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ATTENBOROUGH: The natural world is
full of extraordinary animals

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with amazing life histories.

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Yet certain stories are more intriguing
than most.

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The mysteries of a butterfly's
life cycle

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or the strange biology
of the emperor penguin.

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some of these creatures were surrounded
by myth and misunderstandings

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for a very long time.

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And some have only recently revealed
their secrets.

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These are the animals that stand out
from the crowd.

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The curiosities 1 find most fascinating
of all.

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some of our most familiar animals
puzzled scientific minds

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for a surprisingly long time.

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The mysterious comings and goings of
barn swallows led

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to some far-fetched ideas.

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While the life cycle of the painted lady
butterfly took centuries to unravel.

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Swallows have successfully nested
and raised their young in this barn

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for several years.

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These chicks will soon leave the nest
and make their first exploratory flights

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around the farm.

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But in a few weeks' time,
they will suddenly vanish.

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Where do they go to?

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In the past, that gave rise to some
extraordinary speculations.

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In fact, in the 1 8th century,
it became a very long-running debate

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headed by some well-known
church figures.

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And swallows are not the only birds that
appear and disappear

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with the changing seasons.

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For centuries people speculated about
where such birds go.

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One explanation was that some birds
changed into others

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by growing different adult plumage.

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Perhaps the redstart turned
into a robin.

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Or the garden warbler into a blackcap.

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since these species were seldom present
at the same time,

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the explanation
seemed entirely plausible.

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The barnacle goose was another mystery.

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Each winter, huge, noisy flocks of them
appear on European shores,

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apparently from out of nowhere.

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No one had ever seen them build a nest
or raise young.

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The barnacle goose gave rise to some
extraordinary folklore,

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as this medieval illustration shows.

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It was thought that the geese grew
on underwater trees,

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starting life as small marine
creatures called goose barnacles.

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Goose barnacles do, of course, exist.
They're small, shelled marine organisms

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with what looks like the head,
which is in fact enclosed by a shell,

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attached by a stalk, which was thought
to resemble the neck of a bird,

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to a bit of wood or a rock.

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The confusion about the nature
of the barnacle goose

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was put to good use by some.

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since it was unclear whether it was
a bird, a fish, or some other creature,

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you could surely be allowed to eat it
on days when

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meat was forbidden by the church.

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But the most commonly held belief

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was that birds disappear in winter
because they hibernated.

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swallows, and their close relatives,
the swifts and martens,

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were thought to do so in mud
at the bottom of ponds and rivers.

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And it's easy to see how this idea
originated because the birds spend

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much of their time near water,

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skimming low over the surface,
hunting for insects or taking a drink.

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1t wasn't until the Middle Ages that
another theory was proposed,

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that some birds may migrate.

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And one of its strongest proponents
was an influential religious leader.

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Frederick II of Hohenstaufen was
a powerful Holy Roman Emperor

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and known for his unorthodox views.

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He ignored the philosophy of the church

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and based his knowledge
of natural history

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on direct observation,
rather than what was ordained.

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Frederick was also a keen falconer, and
he wrote this book, The Art of Falconry.

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And in it, surprisingly,
there are entire chapters

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on the migration of birds.

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His confidence came from the
fact that, unlike his contemporaries,

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and those before him, he had actually
observed birds in the field for himself.

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He had no doubt about the migration
and so little patience for the myths

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surrounding the barnacle goose.

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He considered the story
to be quite ridiculous

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and argued that the birds
simply breed in distant lands.

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His views started a debate that split
people into two camps.

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Those believing in the old hibernation
theory and those who supported the idea

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that birds migrate.

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This was the start of a new era

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which was to sweep away myths
and focus instead on facts

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and careful observation.

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And across Europe, the evidence for bird
migration started to accumulate.

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1n Germany, a 1 2th-century monk is said
to have taken a swallow from its nest

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and attached a parchment note
to its leg that read,

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''Oh, swallow, where do you live
in winter?''

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The following spring, the bird returned
with a note saying, ''1n Asia.

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''1n the home of Petrus. ''
That is, 1srael.

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The story may not have been true,
but it certainly gave the right hint.

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In the early 1 6th century, a bishop from
Sweden called Olaus Magnus

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reignited the debate about swallows
with this picture.

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He claimed that in winter, fishermen
often drew up swallows in their nets

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hanging together in a mass.

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This astonishing assertion provided
ample fuel for the anti-migration lobby.

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And, unlikely as it was, the view that
swallows spent their winters underwater

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became increasingly entrenched.

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By the 1 8th century, the debate about
migration versus hibernation

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had come to a head.

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And, across the continent,
opinions were divided.

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But new evidence was about to come
from an unusual source.

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Edward Jenner was an English country
doctor who also had a deep interest

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in natural history.

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He noted that although swallows often
splash in water as they skim across it,

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they never immerse themselves.

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Were they to do so, he suggested,
their wings would become so wet

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that they would be unable to fly.

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To test his idea,Jenner reportedly held
a swift underwater for two minutes.

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Not surprisingly, it died.

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jenner went on
to devise another experiment

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to discover where the birds go.

