Natural Curiosities (2013–…): Season 4, Episode 5 - Incredible Shells - full transcript

The natural world is full
of extraordinary animals

with amazing life histories.

Yet certain stories
are more intriguing than others.

The mysteries of a butterfly's
life cycle

or the strange biology
of the Emperor Penguin.

Some of these creatures were
surrounded by fantastic myths

and misunderstandings.

Others have only recently
revealed their secrets.

These are the creatures
that stand out from the crowd -

the curiosities that I find
particularly fascinating.

In this programme, we investigate
two types of shell.



The shells of tortoises and turtles,
that act as body armour,

and the shells of birds
that protect their growing chicks

until they hatch.

How did these different shells
evolve

and do they offer more
than just protection?

Tortoises and turtles have shells
that enclose their bodies almost

completely, leaving only the legs
and the head sticking out.

It's a unique body plan that has
been around for a very long time.

Tortoises and turtles
are very ancient creatures.

They have an ancestry which
stretches back 200 million years

and individuals like this one
from the island of Aldabra

in the Indian Ocean
can live for 150 years.

Their shell is obviously
very good protection,

but it does bring disadvantages.



It's awkward to get about, hence
the legendary speed of the tortoise,

and also, of course, it can make
mating quite tricky.

But this is much more than
a lifeless, boney box.

This is part of the living skeleton
of the animal.

There are several hundred species
of tortoise and turtle

with a great variety
of beautiful shells.

Tortoises live on land and turtles
are almost exclusively aquatic,

but the two groups
have much in common.

The shells of both are covered
with horny plates

that get bigger as the bone
beneath them grows.

In doing so, the plates
develop rings of ridges

rather like the growth rings
in a tree.

But, for a long time,

no-one really understood
how the boney shell itself

came into existence.

The very first fossil tortoises
to be discovered

dated from around
215 million years ago.

The strange thing about them
was that their shell

was pretty well complete.

In fact, they looked very much
like this fossil,

which is 60 million years old.

And its shell, as you can see,
is, in fact, fully formed.

But nobody had discovered fossils

which give us any idea
as to how this sort of shell

might have developed.

Some suggested that the earliest
ancestors of the group

grew thick plates in their skin,
much like those of a crocodile,

and that these gradually expanded
until they joined up

and formed a complete shell.

But that didn't explain how
the shell became a sort of box

into which the animal
could retract its legs and neck

when danger threatened.

But if you look closely at the
structure of a tortoise's shell

and examine the way it forms
as the animal grows,

you can get clues that suggest
a very different origin.

This beautiful little spider
tortoise from Madagascar

has a shell in two parts,
like all tortoises.

The top part is called the carapace

and the lower part is called
the plastron.

And the two are fused together.

The surface of the shell
is covered with these scutes,

which are made of keratin, the same
substance as our own fingernails,

but beneath the scutes,
it's solid bone,

and it's formed in the most
extraordinary way.

The ribs grow backwards and upwards

in order to cover the pelvis
and the shoulders.

So, in a sense, you could say
a tortoise's skeleton

has become turned inside out

with the shoulders and the pelvis
within the ribs.

A tortoise is therefore, in effect,
trapped inside a boney box,

and that, as you might imagine,

makes some things you want to do
rather difficult.

Mating, for example,
will not be easy.

One thing that helps is the fact
that the lower half

of the male shell, the plastron,
has become somewhat concave

so that it fits over
the female's rounded back.

But, even then,
it's obviously not easy.

Imagine making love
in a suit of armour.

Not elegant at all.

So, for a very long time,

the origin of the tortoise's shell
was a puzzle,

and it remained so even to that
greatest of all naturalists,

Charles Darwin.

In the 1830s, Darwin visited
the Galapagos Islands

and was very struck by the giant
tortoises that lived there.

He was particularly fascinated
to see that tortoises

on different islands
had differently-shaped shells.

Those from Floreana and Espanola
islands were like this -

they had a prominent arch
that curved up at the front

so that the neck
could reach upwards.

And those from Santiago Island
were more dome-shaped at the front.

He watched carefully and noticed
that the shell shapes

related to the way
that each type of tortoise fed.

