'capable of speeds of over 40mph.

'And when they hit the water, their versatility really becomes apparent.

'You'd think that their wings would be too cumbersome for diving,

'but they swim with them half closed to reduce turbulence.

'Guillemots can reach depths of over 150m.

'This puts them amongst the deepest divers of all birds.

'To really understand how they can be both skilful flyers

'and impressive divers, you have to look at their bones.'

Now, we've got a specialist flyer, the pigeon here,

and we've got a specialist diver, the penguin.

We've just got a few bones of the wing,

that's enough.

I think probably my favourite way to look at the differences here,

and I love this technique,

is to get a torch and shine it through the bones.

You can see, if I shine it through the pigeon,

this light shines through them perfectly,

and you can really see, they're almost translucent.

And this is what you'd associate with an animal that has

lightweight bones which is essential for flight.

If we go the opposite end of the scale and look at the wing bones

from a penguin, you can barely see that light coming through.

This is because they're incredibly dense bones to counteract buoyancy.

Where will the guillemot fit?

You can see, if you have a good look with a torch,

it's somewhere between the two, it's a happy medium.

You can see through slightly, but it's much more dense.

It's not as dense as the penguin,

but it's definitely more dense than the pigeon.

'This amazing adaptation,

'bones light enough to fly

'and yet heavy enough to dive,

'makes this one of the most impressive birds on the planet.'

My little guillemot here really is the ultimate flyer,

and for that reason, I'm in love with this bird, it's brilliant.

'The skeleton has enabled birds to conquer the sky.

'And also the land.

'And even the sea.

'Next time, we'll discover how bones have evolved to detect prey...'

What you've got, in effect,

is a 40 or 50-tonne rigid swimming radar gun.

'..and to sense the world around us.'

These eyes are so large that each one is larger than

the animal's own brain.