0:00:03 > 0:00:06The natural world is full of extraordinary animals with
0:00:06 > 0:00:09amazing life histories.
0:00:09 > 0:00:13Yet certain stories are more intriguing than most.
0:00:16 > 0:00:19The mysteries of a butterfly's life cycle,
0:00:19 > 0:00:23or the strange biology of the emperor penguin.
0:00:23 > 0:00:26Some of these creatures were surrounded by myth
0:00:26 > 0:00:29and misunderstandings for a very long time.
0:00:30 > 0:00:33And some have only recently revealed their secrets.
0:00:35 > 0:00:38These are the animals that stand out from the crowd,
0:00:38 > 0:00:42the curiosities I find most fascinating of all.
0:00:51 > 0:00:54Some of our most familiar animals puzzled scientific
0:00:54 > 0:00:58minds for a surprisingly long time.
0:00:58 > 0:00:59The mysterious comings
0:00:59 > 0:01:03and goings of barn swallows led to some far-fetched ideas.
0:01:05 > 0:01:09While the life cycle of the painted lady butterfly took centuries
0:01:09 > 0:01:10to unravel.
0:01:12 > 0:01:14But the abilities of some plants
0:01:14 > 0:01:19and animals are so remarkable that they seem to be almost supernatural.
0:01:19 > 0:01:24In this programme, I investigate the shocking power of a fish that
0:01:24 > 0:01:27advanced our understanding of electricity,
0:01:27 > 0:01:31and plants with senses that are surprising modern science.
0:01:33 > 0:01:37How do these extraordinary powers help the organisms that
0:01:37 > 0:01:38produced them?
0:01:45 > 0:01:49The freshwater eel is surrounded by legends.
0:01:49 > 0:01:53The first Europeans to explore the New World heard amazing
0:01:53 > 0:01:55stories about it.
0:01:55 > 0:01:58And when, in the 18th century, specimens of this strange fish
0:01:58 > 0:02:01reached Europe, they created a sensation.
0:02:03 > 0:02:07In 1776, Captain George Baker,
0:02:07 > 0:02:09an American mariner and whaler,
0:02:09 > 0:02:12made the long and difficult journey from South America across a
0:02:12 > 0:02:18raging Atlantic Ocean to bring five live electric eels to London.
0:02:18 > 0:02:21These are two of his actual eels.
0:02:22 > 0:02:26Captain Baker and his five electric eels, or gymnotas as they were known,
0:02:26 > 0:02:30set up shop in the Haymarket and offered two shillings
0:02:30 > 0:02:34and sixpence for a shock, or five shillings for a spark.
0:02:36 > 0:02:39Baker's eels had come all the way from the lower
0:02:39 > 0:02:42reaches of the Amazon and Orinoco rivers,
0:02:42 > 0:02:47where he had heard tales from the locals about their astonishing powers.
0:02:47 > 0:02:51They called these fish "trembladores".
0:02:51 > 0:02:55Humboldt, the famous naturalist and explorer, had described how he
0:02:55 > 0:02:59had witnessed horses being killed by the repeated shocks from these fish.
0:03:01 > 0:03:03And he himself accidentally stepped on one
0:03:03 > 0:03:05and vividly described the effect.
0:03:06 > 0:03:10"With each stroke, you feel an internal vibration that lasts
0:03:10 > 0:03:14"two or three seconds, followed by a painful numbness.
0:03:14 > 0:03:18"All day I felt strong pain in my knees and in all my joints."
0:03:21 > 0:03:25I encountered this remarkable fish in its natural environment
0:03:25 > 0:03:29when I filmed at the same rivers that Humboldt explored.
0:03:29 > 0:03:31There was talk of me swimming with the eel,
0:03:31 > 0:03:35but thankfully we had some technical difficulties with the diving
0:03:35 > 0:03:37equipment that I was supposed to wear,
0:03:37 > 0:03:41and so I stayed safely in a canoe and was able to demonstrate
0:03:41 > 0:03:44another subtler, but equally remarkable, side to this fish.
0:03:47 > 0:03:52The eels were constantly producing electric discharges.
0:03:52 > 0:03:56Somehow they were generating a small, nonstop flowing current.
0:03:56 > 0:03:58ELECTRIC DRONE
0:03:58 > 0:04:01They were also able to sense electricity and were
0:04:01 > 0:04:06attracted to electrical pulses emitted from my underwater detector,
0:04:06 > 0:04:11suggesting that electricity plays a key role in their lives.
0:04:12 > 0:04:14But at the time of their discovery,
0:04:14 > 0:04:18no-one knew the full functions of their extraordinary abilities.
0:04:20 > 0:04:24We now know that the shock was caused by electricity,
0:04:24 > 0:04:29and I can demonstrate it by touching the animal with an electrode.
0:04:29 > 0:04:31Watch.
0:04:31 > 0:04:33There. You see?
0:04:33 > 0:04:36The scope and the lights are flashing up and down.
0:04:36 > 0:04:38Extraordinary.
0:04:38 > 0:04:42But this is only a small indication of the real power of this fish.
0:04:42 > 0:04:44If I were to try and pick it up, I could get
0:04:44 > 0:04:49a jolt of an astonishing 600 volts, which is quite enough to kill me.
0:04:57 > 0:05:01This 1960s educational film illustrated the shock,
0:05:01 > 0:05:04even though the equipment used prevented
0:05:04 > 0:05:07the volunteers from getting its full power.
0:05:07 > 0:05:10They were to join hands and then connected to a live eel.
0:05:12 > 0:05:15WOMAN SCREAMS
0:05:16 > 0:05:20Firm believers in electric eels. Thank you very much.
0:05:21 > 0:05:24You can imagine how startling Baker's electric eels
0:05:24 > 0:05:26were 200 years ago.
