Remarkable Regeneration David Attenborough's Natural Curiosities

The natural world is full of extraordinary animals

with amazing life histories.

Yet certain stories are more intriguing than most.

The mysteries of a butterfly's life cycle

or the strange biology of the emperor penguin.

Some of these creatures

were surrounded by myth and misunderstandings

for a very long time.

And some have only recently revealed their secrets.

These are the animals that stand out from the crowd -

the curiosities I find particularly fascinating.

Salamanders can regenerate entire legs and tails

to replace ones that they lose.

And moose can regrow their enormous antlers every year.

How do these animals regenerate entire body parts

and why isn't it possible

for all animals to do the same?

When I was a boy,

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

It's a fire salamander.

They may look like lizards,

but in fact they're not reptiles,

they're amphibians, with moist skins.

For centuries, mythical stories surrounded these creatures.

It was believed that they were icy-cold animals

that could dwell within fires, unharmed by the heat.

Although their fire-surviving powers may be untrue,

the salamander nonetheless possesses a real natural ability

that is just as extraordinary.

They're able to regrow damaged tails,

legs and other parts of the body

through a process called regeneration.

There are more than 600 different species of salamander.

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

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

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

Salamanders are predators,

and many hunt for small invertebrates

such as slugs and worms.

But sometimes they hunt each other...

with dramatic consequences.

This tiny North American redback salamander

is on the menu of the much bigger seal salamander.

Time to make a retreat.

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

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

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

This ability to replace an entire body part

is unusual among adult vertebrates and seems almost magical.

Regeneration is a subject that fascinates us.

Modern medicine has spent a lot of money and time

studying the ways our own bodies can regenerate tissue.

All living creatures, including humans,

have the ability to repair damaged parts of the body,

but the extent of that repair varies considerably.

As small infants,

we have the ability to regrow the tips of our fingers

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

So animals like salamanders,

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

Regeneration had been known about since ancient times,

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

In the 17th and 18th century,

there was a new wave of scientific discovery.

A brilliant Italian scientist named Lazzaro Spallanzani

made meticulous observations into regeneration

across many different species

and shared his ideas in detailed letters.

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

whom he regularly corresponded with,

to announce that he had discovered tail regeneration in salamanders.

Throughout the following year,

he followed up his initial observations

with numerous experiments to try to understand

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

He found that all species of salamander that he tested

could regrow their tails when injured,

and they did so more rapidly in summer than in winter

and retained this incredible ability throughout their lives.

Spallanzani advocated a radical theory.

He thought that salamanders already possessed

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

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

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

He studied salamander tadpoles

and came up with another, even more interesting idea.

A year after his initial letter,

Spallanzani once again wrote to Charles Bonnet,

this time with detailed descriptions of further experiments

into tail regeneration.

Most notably in this description, he wrote,

"I am almost led to believe

"that the tail regenerates in tadpoles

"are more of an elongation of the old parts

"than a development from a germ."

This suggests that Spallanzani was on the right track,

but the idea that a salamander could regrow a new tail

from seemingly nothing was not well supported,

and Spallanzani was therefore never willing

to pursue the idea further.

However, there's no doubt that his research

helped to lead other scientists closer

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

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

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

in the remarkable growth of new limbs

that we understand better today.

When a limb is lost,

the exposed blood vessels and tissue

contract to quickly stop any bleeding.

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

to protect the body from infection.

Now cells that were once dormant

begin dividing and multiplying to create new ones.

Each cell retains a kind of memory

of the type of tissue it used to be,

so a new cell that regrows from damaged muscle

will always become muscle.

Within weeks, the salamander has a full-grown leg

almost identical to the original.

Although we now know the steps that take place

during the regeneration of body parts,

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

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

responds to injury.

In humans, if an arm is severed,

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

In response, the area becomes swollen

and is covered over with scar tissue,

preventing any new growth occurring.

But in salamanders, the immune system responds differently,

and instead of forming a scar, it triggers regeneration.

Another rather unusual-looking salamander

that lives in the freshwaters of Mexico

sheds new light on how this happens.

Axolotls are among the best regenerators in the natural world,

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

Like us, they have special white blood cells

that consume invading bacteria and damaged tissue

around injuries and wounds.

Researchers removed them, and the results were surprising.

The axolotl was unable to regrow new limbs.

So, white blood cells were part

of the secret of their powers of regeneration.

Understanding the role of the salamander's blood cells

in regrowing limbs

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

and we can't.

All amphibians have tadpoles,

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

But salamanders are able to retrigger that remarkable process.

