0:01:21 > 0:01:24If there's one place in the world where reptiles still rule,
0:01:24 > 0:01:28it's here in the Galapagos Islands.
0:01:28 > 0:01:33The ancestors of the reptiles, the amphibians, had wet, permeable skins.
0:01:34 > 0:01:39As a consequence, they couldn't exist for long away from water.
0:01:39 > 0:01:44Reptiles, however, like these marine iguanas, are not so restricted.
0:01:44 > 0:01:46They can survive in places where amphibians
0:01:46 > 0:01:48would roast to death in minutes.
0:01:48 > 0:01:53Out on the scorching lava fields, the iguanas lie unprotected
0:01:53 > 0:01:55in the ferocious Equatorial sun.
0:01:55 > 0:01:58They can do so because of the major innovation
0:01:58 > 0:02:00made by the first reptiles -
0:02:00 > 0:02:02the nature of their skin.
0:02:02 > 0:02:07It's not moist like a frog's, but tough, covered with scales,
0:02:07 > 0:02:11and, most crucial of all, it's practically watertight.
0:02:11 > 0:02:13This skin has enabled reptiles
0:02:13 > 0:02:16to colonise the hottest and driest places on earth.
0:02:16 > 0:02:17And this is one of them.
0:02:17 > 0:02:20The Namib desert in south-western Africa.
0:02:35 > 0:02:38The sand here gets so hot, it scorches the skin,
0:02:38 > 0:02:42and even the sole of a reptile's foot can get burned,
0:02:42 > 0:02:44so when the sun is too much,
0:02:44 > 0:02:47this little lizard gets relief by gymnastics.
0:03:00 > 0:03:03The scales of the reptiles' skin are dead -
0:03:03 > 0:03:05horny outgrowths, like our fingernails.
0:03:05 > 0:03:09The reptiles use them for all kinds of purposes.
0:03:09 > 0:03:12No bird would want to eat the Australian thorny devil
0:03:12 > 0:03:14with scales like these.
0:03:16 > 0:03:20Scales are used to protect the body against wear and tear.
0:03:20 > 0:03:24Reptile legs stick out at the side rather than give support underneath.
0:03:24 > 0:03:28Although this shingleback drags its belly along the ground,
0:03:28 > 0:03:31its tough, heavy scales prevent damage.
0:03:35 > 0:03:38The scales can be of different sizes.
0:03:38 > 0:03:40Small where the skin needs to be flexible,
0:03:40 > 0:03:45and large and robust, especially on the head, to reinforce the skull.
0:03:48 > 0:03:50In some lizards, the horny skin and scales
0:03:50 > 0:03:53are fashioned into dramatic headgear.
0:04:21 > 0:04:26Sometimes the adornments are designed not just to protect
0:04:26 > 0:04:29but to scare enemies, as in the bearded dragon.
0:04:45 > 0:04:49The Australian frilled lizard, like most of them, is a bluffer.
0:04:49 > 0:04:52Its great ruff is no more than its scaly skin
0:04:52 > 0:04:54supported by bones from the throat.
0:04:54 > 0:04:59In fact, it's a relatively harmless creature with no offensive weapons.
0:04:59 > 0:05:01If its bluff doesn't work...
0:05:03 > 0:05:05..it can run for it.
0:05:29 > 0:05:32The horned iguana from the West Indies
0:05:32 > 0:05:35is one of the most heavily armoured of all lizards.
0:05:35 > 0:05:39Big, powerful, it's the rhinoceros of the reptile world.
0:05:42 > 0:05:44There's one group of lizards which,
0:05:44 > 0:05:47unusually for reptiles, is most active at night.
0:05:47 > 0:05:49The geckos.
0:05:54 > 0:05:58Some of their scales are the most complicated of all.
0:05:58 > 0:06:02Geckos can run up vertical walls, even panes of glass.
0:06:02 > 0:06:05The trick is done by scales on the soles of the feet.
0:06:05 > 0:06:09Each scale is branched and carries hundreds of microscopic hairs,
0:06:09 > 0:06:12invisible to the naked eye.
0:06:12 > 0:06:17The electron microscope shows that each ends in a cluster of tiny hooks
0:06:17 > 0:06:20that enables the gecko to hang on to virtually anything.
0:06:24 > 0:06:26The reptiles' skin is rich in pigment cells
0:06:26 > 0:06:28which can provide marvellous disguises.
0:06:28 > 0:06:31The Madagascan gecko is coloured
0:06:31 > 0:06:34exactly like the bark it always sits on.
0:06:37 > 0:06:40The chameleon can vary the shade of green in its skin
0:06:40 > 0:06:44so that it becomes invisible among leaves.
0:06:46 > 0:06:48The earless dragon from central Australia
0:06:48 > 0:06:50is obvious when it's feeding.
0:06:52 > 0:06:54But when it's motionless,
0:06:54 > 0:06:58it's difficult to distinguish it from pebbles.
0:07:05 > 0:07:07THUNDER ROLLS
0:07:11 > 0:07:15For the biggest scales of all, we go back to the Galapagos Islands
0:07:15 > 0:07:18and look for their most famous inhabitant,
0:07:18 > 0:07:20the giant tortoise.
0:07:20 > 0:07:24Its scales are supported from beneath by massive, bony plates
0:07:24 > 0:07:27so that the animal is as impregnable as a tank.
