David Attenborough's Natural Curiosities (2013) s01e03 Episode Script

SIR DAVID ATTENBOROUGH: Some animals live extremely long lives, but how does their skin help them on their journey to old age? Curious questions like this have always fascinated me.
I've had the good luck to meet some very interesting creatures, but some are particularly unusual.
We've known about some of these animals for centuries.
Others we've discovered more recently.
In this series, I unravel some of their stories and reveal why they're considered natural curiosities.
The elephant and the mole rat are curious creatures.
They're both extremely wrinkled, starting their young lives looking ancient and remaining that way into old age.
Yet they outlive most other animals their size.
What are their secrets? Elephants are truly strange creatures, both in looks and behaviour.
Aristotle described them as "the beast that passeth all others in wit and mind".
But the more we learn about its curious body and behaviour, the more remarkable it appears to be.
The evolution of such a strange-looking creature is no accident.
Its fascinating body is the key to allowing elephants to live a long life.
For elephants, even young ones, it's an advantage to be wrinkly and not at all a sign of old age.
Elephants evolved from mammoths over 55 million years ago.
Today, they're the heaviest land mammals alive and one of the longest-lived, with a life expectancy of about 70 years.
Big creatures usually live a long time, largely because they have slow metabolisms.
However, elephants have particular characteristics that help them reach old age.
One of the most important, a family structure in which the oldest matriarchs pass on vital experience.
And their bodies have developed some special features to deal with the problems of being so big.
Their trunk is one of them.
This surely is the most extraordinary nose possessed by any living creature.
It can be moved with ease and dexterity to gently caress, tear down trees, suck up litres of water.
The trunk is, in fact, a union between the nose and the upper lip and it's highly sensitive, with over 100,000 muscle units in it.
The end of the trunk can move rather like a hand.
This mobile tip allows the elephant to feel and pick up delicate objects, such as a single blade of grass.
The stretched nose is a masterpiece of evolution and key to how the elephant can survive with such a large and curious body.
If they hadn't developed a trunk, elephants couldn't have become so big.
It enables them, in spite of their huge, stocky body, to reach down to the ground to collect food and water.
Fuelling a big body is a full-time job, and an elephant has to consume its own weight in food every 20 days.
One might think this great weight would be a stress on joints and teeth, and wear elephants out before old age, but not so.
Eating vegetation is, of course, very tough on the teeth, and there are some animals that, when their teeth are worn down, simply starve and die.
But elephants can live to 70 years old, and their secret lies in their extraordinary molar teeth.
They have two pairs - two at the top, two at the bottom - and here's one of them.
This is the grinding surface which is capable of shredding twigs and bark and even wood, and, of course, it wears.
But as it wears down, so another tooth is developing within the jaw, which finally emerges and pushes this forward until it actually breaks off and is shed.
Acquiring new teeth in that way enables elephants to remain well fed and healthy into old age.
In elephant society, the older females are invaluable and pass on the wisdom they've gained during their long lives to younger members of the family.
(RUMBLING GROWL) Mature females spend long periods of time listening out for vital sounds of danger and warn the group.
(RUMBLING GROWL) Such sensitivity to sound was the subject of one of the very first animal behaviour experiments.
Someone in France in the early 18th century noted that elephants in menageries appeared to react to faint, distant sounds outside their enclosures.
So, they tested two elephants, Hans and Parkie, and engaged a Paris orchestra to play love music to them.
One elephant was very impressed by the French horn player.
It was reported that, "The animal knelt down before him, "caressed him with his trunk, "and expressed to him in all sorts of pretty ways "the pleasure which it had felt in listening to him.
" We now know that the French horn can produce a low-frequency sound that's very like the rumble that elephants produce, using a similar resonating chamber in their heads.
(RUMBLING GROWL) They can also hear very deep sounds beyond our own hearing.
The oldest experienced females are experts at interpreting them.
Such frequencies create vibrations in the ground that travel a very long way, which the elephants can detect through their feet.
Their feet, in fact, are not as solid as they might look but have special internal cushioning to soften the impact of the animal's weighty footsteps.
For such a large creature, that can be 40 times our weight, this foot seems unfeasibly small.
Its surface area is little more than twice our own feet, but this foot has a surprising structure.
