ATTENBOROUGH: The natural world is full of extraordinary animals
with amazing life histories.
Yet certain stories are more intriguing than most.
The mysteries of a butterfly's life cycle
or the strange biology of the emperor penguin.
some of these creatures were surrounded by myth and misunderstandings
for a very long time.
And some have only recently revealed their secrets.
These are the animals that stand out from the crowd.
The curiosities 1 find most fascinating of all.
some of our most familiar animals puzzled scientific minds
for a surprisingly long time.
The mysterious comings and goings of barn swallows led
to some far-fetched ideas.
While the life cycle of the painted lady butterfly took centuries to unravel.
Swallows have successfully nested and raised their young in this barn
for several years.
These chicks will soon leave the nest and make their first exploratory flights
around the farm.
But in a few weeks' time, they will suddenly vanish.
Where do they go to?
In the past, that gave rise to some extraordinary speculations.
In fact, in the 1 8th century, it became a very long-running debate
headed by some well-known church figures.
And swallows are not the only birds that appear and disappear
with the changing seasons.
For centuries people speculated about where such birds go.
One explanation was that some birds changed into others
by growing different adult plumage.
Perhaps the redstart turned into a robin.
Or the garden warbler into a blackcap.
since these species were seldom present at the same time,
the explanation seemed entirely plausible.
The barnacle goose was another mystery.
Each winter, huge, noisy flocks of them appear on European shores,
apparently from out of nowhere.
No one had ever seen them build a nest or raise young.
The barnacle goose gave rise to some extraordinary folklore,
as this medieval illustration shows.
It was thought that the geese grew on underwater trees,
starting life as small marine creatures called goose barnacles.
Goose barnacles do, of course, exist. They're small, shelled marine organisms
with what looks like the head, which is in fact enclosed by a shell,
attached by a stalk, which was thought to resemble the neck of a bird,
to a bit of wood or a rock.
The confusion about the nature of the barnacle goose
was put to good use by some.
since it was unclear whether it was a bird, a fish, or some other creature,
you could surely be allowed to eat it on days when
meat was forbidden by the church.
But the most commonly held belief
was that birds disappear in winter because they hibernated.
swallows, and their close relatives, the swifts and martens,
were thought to do so in mud at the bottom of ponds and rivers.
And it's easy to see how this idea originated because the birds spend
much of their time near water,
skimming low over the surface, hunting for insects or taking a drink.
1t wasn't until the Middle Ages that another theory was proposed,
that some birds may migrate.
And one of its strongest proponents was an influential religious leader.
Frederick II of Hohenstaufen was a powerful Holy Roman Emperor
and known for his unorthodox views.
He ignored the philosophy of the church
and based his knowledge of natural history
on direct observation, rather than what was ordained.
Frederick was also a keen falconer, and he wrote this book, The Art of Falconry.
And in it, surprisingly, there are entire chapters
on the migration of birds.
His confidence came from the fact that, unlike his contemporaries,
and those before him, he had actually observed birds in the field for himself.
He had no doubt about the migration and so little patience for the myths
surrounding the barnacle goose.
He considered the story to be quite ridiculous
and argued that the birds simply breed in distant lands.
His views started a debate that split people into two camps.
Those believing in the old hibernation theory and those who supported the idea
that birds migrate.
This was the start of a new era
which was to sweep away myths and focus instead on facts
and careful observation.
And across Europe, the evidence for bird migration started to accumulate.
1n Germany, a 1 2th-century monk is said to have taken a swallow from its nest
and attached a parchment note to its leg that read,
''Oh, swallow, where do you live in winter?''
The following spring, the bird returned with a note saying, ''1n Asia.
''1n the home of Petrus. '' That is, 1srael.
The story may not have been true, but it certainly gave the right hint.
In the early 1 6th century, a bishop from Sweden called Olaus Magnus
reignited the debate about swallows with this picture.
He claimed that in winter, fishermen often drew up swallows in their nets
hanging together in a mass.
This astonishing assertion provided ample fuel for the anti-migration lobby.
And, unlikely as it was, the view that swallows spent their winters underwater
became increasingly entrenched.
By the 1 8th century, the debate about migration versus hibernation
had come to a head.
And, across the continent, opinions were divided.
But new evidence was about to come from an unusual source.
Edward Jenner was an English country doctor who also had a deep interest
in natural history.
He noted that although swallows often splash in water as they skim across it,
they never immerse themselves.