0:05:29 > 0:05:30In the 18th century,
0:05:30 > 0:05:33electricity was becoming one of the most fashionable areas
0:05:33 > 0:05:38of scientific investigation, but it was still very poorly understood.
0:05:38 > 0:05:41Very few advances had been made since its discovery 150 years
0:05:41 > 0:05:45earlier by Elizabeth I's personal physician, William Gilbert.
0:05:45 > 0:05:50Gilbert repeated a trick that had been known about since Greek times.
0:05:50 > 0:05:54Rubbing a piece of amber with cat fur, that allowed the amber
0:05:54 > 0:05:58to attract a small object like a feather. Let's give it a try.
0:05:58 > 0:05:59Here is a bit of amber.
0:06:03 > 0:06:04There.
0:06:04 > 0:06:07It had always been assumed that this amber effect was caused
0:06:07 > 0:06:11by magnetism but Gilbert showed that it was something different.
0:06:11 > 0:06:15He named this new force after the Greek word for amber,
0:06:15 > 0:06:20electron, and so electricity was born.
0:06:21 > 0:06:26Londoners of the time developed a fascination for this magical force.
0:06:26 > 0:06:31Showmen staged bizarre spectacles to demonstrate its properties.
0:06:31 > 0:06:35In one, a young boy attached to a friction generator
0:06:35 > 0:06:37attracted small pieces of paper to his hands.
0:06:37 > 0:06:42In another, a gentleman kissed a lady and was repulsed
0:06:42 > 0:06:45by the charge carried through her whalebone corset.
0:06:46 > 0:06:49No-one knew what to do with electricity
0:06:49 > 0:06:53but a better understanding of its nature was slowly emerging.
0:06:54 > 0:06:56More and more ingenious ways were developed
0:06:56 > 0:07:00to create what we now call static electricity.
0:07:00 > 0:07:04And soon it became something more than just a quirk of rubbing amber,
0:07:04 > 0:07:06it became visible as a spark.
0:07:11 > 0:07:14The ability to produce this characteristic blue spark
0:07:14 > 0:07:17along with its invigorating smell became the signature
0:07:17 > 0:07:21of this new force and it prompted scientists to make
0:07:21 > 0:07:24obvious comparisons with other natural phenomena.
0:07:24 > 0:07:26THUNDER
0:07:28 > 0:07:32In the American colonies, Benjamin Franklin bravely,
0:07:32 > 0:07:36or perhaps foolishly, flew kites into thunderstorms and proved that
0:07:36 > 0:07:40lightning and the electric spark were one and the same.
0:07:41 > 0:07:45But there's another common property of lightning and static electricity.
0:07:45 > 0:07:47That is the ability to shock.
0:07:47 > 0:07:51It wasn't long before a comparison was made between the shock from
0:07:51 > 0:07:55the early generators and the shock that could be delivered by a fish.
0:07:58 > 0:08:00The electric eel wasn't the only kind of fish
0:08:00 > 0:08:03known to give humans a powerful jolt.
0:08:03 > 0:08:07The ancient Egyptians knew that the electric catfish could also
0:08:07 > 0:08:11give shocks and they called it the "Thunderer of the Nile".
0:08:12 > 0:08:17And in the nearby Mediterranean lives the torpedo ray.
0:08:17 > 0:08:19Its muscle batteries make it so bulky
0:08:19 > 0:08:22it can't undulate its body like other rays
0:08:22 > 0:08:26but has to propel itself by waving its tail.
0:08:26 > 0:08:28Like the electric eel,
0:08:28 > 0:08:32it uses its discharge to stun the other fish on which it prays.
0:08:34 > 0:08:38Sadly, the pressure of celebrity and having to produce shocks
0:08:38 > 0:08:42and sparks to order exhausted Baker's long-suffering eels
0:08:42 > 0:08:44and they didn't last the winter.
0:08:44 > 0:08:49But two were preserved and expertly dissected by John Hunter,
0:08:49 > 0:08:52a very distinguished Scottish surgeon of the time
0:08:52 > 0:08:56and he found a great number of striped muscular layers
0:08:56 > 0:08:59that proved to be where the electricity was generated.
0:08:59 > 0:09:03They are now referred to as Hunter's organs.
0:09:04 > 0:09:08He found these muscles along the tail and sides of the eels
0:09:08 > 0:09:10arranged in stacks.
0:09:12 > 0:09:15One scientist called Galvani believed that animals
0:09:15 > 0:09:20had their own natural electricity even without these electric organs
0:09:20 > 0:09:25and he tried to prove this by connecting wires to frogs' legs
0:09:25 > 0:09:27and making them twitch.
0:09:27 > 0:09:31He called this phenomenon animal electricity.
0:09:32 > 0:09:36But another scientist called Volta had other ideas.
0:09:36 > 0:09:40He proved that the frog was merely a conductor for electricity
0:09:40 > 0:09:42with a simple experiment.
0:09:43 > 0:09:48Volta replaced Galvani's frog with discs of cloth
0:09:48 > 0:09:50soaked in saltwater or acid
0:09:50 > 0:09:53and sandwiched them between two different metals.
0:09:53 > 0:09:56I can do the same thing with filter paper,
0:09:56 > 0:10:01copper two pence pieces and these simple galvanised zinc washers.
0:10:01 > 0:10:02Watch.
0:10:04 > 0:10:06Tuppenny piece.
0:10:07 > 0:10:08Filter.
0:10:08 > 0:10:10And washer.
0:10:14 > 0:10:16There, nearly 0.6 of a volt.
0:10:17 > 0:10:21But the amount of electricity generated was tiny.
0:10:21 > 0:10:25Certainly not enough to make the sparks seen from eels.
