Sensing the World 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, to eat

and even find a mate.

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.

This time, we'll discover the way

bones allow animals to perceive the world.

Looking at each sense in turn, we'll find out how vertebrates have

evolved to see, hear and smell.

This tiny little bone that is unique to the species has radicalised

the way it feeds, the way it forages, the way it survives.

And even use senses that appear supernatural.

What you've got in effect is a 40 or 50-tonne, rigid, swimming radar gun.

I'm going to reveal the Secrets Of Bones.

I've been building the skeleton of a lowland gorilla, and when

thinking about how it senses the world, it strikes me that there's

one part of its skeleton that's more important than any other - the skull.

Skulls evolved for one function - and that was to house the brain,

at all costs, to protect the brain inside.

But since then they've changed, and they've adapted and evolved

specifically to become a sensory hub. They allow a sense of smell,

hearing and, importantly, the sense of sight.

On the outside, it might look like the weird

and wonderful sensory organs

are formed just from skin and soft tissue,

but that couldn't be further from the truth.

The bone itself is absolutely vital,

and the skull is at the centre of the bony adaptations for sensing.

These adaptations are so clear

that I can often work out how an animal hunts,

navigates and avoids being eaten, just from looking at its bones.

First, I'm going to look at sight, which is

the gorilla's most important sense -

and that's evident by the large orbits or eye sockets in the skull.

Now what they do, they allow these incredibly complex,

delicate sensory structures, the eyes, to be housed

and protected in a way

that won't allow them to be damaged or knocked or squished.

I mean, the last thing you want is your eye to be ruptured.

But more than that, also it allows a direct

transfer of information from the outside world, from the eye,

through these little optic canals, right into the brain itself.

So, at a basic level, the orbits house and protect the delicate eyes.

But there's more to these bony sockets than that.

Where they're placed in the skull plays a key role in sight.

So, in my bag I happen to have two very different skulls -

the first of which is a sheep, and I also have a wolf.

The thing that interests me most is where their eyes are.

Now, on the sheep here you can see the eyes, the eye sockets,

are situated right on the side of the head, really far back.

And that's because this animal spends a lot of its life head down,

on the ground, eating, grazing,

and what's going to happen is something's going to sneak up to it.

By having these eye sockets situated way back

on the side of its head, it can see almost 360 degrees around it

and this gives amazing peripheral vision.

The opposite end of the scale is something like the wolf.

Now the wolf is an apex predator.

It doesn't need to see behind it, nothing's going to sneak up

and eat it, but what it does need is a set of eyes, a set of eye sockets,

at the front of its skull where it has amazing stereoscopic vision.

Now this means it has a huge overlap between what each eye can see

and this gives it great depth perception,

so it can see exactly how far away something is.

And this is the case throughout the animal kingdom.

Prey animals tend to have eyes on the side of their heads.

And predators usually have forward facing eyes to help them hunt.

So the position of the eyes and sockets has become

an evolutionary trade-off for both predator and prey.

As one evolves massive peripheral vision, the other evolves amazing

stereoscopic vision, and what they're both trying to do is out-compete

the other one in terms of staying off the dinner plate and having dinner.

So the orbits can usually tell me whether an animal

is predator or prey.

But that's not all.

The size of those eye sockets gives a clue

about when and where an animal hunts.

I have two skulls here from animals roughly the same size, now they're

both primates and they're about the size of a big kitten, I guess.

The first one is from a marmoset, which is

a monkey from South America, and you can see the orbits,

and therefore the eyes, are roughly the same sort of size

I guess you'd expect from an animal with a body this sort of size.

What's really special is this little fella here.

Now this is a tarsier skull,

and instantly you can see it's got these absolutely massive orbits.

And it tells me that this animal is nocturnal.

These huge orbits house a pair of enormous eyes that

let as much light in as possible, enabling the tarsier

to hunt at night.

Tarsiers have the largest eyes in comparison to body size

of any mammal.

And, remarkably, each eye is larger than their own brain.

If you were to somehow scale my eyes up to be the same size

proportionally as the little tarsier here

then each would be the same size as a grapefruit.

But having such colossal eyes does pose a problem.

The eyes are so large

they can't actually move within their own eye sockets like ours can.

To get around this, the little animal has an amazing

skeletal adaptation where it can move its skull on the top

of its vertebrae, almost 180 degrees in each direction.

