0:00:02 > 0:00:05Some animals live extremely long lives,
0:00:05 > 0:00:10but how does their skin help them on their journey to old age?
0:00:11 > 0:00:15Curious questions like this have always fascinated me.
0:00:16 > 0:00:20I've had the good luck to meet some very interesting creatures,
0:00:20 > 0:00:23but some are particularly unusual.
0:00:24 > 0:00:27We've known about some of these animals for centuries.
0:00:27 > 0:00:30Others we've discovered more recently.
0:00:31 > 0:00:34In this series, I unravel some of their stories
0:00:34 > 0:00:38and reveal why they're considered natural curiosities.
0:00:48 > 0:00:52The elephant and the mole rat are curious creatures.
0:00:53 > 0:00:57They're both extremely wrinkled, starting their young lives
0:00:57 > 0:01:00looking ancient and remaining that way into old age.
0:01:01 > 0:01:05Yet they outlive most other animals their size.
0:01:05 > 0:01:07What are their secrets?
0:01:20 > 0:01:23Elephants are truly strange creatures,
0:01:23 > 0:01:28both in looks and behaviour. Aristotle described them as
0:01:28 > 0:01:33"the beast that passeth all others in wit and mind".
0:01:33 > 0:01:36But the more we learn about its curious body and behaviour
0:01:36 > 0:01:39the more remarkable it appears to be. The evolution
0:01:39 > 0:01:42of such a strange-looking creature is no accident.
0:01:42 > 0:01:45Its fascinating body is the key
0:01:45 > 0:01:48to allowing elephants to live a long life.
0:01:48 > 0:01:50For elephants - even young ones -
0:01:50 > 0:01:54it's an advantage to be wrinkly and not at all a sign of old age.
0:01:57 > 0:02:02Elephants evolved from mammoths over 55 million years ago.
0:02:02 > 0:02:05Today, they're the heaviest land mammals alive
0:02:05 > 0:02:10and one of the longest lived, with a life expectancy of about 70 years.
0:02:12 > 0:02:14Big creatures usually live a long time
0:02:14 > 0:02:17largely because they have slow metabolisms.
0:02:17 > 0:02:20However, elephants have particular characteristics
0:02:20 > 0:02:23that help them reach old age.
0:02:23 > 0:02:25One of the most important, a family structure
0:02:25 > 0:02:29in which the oldest matriarchs pass on vital experience.
0:02:30 > 0:02:33And their bodies have developed some special features
0:02:33 > 0:02:36to deal with the problems of being so big.
0:02:36 > 0:02:38Their trunk is one of them.
0:02:40 > 0:02:44This surely is the most extraordinary nose
0:02:44 > 0:02:47possessed by any living creature.
0:02:48 > 0:02:53It can be moved with ease and dexterity to gently caress,
0:02:53 > 0:02:56tear down trees, suck up litres of water.
0:02:56 > 0:03:01The trunk is, in fact, a union between the nose and the upper lip
0:03:01 > 0:03:06and it's highly sensitive, with over 100,000 muscle units in it.
0:03:06 > 0:03:10The end of the trunk can move rather like a hand.
0:03:10 > 0:03:14This mobile tip allows the elephant to feel
0:03:14 > 0:03:18and pick up delicate objects such as a single blade of grass.
0:03:19 > 0:03:24The stretched nose is a masterpiece of evolution and key
0:03:24 > 0:03:29to how the elephant can survive with such a large and curious body.
0:03:32 > 0:03:34If they hadn't developed a trunk,
0:03:34 > 0:03:37elephants couldn't have become so big.
0:03:38 > 0:03:41It enables them, in spite of their huge stocky body,
0:03:41 > 0:03:45to reach down to the ground to collect food and water.
0:03:46 > 0:03:49Fuelling a big body is a full-time job
0:03:49 > 0:03:54and an elephant has to consume its own weight in food every 20 days.
0:03:55 > 0:03:57One might think this great weight
0:03:57 > 0:04:00would be a stress on joints and teeth,
0:04:00 > 0:04:04and wear elephants out before old age, but not so.
0:04:05 > 0:04:09Eating vegetation is, of course, very tough on the teeth
0:04:09 > 0:04:12and there are some animals that, when their teeth are worn down,
0:04:12 > 0:04:15simply starve and die.
0:04:15 > 0:04:19But elephants can live to 70 years old
0:04:19 > 0:04:24and their secret lies in their extraordinary molar teeth.
0:04:24 > 0:04:27They have two pairs - two at the top, two at the bottom
0:04:27 > 0:04:29and here's one of them.