0:07:32 > 0:07:36The mountain slopes where they live are dry for much of the year,
0:07:36 > 0:07:38but their watertight skin
0:07:38 > 0:07:41keeps their liquid demands to a minimum.
0:07:41 > 0:07:44But even so, they have to top up this water,
0:07:44 > 0:07:49and this is one of the few waterholes in the crater.
0:07:49 > 0:07:51These few are just wallowing in the mud,
0:07:51 > 0:07:54but at this moment, a shower has started,
0:07:54 > 0:07:57and it'll fill little puddles here in the mud,
0:07:57 > 0:08:03and soon tortoises from all over the crater will stream down here
0:08:03 > 0:08:05to sip in those puddles.
0:08:20 > 0:08:23Since the reptiles were the first backboned animals
0:08:23 > 0:08:26to live completely away from water,
0:08:26 > 0:08:30they must have been the first creatures to develop real thirsts.
0:08:31 > 0:08:34And they have vast capacities.
0:08:34 > 0:08:38They can store several gallons of liquid in their bodies.
0:08:38 > 0:08:40So when water is about, they make the best of it
0:08:40 > 0:08:42and fill their reserve tanks.
0:08:42 > 0:08:46This may well be their last drink for months.
0:08:53 > 0:08:56The tortoises are an extremely ancient group.
0:08:56 > 0:09:00They appeared right at the dawn of the age of reptiles,
0:09:00 > 0:09:02some 180 million years ago.
0:09:02 > 0:09:06They saw the dinosaurs come and vanish.
0:09:06 > 0:09:10And they continued, as far as we can judge from their fossils,
0:09:10 > 0:09:13almost unchanged right until today.
0:09:30 > 0:09:36Their armoured shell may be unwieldy, but it's a most successful defence.
0:09:36 > 0:09:39Inside it, nothing can reach them.
0:09:56 > 0:10:01After a night of rain, the pools are full of water.
0:10:07 > 0:10:11They're also full of wallowing tortoises.
0:10:11 > 0:10:14Why, we just don't know, although some say
0:10:14 > 0:10:17it's a way of keeping warm at night and cool by day.
0:10:20 > 0:10:24Temperature control is something all reptiles must achieve,
0:10:24 > 0:10:26and some on the Galapagos
0:10:26 > 0:10:28must cope with the scorching Equatorial sun
0:10:28 > 0:10:31without the benefit of fresh water.
0:10:37 > 0:10:42Marine iguanas live down on the hot, black lava rocks.
0:10:42 > 0:10:47Although their skin is watertight, it's a very poor insulator.
0:10:47 > 0:10:49That may seem to be a limitation in this heat,
0:10:49 > 0:10:52but they've turned it to their advantage.
0:11:00 > 0:11:05We tend to think of reptiles as sluggish, cold-blooded creatures.
0:11:05 > 0:11:07But that's a mistaken view.
0:11:07 > 0:11:11Some of them, like these marine iguanas, for example,
0:11:11 > 0:11:15can maintain a higher working body temperature than us.
0:11:15 > 0:11:18In fact, the misleading term "cold-blooded"
0:11:18 > 0:11:21simply means the animals can't generate
0:11:21 > 0:11:24their own body heat internally.
0:11:24 > 0:11:28Instead, they get it from the sun by sunbathing.
0:11:28 > 0:11:33What's more, they can control the amount of heat they absorb
0:11:33 > 0:11:35to within very fine limits.
0:11:42 > 0:11:44At the moment, it's early morning.
0:11:44 > 0:11:47The night has been relatively cold
0:11:47 > 0:11:49and the iguanas are out on the rocks
0:11:49 > 0:11:52soaking up all the heat the sun provides.
0:11:52 > 0:11:53For until they're warm,
0:11:53 > 0:11:57their body chemistry won't produce the power they need to be active.
0:12:09 > 0:12:12But overheating can be as dangerous as chilling.
0:12:12 > 0:12:15The iguanas can't cool themselves by sweating,
0:12:15 > 0:12:20for the reptiles' skin hasn't got any sweat glands.
0:12:20 > 0:12:24So when, around midday, the sun gets too warm for comfort,
0:12:24 > 0:12:29they move down into clefts in the rocks and hang there in cool shadow.
0:12:37 > 0:12:40By choosing their resting places with care,
0:12:40 > 0:12:43they can keep their body temperature
0:12:43 > 0:12:46very close to 37 degrees centigrade at all times.
0:12:49 > 0:12:54When they've reached their working temperature, they can go for a swim.
0:12:59 > 0:13:05These reptiles feed on seaweed, and some of it they get by diving.
0:13:05 > 0:13:08But the sun-warmed iguanas have a problem.
0:13:09 > 0:13:12The sea is particularly cold here.
0:13:12 > 0:13:15A current comes straight up from the Antarctic,
0:13:15 > 0:13:17and it's easy to get chilled and torpid.
0:13:17 > 0:13:20So they overcome the difficulty
0:13:20 > 0:13:24by withdrawing warm blood into the centre of their body.
0:13:24 > 0:13:27It simply delays the cooling process.
0:13:39 > 0:13:42However, the marine iguanas must not stay out too long.
0:13:44 > 0:13:48Should they become over-chilled, they will lose their energy
0:13:48 > 0:13:50and no longer have the strength
0:13:50 > 0:13:53to cling to the rocks and resist the waves.