It does this by having specialised joints between

its neck vertebrae so that it can rotate its head right around

and see in all directions.

Like an owl.

Many animals that operate in low light conditions rely on

extra large eye sockets, like sea lions,

that fish in murky water,

or the tiger, which hunts in dark forests.

It even translates to humans.

Polar regions receive much less light than equatorial areas,

so people with Arctic ancestry can have eye sockets

20% larger than those from the equator.

So, just looking at a skull can reveal how

an animal senses the world.

Whilst the tarsier relies on sight,

other vertebrates depend on different senses.

I'm on my way to see an animal with extraordinary hearing,

thanks again to the unusual structure of its skull.

And to demonstrate just how effective this adaptation is,

I've got a test for the animal in question.

I've got three buzzers here and I'm

going to hide them in three different locations amongst these leaves.

These buzzers are controlled by this little box here

and each of these buttons controls an individual buzzer.

When I press it...

BUZZING

..even though I've only just hidden these buzzers,

already I'm having huge difficulties in deciding

where each buzzer is and which one is buzzing.

That's because if I needed to find them I'd actually have to go

and look for them because, as a human, my main sense is my vision.

But there's one animal that can hear much better than I can

and it will be able to find these buzzers instantly.

BUZZING

Perfect, straight to the buzzer.

This is a great grey owl.

This is a hand-reared owl,

kept here at the International Centre For Birds Of Prey.

I'm going to see if he can go to the second buzzer.

BUZZING

He can't see these buzzers, they're too well hidden.

So he's relying totally on his ears.

He got there, you found your second buzzer!

You're such a clever bird!

He's been trained to come to the buzzers to illustrate just

how accurate his hearing is by curator Holly Cale.

We've seen already that this bird has an amazing ability

to hear, is that its main sense?

It's definitely up there, it's probably its most

important sense, but the eyesight is also very good.

They're very good at seeing fine changes down in the undergrowth,

they combine that with the hearing, when they need to.

Great grey owls usually hunt by perching on branches or

tree tops, watching and listening for prey below.

But the Arctic habitat of these owls means their prey is often

hidden under the snow, rendering their eyesight useless.

And that's why they've developed such sensitive hearing - they can

detect a tiny mouse under half a metre of snow

from over ten metres away.

Underneath all of the insulation that he needs to stay

warm in the Arctic he's actually not a huge owl.

The most striking thing about it is he's got this beautiful round facial

disc and that's there to funnel sound into the ears as best he can.

If I can pop by finger in the side here, very gently, roughly where

his ear is and stop when I get to his skull -

there we go - that shows you he's got a good inch of insulation.

A lot of feather.

And an inch of facial disc angling and funnelling sound into those ears

But this facial disc is just part of what gives

the great grey owl its auditory prowess.

Like most vertebrates, it has ear openings

on either side of its skull.

This means that sound reaches the two ears at

slightly different times,

allowing it to detect the direction the noise is coming from.

What's special about certain species of owl is that

one ear is slightly higher than the other.

This asymmetry means the height of a sound can be pinpointed,

making their hearing even more accurate.

He's being very well-behaved but he's constantly looking around,

he's restless it seems - is he listening all the time?

He is constantly aware of what's going on around him,

so every time there's a noise, a background noise,

bits and pieces going on, he'll turn his head,

he'll face his facial disc to where he thinks that noise is coming from

to get a better idea of what's going on in his surroundings.

Is it important, is it something he needs to worry about,

is it something worth hunting?

I genuinely think I'm in love with this guy.

He's such a wonderful little character,

but he's like this hunting, flying, predatory satellite dish.

He's perfect, isn't he?

He is, all of those things combine to make him a little star.

Whilst an asymmetrical skull allows these owls to isolate

sounds more accurately,

bone has an even more fundamental role to play in hearing.

Because without it,

most vertebrates wouldn't hear much of anything at all.

As a sound wave hits the inner ear,

99.9% of its energy would be reflected away -

and almost all sounds would go unheard - if it wasn't for bones.

Most vertebrates have developed tiny, delicate ear-bones -

or ossicles.

And mammals have three of them.

These are the human ossicles,

and as well as being very fragile,

they're the smallest bones

in our body.

They're made up of

the malleus,

incus and stapes.

These bones work together to form a vibrating chain, passing

sound waves from the malleus

to the incus to the stapes.