0:04:29 > 0:04:33This is the grinding surface which is capable
0:04:33 > 0:04:39of shredding twigs and bark and even wood and, of course, it wears.
0:04:39 > 0:04:44But as it wears down, so another tooth is developing within the jaw,
0:04:44 > 0:04:47which finally emerges and pushes this forward
0:04:47 > 0:04:51until it actually breaks off and is shed.
0:04:53 > 0:04:56Acquiring new teeth in that way enables elephants
0:04:56 > 0:04:59to remain well-fed and healthy into old age.
0:05:01 > 0:05:04In elephant society, the older females are invaluable
0:05:04 > 0:05:07and pass on the wisdom they've gained during their long lives
0:05:07 > 0:05:09to younger members of the family.
0:05:14 > 0:05:18Mature females spend long periods of time listening out
0:05:18 > 0:05:21for vital sounds of danger and warn the group.
0:05:23 > 0:05:26Such sensitivity to sound was the subject
0:05:26 > 0:05:30of one of the very first animal behaviour experiments.
0:05:33 > 0:05:37Someone in France in the early 18th century noted that
0:05:37 > 0:05:42elephants in menageries appeared to react to faint distant sounds
0:05:42 > 0:05:45outside their enclosures.
0:05:45 > 0:05:49So they tested two elephants - Hans and Parkie -
0:05:49 > 0:05:53and engaged a Paris orchestra to play love music to them.
0:05:54 > 0:05:58One elephant was very impressed by the French horn player.
0:05:58 > 0:06:03It was reported that, "The animal knelt down before him,
0:06:03 > 0:06:06"caressed him with his trunk, and expressed to him in all sorts
0:06:06 > 0:06:11"of pretty ways the pleasure which it had felt in listening to him."
0:06:14 > 0:06:19We now know that the French horn can produce a low-frequency sound
0:06:19 > 0:06:22that's very like the rumble that elephants produce
0:06:22 > 0:06:25using a similar resonating chamber in their heads.
0:06:25 > 0:06:27LOW RUMBLE
0:06:29 > 0:06:34They can also hear very deep sounds, beyond our own hearing.
0:06:34 > 0:06:39The oldest, experienced females are experts at interpreting them.
0:06:40 > 0:06:44Such frequencies create vibrations in the ground that travel
0:06:44 > 0:06:48a very long way which the elephants can detect through their feet.
0:06:50 > 0:06:53Their feet, in fact, are not as solid as they might look,
0:06:53 > 0:06:55but have special internal cushioning
0:06:55 > 0:06:58to soften the impact of the animal's weighty footsteps.
0:07:00 > 0:07:05For such a large creature, that can be 40 times our weight,
0:07:05 > 0:07:09this foot seems unfeasibly small.
0:07:09 > 0:07:12Its surface area is little more than twice our own feet
0:07:12 > 0:07:16but this foot has a surprising structure.
0:07:16 > 0:07:19The elephant walks on five toes
0:07:19 > 0:07:24and the back part of its foot consists of a highly spongy heel.
0:07:24 > 0:07:29The raised heel can compress and expand to absorb shock
0:07:29 > 0:07:32and shield the other heavy bones in the body from pressure.
0:07:32 > 0:07:37It's as if the elephant were wearing a high-heeled training shoe.
0:07:37 > 0:07:41When an elephant runs, it bounces on this spongy heel
0:07:41 > 0:07:47and its leg bones act like pogo sticks to push the animal upwards.
0:07:49 > 0:07:53This system protects the bones and inner tissues
0:07:53 > 0:07:56and wild elephants rarely get arthritis.
0:07:59 > 0:08:03Despite their large size, they live active physical lives
0:08:03 > 0:08:05without too much damage to their bodies.
0:08:08 > 0:08:12Males, as they mature, usually go off to live by themselves,
0:08:12 > 0:08:14but the females stay with the family group
0:08:14 > 0:08:18and play a very important part in guiding the younger ones.
0:08:22 > 0:08:25Young elephants tend to look old, even at the start of their lives,
0:08:25 > 0:08:27because of their wrinkly skin.
0:08:28 > 0:08:32But for elephants, wrinkles are not signs of ageing.
0:08:32 > 0:08:33On the contrary,
0:08:33 > 0:08:37they're extremely important for an elephant's very survival.
0:08:38 > 0:08:43The elephant's thick, creased skin has been the subject of much debate
0:08:43 > 0:08:47over the years and early anatomists had some novel ideas about it.
0:08:47 > 0:08:51Many believed that the elephant could actually move its skin
0:08:51 > 0:08:53to crush flies between the wrinkles.