We, too, undergo extraordinary development in the womb.

Maybe like the salamander,

there is a way of us retaining this ability

into our adult lives as well.

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

to enhance its chances of survival.

While we don't yet know all the answers,

it's likely that this incredible creature

could revolutionise modern medicine

and the way we treat injuries.

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

regrow their enormous new antlers every year

and discover what happens when regeneration goes wrong.

This impressive skeleton

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

It's an Irish elk.

Its antlers are enormous -

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

and they weigh 40 kilos.

An Irishman named Dr Molyneux

first scientifically described the elk in 1697

from specimens taken out of an Irish peat bog.

Some believed that this elk was a large moose

and were convinced living specimens could be found elsewhere

across Europe and Russia.

But not everyone agreed,

and a debate about the life of this creature

would continue for more than a hundred years.

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

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

Both have very impressive antlers.

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

Unlike horns, which are permanent structures,

they are shed and replaced every year.

But how can deer regrow huge chunks of bone,

something no other mammal can do?

Moose, like this young bull behind me,

start growing their new antlers

immediately after they shed their old ones.

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

known as pedicles,

and they have a soft, furry covering called velvet.

This is vital to their amazing powers of regeneration.

Blood vessels at the base start the growth,

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

Then blood vessels within the velvet take over

and transport nutrients and growth hormones to the growing tips.

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

making it the fastest-growing bone of any animal.

Once at full size, the velvet is shed.

The animal rubs its head against a tree

to encourage the thin velvet to fall off.

It may look gruesome,

but it's a natural part of the annual cycle

and does the animal no harm.

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

It's a question that baffled early naturalists,

until Charles Darwin suggested

it may be to do with attracting the opposite sex.

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

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

But as they get older and their body size increases,

so the antlers will also increase,

eventually becoming impressive ornaments

with which to compete for females.

Those with the biggest antlers

are certainly more attractive to the females.

Maybe they are an indicator of fitness and strength.

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

during the breeding season.

This is a time when a bull

must protect his harem and see off competitors.

Competing males tilt their heads

to show off their antlers to their best advantage.

But if the bulls are equally matched,

then the competitors fight.

The winner then gains access to the females.

The benefits of such a victory are huge.

But to get to that point,

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

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

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

The sheer size of these antlers

has led to some to argue that

they were unlikely to have been used in physical combat.

Unlike other deer, the antlers of the Irish elk

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

so that if a bull looked straight ahead

it would be at its biggest and most impressive.

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

and attract females without actually fighting.

So, although the Irish elk was armed

with what appear to be enormous weapons,

it seems they were mostly for show.

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

Fighting is always a risky business

and will often result in serious injuries.

After the breeding season, the antlers are discarded.

Moose shed theirs in the winter,

whereas smaller deer keep theirs until the next spring.

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

to carry throughout the winter.

But why are antlers shed at all?

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

If a moose damages an antler during a fight,

it will lose its chance of mating for that season.

By shedding and regrowing their antlers each year,

bulls ensure that they stay in the mating game.

Just before antlers are shed,

minerals within them are reabsorbed from the base,

weakening the structure so that they eventually fall off.

The flesh underneath is exposed,

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

Experiments have shown that the skin lesion

that forms over the open wound

creates a connection with the underlying tissue

that is crucial to regeneration.

If this connection isn't made,

the production of velvet will be interrupted, and the antlers

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

So what about the Irish elk?

Could the problems of regenerating such gigantic antlers

have determined its fate?

The French scientist Georges Cuvier was keen to demonstrate

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

To prove his point, Cuvier undertook a detailed examination

of Irish elk fossils.

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

that could no longer be found alive.

And so the Irish elk was one of the first animals

to be recognised as being extinct.

Georges Cuvier had solved the question

of whether or not the Irish elk and moose

were one and the same creature.

But why did the Irish elk die out?

Cuvier suggested that evolution had set it on a course

of ever-increasing growth,

and that eventually the antlers became so large

that the poor animal could not even lift its neck.

He may not have been that far from the truth.

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

put a strain on their bodies.

A significant proportion of minerals within their bones

were extracted and moved into their growing antlers.

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

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

It was gamble that worked for thousands of years.

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

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

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

and permanently weakened the skeleton.

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

with females no longer able to produce young every year.

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

finally vanished from the landscape,

and in its place the moose has become

the largest deer on Earth today.

So, while regeneration can give the salamander

a second chance to a full life,

the yearly regeneration of antlers in male moose

is a risky strategy...

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