Were they to do so, he suggested, their wings would become so wet
that they would be unable to fly.
To test his idea,Jenner reportedly held a swift underwater for two minutes.
Not surprisingly, it died.
jenner went on to devise another experiment
to discover where the birds go.
He took 1 2 swifts from their nests and marked them
by taking off two of their claws.
The following year, some of the birds he'd marked were caught again in exactly
the same spot.
Although jenner could not discover where his swifts had been over the winter,
he was the first to show that they returned to use the same breeding sites
in the following years.
And we now know that this is true for swallows as well.
About the same time, across the Channel, a German bird enthusiast
had come up with a similar idea.
johann Frisch caught several birds near his house and attached to their legs
woollen threads like this, which he'd dipped in red watercolour.
He predicted that if swallows really did spend the winter
at the bottom of lakes,
the red colour would be washed off.
The following spring, Frisch's swallows returned and the threads were unchanged.
It was a very simple, but very effective experiment.
Evidence against the hibernation theory continued to mount.
And, eventually, a new technique put the final nail in its coffin,
systematic bird ringing.
This bird has just been fitted with its own individual marker,
a small metal ring on its leg
with a unique code of numbers.
It's part of a national scheme that's been running for over 1 00 years.
And provides scientists with invaluable data on bird movements.
Early in the 20th century, the study of migration really took off.
Birds were recovered on their breeding and wintering grounds,
and often en route, too.
six hundred years after Frederick Von Hohenstaufen had first
started the debate, real evidence was beginning to accumulate.
In the summer of 1 91 1 , a metal ring just like this one was clipped onto the leg
of a young swallow in Staffordshire.
The number on the ring was B830.
Eighteen months later, the same bird was caught by a farmer in South Africa.
Here, at last, was the indisputable proof that swallows migrate
and spend the winter thousands of miles away.
Off you go. There we are.
Today, of course, we know that the swallow's migration is one of the most
impressive in all the animal kingdom.
1t takes it across the largest desert in the world, the sahara.
1t's a gruelling and dangerous journey,
and many die on the way from exhaustion or starvation.
They travel for nearly four months,
covering nearly 1 0,000 kilometres and eventually reach southern Africa.
And bird ringing also helped to dispel the myth of the barnacle goose.
1n the 1 960s, a Norwegian expedition ringed geese
nesting on the Arctic island of spitzbergen.
That autumn, some of the same birds were sighted on the west coast of scotland,
some 2,000 kilometres away.
Frederick Von Hohenstaufen had been proved to be absolutely correct.
It took centuries to discover the truth behind the swallows' seasonal movements.
But, in their time, they baffled the minds of many great naturalists.
And started one of the longest running of all scientific debates.
But, in the end, the true story proved to be even more extraordinary
than the fantastic myths that were invented to explain it.
Just like the swallow, the painted lady butterfly seems to appear
magically out of nowhere.
And that started some extraordinary ideas and controversies.
The painted lady is one of our largest butterflies
and a familiar summer visitor to our gardens.
And yet its appearance and disappearance each year has puzzled us for centuries.
It's only now that we're beginning to understand its extraordinary life cycle
and discover where it vanishes each year.
Early naturalists were confused by the sudden appearance of painted ladies
each spring, because they were unaware of the connection between butterflies
and caterpillars.
For a very long time, it was widely believed that butterflies arise
from rotting material by what was called spontaneous generation.
In the 1 830s, a German scientist named Renous
was arrested for heresy for claiming that he could change caterpillars
into butterflies.
Arresting someone for something now known to be common knowledge
may seem rather extreme.
But, at the time, many still believed that caterpillars and butterflies
were completely different creatures, created by the hand of God.
Needless to say, people had been well aware of the existence of both
butterflies and caterpillars since the earliest times.
But the thought that any two were related, let alone the same species,
seemed impossible.
And it's easy to see why.
Not only do caterpillars and butterflies
look like very different types of animals,
but the colours and patterns of a caterpillar don't match up
with those of its adult form.
The only way to know which larva and which butterfly go together
is to keep caterpillars and watch them turn into butterflies.
But it wasn't until the 1 7th century that anyone left a record of doing that.
One of the first was a remarkable woman named Maria Sibylla Merian.
Merian was born in Germany
at a time when women still had little formal education,
and no role in the scientific world.
But she was an accomplished artist
and painted plants and insects she saw around her.
To do that, she kept caterpillars, fed them on leaves
and watched them turn into butterflies.
