Sex 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.

In this series,

I've been exploring the natural world...

from the inside out.

So far on my journey we've seen how a single, basic body plan

has given rise to vertebrates of

practically every possible shape and size.

Bones have evolved for running...

..flying...

hunting...and even sensing the world.

But there is one significant and defining power

more important than anything else,

and bones have a crucial part to play.

And that is sex.

'This time, we'll discover just how important the skeleton is

'in the race to reproduce.

'From courtship...'

This skull blows my mind.

It looks like an alien, but there's nothing extraterrestrial about it.

'..to competition...'

When this weight hits the ground, that's approximately the same impact

as two bighorn sheep smashing their heads together.

'..and finally copulation.'

It's the largest penis bone on earth.

I'm going to reveal the Secrets Of Bones.

Sex has had a dramatic impact on the vertebrate skeleton.

And, in the struggle to reproduce,

animals have evolved some extreme skeletal adaptations

to maximise their genetic success.

To begin, I'm going to look at the lengths vertebrates go to

to stand out in a crowd.

Attraction isn't always about bright colours, big feathers

or some impressive dancing.

It runs bone deep, too.

And for many vertebrates, the skull is crucial during courtship.

Take this gorilla skull here, for instance.

It's got this amazing structure on the back of the skull.

Now this the sagittal crest.

And when I see one of these usually it tells me

that there are massive muscle attachments which go right down

the side of the face and to the jaw.

These guys have a very heavy, fibrous diet

and need to do what we call industrial processing of their food

where they chew and chew and chew.

But if this adaptation has evolved purely to help gorillas survive,

you'd expect to find it in both sexes.

Although female gorillas have a practically identical diet,

by comparison, their sagittal crests are much smaller.

It's only in the big, top males

you see a crest like this which is just so prominent.

And I think I know why.

In adult male gorillas,

the bony ridge acts as a base for a layer of fatty tissue,

creating a huge crest.

But this isn't just for eating,

it's also for display...

like a peacock's tail.

Males with bigger crests tend to attract more mates.

It's thought that a large ridge is an indicator

of a strong, healthy individual.

In the natural world, females are much more likely to select a male

who displays signs of superior genetic fitness.

This is sexual selection.

And it's driven the skeleton to adapt in some extraordinary ways.

Here I've got another fascinating skull.

Now this is from an apex predator - it's from a lion.

These canines are massive

and they're perfectly built to kill.

Lions can attack and kill animals much larger than themselves.

And their teeth are essential for the job.

Look at the size of these canines -

they're almost twice the size of the lion's.

But the biggest thing this is likely to hunt is a bug.

And it's far more at home eating fruits from the forest floor.

Now, this is the skull of a mandrill,

the largest species of monkey in the world.

Now, these big canines do have a very important function,

but it has nothing to do with hunting.

Although quite useful when stripping fruit,

they're surplus to requirements.

In mandrill females, canines grow to around a centimetre in length.

But in males, they can reach up to six times longer.

Their teeth have evolved to be this long,

not for eating, but for a different reason - reproduction.

In mandrills, big teeth indicate healthy genes.

So, for the much smaller females, size does matter.

Because only when a male's canines exceed 3cm

does he have any chance of being selected for mating.

The mandrill has successfully changed the primary function

of its teeth to aid courtship.

But one vertebrate has gone even further.

This skull blows my mind.

It looks more like an alien, but there's nothing extraterrestrial

about it, and it is a real animal, I promise,

and it can be found on Earth.

Now, once again, this is a male.

And the most obvious features are these two things here.

Now, these are modified teeth. These are the animal's canines,

but unlike mine or the gorilla's here,

which grow downwards, these instead grow up from the skull.

And when they grow, they curve,

and they keep growing and they keep curving.

This can happen to such an extent that, in some very old males,

they've actually been found to penetrate the bone in the skull.

They go through the brain case and eventually into the brain.

This would spell certain death for the animal.

Attracting a mate is so important to the species,

they effectively risk their lives in order to do so.

Here you can see just who that skull belongs to.

Now, this is a babirusa, an animal from the island of Sulawesi

in Southeast Asia.

Babirusa are members of the pig family.

Large tusks are only found in males,

and appear in adolescence when they are around a year old.

They start by puncturing the upper lip,

and continue to grow throughout the animal's life.

But their specific purpose has been debated for many years.

Local legends say that babirusa use them to hang from branches,

so they can escape big cats or spy on females passing below.

Although this story is far-fetched, females are a factor.

Because for babirusa males, it's all about getting noticed.

Just like the gorilla's crest or the mandrill's canines,

researchers believe that tusk length is a sign of genetic health.

So males with longer, curlier tusks

are more attractive to babirusa females.

Skeletal adaptations play a crucial role in courtship.

But sometimes the biggest challenge is actually locating a mate.

And for one elusive species, brand-new research suggests

that the skeleton could help in tracking down a partner

in the middle of nowhere.

This animal's scientific name, Monodon monoceros,

means one tooth, one horn.

This really is a unique tooth,

and it's the only example in nature of a tooth that spirals

and a tusk that's straight.