The elephant walks on five toes, and the back part of its foot consists of a highly spongy heel.
The raised heel can compress and expand to absorb shock and shield the other heavy bones in the body from pressure.
It's as if the elephant were wearing a high-heeled training shoe.
When an elephant runs, it bounces on this spongy heel, and its leg bones act like pogo sticks to push the animal upwards.
This system protects the bones and inner tissues, and wild elephants rarely get arthritis.
(SNARLING SHRIEK) Despite their large size, they live active physical lives without too much damage to their bodies.
Males, as they mature, usually go off to live by themselves, but the females stay with the family group and play a very important part in guiding the younger ones.
Young elephants tend to look old, even at the start of their lives, because of their wrinkly skin.
But for elephants, wrinkles are not signs of ageing.
On the contrary, they're extremely important for an elephant's very survival.
The elephant's thick, creased skin has been the subject of much debate over the years, and early anatomists had some novel ideas about it.
Many believed that the elephant could actually move its skin to crush flies between the wrinkles.
I may say that was never witnessed in action.
But the skin was thought to be enormously thick and insensitive, but, in fact, it varies across the elephant's body and can be as thick as two or three centimetres around the top of its trunk and along the back, and as thin as paper around the eyes.
Although the skin looks tough and wrinkly, it's remarkably sensitive.
An elephant can feel small flies on its body, even if it can't crush them between its wrinkles.
But these wrinkles really do have an important function.
The patterned crevices hold water, which travels along them all over the body.
Wrinkly skins can retain five to ten times more water than smooth ones, so moisture collected during wallowing stops the skin from dehydrating and overheating for a long time afterwards.
Significantly, African elephants that live in hotter, drier places have more deeply wrinkled skins than Asian elephants.
So, wrinkles, for the elephant, are ways of protecting the skin, not the unwanted consequence of old age.
The elephant was once considered an oddity of nature.
For centuries, we've been fascinated by their large ears, their extraordinary trunks, the stocky feet, the wrinkly skins, but over the years, we've come to understand their significance.
The elephant's unique biology is key to its long-term survival and its ability to seemingly avoid the rigours of old age.
Elephants understandably live a long time because of the slow metabolism of their huge bodies.
(RUMBLING GROWL) But small naked mole rats live much longer than any other mammal of a comparable size.
Why? Could it be that the body of this bizarre little creature holds the secret of eternal youth? When a German naturalist, Wilhelm Rüppell, discovered a lone, hairless, wrinkled naked mole rat in 1842 in Ethiopia, he was convinced that he had stumbled across a decrepit old individual and he gave it the name "Heterocephalus glaber", which, loosely translated, means a smooth-skinned animal with an oddly shaped head.
He noted that the form of the body, because of its hairlessness, gives an unpleasant impression.
It does.
For the next 40 years, these bizarre-looking creatures were largely ignored by scientists.
Then, in 1885, a British zoologist in London's Natural History Museum called Oldfield Thomas decided to examine in detail the museum's specimens that had been sitting in store for decades.
Here we can see some of his drawings.
Thomas declared that the weird animal described by Rüppell was, in fact, normal.
We now know that all mole rats look like this, whatever their age.
However, what those earlier naturalists couldn't have known was that they had chanced upon a mammal that would fascinate and intrigue scientists for the next 150 years, a creature that might even shed light on the secrets of ageing and longevity.
Its body hardly seemed to alter, no matter how long it lived.
Old mole rats stayed physically young throughout their lives.
And, not only that - the strangest discovery of all was that they sometimes lived for almost 30 years.
The life span of animals varies enormously.
Amongst mammals, a tiny little shrew like this lives just two or so years, while a giant whale can reach the age of 100.
Lifestyle is an important factor in defining life span.
A shrew has a fast and furious life, producing many young, of which few survive.
Whales, on the other hand, breed slowly and don't have many predators.
Generally, big animals live longer.
So, it's very odd indeed that mole rats live up to nine times longer than any other similar-sized rodent.
Why? In the 1960s, more than 100 years after their discovery, scientists started keeping the animals in laboratories to try and answer that question.
The results were confusing.
The mole rats lived in colonies, and only a few females ever reproduced.
Around that time, an evolutionary biologist called Richard Alexander was studying the way colonial insects, such as termites, organised their colonies.