0:14:04 > 0:14:08By mid-afternoon, they're all back on the sunbathing rocks,
0:14:08 > 0:14:10eager to get warmed up again.
0:14:22 > 0:14:24To get warm quickly, you need to expose
0:14:24 > 0:14:28as much as possible of your surface to the sun and warm rocks.
0:14:28 > 0:14:32So the most recently emerged iguanas slump out, spread-eagled,
0:14:32 > 0:14:36just like exhausted human bathers after a cold swim.
0:14:36 > 0:14:38It's vital for them to warm up
0:14:38 > 0:14:42because without warmth, they cannot digest their meals.
0:14:48 > 0:14:51This is where they will all congregate as the day cools
0:14:51 > 0:14:56to collect the last rays of the sinking sun.
0:14:56 > 0:14:59One advantage of generating your own body heat internally,
0:14:59 > 0:15:03as we and all mammals do, is that, when the sun goes down,
0:15:03 > 0:15:07we can remain active and we can live in cold climates.
0:15:07 > 0:15:11But the price we pay for those privileges is very high.
0:15:11 > 0:15:14Something like 80% of the energy in the food we eat
0:15:14 > 0:15:17goes to maintaining our body temperatures
0:15:17 > 0:15:19at around 38 degrees centigrade.
0:15:19 > 0:15:23The reptiles' system, getting heat directly from the sun,
0:15:23 > 0:15:24is much more economical.
0:15:24 > 0:15:27A reptile can survive on 10% of the food
0:15:27 > 0:15:31a mammal of a similar size would require.
0:15:31 > 0:15:33And so, although they can't live in the Arctic,
0:15:33 > 0:15:36they can survive on low-calorie foods,
0:15:36 > 0:15:40like the seaweed these marine iguanas eat,
0:15:40 > 0:15:45and live in great numbers in places where food is very scarce,
0:15:45 > 0:15:46such as deserts.
0:16:08 > 0:16:12A chameleon, for example, can flourish in barren areas,
0:16:12 > 0:16:14provided that every few days
0:16:14 > 0:16:18it catches one reasonably-sized insect.
0:16:29 > 0:16:32The chameleon's talent for changing colour
0:16:32 > 0:16:35serves not only to give it different disguises
0:16:35 > 0:16:37but to express its emotions.
0:16:37 > 0:16:39It goes black with rage
0:16:39 > 0:16:42and becomes brightly coloured when courting,
0:16:42 > 0:16:45as many reptiles do.
0:16:45 > 0:16:48Anolis lizards display with extensible throat pouches,
0:16:48 > 0:16:52each species with its own particular colour.
0:16:59 > 0:17:03Nodding reinforces the effect. It's like waving a flag.
0:17:06 > 0:17:10This is the rare green iguana from Fiji.
0:17:10 > 0:17:13His courting colours are permanently on display,
0:17:13 > 0:17:15for only the male has black stripes.
0:17:23 > 0:17:26He doesn't have a throat pouch, but, like many lizards,
0:17:26 > 0:17:32uses head nodding to signal his status as a male in breeding fettle.
0:17:35 > 0:17:40By comparison, the female is plainer and not nearly so demonstrative.
0:17:45 > 0:17:47The Galapagos iguanas also nod.
0:17:47 > 0:17:52For them, as for others, the gesture serves a double purpose -
0:17:52 > 0:17:56not only to impress the females, but to warn off rival males.
0:17:56 > 0:18:00Each species has its own rhythm.
0:18:00 > 0:18:04The land iguanas have a slightly different language.
0:18:06 > 0:18:07A nose-to-nose nodding session
0:18:07 > 0:18:11is enough to settle a territorial dispute between these males.
0:18:37 > 0:18:42Eventually, all these displays lead to the desired consummation.
0:19:05 > 0:19:08The reptiles were the first vertebrates
0:19:08 > 0:19:11for whom internal fertilisation was essential.
0:19:11 > 0:19:15Their immediate ancestors, the amphibians, had no need for it.
0:19:15 > 0:19:17They mated in water,
0:19:17 > 0:19:20and sperms shed from the body could swim to the eggs.
0:19:20 > 0:19:24But out on dry land, the male reptiles had to find
0:19:24 > 0:19:27some other way of ensuring that sperm met egg -
0:19:27 > 0:19:30by placing it inside the female's body.
0:19:30 > 0:19:35And it sometimes seems that that process for these antique creatures
0:19:35 > 0:19:39is still a very clumsy and laborious business.
0:19:42 > 0:19:44In due course, that produces
0:19:44 > 0:19:47the second of the great reptilian innovations.
0:19:47 > 0:19:50Their waterproof skins had enabled them
0:19:50 > 0:19:53to live in the driest places on Earth.
0:19:53 > 0:19:56But that was of limited value if they had to retreat
0:19:56 > 0:20:01to find open water in which to lay eggs, as the amphibians have to do.
0:20:01 > 0:20:07The reptiles solved that problem by producing this.
0:20:07 > 0:20:09The waterproof egg.
0:20:09 > 0:20:14In effect, it's a tiny pond encapsulated in parchment and shell
0:20:14 > 0:20:18in which their young can pass through what amounts to the tadpole stage.
0:20:50 > 0:20:54The Fijian green iguana lays only two or three eggs,
0:20:54 > 0:20:58burying them with care in the ground.