0:10:25 > 0:10:28Unlike Galvani, Volta saw no distinction
0:10:28 > 0:10:32between animal electricity and his new electricity from metals
0:10:32 > 0:10:37so he now looked at animals to see how he might amplify his new device.
0:10:40 > 0:10:43Was it significant that the muscles
0:10:43 > 0:10:47producing the electric power in the eels were arranged in stacks?
0:10:49 > 0:10:54Volta decided to add more stacks to his electric pile.
0:10:54 > 0:10:57We call this way of connecting electric cells together
0:10:57 > 0:11:01"in series", and we now know that it increases the voltage.
0:11:01 > 0:11:04But Volta was about to find this out for the first time.
0:11:04 > 0:11:07He piled up his tiny cells like the bands of muscle
0:11:07 > 0:11:08in an electric fish.
0:11:08 > 0:11:11Here I've got ten pairs.
0:11:11 > 0:11:12And just watch.
0:11:15 > 0:11:16Nearly six volts.
0:11:16 > 0:11:18Wonderful.
0:11:18 > 0:11:22Volta could now produce heat, shocks and even sparks
0:11:22 > 0:11:27from electricity in a continuous never-ending stream.
0:11:28 > 0:11:33He had made the first battery, partly inspired by the electric eel.
0:11:34 > 0:11:38The pieces of the puzzle had come together and the eel's example
0:11:38 > 0:11:41had helped to advance our understanding of electricity.
0:11:42 > 0:11:45Eels, in fact, contain natural batteries.
0:11:45 > 0:11:48Stacks of special muscles.
0:11:49 > 0:11:52It's amazing to think when electricity is so much
0:11:52 > 0:11:55a part of our lives today that before Volta
0:11:55 > 0:11:59the only source of electricity was lightning,
0:11:59 > 0:12:01a few static generators
0:12:01 > 0:12:04and fish like this incredible electric eel.
0:12:09 > 0:12:13Understanding how electric eels managed to find their way around
0:12:13 > 0:12:16revealed a hitherto unknown animal sense.
0:12:18 > 0:12:21But it's not just animals that have surprised us.
0:12:22 > 0:12:25We're now discovering that plants too
0:12:25 > 0:12:29have intriguing abilities that are still mysterious.
0:12:32 > 0:12:35We think of plants as passive, still and silent.
0:12:35 > 0:12:39But they may have more in common with animals than you might think.
0:12:44 > 0:12:48New research suggests that they have surprising abilities.
0:12:48 > 0:12:51It depends on how you look at them.
0:12:53 > 0:12:57I first started seeing plants in a different light
0:12:57 > 0:13:00when making a series called The Private Life of Plants.
0:13:01 > 0:13:06We used time-lapse photography to reveal the way they move.
0:13:07 > 0:13:10The bramble spreads aggressively - seemingly unstoppable.
0:13:11 > 0:13:15Other plants pulsed to the rhythms of day and night.
0:13:15 > 0:13:18And flower buds explode like fireworks.
0:13:21 > 0:13:25So, with speeded up film, we had been able to translate
0:13:25 > 0:13:27their time into ours
0:13:27 > 0:13:30and to realise that they're constantly on the move.
0:13:37 > 0:13:41200 years ago, one plant that moved very quickly indeed
0:13:41 > 0:13:45attracted the attention of a great scientific mind.
0:13:45 > 0:13:48It appeared to behave like an animal
0:13:48 > 0:13:51and could move fast enough to catch its own food.
0:13:52 > 0:13:56Charles Darwin was fascinated by the Venus flytrap.
0:13:56 > 0:14:00He called it one of the most wonderful plants in the world.
0:14:00 > 0:14:04He recognised that it could move in a very different way
0:14:04 > 0:14:06to that of plant growth.
0:14:06 > 0:14:10This movement was not only fast but also repeatable.
0:14:10 > 0:14:12Darwin experimented and found that the traps
0:14:12 > 0:14:14are not triggered by raindrops
0:14:14 > 0:14:18but only by a very particular stimulation of the leaf hairs,
0:14:18 > 0:14:20such as an insect might make.
0:14:20 > 0:14:25But what intrigued him most was the speed of the reaction.
0:14:25 > 0:14:29He sent one of these flytraps to a friend, Dr Burdon-Sanderson,
0:14:29 > 0:14:33who was performing groundbreaking work on muscles and electricity.
0:14:33 > 0:14:37His tests confirmed that the tiny electrical discharge
0:14:37 > 0:14:41caused by an animal muscle cell contracting was almost identical
0:14:41 > 0:14:45to those signals obtained by attaching electrodes to the flytrap
0:14:45 > 0:14:47when it was shutting.
0:14:48 > 0:14:50Although plants have no muscles,
0:14:50 > 0:14:55electrical stimulation enables them to move in a similar way to animals.
0:14:57 > 0:15:01Electrical signals cause cells to change the pressure of sap
0:15:01 > 0:15:04in their leaves, so creating movement.
0:15:05 > 0:15:08As a result, some plants, like animals,
0:15:08 > 0:15:10can actively catch their prey.
0:15:13 > 0:15:18Recently it's been discovered that other plants use electricity too
0:15:18 > 0:15:20but for a very different purpose.
0:15:22 > 0:15:27Plants are rooted to the ground and have a small negative charge.
0:15:27 > 0:15:31The higher up the plant you go, the greater the electric charge.
0:15:31 > 0:15:34This creates an electric field around the flower.
0:15:34 > 0:15:39We can't see it but these electrodes are picking up the energy
0:15:39 > 0:15:43of this tiny field and converting it into the sound that we can hear.
0:15:44 > 0:15:48Bees, on the other hand, have a positive charge.
0:15:48 > 0:15:50Friction whilst flying causes them
0:15:50 > 0:15:54to lose electrons, leaving them electrically charged.
