Remarkable Regeneration David Attenborough's Natural Curiosities


Remarkable Regeneration

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The natural world is full of extraordinary animals

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with amazing life histories.

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Yet certain stories are more intriguing than most.

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The mysteries of a butterfly's life cycle

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or the strange biology of the emperor penguin.

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Some of these creatures

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were surrounded by myth and misunderstandings

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for a very long time.

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And some have only recently revealed their secrets.

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These are the animals that stand out from the crowd -

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the curiosities I find particularly fascinating.

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Salamanders can regenerate entire legs and tails

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to replace ones that they lose.

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And moose can regrow their enormous antlers every year.

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How do these animals regenerate entire body parts

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and why isn't it possible

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for all animals to do the same?

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When I was a boy,

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my father gave me one of these for my eighth birthday.

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It's a fire salamander.

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They may look like lizards,

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but in fact they're not reptiles,

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they're amphibians, with moist skins.

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For centuries, mythical stories surrounded these creatures.

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It was believed that they were icy-cold animals

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that could dwell within fires, unharmed by the heat.

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Although their fire-surviving powers may be untrue,

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the salamander nonetheless possesses a real natural ability

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that is just as extraordinary.

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They're able to regrow damaged tails,

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legs and other parts of the body

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through a process called regeneration.

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There are more than 600 different species of salamander.

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They range in size from just a couple of centimetres up to

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the world's largest amphibian, the Chinese giant salamander,

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that can grow to over a metre and a half in length.

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Salamanders are predators,

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and many hunt for small invertebrates

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such as slugs and worms.

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But sometimes they hunt each other...

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with dramatic consequences.

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This tiny North American redback salamander

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is on the menu of the much bigger seal salamander.

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Time to make a retreat.

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This may looking shocking, but the redback isn't badly injured.

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A weak point in its skin allows its tail to break off easily.

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Incredibly, it will regrow a new tail in just a matter of weeks.

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This ability to replace an entire body part

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is unusual among adult vertebrates and seems almost magical.

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Regeneration is a subject that fascinates us.

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Modern medicine has spent a lot of money and time

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studying the ways our own bodies can regenerate tissue.

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All living creatures, including humans,

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have the ability to repair damaged parts of the body,

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but the extent of that repair varies considerably.

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As small infants,

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we have the ability to regrow the tips of our fingers

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if they're severed, but we lose this ability as we age.

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So animals like salamanders,

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with their super-regenerative powers, seem intriguing to us.

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Regeneration had been known about since ancient times,

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but for a long time no-one understood how it happened.

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In the 17th and 18th century,

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there was a new wave of scientific discovery.

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A brilliant Italian scientist named Lazzaro Spallanzani

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made meticulous observations into regeneration

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across many different species

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and shared his ideas in detailed letters.

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In November 1765, he wrote to the eminent scientist Charles Bonnet,

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whom he regularly corresponded with,

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to announce that he had discovered tail regeneration in salamanders.

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Throughout the following year,

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he followed up his initial observations

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with numerous experiments to try to understand

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how the salamander could regrow a tail just like the original.

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He found that all species of salamander that he tested

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could regrow their tails when injured,

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and they did so more rapidly in summer than in winter

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and retained this incredible ability throughout their lives.

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Spallanzani advocated a radical theory.

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He thought that salamanders already possessed

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a number of miniature spare parts at the base of each limb

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that could grow in size to replace a lost or damaged one.

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He was unable to prove this theory, but he didn't give up.

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He studied salamander tadpoles

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and came up with another, even more interesting idea.

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A year after his initial letter,

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Spallanzani once again wrote to Charles Bonnet,

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this time with detailed descriptions of further experiments

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into tail regeneration.

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Most notably in this description, he wrote,

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"I am almost led to believe

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"that the tail regenerates in tadpoles

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"are more of an elongation of the old parts

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"than a development from a germ."

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This suggests that Spallanzani was on the right track,

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but the idea that a salamander could regrow a new tail

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from seemingly nothing was not well supported,

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and Spallanzani was therefore never willing

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to pursue the idea further.

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However, there's no doubt that his research

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helped to lead other scientists closer

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towards proving what really happens when a salamander regrows its tail.