This is the narwhal.

Known as the Arctic unicorn, these strange and secretive members

of the whale family are found in the outer reaches

of the northern oceans.

Their remote existence has meant they are very difficult to study.

Over the centuries, this has led to many conflicting theories

about the function of their impressive tusk.

Some thought it was an ice breaking tool.

Others believed it was a weapon for jousting in the open ocean.

People also thought it was a feeding device, and this makes sense -

you can imagine the narwhal swimming through the water,

finding a fish, spearing it.

But then it's stuck and it can't get the fish off,

so this idea doesn't work either.

What we now think is that this wonderful tusk

is some sort of sensory organ.

Scientists at Harvard University

believe that males use their sensitive tusks to find females

in this icy wilderness.

The two sexes live apart,

only seeking each other out in the mating season.

The Harvard team believes that minute sensory pits

along the outside of the tusk are the key.

There are as many as 2,500 in one square millimetre,

and they're thought to sense slight changes in pressure,

temperature and even salinity.

Early evidence suggests that by detecting subtle gradient changes

in the water particles,

males can home in on female pods in the vast, open ocean.

Even now, we're just beginning to understand how important

the skeleton is in the quest to reproduce.

For most vertebrates, finding a mate is only half the story.

Fighting off rival suitors is just as important.

As males compete for access to females,

sexual selection has shaped bones into weapons for combat.

Antlers, horns, and sheer bulk are all important

when it comes to winning a mate.

But you don't need to be huge to go into battle.

Hidden in the vaults of London's Horniman Museum

is a miniature fighter with some pretty impressive headgear.

This may look like a mini triceratops,

but it's actually a chameleon.

These three horns are real bone,

and the males use these for a spot of jousting.

Jackson's chameleons live in the forests of East Africa,

and, even though they're reptiles,

their horns are made of exactly the same material

as you'd find on a bull.

A bony core with a keratin sheath wrapped around it for strength.

Although tiny, males can be highly aggressive,

resulting in intense physical combat.

These featherweight fighters use their imposing horns

to push each other out of the tree tops

and secure mating rights with nearby females.

It may seem extreme, but the stakes are high.

In this battle to reproduce, there can be only one winner.

But, as impressive as this little guy is,

when it comes to using your head,

there is one animal that wins hands down.

Bighorn sheep use their skulls in a way that is simply mind-blowing.

In the Rocky Mountains,

when the mating season arrives, rams gather...

..and battle begins.

Usually, it's the stronger, older rams who come out on top.

The prize is the opportunity to mate.

These brutal tournaments can last all day

with skulls colliding at 20mph.

Their horns can weigh in excess of 14kg -

more than the rest of the skeleton combined.

But what's really amazing is that the key to this animal's success

is not just these enormous horns.

The impact force of the two skulls colliding

can reach almost 3,500 newtons.

That's the equivalent of hitting a baseball

at over 140 kilometres an hour.

3,500 newtons is an impressive force to take head-on.

And, just like a baseball,

the strength of the bighorn sheep skull is all in the stitching.

Which means they can take a lot more impact than you might think.

Throughout the animal kingdom, skulls aren't just one single bone,

but many separate plates held together by stitches or sutures.

In most species, the gap between the plates

becomes fused once the bone stops growing.

But in bighorns, things are very different.

Like all skulls, they're made up from numerous individual plates,

but the sutures play a unique role.

At the moment of impact, the force transmits through the skull.

The sutures keep the joins between each of the plates flexible

so they can move freely.

The sutures act a bit like springs, absorbing the shock.

The bony sutures in male bighorns are also more complex

than in many other vertebrates, which makes them immensely strong

and helps prevent the plates separating as the skulls collide.

And when you replace a bighorn skull with one from an ordinary sheep,

you can see exactly what I mean.

To show just how special bighorn sheep skulls are,

I've devised an experiment to illustrate what would happen

to an ordinary sheep skull

without the bighorn's specialised bony protection.

OK, so I've been doing the maths

and I've figured out that if I drop a 10kg weight

from three-and-a-half metres,

when this weight hits the ground, that's approximately the same impact

as two bighorn sheep smashing their heads together.

I've put an old skull from a regular sheep on the ground.

Obviously, things would be slightly different

if it were still attached to a living animal.

But it should give an idea of how an ordinary sheep skull

measures up against the forces exerted on a bighorn.

The sutures in regular sheep

don't have the same shock-absorbing capabilities of a bighorn.

3,500 newtons of force causes countless fractures

across the entire skull.

Shattering it into thousands of tiny fragments.

Whereas in the skull of a bighorn,

the same impact is a daily occurrence.

The skeleton has evolved to play a crucial role

in maximising reproductive success.

From courtship, to competing for mates

and even during copulation.

But one bone in particular has evolved to increase

the chances of fertilisation.

Dr Sam Turvey from the Zoological Society of London

is here to show me which bone that is and how it functions.

Dr Turvey is an expert in vertebrate evolution

and is going to kick things off by testing my knowledge.

What is it, then?

Well, I do know what this is.