0:08:53 > 0:08:57I may say that was never witnessed in action.
0:08:57 > 0:09:01But the skin was thought to be enormously thick and insensitive,
0:09:01 > 0:09:04but, in fact, it varies across the elephant's body
0:09:04 > 0:09:06and can be as thick as two or three centimetres
0:09:06 > 0:09:09around the top of its trunk and along the back,
0:09:09 > 0:09:12and as thin as paper around the eyes.
0:09:12 > 0:09:16Although the skin looks tough and wrinkly, it's remarkably sensitive.
0:09:16 > 0:09:19An elephant can feel small flies on its body
0:09:19 > 0:09:22even if it can't crush them between its wrinkles.
0:09:24 > 0:09:29But these wrinkles really do have an important function.
0:09:29 > 0:09:31The patterned crevices hold water,
0:09:31 > 0:09:34which travels along them all over the body.
0:09:34 > 0:09:39Wrinkly skins can retain five to ten times more water than smooth ones,
0:09:39 > 0:09:43so moisture collected during wallowing stops the skin
0:09:43 > 0:09:47from dehydrating and overheating for a long time afterwards.
0:09:49 > 0:09:53Significantly, African elephants that live in hotter, drier places
0:09:53 > 0:09:59have more deeply wrinkled skins than Asian elephants.
0:09:59 > 0:10:04So wrinkles, for the elephant, are ways of protecting the skin -
0:10:04 > 0:10:07not the unwanted consequence of old age.
0:10:10 > 0:10:14The elephant was once considered an oddity of nature.
0:10:15 > 0:10:19For centuries, we've been fascinated by their large ears,
0:10:19 > 0:10:24their extraordinary trunks, the stocky feet, the wrinkly skins,
0:10:24 > 0:10:28but over the years, we've come to understand their significance.
0:10:28 > 0:10:32The elephant's unique biology is key to its long-term survival
0:10:32 > 0:10:37and its ability to seemingly avoid the rigours of old age.
0:10:43 > 0:10:47Elephants understandably live a long time
0:10:47 > 0:10:50because of the slow metabolism of their huge bodies.
0:10:52 > 0:10:55But small naked mole rats live much longer
0:10:55 > 0:10:59than any other mammal of a comparable size. Why?
0:11:01 > 0:11:04Could it be that the body of this bizarre little creature
0:11:04 > 0:11:07holds the secret of eternal youth?
0:11:10 > 0:11:17When a German naturalist, Wilhelm Ruppell, discovered a lone,
0:11:17 > 0:11:22hairless, wrinkled naked mole rat in 1842 in Ethiopia,
0:11:22 > 0:11:25he was convinced that he had stumbled across
0:11:25 > 0:11:27a decrepit old individual
0:11:27 > 0:11:31and he gave it the name "heterocephalus glaber",
0:11:31 > 0:11:33which, loosely translated, means
0:11:33 > 0:11:36a smooth-skinned animal with an oddly-shaped head.
0:11:37 > 0:11:42He noted that the form of the body, because of its hairlessness,
0:11:42 > 0:11:44gives an unpleasant impression.
0:11:45 > 0:11:46It does.
0:11:49 > 0:11:51For the next 40 years,
0:11:51 > 0:11:56these bizarre-looking creatures were largely ignored by scientists.
0:11:56 > 0:12:01Then, in 1885, a British zoologist in London's Natural History Museum
0:12:01 > 0:12:05called Oldfield Thomas decided to examine in detail
0:12:05 > 0:12:09the museum's specimens that had been sitting in store for decades.
0:12:11 > 0:12:16Here we can see some of his drawings. Thomas declared that
0:12:16 > 0:12:20the weird animal described by Ruppell was, in fact, normal.
0:12:20 > 0:12:25We now know that all mole rats look like this, whatever their age.
0:12:25 > 0:12:28However, what those earlier naturalists couldn't have known
0:12:28 > 0:12:32was that they had chanced upon a mammal that would fascinate
0:12:32 > 0:12:36and intrigue scientists for the next 150 years.
0:12:36 > 0:12:39A creature that might even shed light
0:12:39 > 0:12:42on the secrets of ageing and longevity.
0:12:44 > 0:12:48Its body hardly seemed to alter, no matter how long it lived.
0:12:50 > 0:12:54Old mole rats stayed physically young throughout their lives.
0:12:55 > 0:12:58And not only that - the strangest discovery of all
0:12:58 > 0:13:03was that they sometimes lived for almost 30 years.