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In fact, Spallanzani's rough sketches did make sense,

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and they were the first to describe some of the vital processes

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in the remarkable growth of new limbs

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that we understand better today.

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When a limb is lost,

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the exposed blood vessels and tissue

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contract to quickly stop any bleeding.

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Then, skin from the edges begins to grow across the damaged area

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to protect the body from infection.

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Now cells that were once dormant

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begin dividing and multiplying to create new ones.

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Each cell retains a kind of memory

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of the type of tissue it used to be,

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so a new cell that regrows from damaged muscle

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will always become muscle.

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Within weeks, the salamander has a full-grown leg

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almost identical to the original.

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Although we now know the steps that take place

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during the regeneration of body parts,

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we still don't fully understand what triggers this kind of response.

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But it seems the answer may lie in how the salamander's body

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responds to injury.

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In humans, if an arm is severed,

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the cells die, alerting the immune system to the problem.

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In response, the area becomes swollen

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and is covered over with scar tissue,

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preventing any new growth occurring.

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But in salamanders, the immune system responds differently,

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and instead of forming a scar, it triggers regeneration.

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Another rather unusual-looking salamander

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that lives in the freshwaters of Mexico

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sheds new light on how this happens.

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Axolotls are among the best regenerators in the natural world,

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and scientists wondered if their blood played a role in the process.

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Like us, they have special white blood cells

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that consume invading bacteria and damaged tissue

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around injuries and wounds.

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Researchers removed them, and the results were surprising.

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The axolotl was unable to regrow new limbs.

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So, white blood cells were part

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of the secret of their powers of regeneration.

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Understanding the role of the salamander's blood cells

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in regrowing limbs

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could be a step towards discovering why they can regenerate body parts

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and we can't.

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All amphibians have tadpoles,

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which develop limbs and enable them to move on to land.

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But salamanders are able to retrigger that remarkable process.

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We, too, undergo extraordinary development in the womb.

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Maybe like the salamander,

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there is a way of us retaining this ability

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into our adult lives as well.

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The salamander has a truly amazing ability to regrow complex body parts

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to enhance its chances of survival.

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While we don't yet know all the answers,

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it's likely that this incredible creature

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could revolutionise modern medicine

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and the way we treat injuries.

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Next, we uncover the secret behind how moose and other deer

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regrow their enormous new antlers every year

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and discover what happens when regeneration goes wrong.

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This impressive skeleton

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belonged to one of the biggest deer to ever live on the planet.

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It's an Irish elk.

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Its antlers are enormous -

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they're almost four metres, 12 feet, across,

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and they weigh 40 kilos.

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An Irishman named Dr Molyneux

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first scientifically described the elk in 1697

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from specimens taken out of an Irish peat bog.

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Some believed that this elk was a large moose

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and were convinced living specimens could be found elsewhere

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across Europe and Russia.

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But not everyone agreed,

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and a debate about the life of this creature

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would continue for more than a hundred years.

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The skeleton of an Irish elk looks very similar to that of a moose.

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So it's easy to see why many believed them to be the same animal.

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Both have very impressive antlers.

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Antlers are only found in the deer family and are made of bone.

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Unlike horns, which are permanent structures,

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they are shed and replaced every year.

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But how can deer regrow huge chunks of bone,

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something no other mammal can do?

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Moose, like this young bull behind me,

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start growing their new antlers

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immediately after they shed their old ones.

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The antlers first appear on little bumps on either side of the head

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known as pedicles,

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and they have a soft, furry covering called velvet.

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This is vital to their amazing powers of regeneration.

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Blood vessels at the base start the growth,

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but as the antler gets longer, this blood supply is cut off.

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Then blood vessels within the velvet take over

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and transport nutrients and growth hormones to the growing tips.

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In older males, the antlers can grow at a rate of two centimetres a day,

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making it the fastest-growing bone of any animal.

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Once at full size, the velvet is shed.

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The animal rubs its head against a tree

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to encourage the thin velvet to fall off.

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It may look gruesome,

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but it's a natural part of the annual cycle

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and does the animal no harm.

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But why should a huge set of antlers be regrown every year?

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It's a question that baffled early naturalists,

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until Charles Darwin suggested

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it may be to do with attracting the opposite sex.