This is a walrus baculum.

-So, a penis bone.

-That's right, it is a penis bone.

And I know it's the largest penis bone on Earth.

I don't know much more than that. I know we don't have them...

-That's right.

-..but that's about it.

But we're in a minority there - 86% of mammal species alive today

have got penis bones of some kind of shape or size.

There's various different hypotheses for what they might or might not do.

Certainly, they can structurally support and maintain an erection,

and it can definitely allow longer sex and more frequent sex.

So, for example, lions with their bacula will have sex

-potentially up to 100 times a day.

-One day?

-In one day.

And, also, it will provide other kinds of structural integrity,

as well. So it will help keep the urethra open during sex,

so preventing the tube through which sperm passes

from being kind of being compressed and collapsed while sex is happening.

So they're really useful.

And also they are arguably the most variable single bone

shown in any vertebrate.

So there's a vast range of different sizes and structures and shapes.

This one's broken, as well. That's incredible -

that it's actually survived and it's healed, as well.

We can only hope that it broke and re-healed outside the mating season!

I hope! Is it true that walrus penises, walrus penis bones,

-were used as clubs?

-It is true, yes.

When walruses were being hunted by people like the Viking colonists,

the Norse in Greenland hundreds of years ago,

they'd also harvest the bacula from the walruses at the same time

and whittle them and carve them and turn them into axe handles and clubs.

So people would have been killed by being bashed on the head

with bacula, potentially.

Imagine being beaten to death with a penis bone.

-What's in the bag?

-Let's have a look.

Well, I've got a test for you, Mr Bones.

Can you tell me which bacula are from which species?

I can identify one.

This one is mine. Not mine!

-It is mine.

-It is yours.

-I found this attached

to a very big, dead grey seal up in Scotland last year.

But just having this next to the walrus, it's massively different.

There are very closely-related species. They're both pinnipeds, and obviously grey seals are,

overall body size wise, smaller than walruses.

-But not by that kind of scale.

-This was still seven-and-a-half foot long, this animal.

Walruses are up in the High Arctic,

and grey seals are further down in temperate regions.

But in warmer environments, you might get animals congregating together

a bit more. Whereas up in the Arctic, there's very low resources.

Animals are far more widely dispersed and so if a male walrus

encounters a female walrus, he can't be certain that female walrus

hasn't already mated with another male more recently.

-So if you've got a large baculum, it can help...

-Increase your chances.

Yeah, basically, potentially, you'd have more sperm going

into the female, a lot greater chance of fertilising the egg.

So, typically, polar species are more likely to have larger bacula.

So what else we got?

Well...

I don't know, they're both incredibly big, but...

So these two... It's interesting that you picked those two up,

because they're clearly morphologically quite similar

to each other, and, I'll tell you now,

they are from very closely-related species.

I want to say bear purely because of the size and...

OK. And bears are carnivores which do have bacula.

I'm going to say brown bear and polar bear.

That's right. In fact, this is from a Kodiak bear,

which is the largest subspecies of brown bear,

which is pretty much the same size as a polar bear.

But still, there's a massive difference in size and shape.

So, again, this is probably the relationship

between polar environment and more temperate environment,

and the relationship between needing larger bacula in polar environments.

-I've been buying myself some time.

-Yeah, I've noticed that.

Tell me which way round it goes first.

-That way, attaching...

-So this is kind of like the pelvis here.

-I don't know.

-Well, shall I tell you what it's also called?

-Give me a clue.

-It's called a Texas Toothpick.

So it's a geographical clue. What might live in Texas?

It's a carnivore that lives in Texas,

that's got a baculum that big.

I'm going to go out on a limb and say raccoon.

-It is a raccoon baculum.

-Really?

-Full marks, Mr Bones.

-That's such a weird shape, as well.

It is. Again, no-one's really quite sure exactly why they're that shape.

The suggestion is that it can be a kind of lock-and-key hypothesis.

That could help it kind of slot into the female pelvis a bit better.

So, it's interesting, isn't it?

For a group of relatively closely related mammals,

they're all within the Carnivora,

the variation in size and structure is quite remarkable.

In fact, bacula are sometimes used as good taxonomic indicators -

ie you can differentiate species,

sometimes solely on the basis of their baculum morphology.

The thing that I like most is that you can tell so much from,

effectively, one little bone - whether it's behaviour,

or particular niches in which the animal lives

or about the animal itself.

And, for me, this sums up perfectly why I love bones.

Sex has shaped the vertebrate skeleton -

from large and dramatic adaptations to the more cryptic and understated.

This remarkable diversity has stemmed from one bony blueprint.

These animals all have essentially the same basic skeleton.

A skeleton that has enabled vertebrates to move...

..to sense the world,

to feed, and to thrive in every habitat on Earth.

And the extraordinary secrets of how each species lives its life

are hidden in their bones.

Through looking closely at their skeletons,

you can see an animal's entire life story from the inside out.

With the incredible diversity of life that we have on our planet,

it's amazing that one group of animals has come to dominate

the land, the sea and the sky.

And this is all thanks to the secrets of bones.