Merian produced hundreds of beautiful paintings of butterflies in their stages
of development, along with the plants on which they feed.
Her drawings are so exquisite and detailed that they still rank among
the best in the world.
Among the things she observed with great care were things like this.
A curious, yet strangely beautiful object.
1t's a chrysalis.
The intermediate stage between a caterpillar and a butterfly.
she was one of the first to record the remarkable change that takes place
in the chrysalis.
1t's one of nature's most extraordinary transformations.
At the age of 52, she sailed from Europe to South America
on a two-year expedition to study insects in the tropical jungles
of Suriname.
It was an exceptional journey for any naturalist at the time,
and particularly for a woman.
When she returned, she produced this beautiful book.
It turned out to be popular, because it was one of the few to be published
not in the scientific language of Latin, but in Dutch.
Because of this, her work was largely dismissed by scientists of the time.
But Merian was one of the first naturalists to correctly connect
the caterpillar with its pupa and the adult form.
Today, Merian's book is widely recognised as a pioneering work
of scientific observation.
And it put an end to the idea of spontaneous generation.
Around the same time, further evidence for the connection between butterflies
and caterpillars came from a different source.
In 1 669, a Dutch scientist by the name of jan Swammerdam
published the results of experiments which would finally prove
that the caterpillar and butterfly are one and the same animal.
Swammerdam was a master of the miniature and dissected the caterpillars and pupae
of butterflies and moths under a microscope.
With a steady hand and endless patience, he carefully cut into the layers of skin
with tiny scissors and what he discovered was truly astonishing.
He found some of the body parts of a butterfly.
The structures were fragile and not complete, but swammerdam had proved
that caterpillar and butterfly are indeed one and the same animal.
We now know that without the caterpillar there can be no butterfly.
Yet for a very long time, the painted lady seemed to be an exception.
Every spring, the adult butterflies would appear across Britain,
without any sightings of their caterpillars.
While some butterflies hibernate in Britain,
there was no sign of painted ladies doing so.
some speculated that they flew to warmer climates,
as birds do.
But how could a tiny insect cross the English Channel?
1n the 20th century, swarms of butterflies moving across Europe
finally provided evidence that painted ladies do indeed cross the sea.
And they were found to fly all the way from North Africa to Britain.
But there were almost no records of painted ladies making
the reverse trip south.
so, for years, it was thought that Britain must be a dead end
for the most northerly stragglers.
And then, in 2009, the public was asked to help solve the mystery.
Among 1 2,000 sightings, there were reports of painted ladies
flying out to sea in the autumn.
And a radar station detected them flying south at heights of 500 metres,
way beyond the sight of human eyes.
We now know that the painted ladies' migration
is a round trip of over 1 2,000 kilometres.
But it's not made by any one individual.
Each only flies part of the way, passing on the migratory battle
to the next generation. 1t's like a relay race,
with up to six generations of butterflies involved.
The painted ladies' epic journey from one continent to the next,
would be a truly astonishing feat for any animal.
But for a tiny creature like this, it seems really extraordinary.
How does it battle the wind and the weather,
and navigate across vast bodies of water?
And with no single individual ever undertaking the whole migration,
how do they find the way?
1t seems that painted ladies are pre-programmed
to either fly north or south.
And this is determined whilst they're still caterpillars,
possibly by temperature and day length.
And also by the plants they feed on.
But how does this information get passed on from caterpillar to butterfly?
The answer may be hidden within the chrysalis.
Recently, CTscanners have allowed us to look inside a pupa.
They reveal that some organs remain intact during the transformation.
A one-day-old pupa clearly shows the gut and breathing tubes
which only change slightly as the chrysalis develops.
Could it be that the brain or nerves also remain intact
and that memories are passed on?
Recent experiments in the lab appear to support this idea.
scientists taught caterpillars to avoid specific smells
by linking them with an unpleasant reaction.
Later on, as adults, the same individuals remembered these smells
and chose to keep away from them.
1f the experiences of a caterpillar can be carried over to the adult,
then maybe cues for migration can also be passed on.
Although we've unravelled much of the painted ladies' life cycle,
many questions remain.
How far does each individual travel?
And do offspring follow similar routes to their ancestors?
One day, we may know the answers. But, for now,
they remain some of the unsolved mysteries of nature.
The arrival each spring of our painted lady butterflies and our swallows
never ceases to delight us.
But now we also understand the extraordinary journeys they undertake
when they disappear again at the end of summer.