0:15:54 > 0:15:58As a bee approaches a flower, the charge fields around the flower
0:15:58 > 0:16:01and the bee interact, and the sound changes...
0:16:01 > 0:16:02FALTERING ELECTRONIC BUZZ
0:16:02 > 0:16:04..there.
0:16:04 > 0:16:06And when it lands, the positive
0:16:06 > 0:16:10and negative fields immediately cancel each other out.
0:16:10 > 0:16:13As this happens, there are two very surprising consequences.
0:16:13 > 0:16:18Firstly, the plant's negatively charged pollen actually
0:16:18 > 0:16:22jumps across onto the positively charged bee.
0:16:22 > 0:16:26Secondly, the plant has a changed electrical field
0:16:26 > 0:16:30and when another bee comes along, it detects this altered
0:16:30 > 0:16:33electrical signature and avoids the flower.
0:16:33 > 0:16:37The plant is, in effect, telling the bee that it has no nectar
0:16:37 > 0:16:39and to come back later.
0:16:40 > 0:16:44When the flower has refilled its stores of nectar, it creates
0:16:44 > 0:16:49a new electric charge which attracts another passing bee.
0:16:49 > 0:16:54This simple on/off signal benefits both the bee and the flower,
0:16:54 > 0:16:57but it does have its limitations.
0:16:58 > 0:17:00The electrical field is tiny,
0:17:00 > 0:17:03so insects can only detect it at close quarters.
0:17:04 > 0:17:08But flowers can also draw attention to themselves over much
0:17:08 > 0:17:13greater distances and they do this by floating messages in the air.
0:17:15 > 0:17:18The perfume of a flower is not just a pleasant smell,
0:17:18 > 0:17:23it's also the primary way in which plants communicate with insects.
0:17:23 > 0:17:27A rose can contain over 400 chemical compounds and a bee
0:17:27 > 0:17:32can recognise a particular combination from over a mile away.
0:17:32 > 0:17:35The very latest research has discovered
0:17:35 > 0:17:39that 90% of the chemicals made by plants, are also
0:17:39 > 0:17:42produced by insects and that is no coincidence.
0:17:45 > 0:17:50Most flowers produce scent to persuade insects to visit them,
0:17:51 > 0:17:55but others use it in a more sophisticated way...
0:17:55 > 0:17:56for protection.
0:17:57 > 0:18:01Cabbages communicate with each other using smell.
0:18:03 > 0:18:06When the leaves of one plant are being attacked by caterpillars,
0:18:06 > 0:18:10it releases a scent which warns its neighbours.
0:18:10 > 0:18:14They then produce chemicals in their leaves that caterpillars
0:18:14 > 0:18:16don't like and so they avoid being eaten.
0:18:18 > 0:18:21And scent also serves to call in the cavalry.
0:18:22 > 0:18:26Leaves that are under attack give off a chemical alarm signal that
0:18:26 > 0:18:31attracts wasps which obligingly pick off the caterpillar attackers.
0:18:35 > 0:18:39So, vegetables, fruits, leaves and flowers are constantly
0:18:39 > 0:18:45communicating with each other using touch, vision and smell.
0:18:45 > 0:18:49They seem to exploit all the senses, apart, that is, from hearing.
0:18:51 > 0:18:55But there are old stories that one particular plant is able to
0:18:55 > 0:18:57produce a very strange sound.
0:18:59 > 0:19:03Hundreds of years ago, a plant with a root that was thought to
0:19:03 > 0:19:08resemble a human body was said to emit a sound that could kill.
0:19:08 > 0:19:10The root was known to have strong anaesthetic
0:19:10 > 0:19:14and hallucinogenic properties. And in the first century AD,
0:19:14 > 0:19:19it was called a mandragora or mandrake as it's now known.
0:19:19 > 0:19:23It was associated with magic and the supernatural
0:19:23 > 0:19:27and was thought to derive power from a demon that emitted a dreadful
0:19:27 > 0:19:30and fatal shriek if the plant was uprooted.
0:19:31 > 0:19:34Fortunately, there were creative ways of avoiding
0:19:34 > 0:19:36death from the killer sound.
0:19:36 > 0:19:39One account advised plugging one's ears
0:19:39 > 0:19:43and then tying a starving dog to the mandrake plant.
0:19:43 > 0:19:48And then, as the dog lunged for food, the plant would be uprooted.
0:19:48 > 0:19:51The dog would tragically die from the mandrake's shriek
0:19:51 > 0:19:53but the man would survive.
0:19:56 > 0:20:00This particular story may have arisen because drinks made with
0:20:00 > 0:20:03the mandrake root can produce hallucinations.
0:20:07 > 0:20:11But we're just beginning to realise that the sensory abilities
0:20:11 > 0:20:15of a root could be as sophisticated as the rest of the plant.
0:20:20 > 0:20:24Latest research suggests that roots are communicating underground.
0:20:27 > 0:20:31And we now have the technology to eavesdrop on the roots' world.
0:20:34 > 0:20:39Believe it or not, the roots of these corn seedlings can make
0:20:39 > 0:20:41and sense sound.
0:20:41 > 0:20:46The noise is very quiet but we can hear it with this equipment,
0:20:46 > 0:20:50if I place a corn seedling in front of a laser beam Like this.
0:20:55 > 0:20:59Now the sound vibration can be detected
0:20:59 > 0:21:01and we can hear it through a speaker...
0:21:02 > 0:21:03CRACKLING
0:21:03 > 0:21:05..there.
0:21:05 > 0:21:10That strange crackling is the sound of corn roots growing.
0:21:10 > 0:21:13It can be seen as pulses on the screen.
0:21:13 > 0:21:16It's been shown, too, that the corn roots respond to the sound
0:21:16 > 0:21:19when it's played back to them.