They have a single breeding female who produces huge numbers of non-breeding workers, a system called eusociality.
He speculated that, if there were such things as a eusocial mammal, it too, like termites, would live underground in hard soil.
He was right.
The naked mole rat perfectly fits Alexander's description of what a eusocial animal should be like.
There it is.
It lives underground in large social groups and digs for tubers in exceptionally hard soil.
Physically, it's evolved for a life below ground.
It has a long, thin body with short legs that suit life in a tunnel.
Its enlarged, strong teeth are used for digging.
Its skull is strong, the head quite large, lips closed behind its teeth to stop any soil going into its mouth.
Also it's almost entirely bald, except for a few sensory hairs.
Could it be that these extraordinary characteristics have something to do with their ability to live very, very long lives? They're certainly key to the mole rat's unusual life underground.
The queen is at the heart of the colony.
She mates with just two or three males and produces babies in huge litters, sometimes of more than 20.
The workers feed the queen, care for the young and guard the tunnels.
Their role is essential.
The colony would not survive if all its members didn't work together.
The tubers that they eat are hard to find on the dry African plains, and the workers have to dig miles of tunnels in their search for them.
The fact that they don't breed might seem hard, but their mother, the queen, does, and her DNA is virtually identical to theirs, and, by working together, the colony can live in places where an individual mole rat could not.
But this still doesn't explain why these creatures live so long.
More recently, another adaptation to life underground threw up a clue.
Fossil records show that mole rats started living underground about 24 million years ago.
Not surprisingly, they're now highly adapted to a life in dark and humid tunnels.
Conditions in a sealed, two-metre-deep tunnel system don't fluctuate greatly, and, maybe because of this, mole rats have lost the ability to regulate their own body temperature.
So, to prevent getting chilled, they huddle together in groups.
They also, like reptiles, absorb heat by basking in the warmer shallow surface tunnels.
Being hairless might be an advantage for an animal that's essentially cold-blooded and needs to get some of its heat from its surrounding, and that may explain why naked mole rats are virtually bald.
But why are not other warm-blooded mammals that live underground also bald? Badgers, for example, have hairy coats.
(GROWLS) Well, badgers come above ground to feed and then they need their hairy coats to keep warm.
Naked mole rats, on the other hand, never see the light of day.
Nonetheless, one might think that being soft-skinned and bald is a huge disadvantage, for mole rats live in stuffy, insanitary conditions.
Mole rats colonies can contain several hundred individuals, and conditions underground are dark and dank and often quite toxic.
Oxygen levels can be very low and carbon dioxide high.
Yet, mysteriously, mole rats show no discomfort and suffer very little from disease.
This tolerance to such hostile conditions may also be related to their strange wrinkled skin and the cells below it.
Apparently, they lack a key neurotransmitter, called substance P, that is normally responsible for sending pain signals to the central nervous system.
This may explain their ability to survive the toxic conditions underground without stress and damage to their bodies.
It could also be one of the secrets of their youthful appearance, if you can call it that, and even their longevity.
Most animals react strongly to pain, and this can damage their bodies.
In mole rats, this effect is eliminated by cutting out the pain response.
Incredibly, no mole rat has ever been found with cancer.
But even if a normal animal survives disease, it still ages.
This is largely due to other chemicals in the body called oxidising agents.
They build up with time and break down the body tissues.
This leads to the telltale signs of old age.
Incredibly, mole rats appear to have no physical reaction to high levels of oxidising agents.
They grow very old, yet they don't physically age.
In wild mole rats, the queen is the most long-lived, and one of them here is 24 years old, yet she still has the body of a two-year-old.
No-one is sure how mole rats avoid the symptoms of old age, but a unique physiology evolved in response to the underground life has created an animal that is almost supernatural.
Here's a creature that's seemingly impervious to pain and with an iron constitution.
It's virtually cold-blooded, with a slow metabolism, and has evolved an unusual mix of strategies to deal with its challenging lifestyle.
In the future, these remarkable animals may help us solve some of our own problems, such as pain control, degenerative disease, and how we might avoid old age and wrinkly skins.
Here is a natural curiosity that is well worth pursuing.
Both elephants and mole rats remain much the same as they grow old.
And, surprisingly, the small naked mole rat lives, relatively speaking, even longer than the elephant.