Here was evidence that differing
environments could,

over a long period of time,

influence the shape
of an animal's body.

An observation that was going to be
an important clue

in unravelling the mysteries
of evolution.

On the islands with humid highlands,
tortoises had short necks.

Here, vegetation is low
and close to the ground

so the tortoises have no need
to reach up to feed

and their shells are dome-shaped.

The neighbouring islands,
however, are much drier,

with harder-to-reach leaves.

Darwin noticed that here the
tortoises had very long necks

and a peak in the front of the shell

that allowed them to crane their
necks upwards to reach leaves.

If all of these tortoises
were related,

then they must have changed
over time.

The variations in shape enabled
different groups of tortoises

to survive in different
island environments.

But, sadly, their shells
couldn't protect them

from one great predator.

While Darwin was studying
Galapagos tortoises,

the seamen had a very different
interest in them.

The animals were a convenient source
of live food onboard ship

and Darwin watched
as they were carried away.

In his journals, he wrote that
single vessels had taken away

as many as 700

and that the ship's company
of a frigate, some years since,

brought down in one day
200 tortoises to the beach.

Tortoises could survive for
a very long time on just water,

with little or no food,

and so could be slaughtered
after weeks at sea

to provide welcome fresh meat.

Between 1831 and 1868,

79 whaling vessels collected
over 13,000 Galapagos tortoises.

Although the Galapagos tortoise
had given clues to Darwin

about the processes of evolution,

the origin of tortoises in general
was still a mystery

and it remained that way
for a very long time.

Then, in 2008, a new clue emerged.

A 220 million-year-old
fossil tortoise,

that predated those
with fully formed shells,

was discovered in China.

It was named Odontochelys,

and although it was adult,
its shell was still incomplete.

It had the beginnings of a plastron
and, on its upper side,

broad ribs that were starting
to form the upper shell.

So here was an important stepping
stone in the biological journey

that would, in time, lead to
tortoises with complete shells.

But this change came at a cost.

When you look inside the shell
of a tortoise or a turtle,

it seems remarkably empty.

Here are the leg bones,
but where are the ribs?

And the answer is that they
have become amalgamated

with the top of the shell

to give it great strength.

But that brings a problem.

We use our ribs to help us make
the lungs work like bellows.

Breathing in and out.

But a tortoise and a turtle
can't do that.

Instead, they have a kind of
muscular sling

which gives a kind of
internal diaphragm,

which moves the lungs in and out.

Rather awkward, you might think,

but a small price to pay
for the great strength

that the ribs now give
to the top of the shell.

The tortoise's shell is a wonderful
example of a multipurpose structure.

It acts as an external protection...

..but also as a solar panel
collecting heat from the sun.

Tortoises are cold-blooded,

so that ability is very valuable
and important to them.

A turtle's shell obviously gives it
very good protection,

but it's not just inert bone,
it's more than that.

On the underside of this,
there are great areas

which act as reservoirs
for important minerals.

Sodium, potassium, magnesium
and phosphate.

And when those minerals are needed
for the workings of the body,

they can be withdrawn
from those reservoirs

with the help of blood
and the body fluids.

The shell's natural
chemical reservoir

is particularly valuable
for sea turtles.

When they dive, they hold their
breath for long periods

without suffering from the dangerous
build-up of toxic chemicals.

This is partly because their shell
can store and release

the minerals that prevent damage
to the body tissues.

The shell is also a survival tool.

he North American freshwater
painted turtle is forced, in winter,

to spend months underwater because
their pools are covered by ice.

The oxygen in their body
then falls to very low levels

and that leads to lactic acid
accumulating in their tissues.

But their shell then releases
carbonates

which neutralise the acid
and store it out of harm's way.

As a result, turtles can survive
submerged in oxygen-poor water

for up to three years.

So, the shell, in fact, provides
much more than physical protection.

Understanding its complex
evolutionary history has taken time

but, just recently,
a chance discovery

has helped complete the picture.

In the year 2013,

scientists working in the Denver
Museum of Nature and Science

re-examined a fossil,
a 260 million-year-old fossil,

that had been dug up
in the previous century,

given the name Eunotosaurus,

and then put in the storage vaults
and forgotten.

