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The natural world is full of extraordinary animals with
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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
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and misunderstandings 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 I find most fascinating of all.
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Some of our most familiar animals puzzled scientific
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minds for a surprisingly long time.
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The mysterious comings
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and goings of barn swallows led to some far-fetched ideas.
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While the life cycle of the painted lady butterfly took centuries
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to unravel.
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But the abilities of some plants
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and animals are so remarkable that they seem to be almost supernatural.
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In this programme, I investigate the shocking power of a fish that
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advanced our understanding of electricity,
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and plants with senses that are surprising modern science.
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How do these extraordinary powers help the organisms that
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produced them?
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The freshwater eel is surrounded by legends.
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The first Europeans to explore the New World heard amazing
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stories about it.
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And when, in the 18th century, specimens of this strange fish
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reached Europe, they created a sensation.
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In 1776, Captain George Baker,
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an American mariner and whaler,
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made the long and difficult journey from South America across a
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raging Atlantic Ocean to bring five live electric eels to London.
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These are two of his actual eels.
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Captain Baker and his five electric eels, or gymnotas as they were known,
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set up shop in the Haymarket and offered two shillings
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and sixpence for a shock, or five shillings for a spark.
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Baker's eels had come all the way from the lower
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reaches of the Amazon and Orinoco rivers,
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where he had heard tales from the locals about their astonishing powers.
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They called these fish "trembladores".
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Humboldt, the famous naturalist and explorer, had described how he
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had witnessed horses being killed by the repeated shocks from these fish.
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And he himself accidentally stepped on one
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and vividly described the effect.
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"With each stroke, you feel an internal vibration that lasts
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"two or three seconds, followed by a painful numbness.
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"All day I felt strong pain in my knees and in all my joints."
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I encountered this remarkable fish in its natural environment
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when I filmed at the same rivers that Humboldt explored.
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There was talk of me swimming with the eel,
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but thankfully we had some technical difficulties with the diving
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equipment that I was supposed to wear,
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and so I stayed safely in a canoe and was able to demonstrate
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another subtler, but equally remarkable, side to this fish.
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The eels were constantly producing electric discharges.
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Somehow they were generating a small, nonstop flowing current.
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ELECTRIC DRONE
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They were also able to sense electricity and were
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attracted to electrical pulses emitted from my underwater detector,
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suggesting that electricity plays a key role in their lives.
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But at the time of their discovery,
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no-one knew the full functions of their extraordinary abilities.
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We now know that the shock was caused by electricity,
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and I can demonstrate it by touching the animal with an electrode.
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Watch.
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There. You see?
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The scope and the lights are flashing up and down.
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Extraordinary.
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But this is only a small indication of the real power of this fish.
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If I were to try and pick it up, I could get
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a jolt of an astonishing 600 volts, which is quite enough to kill me.
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This 1960s educational film illustrated the shock,
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even though the equipment used prevented
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the volunteers from getting its full power.
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They were to join hands and then connected to a live eel.
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WOMAN SCREAMS
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Firm believers in electric eels. Thank you very much.
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You can imagine how startling Baker's electric eels
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were 200 years ago.
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In the 18th century,
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electricity was becoming one of the most fashionable areas
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of scientific investigation, but it was still very poorly understood.
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Very few advances had been made since its discovery 150 years
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earlier by Elizabeth I's personal physician, William Gilbert.
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Gilbert repeated a trick that had been known about since Greek times.
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Rubbing a piece of amber with cat fur, that allowed the amber
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to attract a small object like a feather. Let's give it a try.
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Here is a bit of amber.
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There.
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It had always been assumed that this amber effect was caused
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by magnetism but Gilbert showed that it was something different.
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He named this new force after the Greek word for amber,
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electron, and so electricity was born.
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Londoners of the time developed a fascination for this magical force.
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Showmen staged bizarre spectacles to demonstrate its properties.
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In one, a young boy attached to a friction generator
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attracted small pieces of paper to his hands.
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In another, a gentleman kissed a lady and was repulsed
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by the charge carried through her whalebone corset.
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No-one knew what to do with electricity
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but a better understanding of its nature was slowly emerging.
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More and more ingenious ways were developed
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to create what we now call static electricity.
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And soon it became something more than just a quirk of rubbing amber,
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it became visible as a spark.
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The ability to produce this characteristic blue spark
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along with its invigorating smell became the signature
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of this new force and it prompted scientists to make
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obvious comparisons with other natural phenomena.
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THUNDER
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In the American colonies, Benjamin Franklin bravely,
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or perhaps foolishly, flew kites into thunderstorms and proved that
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lightning and the electric spark were one and the same.
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But there's another common property of lightning and static electricity.
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That is the ability to shock.
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It wasn't long before a comparison was made between the shock from
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the early generators and the shock that could be delivered by a fish.
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The electric eel wasn't the only kind of fish
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known to give humans a powerful jolt.
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The ancient Egyptians knew that the electric catfish could also
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give shocks and they called it the "Thunderer of the Nile".
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And in the nearby Mediterranean lives the torpedo ray.
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Its muscle batteries make it so bulky
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it can't undulate its body like other rays
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but has to propel itself by waving its tail.
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Like the electric eel,
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it uses its discharge to stun the other fish on which it prays.
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Sadly, the pressure of celebrity and having to produce shocks
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and sparks to order exhausted Baker's long-suffering eels
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and they didn't last the winter.