And because the ossicles are arranged as a system of levers, a small

force at one end becomes a larger force at the other, so not only is

a sound wave passed through to the inner ear, the sound is amplified.

And what's more, the composition of these tiny bones is different

to every other bone in the human skeleton.

Bone is essentially made up of two components -

an organic part - collagen - which provides the flexibility

and a mineral one - calcium phosphate - which gives the bone

rigidity.

These different compounds are found in varying degrees in almost

every bone in our body.

Now, these little bones here are really mineral rich,

and this makes them really hard but quite fragile.

This would be useless in something like our femur, in our thigh,

because with all that weight bearing and twisting it would simply shatter.

But whereas these little bones are protected deep within the skull,

by being very hard allows them to transfer and conduct sound perfectly.

So the chemical composition of bone

and the way the three ossicles work together

makes an extremely efficient hearing system, transmitting 60% of

the sound energy that hits the eardrum to the inner ear.

Whilst most vertebrates have just one ossicle,

only mammals have three,

helping them have some of the sharpest hearing on the planet.

There's another key sense housed in the skull,

which has more of a connection with bone than might first appear.

The sense of smell.

All skulls have an opening for the nostrils.

They're even found on birds' beaks.

Nostrils occur in different positions,

just like the eye sockets and ear openings.

And one animal has taken this to the extreme.

Kiwis are the only birds

with nostrils right on the tip of the beak.

They're nocturnal, and virtually blind

so rely on their sense of smell to find food.

As they walk, kiwis tap the ground with their beak, probing the soil

to sniff out their prey - earthworms,

insects, fallen fruits and seeds.

Having nostrils at the end of the beak means that,

when poked underground, they can smell an earthworm 15cm down.

Even at this basic level,

changes to the skeleton help the kiwi detect its prey.

But hidden inside the skull is another bony structure that

can turn an animal's sniffing into a supersense.

Here at the Oxford Museum of Natural History,

there's a perfect example.

If you look inside the nose here you can see this elaborate,

honeycomb-like structure.

These structures are actually very delicate bones,

known as turbinates.

These turbinates, along with this really long muzzle here,

tells me that this animal has an incredibly good sense of smell.

This is the skull from a polar bear.

Their eyesight is about the same as ours

but it's estimated their sense of smell is 100 times greater.

Polar bears have been reported as travelling 20 kilometres

in a straight line to reach a carcass,

which they've located by following their nose.

By sniffing the ice,

they can detect where a seal is using a breathing hole.

And can even find a seal pup hidden under a thick layer of ice

from over a kilometre away.

And it's because their prey is spread out over huge distances,

that a polar bear's sense of smell needs to be exceptional.

This extraordinary ability is down to the turbinate bones

in their nose, which form a sophisticated system for smelling.

The turbinates are separated into three very distinct areas, the first

of which are the maxilloturbinates, and they're at the front,

and are actually responsible for warming air as it enters the nose.

This is kind of essential if you're living up in the Arctic.

Behind those you have the nasoturbinates

and the ethmoturbinates

and these are the ones that are associated with a sense of smell.

This delicate bony structure is covered in sensory cells

which detect smell and transmit information to the brain.

In the polar bear, the turbinates' large size

and intricate honeycomb structure provides a huge surface area

to house a vast number of these sensory cells.

And this is what's key to giving the polar bear

such an amazing sense of smell.

There's one last nose which has to be the most bizarre

when it comes to bony adaptations for smelling.

This nose is unique

and very few people have ever seen it,

because it belongs to one of the rarest mammals on

the planet, found only on the island of Hispaniola in the Caribbean.

I've come to the Zoological Society of London to meet

Dr Sam Turvey, who can show me what this nose can do and the animal

it belongs to.

It's an animal called Hispaniolan Solenodon.

And they are a type of insectivorous mammal

they are distantly related to shrews.

They're very distinct from anything,

they diverged from all other living mammals about 76 million

years ago, that's during the time of the dinosaurs.

Looking at its snout, it looks like it's broken it,

but I'm guessing that's not the case, it's got this kink in the middle - what's going on?

We know very little about solenodons,

they're extremely threatened with extinction and there have

been very few ecological studies conducted on them.

But what we do know is that they're active at night

and also at dawn and dusk.

In fact, it will navigate around and find its prey

using that very, very elongated snout.

If you're lucky enough to see one in the wild you'll

see the snout's constantly twitching around like this,

so they're almost comic, very cute looking characters if you see them.