0:21:07 > 0:21:10From all reptile eggs, the young clamber out, fully formed,
0:21:10 > 0:21:13virtually exact miniatures of their parents
0:21:13 > 0:21:15and ready for immediate action.
0:21:25 > 0:21:27These are baby skinks.
0:21:34 > 0:21:35From an egg like this
0:21:35 > 0:21:39there once hatched one of the most spectacular reptiles of all.
0:21:48 > 0:21:52For this is the fossilised egg of a dinosaur.
0:21:52 > 0:21:55There can be no question of the success of these early reptiles.
0:21:55 > 0:21:59They dominated the world for 130 million years.
0:21:59 > 0:22:04During that time, they developed into all kinds of shapes and sizes.
0:22:15 > 0:22:17Many took to the air,
0:22:17 > 0:22:21and some were the biggest flying animals to have existed,
0:22:21 > 0:22:25with a wingspan as big as a small aeroplane.
0:22:29 > 0:22:31Others even returned to the sea.
0:22:31 > 0:22:35Some rowed themselves along with huge flippers,
0:22:35 > 0:22:39while others sculled with their tails as dolphins do today.
0:22:41 > 0:22:45And turtles grew to the size of a small boat.
0:22:47 > 0:22:50From eggs hatched the dinosaur dynasty,
0:22:50 > 0:22:54which was the most spectacular demonstration of reptilian success.
0:22:56 > 0:22:59Deinonychus.
0:22:59 > 0:23:01Tyrannosaurus rex.
0:23:04 > 0:23:08The three-horned dinosaur, triceratops.
0:23:08 > 0:23:11Stegosaurus, parasaurolophus.
0:23:11 > 0:23:15And brachiosaurus, as big as a house.
0:23:15 > 0:23:18These were among the most impressive animals ever to tread the earth.
0:23:18 > 0:23:21And we know them from their bones.
0:23:22 > 0:23:27This is the richest deposit of dinosaur bones yet discovered.
0:23:27 > 0:23:29It's in Utah, in the western United States,
0:23:29 > 0:23:32and 140 million years ago,
0:23:32 > 0:23:35when the dinosaurs were at their prime,
0:23:35 > 0:23:40this great cliff face, which is now tilted, lay horizontally.
0:23:40 > 0:23:43This stone was the loose sand and gravel
0:23:43 > 0:23:47of a sandbank in the middle of a wide river.
0:23:47 > 0:23:54Down that river floated the great, bloated, rotting carcasses
0:23:54 > 0:23:56of huge dinosaurs.
0:23:56 > 0:24:00And many of them got stranded on this river bank,
0:24:00 > 0:24:02and here their bones lie.
0:24:02 > 0:24:07The layer in which they lie is only about 12 feet thick.
0:24:07 > 0:24:12It's thought it was all laid down in the space of 100 years or so,
0:24:12 > 0:24:16which gives some idea of how abundant they must have been at the time.
0:24:16 > 0:24:20The long bones - the leg bones and the shoulder blades
0:24:20 > 0:24:22and tails and backbones -
0:24:22 > 0:24:26are all roughly pointing in this direction.
0:24:26 > 0:24:28That makes it pretty clear
0:24:28 > 0:24:31that the river current flowed this way.
0:24:39 > 0:24:41This is a plate from a young stegosaurus,
0:24:41 > 0:24:43the one with two rows of blades on its back.
0:24:47 > 0:24:51And this the tooth of a savage allosaurus.
0:24:54 > 0:24:5814 different species of dinosaur have been found in this quarry,
0:24:58 > 0:25:03ranging from tiny creatures no bigger than a chicken
0:25:03 > 0:25:05to real monsters like the animal to whom
0:25:05 > 0:25:08this enormous thigh bone belonged,
0:25:08 > 0:25:10which was one of the biggest land-living animals
0:25:10 > 0:25:12the world has seen.
0:25:12 > 0:25:15Most of the bones left in this quarry
0:25:15 > 0:25:19come from carcasses dismembered by the river
0:25:19 > 0:25:21or by scavenging reptiles,
0:25:21 > 0:25:25but by the time quarrying finished here in the 1920s,
0:25:25 > 0:25:28over 30 near-complete skeletons had been taken away,
0:25:28 > 0:25:32and many of the most beautiful and impressive dinosaur skeletons
0:25:32 > 0:25:36in our museums today come from this quarry
0:25:36 > 0:25:39or from others working in the same formation.
0:25:41 > 0:25:45A brontosaurus, one of the biggest of all.
0:25:45 > 0:25:49Over 60 feet long and weighing in life about 30 tons,
0:25:49 > 0:25:53about the same as three full-grown bull elephants.
0:25:53 > 0:25:55The question immediately arises -
0:25:55 > 0:25:58why did these vegetable-eating dinosaurs
0:25:58 > 0:26:01grow to such a gigantic size?
0:26:01 > 0:26:04There are at least two possible answers.
0:26:04 > 0:26:09The first concerns their food, which was cycads and ferns.
0:26:09 > 0:26:13Those sort of plants are tough and fibrous
0:26:13 > 0:26:16and take a great deal of digestion.
0:26:16 > 0:26:18The dinosaurs only had relatively feeble teeth
0:26:18 > 0:26:20which weren't much good at mastication,
0:26:20 > 0:26:23so they had to have huge stomachs
0:26:23 > 0:26:27which would serve as fermentation vats
0:26:27 > 0:26:30in which the food could be kept for long periods of time
0:26:30 > 0:26:31while it was digested.