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But two were preserved and expertly dissected by John Hunter,
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a very distinguished Scottish surgeon of the time
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and he found a great number of striped muscular layers
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that proved to be where the electricity was generated.
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They are now referred to as Hunter's organs.
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He found these muscles along the tail and sides of the eels
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arranged in stacks.
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One scientist called Galvani believed that animals
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had their own natural electricity even without these electric organs
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and he tried to prove this by connecting wires to frogs' legs
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and making them twitch.
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He called this phenomenon animal electricity.
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But another scientist called Volta had other ideas.
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He proved that the frog was merely a conductor for electricity
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with a simple experiment.
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Volta replaced Galvani's frog with discs of cloth
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soaked in saltwater or acid
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and sandwiched them between two different metals.
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I can do the same thing with filter paper,
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copper two pence pieces and these simple galvanised zinc washers.
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Watch.
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Tuppenny piece.
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Filter.
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And washer.
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There, nearly 0.6 of a volt.
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But the amount of electricity generated was tiny.
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Certainly not enough to make the sparks seen from eels.
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Unlike Galvani, Volta saw no distinction
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between animal electricity and his new electricity from metals
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so he now looked at animals to see how he might amplify his new device.
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Was it significant that the muscles
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producing the electric power in the eels were arranged in stacks?
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Volta decided to add more stacks to his electric pile.
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We call this way of connecting electric cells together
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"in series", and we now know that it increases the voltage.
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But Volta was about to find this out for the first time.
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He piled up his tiny cells like the bands of muscle
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in an electric fish.
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Here I've got ten pairs.
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And just watch.
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Nearly six volts.
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Wonderful.
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Volta could now produce heat, shocks and even sparks
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from electricity in a continuous never-ending stream.
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He had made the first battery, partly inspired by the electric eel.
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The pieces of the puzzle had come together and the eel's example
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had helped to advance our understanding of electricity.
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Eels, in fact, contain natural batteries.
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Stacks of special muscles.
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It's amazing to think when electricity is so much
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a part of our lives today that before Volta
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the only source of electricity was lightning,
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a few static generators
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and fish like this incredible electric eel.
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Understanding how electric eels managed to find their way around
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revealed a hitherto unknown animal sense.
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But it's not just animals that have surprised us.
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We're now discovering that plants too
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have intriguing abilities that are still mysterious.
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We think of plants as passive, still and silent.
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But they may have more in common with animals than you might think.
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New research suggests that they have surprising abilities.
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It depends on how you look at them.
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I first started seeing plants in a different light
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when making a series called The Private Life of Plants.
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We used time-lapse photography to reveal the way they move.
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The bramble spreads aggressively - seemingly unstoppable.
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Other plants pulsed to the rhythms of day and night.
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And flower buds explode like fireworks.
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So, with speeded up film, we had been able to translate
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their time into ours
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and to realise that they're constantly on the move.
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200 years ago, one plant that moved very quickly indeed
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attracted the attention of a great scientific mind.
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It appeared to behave like an animal
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and could move fast enough to catch its own food.
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Charles Darwin was fascinated by the Venus flytrap.
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He called it one of the most wonderful plants in the world.
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He recognised that it could move in a very different way
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to that of plant growth.
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This movement was not only fast but also repeatable.
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Darwin experimented and found that the traps
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are not triggered by raindrops
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but only by a very particular stimulation of the leaf hairs,
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such as an insect might make.
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But what intrigued him most was the speed of the reaction.
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He sent one of these flytraps to a friend, Dr Burdon-Sanderson,
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who was performing groundbreaking work on muscles and electricity.
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His tests confirmed that the tiny electrical discharge
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caused by an animal muscle cell contracting was almost identical
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to those signals obtained by attaching electrodes to the flytrap
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when it was shutting.
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Although plants have no muscles,
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electrical stimulation enables them to move in a similar way to animals.
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Electrical signals cause cells to change the pressure of sap
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in their leaves, so creating movement.
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As a result, some plants, like animals,
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can actively catch their prey.
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Recently it's been discovered that other plants use electricity too
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but for a very different purpose.
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Plants are rooted to the ground and have a small negative charge.
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The higher up the plant you go, the greater the electric charge.
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This creates an electric field around the flower.
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We can't see it but these electrodes are picking up the energy
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of this tiny field and converting it into the sound that we can hear.
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Bees, on the other hand, have a positive charge.
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Friction whilst flying causes them
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to lose electrons, leaving them electrically charged.
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As a bee approaches a flower, the charge fields around the flower
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and the bee interact, and the sound changes...
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FALTERING ELECTRONIC BUZZ
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..there.
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And when it lands, the positive
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and negative fields immediately cancel each other out.
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As this happens, there are two very surprising consequences.
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Firstly, the plant's negatively charged pollen actually
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jumps across onto the positively charged bee.
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Secondly, the plant has a changed electrical field
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and when another bee comes along, it detects this altered
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electrical signature and avoids the flower.
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The plant is, in effect, telling the bee that it has no nectar
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and to come back later.
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When the flower has refilled its stores of nectar, it creates
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a new electric charge which attracts another passing bee.
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This simple on/off signal benefits both the bee and the flower,
256
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but it does have its limitations.
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The electrical field is tiny,
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so insects can only detect it at close quarters.
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But flowers can also draw attention to themselves over much
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greater distances and they do this by floating messages in the air.
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The perfume of a flower is not just a pleasant smell,
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it's also the primary way in which plants communicate with insects.
