Episode 3 David Attenborough's Natural Curiosities


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Some animals live extremely long lives,

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but how does their skin help them on their journey to old age?

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Curious questions like this have always fascinated me.

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I've had the good luck to meet some very interesting creatures,

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but some are particularly unusual.

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We've known about some of these animals for centuries.

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Others we've discovered more recently.

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In this series, I unravel some of their stories

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and reveal why they're considered natural curiosities.

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The elephant and the mole rat are curious creatures.

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They're both extremely wrinkled, starting their young lives

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looking ancient and remaining that way into old age.

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Yet they outlive most other animals their size.

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What are their secrets?

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Elephants are truly strange creatures,

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both in looks and behaviour. Aristotle described them as

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"the beast that passeth all others in wit and mind".

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But the more we learn about its curious body and behaviour

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the more remarkable it appears to be. The evolution

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of such a strange-looking creature is no accident.

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Its fascinating body is the key

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to allowing elephants to live a long life.

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For elephants - even young ones -

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it's an advantage to be wrinkly and not at all a sign of old age.

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Elephants evolved from mammoths over 55 million years ago.

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Today, they're the heaviest land mammals alive

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and one of the longest lived, with a life expectancy of about 70 years.

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Big creatures usually live a long time

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largely because they have slow metabolisms.

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However, elephants have particular characteristics

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that help them reach old age.

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One of the most important, a family structure

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in which the oldest matriarchs pass on vital experience.

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And their bodies have developed some special features

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to deal with the problems of being so big.

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Their trunk is one of them.

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This surely is the most extraordinary nose

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possessed by any living creature.

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It can be moved with ease and dexterity to gently caress,

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tear down trees, suck up litres of water.

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The trunk is, in fact, a union between the nose and the upper lip

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and it's highly sensitive, with over 100,000 muscle units in it.

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The end of the trunk can move rather like a hand.

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This mobile tip allows the elephant to feel

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and pick up delicate objects such as a single blade of grass.

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The stretched nose is a masterpiece of evolution and key

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to how the elephant can survive with such a large and curious body.

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If they hadn't developed a trunk,

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elephants couldn't have become so big.

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It enables them, in spite of their huge stocky body,

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to reach down to the ground to collect food and water.

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Fuelling a big body is a full-time job

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and an elephant has to consume its own weight in food every 20 days.

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One might think this great weight

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would be a stress on joints and teeth,

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and wear elephants out before old age, but not so.

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Eating vegetation is, of course, very tough on the teeth

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and there are some animals that, when their teeth are worn down,

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simply starve and die.

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But elephants can live to 70 years old

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and their secret lies in their extraordinary molar teeth.

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They have two pairs - two at the top, two at the bottom

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and here's one of them.

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This is the grinding surface which is capable

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of shredding twigs and bark and even wood and, of course, it wears.

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But as it wears down, so another tooth is developing within the jaw,

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which finally emerges and pushes this forward

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until it actually breaks off and is shed.

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Acquiring new teeth in that way enables elephants

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to remain well-fed and healthy into old age.

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In elephant society, the older females are invaluable

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and pass on the wisdom they've gained during their long lives

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to younger members of the family.

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Mature females spend long periods of time listening out

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for vital sounds of danger and warn the group.

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Such sensitivity to sound was the subject

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of one of the very first animal behaviour experiments.

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Someone in France in the early 18th century noted that

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elephants in menageries appeared to react to faint distant sounds

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outside their enclosures.

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So they tested two elephants - Hans and Parkie -

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and engaged a Paris orchestra to play love music to them.

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One elephant was very impressed by the French horn player.

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It was reported that, "The animal knelt down before him,

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"caressed him with his trunk, and expressed to him in all sorts

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"of pretty ways the pleasure which it had felt in listening to him."

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We now know that the French horn can produce a low-frequency sound

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that's very like the rumble that elephants produce

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using a similar resonating chamber in their heads.

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LOW RUMBLE

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They can also hear very deep sounds, beyond our own hearing.

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The oldest, experienced females are experts at interpreting them.

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Such frequencies create vibrations in the ground that travel

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a very long way which the elephants can detect through their feet.

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Their feet, in fact, are not as solid as they might look,

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but have special internal cushioning

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to soften the impact of the animal's weighty footsteps.

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For such a large creature, that can be 40 times our weight,

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this foot seems unfeasibly small.

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Its surface area is little more than twice our own feet

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but this foot has a surprising structure.

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The elephant walks on five toes

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and the back part of its foot consists of a highly spongy heel.