0:26:31 > 0:26:36A huge stomach requires a huge body to carry it.
0:26:36 > 0:26:41The second reason concerns that recurring problem for all reptiles -
0:26:41 > 0:26:43temperature control.
0:26:43 > 0:26:47The bigger your body, the less susceptible it is
0:26:47 > 0:26:52to variations in temperature, because it retains its heat longer.
0:26:52 > 0:26:56Temperature control may be the reason
0:26:56 > 0:27:00for the bizarre body of another famous dinosaur,
0:27:00 > 0:27:02the stegosaurus.
0:27:02 > 0:27:05It used to be said that these plates were a kind of armour.
0:27:05 > 0:27:08But close examination has shown
0:27:08 > 0:27:12that they were covered with a skin thick with blood.
0:27:12 > 0:27:15So if the animal were broadside onto the sun,
0:27:15 > 0:27:19they would serve as solar panels, rapidly warming the blood.
0:27:19 > 0:27:23And if it were overheated and faced into the sun,
0:27:23 > 0:27:26they would serve as cooling radiators.
0:27:26 > 0:27:29Such an ability to influence temperature
0:27:29 > 0:27:30could have been invaluable.
0:27:30 > 0:27:34Certainly, for a plant eater to be sluggish on a cool morning
0:27:34 > 0:27:36could've been disastrous.
0:27:48 > 0:27:51Down in Texas, the muds of an estuary,
0:27:51 > 0:27:54now turned to rock and forming a river bed,
0:27:54 > 0:27:57preserve a vivid record of these creatures.
0:28:00 > 0:28:04This is the footprint of a flesh-eating dinosaur, a hunter,
0:28:04 > 0:28:07with huge talons on its two feet.
0:28:07 > 0:28:13It stood on its two feet, upright, about 10 or 12 feet tall,
0:28:13 > 0:28:17with its tail on the ground, which here has ploughed into the mud,
0:28:17 > 0:28:20throwing up this great furrow.
0:28:20 > 0:28:23Here are two more of them.
0:28:23 > 0:28:27From their depth, we can get an idea of the animal's great weight.
0:28:30 > 0:28:35The line of tracks continues across the rock of the river bed.
0:28:40 > 0:28:42Further down are the tracks of the reptile
0:28:42 > 0:28:44it may well have been stalking,
0:28:44 > 0:28:49one of those huge vegetarians, with footprints a yard across.
0:28:52 > 0:28:55Farther north, in the badlands of Montana,
0:28:55 > 0:28:58the muds and sands over which the dinosaurs roamed
0:28:58 > 0:29:01form cliffs of crumbling rocks.
0:29:08 > 0:29:13And here, weathering out near the top of this cliff,
0:29:13 > 0:29:18is the skull of one of the most dramatic of all dinosaurs -
0:29:18 > 0:29:21triceratops, the three-horned dinosaur.
0:29:21 > 0:29:23Here's one horn. Here's the second horn,
0:29:23 > 0:29:25which has already weathered away.
0:29:25 > 0:29:27And its nose is pointing that way,
0:29:27 > 0:29:33so its third horn on the tip of the nose is still in this rock.
0:29:33 > 0:29:38Here is its eye. It has this great bony frill extending over its neck.
0:29:38 > 0:29:44In life, it was an immense creature, weighing eight to ten tons,
0:29:44 > 0:29:4620 or 30 feet long.
0:29:46 > 0:29:51It was a vegetarian, champing up with powerful, scissor-like jaws
0:29:51 > 0:29:55the cycads that grew in this neighbourhood.
0:29:55 > 0:30:00And this head alone weighed about two tons.
0:30:00 > 0:30:03The bony frill, while doubtless protecting the neck,
0:30:03 > 0:30:07also carried a great band of muscles,
0:30:07 > 0:30:11helping to manipulate and move this heavy head.
0:30:11 > 0:30:13Its brain was comparatively large,
0:30:13 > 0:30:17one of the largest of all dinosaurs' brains,
0:30:17 > 0:30:21weighing about two pounds, but that didn't save it.
0:30:21 > 0:30:24This beast was one of the last of its kind.
0:30:26 > 0:30:28On a geological timescale,
0:30:28 > 0:30:32the disappearance of the dinosaurs was extraordinarily abrupt.
0:30:32 > 0:30:36It's marked in the most dramatic way in these cliffs.
0:30:36 > 0:30:40Among the yellow and red sandstones and clays,
0:30:40 > 0:30:44there is this thin layer of coal along which I'm walking.
0:30:44 > 0:30:48Its black line rules an end to the reign of the dinosaurs.
0:30:48 > 0:30:52In the beds beneath it, there have been found in this area
0:30:52 > 0:30:57the remains of at least nine or ten different species of dinosaur.
0:30:57 > 0:31:00Above it, there are none.
0:31:00 > 0:31:02Even though the end may have taken
0:31:02 > 0:31:05tens of thousands of years to be complete,
0:31:05 > 0:31:09it was nonetheless extraordinarily abrupt and wholesale.
0:31:10 > 0:31:12What on earth could have brought it about?
0:31:15 > 0:31:17There have been dozens of suggestions.
0:31:17 > 0:31:22The more extreme require some kind of global catastrophe,
0:31:22 > 0:31:24but they're unlikely to be correct
0:31:24 > 0:31:28because it was only the dinosaurs that disappeared, not all reptiles.