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00:17:23,000 --> 00:17:27,600
A rose can contain over 400 chemical compounds and a bee
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can recognise a particular combination from over a mile away.
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The very latest research has discovered
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that 90% of the chemicals made by plants, are also
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produced by insects and that is no coincidence.
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00:17:45,960 --> 00:17:50,160
Most flowers produce scent to persuade insects to visit them,
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but others use it in a more sophisticated way...
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for protection.
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00:17:57,800 --> 00:18:01,440
Cabbages communicate with each other using smell.
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When the leaves of one plant are being attacked by caterpillars,
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it releases a scent which warns its neighbours.
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They then produce chemicals in their leaves that caterpillars
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don't like and so they avoid being eaten.
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And scent also serves to call in the cavalry.
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Leaves that are under attack give off a chemical alarm signal that
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attracts wasps which obligingly pick off the caterpillar attackers.
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00:18:35,280 --> 00:18:39,760
So, vegetables, fruits, leaves and flowers are constantly
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communicating with each other using touch, vision and smell.
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They seem to exploit all the senses, apart, that is, from hearing.
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But there are old stories that one particular plant is able to
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produce a very strange sound.
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00:18:59,440 --> 00:19:03,200
Hundreds of years ago, a plant with a root that was thought to
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00:19:03,200 --> 00:19:08,560
resemble a human body was said to emit a sound that could kill.
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The root was known to have strong anaesthetic
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00:19:10,920 --> 00:19:14,800
and hallucinogenic properties. And in the first century AD,
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it was called a mandragora or mandrake as it's now known.
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It was associated with magic and the supernatural
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00:19:23,000 --> 00:19:27,600
and was thought to derive power from a demon that emitted a dreadful
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and fatal shriek if the plant was uprooted.
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Fortunately, there were creative ways of avoiding
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death from the killer sound.
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One account advised plugging one's ears
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00:19:39,080 --> 00:19:43,760
and then tying a starving dog to the mandrake plant.
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00:19:43,760 --> 00:19:48,640
And then, as the dog lunged for food, the plant would be uprooted.
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The dog would tragically die from the mandrake's shriek
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but the man would survive.
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This particular story may have arisen because drinks made with
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00:20:00,640 --> 00:20:03,640
the mandrake root can produce hallucinations.
301
00:20:07,760 --> 00:20:11,200
But we're just beginning to realise that the sensory abilities
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00:20:11,200 --> 00:20:15,040
of a root could be as sophisticated as the rest of the plant.
303
00:20:20,800 --> 00:20:24,920
Latest research suggests that roots are communicating underground.
304
00:20:27,480 --> 00:20:31,320
And we now have the technology to eavesdrop on the roots' world.
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00:20:34,200 --> 00:20:39,280
Believe it or not, the roots of these corn seedlings can make
306
00:20:39,280 --> 00:20:41,680
and sense sound.
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00:20:41,680 --> 00:20:46,440
The noise is very quiet but we can hear it with this equipment,
308
00:20:46,440 --> 00:20:50,560
if I place a corn seedling in front of a laser beam Like this.
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00:20:55,680 --> 00:20:59,200
Now the sound vibration can be detected
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00:20:59,200 --> 00:21:01,120
and we can hear it through a speaker...
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00:21:02,360 --> 00:21:03,920
CRACKLING
312
00:21:03,920 --> 00:21:05,400
..there.
313
00:21:05,400 --> 00:21:10,240
That strange crackling is the sound of corn roots growing.
314
00:21:10,240 --> 00:21:13,400
It can be seen as pulses on the screen.
315
00:21:13,400 --> 00:21:16,840
It's been shown, too, that the corn roots respond to the sound
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00:21:16,840 --> 00:21:19,080
when it's played back to them.
317
00:21:19,080 --> 00:21:22,560
Time-lapse footage shot over just a few hours clearly shows
318
00:21:22,560 --> 00:21:26,280
the roots growing towards the tiny speakers that emit the sound.
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00:21:27,360 --> 00:21:29,040
There is much speculation
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00:21:29,040 --> 00:21:32,080
about the purpose of this curious phenomenon.
321
00:21:32,080 --> 00:21:36,600
Perhaps it helps roots avoid growing into hard objects or being too
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00:21:36,600 --> 00:21:39,160
close to competing plants.
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00:21:39,160 --> 00:21:42,760
It could act like simple echolocation,
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00:21:42,760 --> 00:21:46,280
we just don't know but it's the first clear evidence that
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00:21:46,280 --> 00:21:49,280
plants have a rudimentary form of hearing
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00:21:49,280 --> 00:21:53,760
and might even be communicating underground using sound.
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00:21:53,760 --> 00:21:57,600
Sensitive equipment is creating a new window into the plant world
328
00:21:57,600 --> 00:22:00,960
and it seems that, like animals, they have a sophisticated
329
00:22:00,960 --> 00:22:04,880
sense of their environment and possess abilities that not
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00:22:04,880 --> 00:22:08,560
so long ago, we would have thought of as supernatural.
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00:22:16,840 --> 00:22:22,360
BIRDSONG
332
00:22:27,840 --> 00:22:29,800
Swallows have successfully nested
333
00:22:29,800 --> 00:22:33,000
and raised their young in this barn for several years.
334
00:22:33,000 --> 00:22:36,320
These chicks will soon leave the nest and make their first
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00:22:36,320 --> 00:22:38,960
exploratory flights around the farm
336
00:22:38,960 --> 00:22:42,840
but in a few weeks' time they will suddenly vanish.
