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