The selenedon is ground-based,

hunting mostly insects and other invertebrates.

It uses its snout to explore cracks

and crevices where its prey hides.

It then shoves its nose into the soil to retrieve its food,

creating holes in the ground called "nose pokes".

This strong, flexible snout is down to its peculiar skeletal structure.

And the only way to see that properly is to X-ray a rare

specimen the institute has in its collection.

So, have you had this X-rayed before?

No, it's the first time it's been X-rayed

and I can't think of many or any other times solenodons have

ever been X-rayed, so it's really interesting to see what we find.

You can see here this white area, that's

the bone of the skull and this greyer shadow, that's the soft

tissue, so you've got the snout coming down here, and this

little thing here, that is the key to the solendon's flexible snout.

What do you think that is?

I'm guessing it's an extra bone.

Yes, it's an os proboscis or a nose bone. And it's the only mammal

in the world, as far as we know, to have this unique bone and this

is what gives the solenodon that little extra edge in having

a really flexible wiggly snout.

You can see it's a ball and socket joint.

So like I get in my shoulder or my hip, it's the same thing there.

That's brilliant.

Yes, and it provides both support for this large, heavy snout

but also flexibility and leverage at the same time.

So this tiny little bone that's unique to this species

has really radicalised the way it feeds, the way it forages,

the way it survives.

It's so nice to see it as well because I've been

working on these species for so many years and I've never seen

a nice X-ray exposure of this, it's the first time for me, it's lovely.

Really interesting.

Having looked at sight, sound and smell, it might

seem that's the end of the story for bones and senses.

But some animals use bone to take their senses

to a whole new level.

This is the mighty sperm whale - a multiple record breaker.

It's the largest of the toothed whales, with some males reaching

20m in length.

What's more, it's the deepest diving mammal,

reaching depths of 3,000m - that's two miles down.

And it's during these super deep dives that it uses its

extraordinary sensory capabilities.

At those depths, it's almost pitch black,

so sperm whales navigate and hunt using echolocation.

And it's their bones that enable them do this.

The sperm whale has a truly massive head filled with a specialised oil

called spermecetti.

Now, at the front of the head, round about here somewhere, it would

produce a series of very quick pulsed clicks.

These travel back through this spermecetti to this part of the skull

and this is very concave,

it's effectively the whale's forehead, I guess.

Once these pulses are channelled and focused they shoot out.

The clicks are the loudest sounds ever recorded from an animal

and can travel for ten kilometres.

If they hit something,

the pulses bounce back towards the whale.

The whale doesn't receive these echoed return pulses

in the top of its head again.

Instead, it receives them in the lower jaw, in this area here.

The lower jaw has evolved to have this grooved channel running

all the way through the bone,

and this is filled with a jelly-like, fatty substance.

It's this that picks up these returned echoes.

It transfers it through this channel, right through the lower jaw

into this area here, and eventually into the inner ear.

After that it goes into the brain,

and this is where the animal builds up a 3D picture

of the world around it.

And there's one further skeletal adaptation which makes

this system even better.

It's in the neck vertebrae. Now, like most mammals,

sperm whales have seven vertebrae in their neck, just like we do.

But the special adaptation here is that most of them

are fused together in one large bone, you can just see there.

This serves to hold the whole head rigid.

This makes sense when you think that it's got a massive head with

a really sensitive sensory organ attached to that.

When it's firing out these little pulses and receiving the echoes,

the last thing it wants is a head that's all over the place and wobbly.

By being held in one position ensures that these

pulses are received as accurately as possible.

So what you've got in effect is a 40 or 50-tonne, rigid,

swimming radar gun.

It's a combination of skeletal adaptations

which add up to create a deadly and sophisticated sensory capability.

This is just one of the countless methods vertebrates use

to sense the world.

Underneath muscle and soft tissue, bone is evolving

to enable them to do this in an ever increasing number of ways.

Be that with enormous eye orbits and a specialised neck joint,

asymmetrical ear openings, or complicated nasal turbinates.

Bone is vital for finding food, detecting predators and navigation.

Next time, we uncover how the skeleton is essential

for capturing and devouring food.

From the enormous...

Each one of these molars can weigh up to 5kg.

..to the bizarre...

It's more alien than it is animal, and it's one massive killing

machine head, with a little tail.

..as we delve even deeper into the Secrets Of Bones.