0:31:28 > 0:31:30A more reasonable idea is that
0:31:30 > 0:31:34it was the rise of the warm-blooded furry mammals
0:31:34 > 0:31:37that caused the disappearance of the dinosaurs.
0:31:37 > 0:31:40We can test the likelihood of that theory
0:31:40 > 0:31:43by looking at the anthills around here.
0:31:48 > 0:31:52The ants in this part of the world roof their nests with gravel.
0:31:52 > 0:31:57Amongst the chips of stone they laboriously haul here
0:31:57 > 0:32:00are things like this.
0:32:01 > 0:32:05This is the tooth of a tiny mammal.
0:32:05 > 0:32:10A small, shrew-like creature, and that was the largest mammal
0:32:10 > 0:32:14that existed in this dinosaur-dominated part of the world.
0:32:14 > 0:32:17It's inconceivable that such tiny creatures
0:32:17 > 0:32:21could have offered any competition with the dinosaurs.
0:32:21 > 0:32:25No, there are better answers to the problem than that.
0:32:39 > 0:32:42The America across which the dinosaurs roamed
0:32:42 > 0:32:44was covered with thick jungle.
0:32:44 > 0:32:46But this fossilised tree,
0:32:46 > 0:32:50which was alive just after the last dinosaurs disappeared,
0:32:50 > 0:32:52is not of a jungle tree.
0:32:52 > 0:32:54This is a redwood, a sequoia,
0:32:54 > 0:32:57a tree which now, and almost certainly then,
0:32:57 > 0:32:59preferred a cool climate,
0:32:59 > 0:33:02and it's just one piece of a body of evidence we have
0:33:02 > 0:33:06which goes to show that about 63 million years ago,
0:33:06 > 0:33:10almost simultaneous with the disappearance of the dinosaurs,
0:33:10 > 0:33:13the world went through a great climate change.
0:33:13 > 0:33:15It got colder.
0:33:15 > 0:33:17In a cold climate,
0:33:17 > 0:33:22the absence of a good insulating skin could be lethal.
0:33:22 > 0:33:25While it's true that a big body retains heat for a long time,
0:33:25 > 0:33:27it's also true that such a body
0:33:27 > 0:33:31takes a long time to regain it once it's been lost.
0:33:31 > 0:33:36And so it could be that a succession of cold nights
0:33:36 > 0:33:40would be enough to drain a dinosaur of its heat beyond recovery.
0:33:40 > 0:33:46And so, such a cooling of the climate might, over thousands of years,
0:33:46 > 0:33:49be enough to exterminate the entire race of dinosaurs.
0:33:49 > 0:33:52But of course the effect of cold nights
0:33:52 > 0:33:55is much less on animals that live in the water.
0:33:55 > 0:33:59Water retains heat longer than the air.
0:33:59 > 0:34:03Indeed, there are giant water-living reptiles
0:34:03 > 0:34:06that have survived from the age of the dinosaur
0:34:06 > 0:34:07right until today.
0:34:31 > 0:34:34This is a truly primeval scene.
0:34:44 > 0:34:48Crocodiles have been lazing around swamps like this
0:34:48 > 0:34:52since the beginning of the age of the dinosaurs.
0:34:52 > 0:34:55At seven metres long and weighing three quarters of a ton,
0:34:55 > 0:34:59a bull Nile crocodile is the biggest reptile alive today.
0:35:11 > 0:35:16It's thought that reptiles in general and dinosaurs in particular
0:35:16 > 0:35:20were dull, stupid creatures with only glimmerings of intelligence
0:35:20 > 0:35:23and the simplest of behaviour patterns.
0:35:23 > 0:35:25That's a very mistaken view,
0:35:25 > 0:35:29as recent discoveries about the behaviour of crocodiles have shown.
0:35:29 > 0:35:30Since water holds the heat,
0:35:30 > 0:35:33crocodiles spend the cool nights in the river.
0:35:33 > 0:35:36To them, it's like a warm bath.
0:35:36 > 0:35:38But as the sun rises, they emerge
0:35:38 > 0:35:42to boost their body temperatures in the sun.
0:36:01 > 0:36:03To prevent themselves from overheating,
0:36:03 > 0:36:06they use one trick marine iguanas don't have.
0:36:06 > 0:36:08They gape.
0:36:27 > 0:36:30Their mouth is lined with thinner skin
0:36:30 > 0:36:32than the armoured hide on their body,
0:36:32 > 0:36:36so the blood in the capillaries there is more quickly cooled.
0:36:42 > 0:36:46When, with the sun's help, their bodies are warmed up,
0:36:46 > 0:36:48they can move very fast indeed.
0:36:56 > 0:37:00They don't have lips so their mouths are not watertight.
0:37:00 > 0:37:02If they were to swallow when submerged,
0:37:02 > 0:37:04water would flood down their throats.
0:37:04 > 0:37:09So eating and swallowing has to be done above the surface.
0:37:09 > 0:37:13The surprising complexity of their family life
0:37:13 > 0:37:16may indicate how dinosaurs behaved.
0:37:16 > 0:37:18Crocodiles mate in the water.
0:37:18 > 0:37:22The 40 or so eggs are laid in a hole on the river bank
0:37:22 > 0:37:24and covered with sand.
0:37:26 > 0:37:30Though the parents don't incubate the eggs, they usually remain close by.