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He took 1 2 swifts from their nests
and marked them

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by taking off two of their claws.

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The following year, some of the birds
he'd marked were caught again in exactly

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the same spot.

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Although jenner could not discover where
his swifts had been over the winter,

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he was the first to show that they
returned to use the same breeding sites

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in the following years.

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And we now know that this is true for
swallows as well.

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About the same time, across the Channel,
a German bird enthusiast

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had come up with a similar idea.

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johann Frisch caught several birds near
his house and attached to their legs

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woollen threads like this,
which he'd dipped in red watercolour.

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He predicted that if swallows really
did spend the winter

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at the bottom of lakes,

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the red colour would be washed off.

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The following spring, Frisch's swallows
returned and the threads were unchanged.

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It was a very simple,
but very effective experiment.

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Evidence against the hibernation theory
continued to mount.

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And, eventually, a new technique put
the final nail in its coffin,

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systematic bird ringing.

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This bird has just been fitted with its
own individual marker,

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a small metal ring on its leg

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with a unique code of numbers.

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It's part of a national scheme that's
been running for over 1 00 years.

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And provides scientists with invaluable
data on bird movements.

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Early in the 20th century, the study of
migration really took off.

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Birds were recovered on their breeding
and wintering grounds,

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and often en route, too.

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six hundred years after
Frederick Von Hohenstaufen had first

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started the debate, real evidence
was beginning to accumulate.

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In the summer of 1 91 1 , a metal ring just
like this one was clipped onto the leg

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of a young swallow in Staffordshire.

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The number on the ring was B830.

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Eighteen months later, the same bird was
caught by a farmer in South Africa.

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Here, at last, was the indisputable
proof that swallows migrate

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and spend the winter
thousands of miles away.

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Off you go. There we are.

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Today, of course, we know that the
swallow's migration is one of the most

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impressive in all the animal kingdom.

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1t takes it across the largest desert in
the world, the sahara.

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1t's a gruelling and dangerous journey,

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and many die on the way
from exhaustion or starvation.

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They travel for nearly four months,

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covering nearly 1 0,000 kilometres
and eventually reach southern Africa.

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And bird ringing also helped to dispel
the myth of the barnacle goose.

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1n the 1 960s, a Norwegian expedition
ringed geese

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nesting on the Arctic island
of spitzbergen.

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That autumn, some of the same birds were
sighted on the west coast of scotland,

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some 2,000 kilometres away.

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Frederick Von Hohenstaufen had been
proved to be absolutely correct.

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It took centuries to discover the truth
behind the swallows' seasonal movements.

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But, in their time, they baffled
the minds of many great naturalists.

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And started one of the longest running
of all scientific debates.

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But, in the end, the true story proved
to be even more extraordinary

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than the fantastic myths
that were invented to explain it.

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Just like the swallow, the painted lady
butterfly seems to appear

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magically out of nowhere.

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And that started some
extraordinary ideas and controversies.

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The painted lady is one of
our largest butterflies

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and a familiar summer
visitor to our gardens.

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And yet its appearance and disappearance
each year has puzzled us for centuries.

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It's only now that we're beginning to
understand its extraordinary life cycle

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and discover where it vanishes
each year.

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Early naturalists were confused by the
sudden appearance of painted ladies

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each spring, because they were unaware
of the connection between butterflies

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and caterpillars.

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For a very long time, it was widely
believed that butterflies arise

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from rotting material by
what was called spontaneous generation.

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In the 1 830s, a German scientist
named Renous

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was arrested for heresy for claiming
that he could change caterpillars

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into butterflies.

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Arresting someone for something now
known to be common knowledge

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may seem rather extreme.

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But, at the time, many still believed
that caterpillars and butterflies

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were completely different creatures,
created by the hand of God.

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Needless to say, people had been well
aware of the existence of both

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butterflies and caterpillars
since the earliest times.

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But the thought that any two were
related, let alone the same species,

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seemed impossible.

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And it's easy to see why.

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Not only do caterpillars and butterflies

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look like very different
types of animals,

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but the colours and patterns
of a caterpillar don't match up

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with those of its adult form.

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The only way to know which larva and
which butterfly go together

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is to keep caterpillars and watch them
turn into butterflies.

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But it wasn't until the 1 7th century
that anyone left a record of doing that.

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One of the first was a remarkable woman
named Maria Sibylla Merian.

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Merian was born in Germany

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at a time when women still
had little formal education,

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and no role in the scientific world.

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But she was an accomplished artist

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and painted plants and insects
she saw around her.

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To do that, she kept caterpillars,
fed them on leaves

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and watched them turn into butterflies.

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Merian produced hundreds of beautiful
paintings of butterflies in their stages

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of development, along with the plants
on which they feed.

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Her drawings are so exquisite and
detailed that they still rank among

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the best in the world.

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Among the things she observed with
great care were things like this.

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A curious, yet
strangely beautiful object.

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1t's a chrysalis.

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The intermediate stage between
a caterpillar and a butterfly.

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she was one of the first to record the
remarkable change that takes place

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in the chrysalis.

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1t's one of nature's most
extraordinary transformations.