337
00:22:42,840 --> 00:22:44,800
Where do they go to?
338
00:22:44,800 --> 00:22:48,680
In the past, that gave rise to some extraordinary speculations.
339
00:22:48,680 --> 00:22:52,320
In fact, in the 18th century, it became a very long-running
340
00:22:52,320 --> 00:22:55,560
debate, headed by some well-known Church figures.
341
00:22:57,360 --> 00:23:00,480
And swallows are not the only birds that appear
342
00:23:00,480 --> 00:23:03,680
and disappear with the changing seasons.
343
00:23:03,680 --> 00:23:07,880
For centuries, people speculated about where such birds go.
344
00:23:07,880 --> 00:23:12,760
One explanation was that some birds changed into others by growing
345
00:23:12,760 --> 00:23:15,160
different adult plumage.
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00:23:15,160 --> 00:23:18,960
Perhaps the redstart turned into a robin...
347
00:23:20,800 --> 00:23:23,840
..or the garden warbler into a blackcap.
348
00:23:24,960 --> 00:23:28,200
Since these species where seldom present at the same time
349
00:23:28,200 --> 00:23:30,880
the explanation seemed entirely plausible.
350
00:23:35,040 --> 00:23:37,560
The barnacle goose was another mystery.
351
00:23:39,640 --> 00:23:42,280
Each winter, huge, noisy flocks of them
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00:23:42,280 --> 00:23:45,960
appear on European shores, apparently from out of nowhere.
353
00:23:47,920 --> 00:23:51,760
No-one had ever seen them build a nest or raise young.
354
00:23:56,920 --> 00:24:01,520
The barnacle goose gave rise to some extraordinary folklore as this
355
00:24:01,520 --> 00:24:04,400
mediaeval illustration shows.
356
00:24:04,400 --> 00:24:08,360
It was thought that the geese grew on underwater trees,
357
00:24:08,360 --> 00:24:13,880
starting life as small marine creatures called goose barnacles.
358
00:24:13,880 --> 00:24:16,640
Goose barnacles do, of course, exist, they're small
359
00:24:16,640 --> 00:24:20,600
shelled marine organisms with what looks like the head,
360
00:24:20,600 --> 00:24:25,600
which is in fact enclosed by a shell, attached by a stalk, which
361
00:24:25,600 --> 00:24:30,840
was thought to resemble the neck of a bird, to a bit of wood or a rock.
362
00:24:33,760 --> 00:24:37,560
The confusion about the nature of the barnacle goose was put to
363
00:24:37,560 --> 00:24:39,800
good use by some.
364
00:24:39,800 --> 00:24:41,760
Since it was unclear whether it was a bird,
365
00:24:41,760 --> 00:24:44,360
a fish or some other creature, you could surely be
366
00:24:44,360 --> 00:24:48,400
allowed to eat it on days when meat was forbidden by the church.
367
00:24:50,600 --> 00:24:53,960
But the most commonly held belief was that birds
368
00:24:53,960 --> 00:24:56,920
disappear in winter because they hibernated.
369
00:24:58,080 --> 00:25:01,760
Swallows and their close relatives, the swifts and martins,
370
00:25:01,760 --> 00:25:06,120
were thought to do so in mud at the bottom of ponds and rivers
371
00:25:06,120 --> 00:25:09,680
and it's easy to see how this idea originated
372
00:25:09,680 --> 00:25:13,760
because the birds spent much of their time near water, skimming low
373
00:25:13,760 --> 00:25:16,640
over the surface, hunting for insects or taking a drink.
374
00:25:18,720 --> 00:25:22,920
It wasn't until the Middle Ages that another theory was proposed that
375
00:25:22,920 --> 00:25:25,480
some birds may migrate
376
00:25:25,480 --> 00:25:30,120
and one of its strongest proponents was an influential religious leader.
377
00:25:32,720 --> 00:25:37,080
Frederick the second of Hohenstaufen was a powerful holy
378
00:25:37,080 --> 00:25:40,960
Roman Emperor and known for his unorthodox views.
379
00:25:40,960 --> 00:25:43,440
He ignored the philosophy of the Church
380
00:25:43,440 --> 00:25:47,560
and based his knowledge of natural history on direct observation
381
00:25:47,560 --> 00:25:50,040
rather than what was ordained.
382
00:25:50,040 --> 00:25:54,040
Frederick was also a keen falconer and he wrote this book,
383
00:25:54,040 --> 00:25:56,320
The Art Of Falconry,
384
00:25:56,320 --> 00:25:58,400
and in it, surprisingly,
385
00:25:58,400 --> 00:26:02,600
there are entire chapters on the migration of birds.
386
00:26:02,600 --> 00:26:04,760
His confidence came from the fact that,
387
00:26:04,760 --> 00:26:07,640
unlike his contemporaries and those before him,
388
00:26:07,640 --> 00:26:11,640
he had actually observed birds in the field for himself.
389
00:26:11,640 --> 00:26:13,960
He had no doubt about the migration and so,
390
00:26:13,960 --> 00:26:17,920
little patience for the myths surrounding the barnacle goose.
391
00:26:17,920 --> 00:26:20,680
He considered the story to be quite ridiculous
392
00:26:20,680 --> 00:26:24,080
and argued that the birds simply breed in distant lands.
393
00:26:26,760 --> 00:26:31,360
His views started a debate that split people into two camps,
394
00:26:31,360 --> 00:26:34,000
those believing in the old hibernation theory
395
00:26:34,000 --> 00:26:37,800
and those who supported the idea that birds migrate.
