0:00:03 > 0:00:05The natural world is full of extraordinary animals
0:00:05 > 0:00:09with amazing life histories.
0:00:09 > 0:00:12Yet certain stories are more intriguing than most.
0:00:15 > 0:00:18The mysteries of a butterfly's life cycle
0:00:18 > 0:00:21or the strange biology of the emperor penguin.
0:00:22 > 0:00:24Some of these creatures
0:00:24 > 0:00:26were surrounded by myth and misunderstandings
0:00:26 > 0:00:28for a very long time.
0:00:30 > 0:00:32And 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 particularly fascinating.
0:00:52 > 0:00:56Salamanders can regenerate entire legs and tails
0:00:56 > 0:00:58to replace ones that they lose.
0:01:01 > 0:01:06And moose can regrow their enormous antlers every year.
0:01:08 > 0:01:12How do these animals regenerate entire body parts
0:01:12 > 0:01:13and why isn't it possible
0:01:13 > 0:01:15for all animals to do the same?
0:01:21 > 0:01:23When I was a boy,
0:01:23 > 0:01:26my father gave me one of these for my eighth birthday.
0:01:26 > 0:01:29It's a fire salamander.
0:01:29 > 0:01:32They may look like lizards,
0:01:32 > 0:01:34but in fact they're not reptiles,
0:01:34 > 0:01:37they're amphibians, with moist skins.
0:01:37 > 0:01:41For centuries, mythical stories surrounded these creatures.
0:01:41 > 0:01:45It was believed that they were icy-cold animals
0:01:45 > 0:01:48that could dwell within fires, unharmed by the heat.
0:01:48 > 0:01:53Although their fire-surviving powers may be untrue,
0:01:53 > 0:01:57the salamander nonetheless possesses a real natural ability
0:01:57 > 0:01:59that is just as extraordinary.
0:01:59 > 0:02:03They're able to regrow damaged tails,
0:02:03 > 0:02:05legs and other parts of the body
0:02:05 > 0:02:08through a process called regeneration.
0:02:12 > 0:02:15There are more than 600 different species of salamander.
0:02:17 > 0:02:20They range in size from just a couple of centimetres up to
0:02:20 > 0:02:26the world's largest amphibian, the Chinese giant salamander,
0:02:26 > 0:02:29that can grow to over a metre and a half in length.
0:02:33 > 0:02:34Salamanders are predators,
0:02:34 > 0:02:37and many hunt for small invertebrates
0:02:37 > 0:02:38such as slugs and worms.
0:02:44 > 0:02:47But sometimes they hunt each other...
0:02:47 > 0:02:50with dramatic consequences.
0:02:53 > 0:02:56This tiny North American redback salamander
0:02:56 > 0:03:00is on the menu of the much bigger seal salamander.
0:03:09 > 0:03:12Time to make a retreat.
0:03:16 > 0:03:20This may looking shocking, but the redback isn't badly injured.
0:03:20 > 0:03:25A weak point in its skin allows its tail to break off easily.
0:03:29 > 0:03:34Incredibly, it will regrow a new tail in just a matter of weeks.
0:03:37 > 0:03:40This ability to replace an entire body part
0:03:40 > 0:03:44is unusual among adult vertebrates and seems almost magical.
0:03:46 > 0:03:50Regeneration is a subject that fascinates us.
0:03:50 > 0:03:53Modern medicine has spent a lot of money and time
0:03:53 > 0:03:57studying the ways our own bodies can regenerate tissue.
0:03:57 > 0:03:59All living creatures, including humans,
0:03:59 > 0:04:02have the ability to repair damaged parts of the body,
0:04:02 > 0:04:06but the extent of that repair varies considerably.
0:04:06 > 0:04:08As small infants,
0:04:08 > 0:04:11we have the ability to regrow the tips of our fingers
0:04:11 > 0:04:14if they're severed, but we lose this ability as we age.
0:04:14 > 0:04:17So animals like salamanders,
0:04:17 > 0:04:22with their super-regenerative powers, seem intriguing to us.
0:04:26 > 0:04:29Regeneration had been known about since ancient times,
0:04:29 > 0:04:33but for a long time no-one understood how it happened.
0:04:35 > 0:04:37In the 17th and 18th century,
0:04:37 > 0:04:40there was a new wave of scientific discovery.