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At the age of 52, she sailed from Europe
to South America

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on a two-year expedition to study
insects in the tropical jungles

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of Suriname.

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It was an exceptional journey for
any naturalist at the time,

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and particularly for a woman.

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When she returned, she produced
this beautiful book.

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It turned out to be popular, because it
was one of the few to be published

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not in the scientific
language of Latin, but in Dutch.

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Because of this, her work was largely
dismissed by scientists of the time.

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But Merian was one of the first
naturalists to correctly connect

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the caterpillar with its pupa
and the adult form.

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Today, Merian's book is widely
recognised as a pioneering work

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of scientific observation.

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And it put an end to the idea of
spontaneous generation.

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Around the same time, further evidence
for the connection between butterflies

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and caterpillars came
from a different source.

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In 1 669, a Dutch scientist by the name
of jan Swammerdam

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published the results of experiments
which would finally prove

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that the caterpillar and butterfly
are one and the same animal.

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Swammerdam was a master of the miniature
and dissected the caterpillars and pupae

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of butterflies and moths under
a microscope.

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With a steady hand and endless patience,
he carefully cut into the layers of skin

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with tiny scissors and what he
discovered was truly astonishing.

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He found some of the body parts
of a butterfly.

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The structures were fragile and not
complete, but swammerdam had proved

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that caterpillar and butterfly are
indeed one and the same animal.

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We now know that without the caterpillar
there can be no butterfly.

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Yet for a very long time, the painted
lady seemed to be an exception.

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Every spring, the adult butterflies
would appear across Britain,

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without any sightings
of their caterpillars.

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While some butterflies
hibernate in Britain,

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there was no sign
of painted ladies doing so.

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some speculated that they flew
to warmer climates,

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as birds do.

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But how could a tiny insect cross
the English Channel?

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00:17:31,567 --> 00:17:35,526
1n the 20th century, swarms of
butterflies moving across Europe

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finally provided evidence that painted
ladies do indeed cross the sea.

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And they were found to fly all the
way from North Africa to Britain.

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But there were almost no records of
painted ladies making

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the reverse trip south.

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so, for years, it was thought that
Britain must be a dead end

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for the most northerly stragglers.

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And then, in 2009, the public was asked
to help solve the mystery.

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Among 1 2,000 sightings, there were
reports of painted ladies

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flying out to sea in the autumn.

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And a radar station detected them flying
south at heights of 500 metres,

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way beyond the sight of human eyes.

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We now know that
the painted ladies' migration

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is a round trip of over
1 2,000 kilometres.

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But it's not made by any one individual.

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Each only flies part of the way, passing
on the migratory battle

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to the next generation.
1t's like a relay race,

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with up to six generations of
butterflies involved.

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The painted ladies' epic journey from
one continent to the next,

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would be a truly astonishing feat
for any animal.

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But for a tiny creature like this,
it seems really extraordinary.

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How does it battle the wind
and the weather,

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and navigate across
vast bodies of water?

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00:19:02,287 --> 00:19:05,802
And with no single individual ever
undertaking the whole migration,

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how do they find the way?

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1t seems that painted ladies
are pre-programmed

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to either fly north or south.

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And this is determined whilst they're
still caterpillars,

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possibly by temperature and day length.

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And also by the plants they feed on.

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00:19:25,607 --> 00:19:30,123
But how does this information get passed
on from caterpillar to butterfly?

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The answer may be hidden within
the chrysalis.

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00:19:35,087 --> 00:19:40,286
Recently, CTscanners have allowed us
to look inside a pupa.

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They reveal that some organs remain
intact during the transformation.

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A one-day-old pupa clearly shows the gut
and breathing tubes

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which only change slightly
as the chrysalis develops.

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00:19:59,127 --> 00:20:02,836
Could it be that the brain or nerves
also remain intact

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00:20:03,847 --> 00:20:05,883
and that memories are passed on?

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00:20:07,407 --> 00:20:11,286
Recent experiments in the lab appear
to support this idea.

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00:20:12,047 --> 00:20:16,120
scientists taught caterpillars to avoid
specific smells

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by linking them with
an unpleasant reaction.

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00:20:20,127 --> 00:20:24,325
Later on, as adults, the same
individuals remembered these smells

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and chose to keep away from them.

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1f the experiences of a caterpillar can
be carried over to the adult,

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then maybe cues for migration
can also be passed on.

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00:20:37,487 --> 00:20:40,638
Although we've unravelled much of
the painted ladies' life cycle,

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many questions remain.

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00:20:42,727 --> 00:20:45,446
How far does each individual travel?

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00:20:46,247 --> 00:20:49,398
And do offspring follow similar
routes to their ancestors?

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One day, we may know the answers.
But, for now,

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they remain some of the
unsolved mysteries of nature.

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00:21:01,287 --> 00:21:05,121
The arrival each spring of our painted
lady butterflies and our swallows

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00:21:05,207 --> 00:21:07,516
never ceases to delight us.

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But now we also understand the
extraordinary journeys they undertake

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00:21:11,407 --> 00:21:14,558
when they disappear again at
the end of summer.