0:13:06 > 0:13:10The life span of animals varies enormously.
0:13:10 > 0:13:13Amongst mammals, a tiny little shrew like this
0:13:13 > 0:13:15lives just two or so years,
0:13:15 > 0:13:19while a giant whale can reach the age of 100.
0:13:20 > 0:13:25Lifestyle is an important factor in defining lifespan.
0:13:25 > 0:13:29A shrew has a fast and furious life,
0:13:29 > 0:13:32producing many young, of which few survive.
0:13:32 > 0:13:35Whales on the other hand breed slowly
0:13:35 > 0:13:36and don't have many predators.
0:13:37 > 0:13:41Generally, big animals live longer.
0:13:41 > 0:13:47So it's very odd indeed that mole rats live up to nine times longer
0:13:47 > 0:13:50than any other similar-sized rodent.
0:13:50 > 0:13:52Why?
0:13:52 > 0:13:56In the 1960s, more than 100 years after their discovery,
0:13:56 > 0:14:01scientists started keeping the animals in laboratories
0:14:01 > 0:14:03to try and answer that question.
0:14:03 > 0:14:05The results were confusing.
0:14:05 > 0:14:11The mole rats lived in colonies and only a few females ever reproduced.
0:14:11 > 0:14:15Around that time, an evolutionary biologist called Richard Alexander
0:14:15 > 0:14:18was studying the way colonial insects,
0:14:18 > 0:14:21such as termites, organised their colonies.
0:14:21 > 0:14:25They have a single breeding female who produces huge numbers
0:14:25 > 0:14:29of non-breeding workers, a system called eusociality.
0:14:29 > 0:14:34He speculated that if there were such things as a eusocial mammal,
0:14:34 > 0:14:38it too, like termites, would live underground in hard soil.
0:14:39 > 0:14:40He was right.
0:14:43 > 0:14:48The naked mole rat perfectly fits Alexander's description
0:14:48 > 0:14:51of what a eusocial animal should be like.
0:14:52 > 0:14:53There it is.
0:14:54 > 0:14:58It lives underground in large social groups
0:14:58 > 0:15:02and digs for tubers in exceptionally hard soil.
0:15:03 > 0:15:06Physically, it's evolved for a life below ground.
0:15:06 > 0:15:11It has a long, thin body with short legs that suit life in a tunnel.
0:15:11 > 0:15:14Its enlarged, strong teeth are used for digging,
0:15:14 > 0:15:18its skull is strong, the head quite large,
0:15:18 > 0:15:22lips closed behind its teeth to stop any soil going into its mouth.
0:15:22 > 0:15:28Also it's almost entirely bald, except for a few sensory hairs.
0:15:28 > 0:15:34Could it be that these extraordinary characteristics have something to do
0:15:34 > 0:15:36with their ability to live very, very long lives?
0:15:39 > 0:15:43They're certainly key to the mole rats' unusual life underground.
0:15:45 > 0:15:47The queen is at the heart of the colony.
0:15:47 > 0:15:50She mates with just two or three males
0:15:50 > 0:15:54and produces babies in huge litters, sometimes of more than 20.
0:15:56 > 0:16:01The workers feed the queen, care for the young and guard the tunnels.
0:16:01 > 0:16:02Their role is essential.
0:16:02 > 0:16:06The colony would not survive if all its members didn't work together.
0:16:10 > 0:16:15The tubers that they eat are hard to find on the dry African plains
0:16:15 > 0:16:20and the workers have to dig miles of tunnels in their search for them.
0:16:20 > 0:16:23The fact that they don't breed might seem hard,
0:16:23 > 0:16:25but their mother, the queen, does
0:16:25 > 0:16:29and her DNA is virtually identical to theirs.
0:16:29 > 0:16:33And, by working together, the colony can live in places
0:16:33 > 0:16:35where an individual mole rat could not.
0:16:37 > 0:16:41But this still doesn't explain why these creatures live so long.
0:16:41 > 0:16:46More recently, another adaptation to life underground threw up a clue.
0:16:47 > 0:16:51Fossil records show that mole rats started living underground
0:16:51 > 0:16:53about 24 million years ago.
0:16:53 > 0:16:56Not surprisingly, they're now highly adapted
0:16:56 > 0:16:58to a life in dark and humid tunnels.
0:16:59 > 0:17:02Conditions in a sealed, two-metre-deep tunnel system
0:17:02 > 0:17:06don't fluctuate greatly and, maybe because of this, mole rats
0:17:06 > 0:17:09have lost the ability to regulate their own body temperature.