0:21:19 > 0:21:22Time-lapse footage shot over just a few hours clearly shows
0:21:22 > 0:21:26the roots growing towards the tiny speakers that emit the sound.
0:21:27 > 0:21:29There is much speculation
0:21:29 > 0:21:32about the purpose of this curious phenomenon.
0:21:32 > 0:21:36Perhaps it helps roots avoid growing into hard objects or being too
0:21:36 > 0:21:39close to competing plants.
0:21:39 > 0:21:42It could act like simple echolocation,
0:21:42 > 0:21:46we just don't know but it's the first clear evidence that
0:21:46 > 0:21:49plants have a rudimentary form of hearing
0:21:49 > 0:21:53and might even be communicating underground using sound.
0:21:53 > 0:21:57Sensitive equipment is creating a new window into the plant world
0:21:57 > 0:22:00and it seems that, like animals, they have a sophisticated
0:22:00 > 0:22:04sense of their environment and possess abilities that not
0:22:04 > 0:22:08so long ago, we would have thought of as supernatural.
0:22:16 > 0:22:22BIRDSONG
0:22:27 > 0:22:29Swallows have successfully nested
0:22:29 > 0:22:33and raised their young in this barn for several years.
0:22:33 > 0:22:36These chicks will soon leave the nest and make their first
0:22:36 > 0:22:38exploratory flights around the farm
0:22:38 > 0:22:42but in a few weeks' time they will suddenly vanish.
0:22:42 > 0:22:44Where do they go to?
0:22:44 > 0:22:48In the past, that gave rise to some extraordinary speculations.
0:22:48 > 0:22:52In fact, in the 18th century, it became a very long-running
0:22:52 > 0:22:55debate, headed by some well-known Church figures.
0:22:57 > 0:23:00And swallows are not the only birds that appear
0:23:00 > 0:23:03and disappear with the changing seasons.
0:23:03 > 0:23:07For centuries, people speculated about where such birds go.
0:23:07 > 0:23:12One explanation was that some birds changed into others by growing
0:23:12 > 0:23:15different adult plumage.
0:23:15 > 0:23:18Perhaps the redstart turned into a robin...
0:23:20 > 0:23:23..or the garden warbler into a blackcap.
0:23:24 > 0:23:28Since these species where seldom present at the same time
0:23:28 > 0:23:30the explanation seemed entirely plausible.
0:23:35 > 0:23:37The barnacle goose was another mystery.
0:23:39 > 0:23:42Each winter, huge, noisy flocks of them
0:23:42 > 0:23:45appear on European shores, apparently from out of nowhere.
0:23:47 > 0:23:51No-one had ever seen them build a nest or raise young.
0:23:56 > 0:24:01The barnacle goose gave rise to some extraordinary folklore as this
0:24:01 > 0:24:04mediaeval illustration shows.
0:24:04 > 0:24:08It was thought that the geese grew on underwater trees,
0:24:08 > 0:24:13starting life as small marine creatures called goose barnacles.
0:24:13 > 0:24:16Goose barnacles do, of course, exist, they're small
0:24:16 > 0:24:20shelled marine organisms with what looks like the head,
0:24:20 > 0:24:25which is in fact enclosed by a shell, attached by a stalk, which
0:24:25 > 0:24:30was thought to resemble the neck of a bird, to a bit of wood or a rock.
0:24:33 > 0:24:37The confusion about the nature of the barnacle goose was put to
0:24:37 > 0:24:39good use by some.
0:24:39 > 0:24:41Since it was unclear whether it was a bird,
0:24:41 > 0:24:44a fish or some other creature, you could surely be
0:24:44 > 0:24:48allowed to eat it on days when meat was forbidden by the church.
0:24:50 > 0:24:53But the most commonly held belief was that birds
0:24:53 > 0:24:56disappear in winter because they hibernated.
0:24:58 > 0:25:01Swallows and their close relatives, the swifts and martins,
0:25:01 > 0:25:06were thought to do so in mud at the bottom of ponds and rivers
0:25:06 > 0:25:09and it's easy to see how this idea originated
0:25:09 > 0:25:13because the birds spent much of their time near water, skimming low
0:25:13 > 0:25:16over the surface, hunting for insects or taking a drink.
0:25:18 > 0:25:22It wasn't until the Middle Ages that another theory was proposed that
0:25:22 > 0:25:25some birds may migrate
0:25:25 > 0:25:30and one of its strongest proponents was an influential religious leader.
0:25:32 > 0:25:37Frederick the second of Hohenstaufen was a powerful holy
0:25:37 > 0:25:40Roman Emperor and known for his unorthodox views.
0:25:40 > 0:25:43He ignored the philosophy of the Church
0:25:43 > 0:25:47and based his knowledge of natural history on direct observation
0:25:47 > 0:25:50rather than what was ordained.
0:25:50 > 0:25:54Frederick was also a keen falconer and he wrote this book,
0:25:54 > 0:25:56The Art Of Falconry,
0:25:56 > 0:25:58and in it, surprisingly,
0:25:58 > 0:26:02there are entire chapters on the migration of birds.
0:26:02 > 0:26:04His confidence came from the fact that,
0:26:04 > 0:26:07unlike his contemporaries and those before him,
0:26:07 > 0:26:11he had actually observed birds in the field for himself.
0:26:11 > 0:26:13He had no doubt about the migration and so,
0:26:13 > 0:26:17little patience for the myths surrounding the barnacle goose.
0:26:17 > 0:26:20He considered the story to be quite ridiculous
0:26:20 > 0:26:24and argued that the birds simply breed in distant lands.
0:26:26 > 0:26:31His views started a debate that split people into two camps,
0:26:31 > 0:26:34those believing in the old hibernation theory
0:26:34 > 0:26:37and those who supported the idea that birds migrate.