0:37:30 > 0:37:34After about 90 days, when the eggs are about to hatch,
0:37:34 > 0:37:38the young, buried and still in their shells, begin calling,
0:37:38 > 0:37:41and the female starts to dig them up.
0:37:41 > 0:37:44CHIRPING
0:37:45 > 0:37:49Then a very remarkable thing happens.
0:37:49 > 0:37:53At this time, the mother develops a pouch beneath her chin
0:37:53 > 0:37:55which will hold about seven eggs or young.
0:38:11 > 0:38:13The eggs are about to hatch,
0:38:13 > 0:38:16as she knows from the chirps of the young inside them.
0:38:31 > 0:38:36She's taking them off to a nursery area in another part of the swamp,
0:38:36 > 0:38:37where there's better cover
0:38:37 > 0:38:40than beside the sandy bank where she nests.
0:38:59 > 0:39:04Burying eggs has its drawbacks. They can become damp and chilled.
0:39:04 > 0:39:08Nearby, there's another nest from which no young emerged.
0:39:08 > 0:39:12When the parents weren't looking, a predator dug them up and ate them.
0:39:12 > 0:39:16Now the remains are only food for ants and beetles.
0:39:26 > 0:39:28This is the culprit. A monitor lizard.
0:39:28 > 0:39:31It will take babies as eagerly as eggs.
0:39:31 > 0:39:33With predators like this around,
0:39:33 > 0:39:35it pays crocodiles to guard their clutch.
0:39:43 > 0:39:47The mother returns in the nick of time.
0:40:17 > 0:40:20It's tempting to think the great dinosaurs
0:40:20 > 0:40:24may have cared just as delicately for their babies.
0:40:45 > 0:40:50Even the bull responds to the sight and sounds of the young.
0:41:54 > 0:41:55Like all reptile hatchlings,
0:41:55 > 0:41:58the young are miniature versions of the adults
0:41:58 > 0:42:00and capable of finding food for themselves
0:42:00 > 0:42:02from the moment they leave the shell.
0:42:39 > 0:42:43The babies could be a meal for birds or other crocodiles,
0:42:43 > 0:42:48so the parents watch over them while they perfect their hunting skills.
0:42:51 > 0:42:53Crocodiles, together with tortoises,
0:42:53 > 0:42:55have changed little over the past ages.
0:42:55 > 0:42:58The ancestral reptiles were walkers.
0:42:58 > 0:43:01But the most sophisticated of modern forms
0:43:01 > 0:43:04have changed their style of getting around.
0:43:17 > 0:43:21It all started with lizards. They too are an ancient group,
0:43:21 > 0:43:26but early in their history, they gave rise to a successful family.
0:43:26 > 0:43:30The lizards, for some reason, have a tendency to lose legs.
0:43:30 > 0:43:33Some are still in the process of doing so today,
0:43:33 > 0:43:35and live under rocks or burrow,
0:43:35 > 0:43:38where legs could get in the way.
0:43:38 > 0:43:42The Australian blue-tongued skink has very small legs.
0:43:42 > 0:43:46In this South African skink, they've virtually disappeared.
0:43:48 > 0:43:51And it moves by wriggling.
0:43:56 > 0:44:00The Australian scaly foot, as its name suggests,
0:44:00 > 0:44:02has only a pair of stumps at the rear
0:44:02 > 0:44:05to betray the fact it's a burrowing lizard.
0:44:10 > 0:44:12This grotesque creature
0:44:12 > 0:44:15has all but lost its eyes as well as its legs.
0:44:15 > 0:44:19It's an amphisbaenid and normally lives entirely underground.
0:44:22 > 0:44:23Some 100 million years ago,
0:44:23 > 0:44:26another group of lizards also took to burrowing
0:44:26 > 0:44:28and they lost their legs.
0:44:28 > 0:44:31Some of their descendants came back above ground
0:44:31 > 0:44:32and became snakes.
0:44:34 > 0:44:37Pythons and boas still retain evidence that they once had limbs -
0:44:37 > 0:44:41two tiny spurs where their hind legs once were.
0:44:41 > 0:44:44Without legs, the snakes had to develop techniques
0:44:44 > 0:44:45for getting around.
0:44:45 > 0:44:48And very efficient they are, too.
0:44:50 > 0:44:55The boa's regular method is to throw its body into S-shaped coils
0:44:55 > 0:44:59so its flanks get purchase on irregularities on the surface
0:44:59 > 0:45:01and the scales underneath grip the ground.
0:45:01 > 0:45:06As the coils move backwards, the snake can thrust itself forwards.
0:45:08 > 0:45:11Boas, like most snakes, can also move in a straight line
0:45:11 > 0:45:13by shuffling along on their ribs.
0:45:13 > 0:45:16That's useful for crawling along a branch.
0:45:18 > 0:45:23The puff adder, when stalking prey, also moves on its ribs,
0:45:23 > 0:45:27lifting them in groups and pulling the scales of its underside
0:45:27 > 0:45:28forward and over the rib tips
0:45:28 > 0:45:32so undulations pass down the lower half of the body.
0:45:33 > 0:45:35To me, the most mystifying technique
0:45:35 > 0:45:39is that used by the sidewinder in south-west Africa.
0:46:18 > 0:46:20The key to understanding how it works
0:46:20 > 0:46:23is to watch for the only two places where,
0:46:23 > 0:46:25at any one time, its body touches the ground.