But when they came to look at it,
and this is it,

it had nine flattened ribs

and elongated vertebrae,

just like the first tortoises.

A missing link had been found.

A creature that linked the early,
land-living reptiles

with the first shelled tortoises.

This is an artist's impression
of it,

with the very beginnings
of a shell on its back.

So now the story of how
the tortoise's shell developed

can be told.

The ribs expanded outwards,

enclosing the hip
and the shoulder joints.

They enlarged and fused with
other bones beneath the shell.

Eventually, they widened
and came together...

..and the scales enlarged
to form a coat of horn

on the surface of the box.

Recent scientific research has
suggested that the ancestors

of the tortoises
first developed a shell

as a way of strengthening
their bodies

to help them dig into the ground
in search of food.

But since then, of course,
it's become primarily protective

and very big and heavy
in some instances.

That may have had another effect.

That may have imposed upon tortoises

a slow-moving, low-energy life

which, in the end, has enabled them
to live to such great old age,

as indeed they do.

Next, we look at the shells that
birds provide for their eggs,

which vary so extraordinarily
in size and appearance.

Eggs have been considered by people
and cultures all over the world

to be wondrous, magical objects.

Myths tell of how the world was
created from a cosmic egg

which contained all the material
needed for life.

And the egg's rounded shape
is fundamental to that idea.

The shell has a complex structure.

On the one hand, it must be strong
enough to protect a growing chick.

And on the other, fragile enough
to allow the chick to break out

when the time comes.

That's true of all bird eggs,

including those belonging to
the largest.

The ostrich is a truly
extraordinary bird.

It has wings and feathers
but it can't fly.

It has massive feet,
somewhat like those of a camel.

It has a digestion

that some people used to think
was strong enough to dissolve iron.

And it lays the biggest egg
of any living bird.

It weighs 1.5 kilos - the equivalent
of two dozen chicken eggs.

The shell is three millimetres thick

and so strong that it's said
I could stand on this

without it breaking,
but I'm not going to try.

And it takes 40 days

for a chick developing
inside this fortress

to break out.

So how does a chick grow
while encased by shell,

since it needs to breathe,
feed and stay warm?

Amazingly, it manages to do
all these things

right from the very start.

A bird embryo begins as
a microscopic egg cell,

released on the surface
of a globule of yolk.

In the oviduct, it gathers
the moist albumen, or egg white.

And it's plumped up,
ready to receive the shell.

Layers of calcium carbonate
are sprayed onto the egg membrane

and they form calcite crystals.

A final layer of cuticle is added
and the egg is complete.

Eventually, it emerges.

And what an astonishing
and beautiful thing it is.

Birds of different species
produce characteristically

different shaped eggs.

They range from the almost
spherical eggs laid by owls,

to eggs pointed at one end
of razorbills and guillemots.

A spherical egg has the smallest
surface-to-volume ratio

so will lose the least heat
for its size.

However, such an egg only presents
a small area of shell to be warmed

by an incubating bird.

A longer, more egg-shaped egg,

may be a good compromise between
losing heat and absorbing it.

It used to be thought
that a guillemot's egg

was pointed at one end
because it prevented it

from rolling off a narrow ledge.

But such a shape makes it
easier to incubate.

The egg fits snugly
beneath the bird's body,

with its sharper end
pointing towards the tail

and the fatter end pressed against
the bird's warm stomach.

If a nest holds several eggs,

a pointed shape doesn't work
as well.

A more rounded one enables them
to be packed closely together

and presents a large
incubating surface overall.

So the shapes of eggs vary
to allow effective incubation,

no matter where the nest is located.

But eggs can get too hot and need
protection from the heat of the sun.

Once again, the nature
of the shell can help.

Its colour can serve as a sun block

and prevent too much damaging
ultraviolet light

from reaching the chick inside.

Emus' eggs are a striking
dark green colour.

This, doubtless, camouflages them as
they lie on the ground among leaves.

But it also prevents too much
sunlight from penetrating the shell.

The great range of colours and
patterns laid by different birds

are all derived
from just two pigments.

They may help to camouflage
and protect the chick inside,

but sometimes that variety
has had the opposite effect

and made them much sought after
by collectors.