0:04:42 > 0:04:47A brilliant Italian scientist named Lazzaro Spallanzani
0:04:47 > 0:04:50made meticulous observations into regeneration
0:04:50 > 0:04:52across many different species
0:04:52 > 0:04:56and shared his ideas in detailed letters.
0:04:57 > 0:05:02In November 1765, he wrote to the eminent scientist Charles Bonnet,
0:05:02 > 0:05:04whom he regularly corresponded with,
0:05:04 > 0:05:09to announce that he had discovered tail regeneration in salamanders.
0:05:09 > 0:05:11Throughout the following year,
0:05:11 > 0:05:13he followed up his initial observations
0:05:13 > 0:05:15with numerous experiments to try to understand
0:05:15 > 0:05:19how the salamander could regrow a tail just like the original.
0:05:19 > 0:05:22He found that all species of salamander that he tested
0:05:22 > 0:05:25could regrow their tails when injured,
0:05:25 > 0:05:28and they did so more rapidly in summer than in winter
0:05:28 > 0:05:33and retained this incredible ability throughout their lives.
0:05:34 > 0:05:37Spallanzani advocated a radical theory.
0:05:37 > 0:05:40He thought that salamanders already possessed
0:05:40 > 0:05:44a number of miniature spare parts at the base of each limb
0:05:44 > 0:05:47that could grow in size to replace a lost or damaged one.
0:05:50 > 0:05:55He was unable to prove this theory, but he didn't give up.
0:05:55 > 0:05:57He studied salamander tadpoles
0:05:57 > 0:06:01and came up with another, even more interesting idea.
0:06:01 > 0:06:03A year after his initial letter,
0:06:03 > 0:06:06Spallanzani once again wrote to Charles Bonnet,
0:06:06 > 0:06:09this time with detailed descriptions of further experiments
0:06:09 > 0:06:12into tail regeneration.
0:06:12 > 0:06:15Most notably in this description, he wrote,
0:06:15 > 0:06:17"I am almost led to believe
0:06:17 > 0:06:20"that the tail regenerates in tadpoles
0:06:20 > 0:06:23"are more of an elongation of the old parts
0:06:23 > 0:06:25"than a development from a germ."
0:06:25 > 0:06:30This suggests that Spallanzani was on the right track,
0:06:30 > 0:06:32but the idea that a salamander could regrow a new tail
0:06:32 > 0:06:36from seemingly nothing was not well supported,
0:06:36 > 0:06:38and Spallanzani was therefore never willing
0:06:38 > 0:06:41to pursue the idea further.
0:06:41 > 0:06:43However, there's no doubt that his research
0:06:43 > 0:06:46helped to lead other scientists closer
0:06:46 > 0:06:51towards proving what really happens when a salamander regrows its tail.
0:06:52 > 0:06:57In fact, Spallanzani's rough sketches did make sense,
0:06:57 > 0:07:01and they were the first to describe some of the vital processes
0:07:01 > 0:07:03in the remarkable growth of new limbs
0:07:03 > 0:07:06that we understand better today.
0:07:06 > 0:07:08When a limb is lost,
0:07:08 > 0:07:11the exposed blood vessels and tissue
0:07:11 > 0:07:14contract to quickly stop any bleeding.
0:07:14 > 0:07:18Then, skin from the edges begins to grow across the damaged area
0:07:18 > 0:07:20to protect the body from infection.
0:07:22 > 0:07:25Now cells that were once dormant
0:07:25 > 0:07:28begin dividing and multiplying to create new ones.
0:07:32 > 0:07:34Each cell retains a kind of memory
0:07:34 > 0:07:37of the type of tissue it used to be,
0:07:37 > 0:07:40so a new cell that regrows from damaged muscle
0:07:40 > 0:07:41will always become muscle.
0:07:43 > 0:07:46Within weeks, the salamander has a full-grown leg
0:07:46 > 0:07:48almost identical to the original.
0:07:50 > 0:07:53Although we now know the steps that take place
0:07:53 > 0:07:55during the regeneration of body parts,
0:07:55 > 0:07:59we still don't fully understand what triggers this kind of response.
0:07:59 > 0:08:02But it seems the answer may lie in how the salamander's body
0:08:02 > 0:08:04responds to injury.
0:08:04 > 0:08:07In humans, if an arm is severed,
0:08:07 > 0:08:11the cells die, alerting the immune system to the problem.
