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.