396
00:26:38,840 --> 00:26:43,640
This was the start of a new era which was to sweep away myths
397
00:26:43,640 --> 00:26:47,920
and focus instead on facts and careful observation.
398
00:26:47,920 --> 00:26:52,840
Across Europe, the evidence for bird migration started to accumulate.
399
00:26:54,760 --> 00:26:58,000
In Germany, a 12th century monk is said to have taken
400
00:26:58,000 --> 00:27:02,080
a swallow from its nest and attached a parchment note to its leg
401
00:27:02,080 --> 00:27:05,920
that read, "Oh, swallow, where do you live in winter?"
402
00:27:07,640 --> 00:27:10,920
The following spring the bird returned with a note saying,
403
00:27:10,920 --> 00:27:14,080
"In Asia, in the home of Petrus, that is Israel."
404
00:27:16,400 --> 00:27:20,320
The story may not have been true, but it certainly gave the right hint.
405
00:27:24,560 --> 00:27:29,200
In the early 16th century, a Bishop from Sweden called
406
00:27:29,200 --> 00:27:35,040
Olaus Magnus reignited the debate about swallows with this picture.
407
00:27:35,040 --> 00:27:38,640
He claimed that in winter, fishermen often drew up
408
00:27:38,640 --> 00:27:42,920
swallows in their nets, hanging together in a mass.
409
00:27:42,920 --> 00:27:45,800
This astonishing assertion provided ample fuel
410
00:27:45,800 --> 00:27:49,640
for the anti-migration lobby and, unlikely as it was,
411
00:27:49,640 --> 00:27:52,560
the view that swallows spent their winter underwater
412
00:27:52,560 --> 00:27:54,720
became increasingly entrenched.
413
00:27:58,560 --> 00:28:02,840
By the 18th century, the debate about migration versus hibernation
414
00:28:02,840 --> 00:28:08,360
had come to a head and across the continent opinions were divided.
415
00:28:11,240 --> 00:28:15,240
But new evidence was about to come from an unusual source.
416
00:28:16,520 --> 00:28:20,880
Edward Jenner was an English country doctor who also had a deep
417
00:28:20,880 --> 00:28:23,440
interest in natural history.
418
00:28:23,440 --> 00:28:27,040
He noted that although swallows often splash in water
419
00:28:27,040 --> 00:28:31,280
as they skim across it, they never immerse themselves.
420
00:28:31,280 --> 00:28:34,600
Were they to do so, he suggested, their wings would become
421
00:28:34,600 --> 00:28:37,400
so wet that they would be unable to fly.
422
00:28:38,520 --> 00:28:41,800
To test his idea, Jenner reportedly held a swift
423
00:28:41,800 --> 00:28:44,040
underwater for two minutes.
424
00:28:44,040 --> 00:28:46,040
Not surprisingly, it died.
425
00:28:48,000 --> 00:28:51,560
Jenner went on to devise another experiment to
426
00:28:51,560 --> 00:28:53,360
discover where the birds go.
427
00:28:53,360 --> 00:28:57,000
He took 12 swifts from their nests and marked them
428
00:28:57,000 --> 00:28:59,120
by taking off two of their claws.
429
00:28:59,120 --> 00:29:02,200
The following year, some of the birds he'd marked were caught
430
00:29:02,200 --> 00:29:04,800
again in exactly the same spot.
431
00:29:04,800 --> 00:29:08,240
Although Jenner could not discover where his swifts had been
432
00:29:08,240 --> 00:29:11,800
over the winter, he was the first to show that they returned to use
433
00:29:11,800 --> 00:29:15,040
the same breeding sites in the following years.
434
00:29:15,040 --> 00:29:18,520
And we now know that this is true for swallows as well.
435
00:29:20,000 --> 00:29:24,280
About the same time, across the Channel, a German bird enthusiast
436
00:29:24,280 --> 00:29:26,280
had come up with a similar idea.
437
00:29:28,360 --> 00:29:32,360
Johann Frisch caught several birds near his house and attached
438
00:29:32,360 --> 00:29:36,400
to their legs woollen threads like this which he'd dipped
439
00:29:36,400 --> 00:29:38,120
in red watercolour.
440
00:29:38,120 --> 00:29:40,880
He predicted that if swallows really did spend
441
00:29:40,880 --> 00:29:45,640
the winter at the bottom of lakes, the red colour would be washed off.
442
00:29:45,640 --> 00:29:48,880
The following spring, Frisch's swallows returned
443
00:29:48,880 --> 00:29:51,080
and the threads where unchanged.
444
00:29:51,080 --> 00:29:55,560
It was a very simple but very effective experiment.
445
00:29:55,560 --> 00:29:59,520
Evidence against the hibernation theory continued to mount
446
00:29:59,520 --> 00:30:04,200
and eventually a new technique put the final nail in its coffin...
447
00:30:04,200 --> 00:30:06,360
systematic bird ringing.
448
00:30:09,320 --> 00:30:13,800
This bird has just been fitted with its own individual marker.
449
00:30:13,800 --> 00:30:18,800
A small metal ring on its leg with a unique code of numbers.
450
00:30:18,800 --> 00:30:21,920
It's part of a national scheme that's been running for over 100
451
00:30:21,920 --> 00:30:26,320
years and provides scientists with invaluable data on bird movements.
452
00:30:27,520 --> 00:30:31,800
Early in the 20th century, the study of migration really took off.