0:08:11 > 0:08:14In response, the area becomes swollen
0:08:14 > 0:08:17and is covered over with scar tissue,
0:08:17 > 0:08:20preventing any new growth occurring.
0:08:20 > 0:08:24But in salamanders, the immune system responds differently,
0:08:24 > 0:08:29and instead of forming a scar, it triggers regeneration.
0:08:31 > 0:08:34Another rather unusual-looking salamander
0:08:34 > 0:08:36that lives in the freshwaters of Mexico
0:08:36 > 0:08:39sheds new light on how this happens.
0:08:41 > 0:08:46Axolotls are among the best regenerators in the natural world,
0:08:46 > 0:08:50and scientists wondered if their blood played a role in the process.
0:08:50 > 0:08:54Like us, they have special white blood cells
0:08:54 > 0:08:57that consume invading bacteria and damaged tissue
0:08:57 > 0:09:00around injuries and wounds.
0:09:01 > 0:09:04Researchers removed them, and the results were surprising.
0:09:04 > 0:09:08The axolotl was unable to regrow new limbs.
0:09:10 > 0:09:13So, white blood cells were part
0:09:13 > 0:09:16of the secret of their powers of regeneration.
0:09:16 > 0:09:19Understanding the role of the salamander's blood cells
0:09:19 > 0:09:20in regrowing limbs
0:09:20 > 0:09:25could be a step towards discovering why they can regenerate body parts
0:09:25 > 0:09:27and we can't.
0:09:28 > 0:09:31All amphibians have tadpoles,
0:09:31 > 0:09:34which develop limbs and enable them to move on to land.
0:09:35 > 0:09:40But salamanders are able to retrigger that remarkable process.
0:09:41 > 0:09:45We, too, undergo extraordinary development in the womb.
0:09:45 > 0:09:47Maybe like the salamander,
0:09:47 > 0:09:50there is a way of us retaining this ability
0:09:50 > 0:09:52into our adult lives as well.
0:09:52 > 0:09:59The salamander has a truly amazing ability to regrow complex body parts
0:09:59 > 0:10:02to enhance its chances of survival.
0:10:02 > 0:10:05While we don't yet know all the answers,
0:10:05 > 0:10:07it's likely that this incredible creature
0:10:07 > 0:10:10could revolutionise modern medicine
0:10:10 > 0:10:12and the way we treat injuries.
0:10:17 > 0:10:21Next, we uncover the secret behind how moose and other deer
0:10:21 > 0:10:25regrow their enormous new antlers every year
0:10:25 > 0:10:30and discover what happens when regeneration goes wrong.
0:10:40 > 0:10:42This impressive skeleton
0:10:42 > 0:10:46belonged to one of the biggest deer to ever live on the planet.
0:10:46 > 0:10:48It's an Irish elk.
0:10:48 > 0:10:50Its antlers are enormous -
0:10:50 > 0:10:53they're almost four metres, 12 feet, across,
0:10:53 > 0:10:55and they weigh 40 kilos.
0:10:55 > 0:10:58An Irishman named Dr Molyneux
0:10:58 > 0:11:02first scientifically described the elk in 1697
0:11:02 > 0:11:07from specimens taken out of an Irish peat bog.
0:11:07 > 0:11:10Some believed that this elk was a large moose
0:11:10 > 0:11:13and were convinced living specimens could be found elsewhere
0:11:13 > 0:11:15across Europe and Russia.
0:11:15 > 0:11:17But not everyone agreed,
0:11:17 > 0:11:20and a debate about the life of this creature
0:11:20 > 0:11:23would continue for more than a hundred years.
0:11:25 > 0:11:30The skeleton of an Irish elk looks very similar to that of a moose.
0:11:34 > 0:11:39So it's easy to see why many believed them to be the same animal.
0:11:40 > 0:11:43Both have very impressive antlers.
0:11:46 > 0:11:51Antlers are only found in the deer family and are made of bone.
0:11:52 > 0:11:54Unlike horns, which are permanent structures,
0:11:54 > 0:11:59they are shed and replaced every year.
0:11:59 > 0:12:03But how can deer regrow huge chunks of bone,
0:12:03 > 0:12:06something no other mammal can do?
0:12:09 > 0:12:12Moose, like this young bull behind me,
0:12:12 > 0:12:13start growing their new antlers
0:12:13 > 0:12:16immediately after they shed their old ones.