0:17:09 > 0:17:14So, to prevent getting chilled, they huddle together in groups.
0:17:14 > 0:17:19They also, like reptiles, absorb heat by basking in the warmer,
0:17:19 > 0:17:20shallow surface tunnels.
0:17:22 > 0:17:24Being hairless might be an advantage for an animal
0:17:24 > 0:17:26that's essentially cold-blooded
0:17:26 > 0:17:29and needs to get some of its heat from its surrounding
0:17:29 > 0:17:33and that may explain why naked mole rats are virtually bald.
0:17:34 > 0:17:37But why are not other warm-blooded mammals
0:17:37 > 0:17:40that live underground also bald?
0:17:40 > 0:17:43Badgers, for example, have hairy coats.
0:17:46 > 0:17:50Well, badgers come above ground to feed
0:17:50 > 0:17:53and then they need their hairy coats to keep warm.
0:17:56 > 0:17:59Naked mole rats, on the other hand, never see the light of day.
0:17:59 > 0:18:02Nonetheless, one might think that being soft skinned
0:18:02 > 0:18:05and bald is a huge disadvantage,
0:18:05 > 0:18:08for mole rats live in stuffy insanitary conditions.
0:18:10 > 0:18:13Mole rats colonies can contain several hundred individuals
0:18:13 > 0:18:18and conditions underground are dark and dank and often quite toxic.
0:18:18 > 0:18:22Oxygen levels can be very low and carbon dioxide high.
0:18:22 > 0:18:25Yet, mysteriously, mole rats show no discomfort
0:18:25 > 0:18:28and suffer very little from disease.
0:18:28 > 0:18:31This tolerance to such hostile conditions may also be related
0:18:31 > 0:18:35to their strange wrinkled skin and the cells below it.
0:18:36 > 0:18:40Apparently, they lack a key neuro-transmitter,
0:18:40 > 0:18:44called Substance P, that is normally responsible
0:18:44 > 0:18:47for sending pain signals to the central nervous system.
0:18:48 > 0:18:50This may explain their ability
0:18:50 > 0:18:53to survive the toxic conditions underground
0:18:53 > 0:18:56without stress and damage to their bodies.
0:18:56 > 0:19:00It could also be one of the secrets of their youthful appearance -
0:19:00 > 0:19:03if you can call it that - and even their longevity.
0:19:08 > 0:19:14Most animals react strongly to pain and this can damage their bodies.
0:19:18 > 0:19:21In mole rats, this effect is eliminated
0:19:21 > 0:19:23by cutting out the pain response.
0:19:25 > 0:19:30Incredibly, no mole rat has ever been found with cancer.
0:19:30 > 0:19:35But even if a normal animal survives disease, it still ages.
0:19:35 > 0:19:39This is largely due to other chemicals in the body
0:19:39 > 0:19:41called oxidising agents.
0:19:41 > 0:19:44They build up with time and break down the body tissues.
0:19:46 > 0:19:49This leads to the telltale signs of old age.
0:19:53 > 0:19:58Incredibly, mole rats appear to have no physical reaction
0:19:58 > 0:20:01to high levels of oxidising agents.
0:20:01 > 0:20:05They grow very old, yet they don't physically age.
0:20:07 > 0:20:11In wild mole rats, the queen is the most long lived
0:20:11 > 0:20:14and one of them here is 24 years old,
0:20:14 > 0:20:18yet she still has the body of a two-year-old.
0:20:18 > 0:20:22No-one is sure how mole rats avoid the symptoms of old age,
0:20:22 > 0:20:26but a unique physiology evolved in response to the underground life
0:20:26 > 0:20:30has created an animal that is almost supernatural.
0:20:30 > 0:20:34Here's a creature that's seemingly impervious to pain
0:20:34 > 0:20:36and with an iron constitution.
0:20:36 > 0:20:41It's virtually cold blooded, with a slow metabolism, and has evolved
0:20:41 > 0:20:45an unusual mix of strategies to deal with its challenging lifestyle.
0:20:45 > 0:20:48In the future, these remarkable animals may help us
0:20:48 > 0:20:51solve some of our own problems such as pain control,
0:20:51 > 0:20:53degenerative disease,
0:20:53 > 0:20:57and how we might avoid old age and wrinkly skins.
0:20:58 > 0:21:03Here is a natural curiosity that is well worth pursuing.
0:21:05 > 0:21:10Both elephants and mole rats remain much the same as they grow old.
0:21:10 > 0:21:15And, surprisingly, the small naked mole rat lives, relatively speaking,
0:21:15 > 0:21:18even longer than the elephant.