453
00:30:31,800 --> 00:30:34,640
Birds were recovered on their breeding and wintering grounds
454
00:30:34,640 --> 00:30:37,840
and often en route, too.
455
00:30:37,840 --> 00:30:41,440
600 years after Frederick von Hohenstaufen had first started
456
00:30:41,440 --> 00:30:45,240
the debate, real evidence was beginning to accumulate.
457
00:30:47,560 --> 00:30:51,360
In the summer of 1911, a metal ring just like this one,
458
00:30:51,360 --> 00:30:55,160
was clipped onto the leg of a young swallow in Staffordshire.
459
00:30:55,160 --> 00:30:59,640
The number on the ring was B830.
460
00:30:59,640 --> 00:31:04,280
18 months later, the same bird was caught by a farmer in South Africa.
461
00:31:04,280 --> 00:31:09,240
Here, at last, was the indisputable proof that swallows migrate
462
00:31:09,240 --> 00:31:11,840
and spend the winter thousands of miles away.
463
00:31:13,960 --> 00:31:16,960
Off you go. There we are.
464
00:31:19,880 --> 00:31:23,280
Today, of course, we know that the swallows' migration is
465
00:31:23,280 --> 00:31:26,880
one of the most impressive in all the animal kingdom.
466
00:31:26,880 --> 00:31:31,080
It takes it across the largest desert in the world, the Sahara,
467
00:31:31,080 --> 00:31:33,560
it's a gruelling and dangerous journey
468
00:31:33,560 --> 00:31:38,400
and many die on the way from exhaustion or starvation.
469
00:31:38,400 --> 00:31:43,760
They travel for nearly four months, covering nearly 10,000km
470
00:31:43,760 --> 00:31:46,200
and eventually reach southern Africa.
471
00:31:52,720 --> 00:31:56,280
And bird ringing also helped to dispel the myth of
472
00:31:56,280 --> 00:31:58,080
the barnacle goose.
473
00:31:58,080 --> 00:32:02,640
In the 1960s, a Norwegian expedition, ringed geese nesting
474
00:32:02,640 --> 00:32:07,680
on the Arctic island of Spitsbergen. That autumn, some of the same
475
00:32:07,680 --> 00:32:11,720
birds were sited on the west coast of Scotland, some 2,000km away.
476
00:32:13,520 --> 00:32:18,120
Frederick von Hohenstaufen had been proved to be absolutely correct.
477
00:32:21,760 --> 00:32:26,000
It took centuries to discover the truth behind the swallows'
478
00:32:26,000 --> 00:32:27,960
seasonal movements.
479
00:32:27,960 --> 00:32:32,720
But in their time, they baffled the minds of many great naturalists and
480
00:32:32,720 --> 00:32:36,920
started one of the longest-running of all scientific debates.
481
00:32:36,920 --> 00:32:41,880
But in the end, the true story proved to be even more extraordinary
482
00:32:41,880 --> 00:32:46,280
than the fantastic myths that where invented to explain it.
483
00:32:48,440 --> 00:32:50,160
Just like the swallow,
484
00:32:50,160 --> 00:32:54,160
the painted lady butterfly seems to appear magically out of nowhere
485
00:32:54,160 --> 00:32:57,800
and that started some extraordinary ideas and controversies.
486
00:32:58,960 --> 00:33:02,200
The painted lady is one of our largest butterflies
487
00:33:02,200 --> 00:33:05,080
and a familiar summer visitor to our gardens.
488
00:33:05,080 --> 00:33:06,360
And yet, its appearance
489
00:33:06,360 --> 00:33:10,000
and disappearance each year, has puzzled us for centuries.
490
00:33:10,000 --> 00:33:13,240
It's only now that we're beginning to understand this extraordinary
491
00:33:13,240 --> 00:33:16,520
life cycle and discover where it vanishes each year.
492
00:33:18,080 --> 00:33:20,720
Early naturalists were confused by the sudden
493
00:33:20,720 --> 00:33:23,720
appearance of painted ladies each spring because they were
494
00:33:23,720 --> 00:33:28,960
unaware of the connection between butterflies and caterpillars.
495
00:33:28,960 --> 00:33:32,520
For a very long time it was widely believed that butterflies
496
00:33:32,520 --> 00:33:37,880
arise from rotting material by what was called spontaneous generation.
497
00:33:40,160 --> 00:33:46,360
In the 1830s, a German scientist named Renous was arrested for heresy
498
00:33:46,360 --> 00:33:50,520
for claiming that he could change caterpillars into butterflies.
499
00:33:50,520 --> 00:33:53,600
Arresting someone for something now known to be common knowledge
500
00:33:53,600 --> 00:33:57,280
may seem rather extreme, but at the time, many still believed that
501
00:33:57,280 --> 00:34:00,720
caterpillars and butterflies were completely different creatures,
502
00:34:00,720 --> 00:34:02,440
created by the hand of God.
503
00:34:04,800 --> 00:34:08,520
Needless to say, people had been well aware of the existence of
504
00:34:08,520 --> 00:34:12,240
both butterflies and caterpillars since the earliest times.
505
00:34:14,240 --> 00:34:17,000
But the thought that any two were related,
506
00:34:17,000 --> 00:34:21,200
let alone the same species, seemed impossible...
507
00:34:21,200 --> 00:34:23,960
and it's easy to see why.