0:26:38 > 0:26:43This was the start of a new era which was to sweep away myths
0:26:43 > 0:26:47and focus instead on facts and careful observation.
0:26:47 > 0:26:52Across Europe, the evidence for bird migration started to accumulate.
0:26:54 > 0:26:58In Germany, a 12th century monk is said to have taken
0:26:58 > 0:27:02a swallow from its nest and attached a parchment note to its leg
0:27:02 > 0:27:05that read, "Oh, swallow, where do you live in winter?"
0:27:07 > 0:27:10The following spring the bird returned with a note saying,
0:27:10 > 0:27:14"In Asia, in the home of Petrus, that is Israel."
0:27:16 > 0:27:20The story may not have been true, but it certainly gave the right hint.
0:27:24 > 0:27:29In the early 16th century, a Bishop from Sweden called
0:27:29 > 0:27:35Olaus Magnus reignited the debate about swallows with this picture.
0:27:35 > 0:27:38He claimed that in winter, fishermen often drew up
0:27:38 > 0:27:42swallows in their nets, hanging together in a mass.
0:27:42 > 0:27:45This astonishing assertion provided ample fuel
0:27:45 > 0:27:49for the anti-migration lobby and, unlikely as it was,
0:27:49 > 0:27:52the view that swallows spent their winter underwater
0:27:52 > 0:27:54became increasingly entrenched.
0:27:58 > 0:28:02By the 18th century, the debate about migration versus hibernation
0:28:02 > 0:28:08had come to a head and across the continent opinions were divided.
0:28:11 > 0:28:15But new evidence was about to come from an unusual source.
0:28:16 > 0:28:20Edward Jenner was an English country doctor who also had a deep
0:28:20 > 0:28:23interest in natural history.
0:28:23 > 0:28:27He noted that although swallows often splash in water
0:28:27 > 0:28:31as they skim across it, they never immerse themselves.
0:28:31 > 0:28:34Were they to do so, he suggested, their wings would become
0:28:34 > 0:28:37so wet that they would be unable to fly.
0:28:38 > 0:28:41To test his idea, Jenner reportedly held a swift
0:28:41 > 0:28:44underwater for two minutes.
0:28:44 > 0:28:46Not surprisingly, it died.
0:28:48 > 0:28:51Jenner went on to devise another experiment to
0:28:51 > 0:28:53discover where the birds go.
0:28:53 > 0:28:57He took 12 swifts from their nests and marked them
0:28:57 > 0:28:59by taking off two of their claws.
0:28:59 > 0:29:02The following year, some of the birds he'd marked were caught
0:29:02 > 0:29:04again in exactly the same spot.
0:29:04 > 0:29:08Although Jenner could not discover where his swifts had been
0:29:08 > 0:29:11over the winter, he was the first to show that they returned to use
0:29:11 > 0:29:15the same breeding sites in the following years.
0:29:15 > 0:29:18And we now know that this is true for swallows as well.
0:29:20 > 0:29:24About the same time, across the Channel, a German bird enthusiast
0:29:24 > 0:29:26had come up with a similar idea.
0:29:28 > 0:29:32Johann Frisch caught several birds near his house and attached
0:29:32 > 0:29:36to their legs woollen threads like this which he'd dipped
0:29:36 > 0:29:38in red watercolour.
0:29:38 > 0:29:40He predicted that if swallows really did spend
0:29:40 > 0:29:45the winter at the bottom of lakes, the red colour would be washed off.
0:29:45 > 0:29:48The following spring, Frisch's swallows returned
0:29:48 > 0:29:51and the threads where unchanged.
0:29:51 > 0:29:55It was a very simple but very effective experiment.
0:29:55 > 0:29:59Evidence against the hibernation theory continued to mount
0:29:59 > 0:30:04and eventually a new technique put the final nail in its coffin...
0:30:04 > 0:30:06systematic bird ringing.
0:30:09 > 0:30:13This bird has just been fitted with its own individual marker.
0:30:13 > 0:30:18A small metal ring on its leg with a unique code of numbers.
0:30:18 > 0:30:21It's part of a national scheme that's been running for over 100
0:30:21 > 0:30:26years and provides scientists with invaluable data on bird movements.
0:30:27 > 0:30:31Early in the 20th century, the study of migration really took off.
0:30:31 > 0:30:34Birds were recovered on their breeding and wintering grounds
0:30:34 > 0:30:37and often en route, too.
0:30:37 > 0:30:41600 years after Frederick von Hohenstaufen had first started
0:30:41 > 0:30:45the debate, real evidence was beginning to accumulate.
0:30:47 > 0:30:51In the summer of 1911, a metal ring just like this one,
0:30:51 > 0:30:55was clipped onto the leg of a young swallow in Staffordshire.
0:30:55 > 0:30:59The number on the ring was B830.
0:30:59 > 0:31:0418 months later, the same bird was caught by a farmer in South Africa.
0:31:04 > 0:31:09Here, at last, was the indisputable proof that swallows migrate
0:31:09 > 0:31:11and spend the winter thousands of miles away.
0:31:13 > 0:31:16Off you go. There we are.
0:31:19 > 0:31:23Today, of course, we know that the swallows' migration is
0:31:23 > 0:31:26one of the most impressive in all the animal kingdom.
0:31:26 > 0:31:31It takes it across the largest desert in the world, the Sahara,
0:31:31 > 0:31:33it's a gruelling and dangerous journey
0:31:33 > 0:31:38and many die on the way from exhaustion or starvation.
0:31:38 > 0:31:43They travel for nearly four months, covering nearly 10,000km
0:31:43 > 0:31:46and eventually reach southern Africa.