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The raised heel can compress and expand to absorb shock

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and shield the other heavy bones in the body from pressure.

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It's as if the elephant were wearing a high-heeled training shoe.

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When an elephant runs, it bounces on this spongy heel

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and its leg bones act like pogo sticks to push the animal upwards.

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This system protects the bones and inner tissues

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and wild elephants rarely get arthritis.

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Despite their large size, they live active physical lives

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without too much damage to their bodies.

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Males, as they mature, usually go off to live by themselves,

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but the females stay with the family group

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and play a very important part in guiding the younger ones.

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Young elephants tend to look old, even at the start of their lives,

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because of their wrinkly skin.

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But for elephants, wrinkles are not signs of ageing.

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On the contrary,

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they're extremely important for an elephant's very survival.

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The elephant's thick, creased skin has been the subject of much debate

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over the years and early anatomists had some novel ideas about it.

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Many believed that the elephant could actually move its skin

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to crush flies between the wrinkles.

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I may say that was never witnessed in action.

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But the skin was thought to be enormously thick and insensitive,

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but, in fact, it varies across the elephant's body

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and can be as thick as two or three centimetres

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around the top of its trunk and along the back,

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and as thin as paper around the eyes.

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Although the skin looks tough and wrinkly, it's remarkably sensitive.

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An elephant can feel small flies on its body

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even if it can't crush them between its wrinkles.

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But these wrinkles really do have an important function.

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The patterned crevices hold water,

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which travels along them all over the body.

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Wrinkly skins can retain five to ten times more water than smooth ones,

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so moisture collected during wallowing stops the skin

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from dehydrating and overheating for a long time afterwards.

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Significantly, African elephants that live in hotter, drier places

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have more deeply wrinkled skins than Asian elephants.

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So wrinkles, for the elephant, are ways of protecting the skin -

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not the unwanted consequence of old age.

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The elephant was once considered an oddity of nature.

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For centuries, we've been fascinated by their large ears,

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their extraordinary trunks, the stocky feet, the wrinkly skins,

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but over the years, we've come to understand their significance.

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The elephant's unique biology is key to its long-term survival

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and its ability to seemingly avoid the rigours of old age.

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Elephants understandably live a long time

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because of the slow metabolism of their huge bodies.

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But small naked mole rats live much longer

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than any other mammal of a comparable size. Why?

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Could it be that the body of this bizarre little creature

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holds the secret of eternal youth?

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When a German naturalist, Wilhelm Ruppell, discovered a lone,

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hairless, wrinkled naked mole rat in 1842 in Ethiopia,

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he was convinced that he had stumbled across

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a decrepit old individual

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and he gave it the name "heterocephalus glaber",

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which, loosely translated, means

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a smooth-skinned animal with an oddly-shaped head.

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He noted that the form of the body, because of its hairlessness,

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gives an unpleasant impression.

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It does.

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For the next 40 years,

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these bizarre-looking creatures were largely ignored by scientists.

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Then, in 1885, a British zoologist in London's Natural History Museum

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called Oldfield Thomas decided to examine in detail

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the museum's specimens that had been sitting in store for decades.

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Here we can see some of his drawings. Thomas declared that

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the weird animal described by Ruppell was, in fact, normal.

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We now know that all mole rats look like this, whatever their age.

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However, what those earlier naturalists couldn't have known

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was that they had chanced upon a mammal that would fascinate

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and intrigue scientists for the next 150 years.

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A creature that might even shed light

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on the secrets of ageing and longevity.

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Its body hardly seemed to alter, no matter how long it lived.

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Old mole rats stayed physically young throughout their lives.

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And not only that - the strangest discovery of all

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was that they sometimes lived for almost 30 years.

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The life span of animals varies enormously.

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Amongst mammals, a tiny little shrew like this

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lives just two or so years,

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while a giant whale can reach the age of 100.

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Lifestyle is an important factor in defining lifespan.

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A shrew has a fast and furious life,

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producing many young, of which few survive.

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Whales on the other hand breed slowly

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and don't have many predators.

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Generally, big animals live longer.

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So it's very odd indeed that mole rats live up to nine times longer

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than any other similar-sized rodent.

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Why?

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In the 1960s, more than 100 years after their discovery,

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scientists started keeping the animals in laboratories

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to try and answer that question.

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The results were confusing.

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The mole rats lived in colonies and only a few females ever reproduced.

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Around that time, an evolutionary biologist called Richard Alexander

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was studying the way colonial insects,

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such as termites, organised their colonies.

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They have a single breeding female who produces huge numbers

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of non-breeding workers, a system called eusociality.