508
00:34:26,760 --> 00:34:30,120
Not only do caterpillars and butterflies look like very
509
00:34:30,120 --> 00:34:33,680
different types of animals, but the colours and patterns
510
00:34:33,680 --> 00:34:38,000
of a caterpillar don't match up with those of its adult form.
511
00:34:38,000 --> 00:34:42,520
The only way to know which lava and which butterfly go together
512
00:34:42,520 --> 00:34:46,280
is to keep caterpillars and watch them turn into butterflies.
513
00:34:46,280 --> 00:34:50,040
But it wasn't until the 17th century that anyone left
514
00:34:50,040 --> 00:34:51,640
a record of doing that.
515
00:34:52,720 --> 00:34:57,400
One of the first was a remarkable woman named Maria Sibylla Merian.
516
00:34:58,520 --> 00:35:01,560
Merian was born in Germany at a time
517
00:35:01,560 --> 00:35:04,240
when women still had little formal education
518
00:35:04,240 --> 00:35:08,560
and no role in the scientific world, but she was an accomplished
519
00:35:08,560 --> 00:35:12,840
artist and painted plants and insects she saw around her.
520
00:35:12,840 --> 00:35:17,000
To do that, she kept caterpillars, fed them on leaves
521
00:35:17,000 --> 00:35:19,440
and watched them turn into butterflies.
522
00:35:21,040 --> 00:35:24,560
Merian produced hundreds of beautiful paintings of butterflies
523
00:35:24,560 --> 00:35:26,560
and their stages of development
524
00:35:26,560 --> 00:35:29,000
along with the plants on which they feed.
525
00:35:29,000 --> 00:35:30,840
Her drawings are so exquisite
526
00:35:30,840 --> 00:35:34,280
and detailed that they still rank among the best in the world.
527
00:35:37,320 --> 00:35:41,280
Among the things she observed with great care, were things like this.
528
00:35:42,640 --> 00:35:48,400
A curious, yet strangely beautiful object, it's a chrysalis,
529
00:35:48,400 --> 00:35:52,280
the intermediate stage between a caterpillar and a butterfly.
530
00:35:55,880 --> 00:35:59,080
She was one of the first to record the remarkable change
531
00:35:59,080 --> 00:36:01,080
that takes place in the chrysalis.
532
00:36:02,920 --> 00:36:06,360
It's one of nature's most extraordinary transformations.
533
00:36:11,000 --> 00:36:15,920
At the age of 52, she sailed from Europe to South America on a
534
00:36:15,920 --> 00:36:20,640
two-year expedition to study insects in the tropical jungles of Surinam.
535
00:36:20,640 --> 00:36:23,200
It was an exceptional journey for any naturalist
536
00:36:23,200 --> 00:36:26,480
at the time and particularly for a woman.
537
00:36:26,480 --> 00:36:29,760
When she returned, she produced this beautiful book.
538
00:36:32,280 --> 00:36:34,000
It turned out to be popular
539
00:36:34,000 --> 00:36:36,200
because it was one of the few to be published
540
00:36:36,200 --> 00:36:40,360
not in the scientific language of Latin but in Dutch.
541
00:36:40,360 --> 00:36:41,520
Because of this,
542
00:36:41,520 --> 00:36:45,280
her work was largely dismissed by scientists of the time
543
00:36:45,280 --> 00:36:48,600
but Merian was one of the first naturalists to correctly
544
00:36:48,600 --> 00:36:52,600
connect the caterpillar with its pupa and the adult form.
545
00:36:54,800 --> 00:36:56,920
Today, Merian's book is widely
546
00:36:56,920 --> 00:37:01,440
recognised as a pioneering work of scientific observation
547
00:37:01,440 --> 00:37:06,040
and it put an end to the idea of spontaneous generation.
548
00:37:08,680 --> 00:37:12,240
Around the same time, further evidence for the connection
549
00:37:12,240 --> 00:37:16,280
between butterflies and caterpillars came from a different source.
550
00:37:18,520 --> 00:37:23,240
In 1669, a Dutch scientist by the name of Jan Swammerdam published
551
00:37:23,240 --> 00:37:27,040
the results of experiments which would finally prove that the
552
00:37:27,040 --> 00:37:30,800
caterpillar and butterfly are one and the same animal.
553
00:37:30,800 --> 00:37:33,960
Swammerdam was a master of the miniature and dissected the
554
00:37:33,960 --> 00:37:36,520
caterpillars and pupae of butterflies and moths
555
00:37:36,520 --> 00:37:40,720
under a microscope. With a steady hand and endless patience,
556
00:37:40,720 --> 00:37:44,160
he carefully cut into the layers of skin with tiny scissors
557
00:37:44,160 --> 00:37:47,320
and what he discovered was truly astonishing.
558
00:37:49,600 --> 00:37:54,520
He found some of the body parts of a butterfly.
559
00:37:54,520 --> 00:37:58,920
The structures were fragile and not complete but Swammerdam had proved
560
00:37:58,920 --> 00:38:03,240
that caterpillar and butterfly are, indeed, one and the same animal.
561
00:38:07,920 --> 00:38:12,080
We now know that without the caterpillar, there can be no butterfly.
562
00:38:12,080 --> 00:38:13,360
Yet, for a very long time,
563
00:38:13,360 --> 00:38:16,320
the painted lady seemed to be an exception.
564
00:38:16,320 --> 00:38:19,920
Every spring, the adult butterflies would appear across Britain
565
00:38:19,920 --> 00:38:22,600
without any sightings of their caterpillars.