0:31:52 > 0:31:56And bird ringing also helped to dispel the myth of
0:31:56 > 0:31:58the barnacle goose.
0:31:58 > 0:32:02In the 1960s, a Norwegian expedition, ringed geese nesting
0:32:02 > 0:32:07on the Arctic island of Spitsbergen. That autumn, some of the same
0:32:07 > 0:32:11birds were sited on the west coast of Scotland, some 2,000km away.
0:32:13 > 0:32:18Frederick von Hohenstaufen had been proved to be absolutely correct.
0:32:21 > 0:32:26It took centuries to discover the truth behind the swallows'
0:32:26 > 0:32:27seasonal movements.
0:32:27 > 0:32:32But in their time, they baffled the minds of many great naturalists and
0:32:32 > 0:32:36started one of the longest-running of all scientific debates.
0:32:36 > 0:32:41But in the end, the true story proved to be even more extraordinary
0:32:41 > 0:32:46than the fantastic myths that where invented to explain it.
0:32:48 > 0:32:50Just like the swallow,
0:32:50 > 0:32:54the painted lady butterfly seems to appear magically out of nowhere
0:32:54 > 0:32:57and that started some extraordinary ideas and controversies.
0:32:58 > 0:33:02The painted lady is one of our largest butterflies
0:33:02 > 0:33:05and a familiar summer visitor to our gardens.
0:33:05 > 0:33:06And yet, its appearance
0:33:06 > 0:33:10and disappearance each year, has puzzled us for centuries.
0:33:10 > 0:33:13It's only now that we're beginning to understand this extraordinary
0:33:13 > 0:33:16life cycle and discover where it vanishes each year.
0:33:18 > 0:33:20Early naturalists were confused by the sudden
0:33:20 > 0:33:23appearance of painted ladies each spring because they were
0:33:23 > 0:33:28unaware of the connection between butterflies and caterpillars.
0:33:28 > 0:33:32For a very long time it was widely believed that butterflies
0:33:32 > 0:33:37arise from rotting material by what was called spontaneous generation.
0:33:40 > 0:33:46In the 1830s, a German scientist named Renous was arrested for heresy
0:33:46 > 0:33:50for claiming that he could change caterpillars into butterflies.
0:33:50 > 0:33:53Arresting someone for something now known to be common knowledge
0:33:53 > 0:33:57may seem rather extreme, but at the time, many still believed that
0:33:57 > 0:34:00caterpillars and butterflies were completely different creatures,
0:34:00 > 0:34:02created by the hand of God.
0:34:04 > 0:34:08Needless to say, people had been well aware of the existence of
0:34:08 > 0:34:12both butterflies and caterpillars since the earliest times.
0:34:14 > 0:34:17But the thought that any two were related,
0:34:17 > 0:34:21let alone the same species, seemed impossible...
0:34:21 > 0:34:23and it's easy to see why.
0:34:26 > 0:34:30Not only do caterpillars and butterflies look like very
0:34:30 > 0:34:33different types of animals, but the colours and patterns
0:34:33 > 0:34:38of a caterpillar don't match up with those of its adult form.
0:34:38 > 0:34:42The only way to know which lava and which butterfly go together
0:34:42 > 0:34:46is to keep caterpillars and watch them turn into butterflies.
0:34:46 > 0:34:50But it wasn't until the 17th century that anyone left
0:34:50 > 0:34:51a record of doing that.
0:34:52 > 0:34:57One of the first was a remarkable woman named Maria Sibylla Merian.
0:34:58 > 0:35:01Merian was born in Germany at a time
0:35:01 > 0:35:04when women still had little formal education
0:35:04 > 0:35:08and no role in the scientific world, but she was an accomplished
0:35:08 > 0:35:12artist and painted plants and insects she saw around her.
0:35:12 > 0:35:17To do that, she kept caterpillars, fed them on leaves
0:35:17 > 0:35:19and watched them turn into butterflies.
0:35:21 > 0:35:24Merian produced hundreds of beautiful paintings of butterflies
0:35:24 > 0:35:26and their stages of development
0:35:26 > 0:35:29along with the plants on which they feed.
0:35:29 > 0:35:30Her drawings are so exquisite
0:35:30 > 0:35:34and detailed that they still rank among the best in the world.
0:35:37 > 0:35:41Among the things she observed with great care, were things like this.
0:35:42 > 0:35:48A curious, yet strangely beautiful object, it's a chrysalis,
0:35:48 > 0:35:52the intermediate stage between a caterpillar and a butterfly.
0:35:55 > 0:35:59She was one of the first to record the remarkable change
0:35:59 > 0:36:01that takes place in the chrysalis.
0:36:02 > 0:36:06It's one of nature's most extraordinary transformations.
0:36:11 > 0:36:15At the age of 52, she sailed from Europe to South America on a
0:36:15 > 0:36:20two-year expedition to study insects in the tropical jungles of Surinam.
0:36:20 > 0:36:23It was an exceptional journey for any naturalist
0:36:23 > 0:36:26at the time and particularly for a woman.
0:36:26 > 0:36:29When she returned, she produced this beautiful book.
0:36:32 > 0:36:34It turned out to be popular
0:36:34 > 0:36:36because it was one of the few to be published
0:36:36 > 0:36:40not in the scientific language of Latin but in Dutch.
0:36:40 > 0:36:41Because of this,
0:36:41 > 0:36:45her work was largely dismissed by scientists of the time
0:36:45 > 0:36:48but Merian was one of the first naturalists to correctly
0:36:48 > 0:36:52connect the caterpillar with its pupa and the adult form.
0:36:54 > 0:36:56Today, Merian's book is widely
0:36:56 > 0:37:01recognised as a pioneering work of scientific observation
0:37:01 > 0:37:06and it put an end to the idea of spontaneous generation.