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In the first few years of adulthood, the antlers are small,

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and, as a result, young males remain subordinate to the larger bulls.

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But as they get older and their body size increases,

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so the antlers will also increase,

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eventually becoming impressive ornaments

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with which to compete for females.

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Those with the biggest antlers

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are certainly more attractive to the females.

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Maybe they are an indicator of fitness and strength.

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And it's no coincidence that antlers are at their full size

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during the breeding season.

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This is a time when a bull

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must protect his harem and see off competitors.

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Competing males tilt their heads

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to show off their antlers to their best advantage.

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But if the bulls are equally matched,

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then the competitors fight.

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The winner then gains access to the females.

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The benefits of such a victory are huge.

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But to get to that point,

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every young bull must, for many years, grow and regrow antlers.

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It's a big investment, draining the body of vital resources...

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and no investment was bigger than that of the Irish elk.

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The sheer size of these antlers

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has led to some to argue that

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they were unlikely to have been used in physical combat.

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Unlike other deer, the antlers of the Irish elk

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grew with a large, flat, palm-like plane facing forwards,

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so that if a bull looked straight ahead

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it would be at its biggest and most impressive.

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In this way, they may have been able to intimidate rivals

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and attract females without actually fighting.

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So, although the Irish elk was armed

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with what appear to be enormous weapons,

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it seems they were mostly for show.

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But this strategy may have been an advantage for the large elk.

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Fighting is always a risky business

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and will often result in serious injuries.

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After the breeding season, the antlers are discarded.

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Moose shed theirs in the winter,

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whereas smaller deer keep theirs until the next spring.

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This may be because the moose antlers are such a heavy load

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to carry throughout the winter.

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But why are antlers shed at all?

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Antlers are made of dead bone and can't be repaired.

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If a moose damages an antler during a fight,

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it will lose its chance of mating for that season.

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By shedding and regrowing their antlers each year,

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bulls ensure that they stay in the mating game.

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Just before antlers are shed,

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minerals within them are reabsorbed from the base,

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weakening the structure so that they eventually fall off.

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The flesh underneath is exposed,

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but not for long, as new skin soon covers the wound.

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Experiments have shown that the skin lesion

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that forms over the open wound

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creates a connection with the underlying tissue

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that is crucial to regeneration.

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If this connection isn't made,

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the production of velvet will be interrupted, and the antlers

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will either not grow at all or develop into strange shapes.

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So what about the Irish elk?

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Could the problems of regenerating such gigantic antlers

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have determined its fate?

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The French scientist Georges Cuvier was keen to demonstrate

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that the Irish elk was a unique species that had become extinct.

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To prove his point, Cuvier undertook a detailed examination

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of Irish elk fossils.

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He was able to show that it was indeed a distinct type of deer

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that could no longer be found alive.

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And so the Irish elk was one of the first animals

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to be recognised as being extinct.

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Georges Cuvier had solved the question

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of whether or not the Irish elk and moose

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were one and the same creature.

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But why did the Irish elk die out?

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Cuvier suggested that evolution had set it on a course

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of ever-increasing growth,

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and that eventually the antlers became so large

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that the poor animal could not even lift its neck.

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He may not have been that far from the truth.

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It's now thought that the annual growth of the Irish elk antlers

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put a strain on their bodies.

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A significant proportion of minerals within their bones

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were extracted and moved into their growing antlers.

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This led to a seasonal osteoporosis, with their bones weakening.

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They were in effect robbing one part of their body to boost another.

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It was gamble that worked for thousands of years.

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But around 10,000 years ago, the climate began to warm.

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The nutrient-rich grasses that the elk relied upon began to disappear.

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Growing massive antlers may now have been too much of a drain

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and permanently weakened the skeleton.

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The change in diet may also have affected their ability to breed,

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with females no longer able to produce young every year.

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Whatever the reason, the Irish elk, with its magnificent antlers,

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finally vanished from the landscape,

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and in its place the moose has become

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the largest deer on Earth today.

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So, while regeneration can give the salamander

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a second chance to a full life,

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the yearly regeneration of antlers in male moose

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is a risky strategy...

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but one with huge rewards for those with the best antlers.

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