0:12:16 > 0:12:21The antlers first appear on little bumps on either side of the head
0:12:21 > 0:12:23known as pedicles,
0:12:23 > 0:12:26and they have a soft, furry covering called velvet.
0:12:26 > 0:12:30This is vital to their amazing powers of regeneration.
0:12:32 > 0:12:35Blood vessels at the base start the growth,
0:12:35 > 0:12:40but as the antler gets longer, this blood supply is cut off.
0:12:40 > 0:12:44Then blood vessels within the velvet take over
0:12:44 > 0:12:48and transport nutrients and growth hormones to the growing tips.
0:12:51 > 0:12:56In older males, the antlers can grow at a rate of two centimetres a day,
0:12:56 > 0:13:00making it the fastest-growing bone of any animal.
0:13:05 > 0:13:08Once at full size, the velvet is shed.
0:13:10 > 0:13:12The animal rubs its head against a tree
0:13:12 > 0:13:15to encourage the thin velvet to fall off.
0:13:21 > 0:13:23It may look gruesome,
0:13:23 > 0:13:25but it's a natural part of the annual cycle
0:13:25 > 0:13:27and does the animal no harm.
0:13:32 > 0:13:37But why should a huge set of antlers be regrown every year?
0:13:38 > 0:13:41It's a question that baffled early naturalists,
0:13:41 > 0:13:44until Charles Darwin suggested
0:13:44 > 0:13:47it may be to do with attracting the opposite sex.
0:13:50 > 0:13:53In the first few years of adulthood, the antlers are small,
0:13:53 > 0:13:58and, as a result, young males remain subordinate to the larger bulls.
0:13:58 > 0:14:01But as they get older and their body size increases,
0:14:01 > 0:14:03so the antlers will also increase,
0:14:03 > 0:14:06eventually becoming impressive ornaments
0:14:06 > 0:14:08with which to compete for females.
0:14:16 > 0:14:18Those with the biggest antlers
0:14:18 > 0:14:22are certainly more attractive to the females.
0:14:22 > 0:14:26Maybe they are an indicator of fitness and strength.
0:14:27 > 0:14:32And it's no coincidence that antlers are at their full size
0:14:32 > 0:14:35during the breeding season.
0:14:42 > 0:14:45This is a time when a bull
0:14:45 > 0:14:48must protect his harem and see off competitors.
0:14:54 > 0:14:57Competing males tilt their heads
0:14:57 > 0:14:59to show off their antlers to their best advantage.
0:15:04 > 0:15:06But if the bulls are equally matched,
0:15:06 > 0:15:09then the competitors fight.
0:15:27 > 0:15:30The winner then gains access to the females.
0:15:33 > 0:15:37The benefits of such a victory are huge.
0:15:37 > 0:15:39But to get to that point,
0:15:39 > 0:15:45every young bull must, for many years, grow and regrow antlers.
0:15:45 > 0:15:51It's a big investment, draining the body of vital resources...
0:15:51 > 0:15:55and no investment was bigger than that of the Irish elk.
0:15:57 > 0:15:59The sheer size of these antlers
0:15:59 > 0:16:01has led to some to argue that
0:16:01 > 0:16:04they were unlikely to have been used in physical combat.
0:16:04 > 0:16:07Unlike other deer, the antlers of the Irish elk
0:16:07 > 0:16:11grew with a large, flat, palm-like plane facing forwards,
0:16:11 > 0:16:13so that if a bull looked straight ahead
0:16:13 > 0:16:17it would be at its biggest and most impressive.
0:16:17 > 0:16:20In this way, they may have been able to intimidate rivals
0:16:20 > 0:16:24and attract females without actually fighting.
0:16:26 > 0:16:28So, although the Irish elk was armed
0:16:28 > 0:16:31with what appear to be enormous weapons,
0:16:31 > 0:16:34it seems they were mostly for show.
0:16:34 > 0:16:39But this strategy may have been an advantage for the large elk.
0:16:40 > 0:16:43Fighting is always a risky business
0:16:43 > 0:16:46and will often result in serious injuries.
0:16:49 > 0:16:53After the breeding season, the antlers are discarded.
0:16:53 > 0:16:55Moose shed theirs in the winter,
0:16:55 > 0:16:59whereas smaller deer keep theirs until the next spring.
0:16:59 > 0:17:02This may be because the moose antlers are such a heavy load
0:17:02 > 0:17:04to carry throughout the winter.