0:46:25 > 0:46:30It is taking a series of steps sideways.
0:46:30 > 0:46:35With only two points of contact, the hot sand doesn't burn it.
0:46:38 > 0:46:41With such methods of stalking prey,
0:46:41 > 0:46:44the snakes have become formidable hunters.
0:46:48 > 0:46:52Many snakes have become swimmers and hunt underwater.
0:46:54 > 0:46:56This venomous Florida mud snake
0:46:56 > 0:46:59has caught a siren, a sort of amphibian.
0:46:59 > 0:47:03Some are prepared to catch the most spiky of meals
0:47:03 > 0:47:05and subdue them by throwing coils around them.
0:47:09 > 0:47:12This might seem an impossible mouthful.
0:47:12 > 0:47:16Most snakes have jaws hinged to give them a great gape,
0:47:16 > 0:47:20but that of the egg-eating snake is simply vast.
0:47:51 > 0:47:54Once swallowed, it cracks the egg
0:47:54 > 0:47:58by grinding it against spikes that project into the gut.
0:48:03 > 0:48:07In many parts of the world, snakes flourish in huge numbers.
0:48:07 > 0:48:09They're so unobtrusive,
0:48:09 > 0:48:12it's difficult to appreciate how many there are,
0:48:12 > 0:48:14except on special occasions.
0:48:14 > 0:48:18One such occurs every spring in southern Canada.
0:48:18 > 0:48:21Prairie garter snakes hibernate communally.
0:48:21 > 0:48:25When spring comes, a flood of newly-warm snakes
0:48:25 > 0:48:28spills from the limestone pits where they've wintered.
0:48:28 > 0:48:33As soon as they emerge, they mate, and in the most spectacular fashion.
0:48:35 > 0:48:38Each female leaves a trail of sexual scent,
0:48:38 > 0:48:42which attracts up to 100 of the smaller males,
0:48:42 > 0:48:45which frantically struggle to become her mate.
0:49:25 > 0:49:29These garter snakes are very advanced members of their group.
0:49:29 > 0:49:32Not only have they developed the technique of hibernation
0:49:32 > 0:49:36to live through the winters when the ground is covered in snow,
0:49:36 > 0:49:38but they've managed to overcome
0:49:38 > 0:49:41many of the limitations of egg-laying.
0:49:41 > 0:49:45A few months after this communal mating, the offspring appear.
0:49:58 > 0:50:01Instead of laying and abandoning her eggs as most snakes do,
0:50:01 > 0:50:05the female garter snake becomes a mobile incubator.
0:50:05 > 0:50:08She retains her batch of eggs inside her body,
0:50:08 > 0:50:10thereby protecting them.
0:50:10 > 0:50:13By basking in the sun, she keeps them warm.
0:50:13 > 0:50:17She even contributes to the nourishment of the embryos
0:50:17 > 0:50:20over three months, almost like mammals do,
0:50:20 > 0:50:23until the time comes for them to be born.
0:50:56 > 0:50:59Live bearing is practised by several snakes,
0:50:59 > 0:51:01including some that have a claim
0:51:01 > 0:51:05to be the most highly-evolved reptiles of all.
0:51:05 > 0:51:09You can find them in the deserts of the western United States.
0:51:13 > 0:51:14RATTLING
0:51:14 > 0:51:16Rattlesnakes.
0:51:16 > 0:51:19This is the western diamondback.
0:51:19 > 0:51:21No animal alive can excel these creatures
0:51:21 > 0:51:27when it comes to finding, stalking and dispatching their victims.
0:51:33 > 0:51:36Its scales serve it in several ways.
0:51:36 > 0:51:38Those on its flanks are grooved
0:51:38 > 0:51:40to increase efficiency as heat absorbers.
0:51:47 > 0:51:50Those on its tail are hollow rattles.
0:51:57 > 0:52:01A pit beneath its eye is so sensitive to heat
0:52:01 > 0:52:05that it can detect the body warmth of a small mammal half a metre away.
0:52:08 > 0:52:12The snake's flickering tongue tastes the air.
0:52:12 > 0:52:14This is a Mexican blacktail rattlesnake.
0:52:19 > 0:52:23It collects the smell as molecules in the air
0:52:23 > 0:52:25and then carries them back to a pit
0:52:25 > 0:52:28in the top of its mouth for tasting.
0:52:37 > 0:52:41When it's found its prey, it strikes.
0:52:41 > 0:52:44The huge poisoned fangs hinge forward,
0:52:44 > 0:52:47ready to inject one of the most lethal poisons in the world.
0:53:02 > 0:53:04A rattlesnake can survive here
0:53:04 > 0:53:07on only a dozen or so meals in a year.
0:53:07 > 0:53:10And that's pretty efficient.
0:53:10 > 0:53:13But being cold-blooded, solar-powered,
0:53:13 > 0:53:15does have its limitations.
0:53:15 > 0:53:19No reptile can survive sustained cold,
0:53:19 > 0:53:23so great areas of the world are closed to them.
0:53:23 > 0:53:26But a very long time ago, one group of the reptiles
0:53:26 > 0:53:28evolved an answer to that problem.
0:53:28 > 0:53:32An answer that was based on that versatile thing,
0:53:32 > 0:53:34the reptilian scale.
0:53:36 > 0:53:37A feather.
0:54:24 > 0:54:27Subtitles by Red Bee Media Ltd