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He speculated that if there were such things as a eusocial mammal,

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it too, like termites, would live underground in hard soil.

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He was right.

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The naked mole rat perfectly fits Alexander's description

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of what a eusocial animal should be like.

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There it is.

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It lives underground in large social groups

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and digs for tubers in exceptionally hard soil.

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Physically, it's evolved for a life below ground.

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It has a long, thin body with short legs that suit life in a tunnel.

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Its enlarged, strong teeth are used for digging,

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its skull is strong, the head quite large,

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lips closed behind its teeth to stop any soil going into its mouth.

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Also it's almost entirely bald, except for a few sensory hairs.

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Could it be that these extraordinary characteristics have something to do

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with their ability to live very, very long lives?

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They're certainly key to the mole rats' unusual life underground.

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The queen is at the heart of the colony.

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She mates with just two or three males

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and produces babies in huge litters, sometimes of more than 20.

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The workers feed the queen, care for the young and guard the tunnels.

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Their role is essential.

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The colony would not survive if all its members didn't work together.

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The tubers that they eat are hard to find on the dry African plains

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and the workers have to dig miles of tunnels in their search for them.

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The fact that they don't breed might seem hard,

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but their mother, the queen, does

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and her DNA is virtually identical to theirs.

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And, by working together, the colony can live in places

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where an individual mole rat could not.

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But this still doesn't explain why these creatures live so long.

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More recently, another adaptation to life underground threw up a clue.

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Fossil records show that mole rats started living underground

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about 24 million years ago.

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Not surprisingly, they're now highly adapted

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to a life in dark and humid tunnels.

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Conditions in a sealed, two-metre-deep tunnel system

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don't fluctuate greatly and, maybe because of this, mole rats

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have lost the ability to regulate their own body temperature.

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So, to prevent getting chilled, they huddle together in groups.

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They also, like reptiles, absorb heat by basking in the warmer,

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shallow surface tunnels.

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Being hairless might be an advantage for an animal

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that's essentially cold-blooded

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and needs to get some of its heat from its surrounding

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and that may explain why naked mole rats are virtually bald.

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But why are not other warm-blooded mammals

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that live underground also bald?

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Badgers, for example, have hairy coats.

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Well, badgers come above ground to feed

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and then they need their hairy coats to keep warm.

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Naked mole rats, on the other hand, never see the light of day.

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Nonetheless, one might think that being soft skinned

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and bald is a huge disadvantage,

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for mole rats live in stuffy insanitary conditions.

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Mole rats colonies can contain several hundred individuals

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and conditions underground are dark and dank and often quite toxic.

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Oxygen levels can be very low and carbon dioxide high.

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Yet, mysteriously, mole rats show no discomfort

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and suffer very little from disease.

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This tolerance to such hostile conditions may also be related

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to their strange wrinkled skin and the cells below it.

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Apparently, they lack a key neuro-transmitter,

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called Substance P, that is normally responsible

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for sending pain signals to the central nervous system.

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This may explain their ability

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to survive the toxic conditions underground

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without stress and damage to their bodies.

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It could also be one of the secrets of their youthful appearance -

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if you can call it that - and even their longevity.

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Most animals react strongly to pain and this can damage their bodies.

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In mole rats, this effect is eliminated

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by cutting out the pain response.

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Incredibly, no mole rat has ever been found with cancer.

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But even if a normal animal survives disease, it still ages.

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This is largely due to other chemicals in the body

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called oxidising agents.

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They build up with time and break down the body tissues.

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This leads to the telltale signs of old age.

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Incredibly, mole rats appear to have no physical reaction

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to high levels of oxidising agents.

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They grow very old, yet they don't physically age.

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In wild mole rats, the queen is the most long lived

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and one of them here is 24 years old,

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yet she still has the body of a two-year-old.

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No-one is sure how mole rats avoid the symptoms of old age,

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but a unique physiology evolved in response to the underground life

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has created an animal that is almost supernatural.

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Here's a creature that's seemingly impervious to pain

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and with an iron constitution.

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It's virtually cold blooded, with a slow metabolism, and has evolved

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an unusual mix of strategies to deal with its challenging lifestyle.

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In the future, these remarkable animals may help us

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solve some of our own problems such as pain control,

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degenerative disease,

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and how we might avoid old age and wrinkly skins.

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Here is a natural curiosity that is well worth pursuing.

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Both elephants and mole rats remain much the same as they grow old.

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And, surprisingly, the small naked mole rat lives, relatively speaking,

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even longer than the elephant.

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