0:17:07 > 0:17:10But why are antlers shed at all?
0:17:10 > 0:17:14Antlers are made of dead bone and can't be repaired.
0:17:14 > 0:17:18If a moose damages an antler during a fight,
0:17:18 > 0:17:22it will lose its chance of mating for that season.
0:17:22 > 0:17:25By shedding and regrowing their antlers each year,
0:17:25 > 0:17:30bulls ensure that they stay in the mating game.
0:17:32 > 0:17:34Just before antlers are shed,
0:17:34 > 0:17:38minerals within them are reabsorbed from the base,
0:17:38 > 0:17:41weakening the structure so that they eventually fall off.
0:17:44 > 0:17:47The flesh underneath is exposed,
0:17:47 > 0:17:51but not for long, as new skin soon covers the wound.
0:17:52 > 0:17:55Experiments have shown that the skin lesion
0:17:55 > 0:17:57that forms over the open wound
0:17:57 > 0:18:00creates a connection with the underlying tissue
0:18:00 > 0:18:02that is crucial to regeneration.
0:18:04 > 0:18:07If this connection isn't made,
0:18:07 > 0:18:10the production of velvet will be interrupted, and the antlers
0:18:10 > 0:18:14will either not grow at all or develop into strange shapes.
0:18:19 > 0:18:22So what about the Irish elk?
0:18:22 > 0:18:25Could the problems of regenerating such gigantic antlers
0:18:25 > 0:18:27have determined its fate?
0:18:29 > 0:18:32The French scientist Georges Cuvier was keen to demonstrate
0:18:32 > 0:18:37that the Irish elk was a unique species that had become extinct.
0:18:37 > 0:18:41To prove his point, Cuvier undertook a detailed examination
0:18:41 > 0:18:44of Irish elk fossils.
0:18:44 > 0:18:48He was able to show that it was indeed a distinct type of deer
0:18:48 > 0:18:51that could no longer be found alive.
0:18:51 > 0:18:54And so the Irish elk was one of the first animals
0:18:54 > 0:18:58to be recognised as being extinct.
0:18:59 > 0:19:02Georges Cuvier had solved the question
0:19:02 > 0:19:04of whether or not the Irish elk and moose
0:19:04 > 0:19:06were one and the same creature.
0:19:07 > 0:19:09But why did the Irish elk die out?
0:19:10 > 0:19:14Cuvier suggested that evolution had set it on a course
0:19:14 > 0:19:16of ever-increasing growth,
0:19:16 > 0:19:20and that eventually the antlers became so large
0:19:20 > 0:19:23that the poor animal could not even lift its neck.
0:19:24 > 0:19:27He may not have been that far from the truth.
0:19:29 > 0:19:34It's now thought that the annual growth of the Irish elk antlers
0:19:34 > 0:19:36put a strain on their bodies.
0:19:36 > 0:19:39A significant proportion of minerals within their bones
0:19:39 > 0:19:43were extracted and moved into their growing antlers.
0:19:43 > 0:19:48This led to a seasonal osteoporosis, with their bones weakening.
0:19:48 > 0:19:54They were in effect robbing one part of their body to boost another.
0:19:56 > 0:19:59It was gamble that worked for thousands of years.
0:20:01 > 0:20:04But around 10,000 years ago, the climate began to warm.
0:20:06 > 0:20:12The nutrient-rich grasses that the elk relied upon began to disappear.
0:20:12 > 0:20:16Growing massive antlers may now have been too much of a drain
0:20:16 > 0:20:19and permanently weakened the skeleton.
0:20:21 > 0:20:26The change in diet may also have affected their ability to breed,
0:20:26 > 0:20:30with females no longer able to produce young every year.
0:20:31 > 0:20:36Whatever the reason, the Irish elk, with its magnificent antlers,
0:20:36 > 0:20:39finally vanished from the landscape,
0:20:39 > 0:20:42and in its place the moose has become
0:20:42 > 0:20:44the largest deer on Earth today.
0:20:48 > 0:20:52So, while regeneration can give the salamander
0:20:52 > 0:20:54a second chance to a full life,
0:20:54 > 0:20:58the yearly regeneration of antlers in male moose
0:20:58 > 0:21:00is a risky strategy...
0:21:00 > 0:21:05but one with huge rewards for those with the best antlers.