In the 19th century, an amateur
naturalist and egg collector,

Major Charles Emil Bendire,
serving in the United States Army,

went to extreme lengths
to collect them.

In 1872, while on Army patrol
in Arizona,

he spotted a zone-tailed hawk's
nest,

whose eggs he didn't have.

Leaving his troops,
he climbed the tree

while keeping an eye out
for hostile Indians,

but just as he took an egg
from the nest, he was attacked.

Quickly, he jumped onto his horse,

putting the egg into his mouth
for safekeeping.

He reached camp alive,
but the effort and stress

of not biting the egg
caused his jaw to lock and swell.

It took several men
and a broken tooth

to extract it from his mouth,
but luckily it was still intact.

Eventually, Charles Bendire
assembled an immense collection

of over 8,000 eggs
and donated them all,

including the hawk's egg,
to the Smithsonian Institution.

Egg collectors were obsessed
with the patterns and colours

on the surface of eggshells
and thought them very beautiful,

and so they are.

But these characteristics
have an important function.

Ostriches use communal nests which
may contain as many as 40 eggs.

They're incubated by
the dominant female

who is able to identify
her own eggs

because she sees them
in extraordinary detail.

Eggshells are covered in tiny pores.

A small wren-sized egg,
like this one,

may have about 200.

A chicken's egg, about 2,000.

And ostrich eggs, like these two,
have more than 30,000.

Each tiny pore is open
to let oxygen in

and carbon dioxide
and waste product out.

And that enables the growing chick
to breathe.

These are the chick's lifelines
to the outside world.

This particular empty ostrich shell

has been filled with water
that contained a blue dye,

and that enables you to see
the pores

that cover the shell's surface.

It's thought that a female ostrich
can recognise

her own individual pore pattern
and, in a communal nest,

give preference to her own eggs
during incubation.

The dominant female lays her eggs
towards the centre of the nest

and other females add theirs,
so creating a large clutch.

But only about 20 eggs can be
successfully incubated at one time.

So the dominant female ensures
that her own are in the centre

and in no danger
of being pushed out.

And it's been shown
that it's the pore pattern

that enables her to do that.

It takes about 40 days
for the eggs to hatch,

and, of course, they have to be
strong enough to carry the weight

of the incubating birds, which
take it in turn to sit on the nest.

But then, when the time comes,
they also have to be fragile enough

to allow the chick inside
to break out.

An egg shell appears to need
conflicting characteristics.

It must be robust enough
to protect the chick

but fragile enough to allow it
to escape when the time comes.

And, indeed, it's enormously strong,

so that even predators
find it hard to break.

The Egyptian vulture, however,
has learned how to do it.

You have to give it a sharp blow
to puncture the surface.

And once you've done that,
its strength is largely lost.

So how does a chick
eventually escape?

Well, just at the right time,
the shell weakens.

When the egg is first laid,
the shell is as hard as china.

But as the chick develops, the shell
releases some of its calcium,

which strengthens the chick's
growing bones.

So the shell becomes thinner
as the chick grows stronger,

until it finally becomes brittle

and the chick can at last
peck or kick its way out.

So the egg shell has many complex
ways of first protecting

and then, finally, releasing
its precious inmate.

The shell of an egg gives it
great protection

but its shape and beauty
has also led to its destruction.

In the 19th century, when amateur
naturalist Charles Bendire

was avidly collecting eggs,

there was a huge craze for eggs
that were coloured.

Complete clutches were often
taken from nests.

Today, the law prohibits this,
and collecting is severely punished.

However, some of these
old collections

are now helping conserve
living birds.

Eggs of several species
from as far back as 1850

reveal that the shells
have become thinner

since the start of
the Industrial Revolution.

This was hard evidence
that helped to ban the use

of certain agricultural chemicals
that were clearly responsible.

Since then, the populations
of these birds have recovered.

There's still much to learn
about eggs and their shells,

so the next time you take a
chicken's egg for your breakfast,

marvel at its shell.

Here is an extraordinary
natural structure

that's very much more
than just packaging.

The shells of tortoises and eggs
offer great protection,

but being able to adapt to different
lifestyles and situations

has been the real secret
to their success.