566
00:38:23,600 --> 00:38:27,360
While some butterflies hibernate in Britain, there was no sign
567
00:38:27,360 --> 00:38:28,920
of painted ladies doing so.
568
00:38:29,920 --> 00:38:34,720
Some speculated that they flew to warmer climates as birds do.
569
00:38:35,720 --> 00:38:39,040
But how could a tiny insect cross the English Channel?
570
00:38:40,160 --> 00:38:43,840
In the 20th century, swarms of butterflies moving across Europe
571
00:38:43,840 --> 00:38:47,640
finally provided evidence that painted ladies do, indeed,
572
00:38:47,640 --> 00:38:49,400
cross the sea.
573
00:38:49,400 --> 00:38:53,800
And they were found to fly all the way from North Africa to Britain.
574
00:38:55,000 --> 00:38:58,240
But there were almost no records of painted ladies making
575
00:38:58,240 --> 00:39:00,320
the reverse trip south.
576
00:39:00,320 --> 00:39:03,680
So, for years, it was thought that Britain must be
577
00:39:03,680 --> 00:39:06,640
a dead-end for the most northerly stragglers.
578
00:39:08,440 --> 00:39:14,120
And then, in 2009, the public was asked to help solve the mystery.
579
00:39:14,120 --> 00:39:18,480
Among 12,000 sightings there were reports of painted ladies
580
00:39:18,480 --> 00:39:21,000
flying out to sea in the autumn.
581
00:39:22,400 --> 00:39:25,680
And a radar station detected them flying south
582
00:39:25,680 --> 00:39:30,240
at heights of 500 metres, way beyond the sight of human eyes.
583
00:39:32,360 --> 00:39:36,240
We now know that the painted ladies migration is a round-trip
584
00:39:36,240 --> 00:39:42,200
of over 12,000km. But it's not made by any one individual.
585
00:39:42,200 --> 00:39:45,040
Each only flies part of the way,
586
00:39:45,040 --> 00:39:48,440
passing on the migratory baton to the next generation.
587
00:39:48,440 --> 00:39:53,360
It's like a relay race with up to six generations of butterflies involved.
588
00:39:55,640 --> 00:39:58,920
The painted ladies epic journey from one continent to the next
589
00:39:58,920 --> 00:40:01,960
would be a truly astonishing feature in any animal
590
00:40:01,960 --> 00:40:05,680
but for a tiny creature like this, it seems really extraordinary.
591
00:40:05,680 --> 00:40:07,280
How does it battle the wind
592
00:40:07,280 --> 00:40:10,880
and the weather and navigate across vast bodies of water?
593
00:40:10,880 --> 00:40:14,400
And with no single individual ever undertaking the whole migration,
594
00:40:14,400 --> 00:40:16,000
how do they find the way?
595
00:40:18,520 --> 00:40:23,000
It seems that painted ladies are pre-programmed to either fly
596
00:40:23,000 --> 00:40:26,800
north or south and this is determined whilst
597
00:40:26,800 --> 00:40:29,600
they are still caterpillars, possibly by temperature
598
00:40:29,600 --> 00:40:34,240
and day length and also by the plants they feed on but how
599
00:40:34,240 --> 00:40:39,320
does this information get passed on from caterpillar to butterfly?
600
00:40:39,320 --> 00:40:42,200
The answer may be hidden within the chrysalis.
601
00:40:43,400 --> 00:40:49,280
Recently CT scanners have allowed us to look inside a pupa.
602
00:40:49,280 --> 00:40:54,480
They reveal that some organs remain intact during the transformation.
603
00:40:56,320 --> 00:41:00,320
A one-day-old pupa clearly shows the gut and breathing tubes
604
00:41:00,320 --> 00:41:03,560
which only change slightly as the chrysalis develops.
605
00:41:07,680 --> 00:41:12,000
Could it be that the brain or nerves also remain intact
606
00:41:12,000 --> 00:41:14,360
and that memories are passed on?
607
00:41:15,720 --> 00:41:20,600
Recent experiments in the lab appear to support this idea.
608
00:41:20,600 --> 00:41:24,080
Scientists taught caterpillars to avoid specific
609
00:41:24,080 --> 00:41:27,480
smells by linking them with an unpleasant reaction.
610
00:41:28,520 --> 00:41:32,280
Later on, as adults, the same individuals remembered these
611
00:41:32,280 --> 00:41:35,520
smells and chose to keep away from them.
612
00:41:35,520 --> 00:41:38,760
If the experiences of a caterpillar can be carried over
613
00:41:38,760 --> 00:41:43,600
to the adult, then maybe cues for migration can also be passed on.
614
00:41:45,800 --> 00:41:49,440
Although we've unravelled much of the painted lady's life-cycle,
615
00:41:49,440 --> 00:41:54,760
many questions remain. How far does each individual travel?
616
00:41:54,760 --> 00:41:59,040
And do offspring follow similar routes to their ancestors?
617
00:41:59,040 --> 00:42:02,680
One day we may know the answers but, for now,
618
00:42:02,680 --> 00:42:06,480
they remain some of the unsolved mysteries of nature.
619
00:42:09,560 --> 00:42:12,680
The arrival each spring of our painted lady butterflies
620
00:42:12,680 --> 00:42:15,800
and our swallows never ceases to delight us
621
00:42:15,800 --> 00:42:18,800
but now we also understand the extraordinary journeys
622
00:42:18,800 --> 00:42:22,640
they undertake when they disappear again at the end of summer.
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