0:37:08 > 0:37:12Around the same time, further evidence for the connection
0:37:12 > 0:37:16between butterflies and caterpillars came from a different source.
0:37:18 > 0:37:23In 1669, a Dutch scientist by the name of Jan Swammerdam published
0:37:23 > 0:37:27the results of experiments which would finally prove that the
0:37:27 > 0:37:30caterpillar and butterfly are one and the same animal.
0:37:30 > 0:37:33Swammerdam was a master of the miniature and dissected the
0:37:33 > 0:37:36caterpillars and pupae of butterflies and moths
0:37:36 > 0:37:40under a microscope. With a steady hand and endless patience,
0:37:40 > 0:37:44he carefully cut into the layers of skin with tiny scissors
0:37:44 > 0:37:47and what he discovered was truly astonishing.
0:37:49 > 0:37:54He found some of the body parts of a butterfly.
0:37:54 > 0:37:58The structures were fragile and not complete but Swammerdam had proved
0:37:58 > 0:38:03that caterpillar and butterfly are, indeed, one and the same animal.
0:38:07 > 0:38:12We now know that without the caterpillar, there can be no butterfly.
0:38:12 > 0:38:13Yet, for a very long time,
0:38:13 > 0:38:16the painted lady seemed to be an exception.
0:38:16 > 0:38:19Every spring, the adult butterflies would appear across Britain
0:38:19 > 0:38:22without any sightings of their caterpillars.
0:38:23 > 0:38:27While some butterflies hibernate in Britain, there was no sign
0:38:27 > 0:38:28of painted ladies doing so.
0:38:29 > 0:38:34Some speculated that they flew to warmer climates as birds do.
0:38:35 > 0:38:39But how could a tiny insect cross the English Channel?
0:38:40 > 0:38:43In the 20th century, swarms of butterflies moving across Europe
0:38:43 > 0:38:47finally provided evidence that painted ladies do, indeed,
0:38:47 > 0:38:49cross the sea.
0:38:49 > 0:38:53And they were found to fly all the way from North Africa to Britain.
0:38:55 > 0:38:58But there were almost no records of painted ladies making
0:38:58 > 0:39:00the reverse trip south.
0:39:00 > 0:39:03So, for years, it was thought that Britain must be
0:39:03 > 0:39:06a dead-end for the most northerly stragglers.
0:39:08 > 0:39:14And then, in 2009, the public was asked to help solve the mystery.
0:39:14 > 0:39:18Among 12,000 sightings there were reports of painted ladies
0:39:18 > 0:39:21flying out to sea in the autumn.
0:39:22 > 0:39:25And a radar station detected them flying south
0:39:25 > 0:39:30at heights of 500 metres, way beyond the sight of human eyes.
0:39:32 > 0:39:36We now know that the painted ladies migration is a round-trip
0:39:36 > 0:39:42of over 12,000km. But it's not made by any one individual.
0:39:42 > 0:39:45Each only flies part of the way,
0:39:45 > 0:39:48passing on the migratory baton to the next generation.
0:39:48 > 0:39:53It's like a relay race with up to six generations of butterflies involved.
0:39:55 > 0:39:58The painted ladies epic journey from one continent to the next
0:39:58 > 0:40:01would be a truly astonishing feature in any animal
0:40:01 > 0:40:05but for a tiny creature like this, it seems really extraordinary.
0:40:05 > 0:40:07How does it battle the wind
0:40:07 > 0:40:10and the weather and navigate across vast bodies of water?
0:40:10 > 0:40:14And with no single individual ever undertaking the whole migration,
0:40:14 > 0:40:16how do they find the way?
0:40:18 > 0:40:23It seems that painted ladies are pre-programmed to either fly
0:40:23 > 0:40:26north or south and this is determined whilst
0:40:26 > 0:40:29they are still caterpillars, possibly by temperature
0:40:29 > 0:40:34and day length and also by the plants they feed on but how
0:40:34 > 0:40:39does this information get passed on from caterpillar to butterfly?
0:40:39 > 0:40:42The answer may be hidden within the chrysalis.
0:40:43 > 0:40:49Recently CT scanners have allowed us to look inside a pupa.
0:40:49 > 0:40:54They reveal that some organs remain intact during the transformation.
0:40:56 > 0:41:00A one-day-old pupa clearly shows the gut and breathing tubes
0:41:00 > 0:41:03which only change slightly as the chrysalis develops.
0:41:07 > 0:41:12Could it be that the brain or nerves also remain intact
0:41:12 > 0:41:14and that memories are passed on?
0:41:15 > 0:41:20Recent experiments in the lab appear to support this idea.
0:41:20 > 0:41:24Scientists taught caterpillars to avoid specific
0:41:24 > 0:41:27smells by linking them with an unpleasant reaction.
0:41:28 > 0:41:32Later on, as adults, the same individuals remembered these
0:41:32 > 0:41:35smells and chose to keep away from them.
0:41:35 > 0:41:38If the experiences of a caterpillar can be carried over
0:41:38 > 0:41:43to the adult, then maybe cues for migration can also be passed on.
0:41:45 > 0:41:49Although we've unravelled much of the painted lady's life-cycle,
0:41:49 > 0:41:54many questions remain. How far does each individual travel?
0:41:54 > 0:41:59And do offspring follow similar routes to their ancestors?
0:41:59 > 0:42:02One day we may know the answers but, for now,
0:42:02 > 0:42:06they remain some of the unsolved mysteries of nature.
0:42:09 > 0:42:12The arrival each spring of our painted lady butterflies
0:42:12 > 0:42:15and our swallows never ceases to delight us
0:42:15 > 0:42:18but now we also understand the extraordinary journeys
0:42:18 > 0:42:22they undertake when they disappear again at the end of summer.