0:00:37 > 0:00:41For millions of years, before birds evolved,
0:00:41 > 0:00:44and long before the rise of the mammals,
0:00:44 > 0:00:48cold-blooded animals ruled the world.
0:00:49 > 0:00:53In some places, they still do.
0:00:59 > 0:01:04Some of these reptiles witnessed the dinosaurs come and go.
0:01:09 > 0:01:15Yet in all that time, they themselves remained virtually unchanged.
0:01:19 > 0:01:24Among them were some of the most impressive reptiles alive today.
0:01:24 > 0:01:29They took that most characteristic of reptilian features, the scale,
0:01:29 > 0:01:33to extremes - they turned it into armour.
0:01:34 > 0:01:41That increased their weight, but nevertheless some can still move with extraordinary speed.
0:01:47 > 0:01:50And although they may appear cold and impassive,
0:01:50 > 0:01:53they can nonetheless be passionate...
0:01:54 > 0:01:57..and even affectionate.
0:02:03 > 0:02:07Among them are the biggest of all reptiles alive today.
0:02:07 > 0:02:11They're the crocodiles, the turtles and the tortoises.
0:02:18 > 0:02:25This is a giant Galapagos tortoise and it's climbed all the way up
0:02:25 > 0:02:27the flank of this great volcano,
0:02:27 > 0:02:31and is here wandering around the rim of the crater.
0:02:31 > 0:02:36But why should it come to such a bleak and inhospitable place?
0:02:39 > 0:02:41Tortoises, being reptiles,
0:02:41 > 0:02:45can't generate their own body heat internally as we do.
0:02:45 > 0:02:48Instead, they must get it from their surroundings
0:02:48 > 0:02:53and these particular ones have come up here to warm themselves
0:02:53 > 0:02:57on the hot volcanic rocks among the jets of steam and sulphurous gas.
0:02:59 > 0:03:03They live longer than any other animal on earth...
0:03:03 > 0:03:06well over 150 years.
0:03:08 > 0:03:13They weigh up to a quarter of a tonne and have shells over a metre across.
0:03:13 > 0:03:16They really are giants.
0:03:24 > 0:03:29Having your body encased in shell obviously brings problems.
0:03:29 > 0:03:31One of them is how do you mate?
0:03:31 > 0:03:36Making love in a suit of armour is not easy.
0:03:36 > 0:03:40But the males have a very ingenious solution.
0:03:45 > 0:03:49The underside of their shell is concave,
0:03:49 > 0:03:52so it fits neatly over the domed top of the shell of the female,
0:03:52 > 0:03:54who is somewhat smaller.
0:03:55 > 0:04:00That doesn't make clambering on top of her any easier initially.
0:04:00 > 0:04:01But once the male is up there,
0:04:01 > 0:04:04it will reduce his chance of slipping off.
0:04:18 > 0:04:23The two shells fit together as neatly as two spoons.
0:04:27 > 0:04:30So mating can begin,
0:04:30 > 0:04:34and once started, it can go on for a long time.
0:04:39 > 0:04:43The tortoise's shell is so familiar to us,
0:04:43 > 0:04:48it's easy to forget what an extraordinary construction it is.
0:04:48 > 0:04:50But how did it originate?
0:04:50 > 0:04:53Nearly all reptiles are covered in scales.
0:04:53 > 0:04:57And so were the tortoises' ancestors.
0:04:57 > 0:05:02But then, as they evolved, a radical change took place.
0:05:04 > 0:05:07The ribs expanded outwards
0:05:07 > 0:05:10so that they enclosed the hip and the shoulder joints.
0:05:13 > 0:05:18They enlarged and fused with other bones beneath the skin.
0:05:18 > 0:05:24They widened and eventually, they joined together to form a bony box.
0:05:27 > 0:05:30Above, the scales in the skin enlarged
0:05:30 > 0:05:34to form a continuous shield of horn on the surface of the box.
0:05:34 > 0:05:36And the basic armour was complete.
0:05:40 > 0:05:45By developing a shell, tortoises inevitably sacrificed speed.
0:05:45 > 0:05:49So they can't sprint off and take shelter in a crack
0:05:49 > 0:05:52when danger threatens, but with a shell like that,
0:05:52 > 0:05:55they seldom need to.
0:05:57 > 0:06:03Some have added deluxe features to the basic model.
0:06:03 > 0:06:05This is an eastern box turtle.
0:06:14 > 0:06:17In North America, where it lives,
0:06:17 > 0:06:20there are plenty of would-be predators...
0:06:20 > 0:06:24racoons among them.
0:06:24 > 0:06:27And they have very nimble paws.
0:06:34 > 0:06:36But these turtles don't have to worry,
0:06:36 > 0:06:40because their shell has a special safety feature -
0:06:40 > 0:06:43it has a drawbridge. And when danger threatens,
0:06:43 > 0:06:49the box turtle just retreats into its shell and pulls it up.
0:06:58 > 0:07:00The drawbridge fits so tightly
0:07:00 > 0:07:04there is no crack for the racoon to get its teeth into.
0:07:19 > 0:07:22After a while, most racoons give up.
0:07:27 > 0:07:30Of course, when your head is inside your shell,
0:07:30 > 0:07:34you can't see whether your attacker has gone or not.
0:07:34 > 0:07:37So it's best to check before you emerge fully.
0:07:42 > 0:07:46A shell is an excellent defence against predators,
0:07:46 > 0:07:51but there are other dangers against which it provides no protection whatever.
0:07:54 > 0:07:59It's midsummer, it's nearly midday and it's very, very hot
0:07:59 > 0:08:02and as a consequence, I'm sweating.
0:08:02 > 0:08:08Sweating is a direct response to heat that only mammals can do.
0:08:08 > 0:08:13No reptile, like this gopher tortoise, has got sweat glands,
0:08:13 > 0:08:17but instead, it's got another way of keeping itself cool.
0:08:19 > 0:08:23Most reptiles head for the shade when it gets too hot.
0:08:23 > 0:08:27To see where this gopher tortoise is heading, here in Florida,
0:08:27 > 0:08:30I'm going to use this...
0:08:31 > 0:08:36A remotely controlled mini-camera on wheels with its own lights.
0:08:36 > 0:08:39It can go pretty well anywhere.
0:08:39 > 0:08:43The gopher tortoise is heading for home.
0:08:48 > 0:08:53And with luck, I'll be able to follow it - the tortoise -
0:08:53 > 0:08:58as it goes down into its burrow.
0:08:58 > 0:09:03And that camera has also got a thermometer mounted on it.
0:09:03 > 0:09:07And already I can see the temperature is beginning to drop.
0:09:09 > 0:09:14The further down the burrow we go, the cooler it gets.
0:09:14 > 0:09:17Now, where's the tortoise?
0:09:18 > 0:09:21There it is!
0:09:21 > 0:09:23We're right behind!
0:09:26 > 0:09:28We're now a couple of metres in,
0:09:28 > 0:09:33but the burrow could go on for some 50 feet - 17 metres.
0:09:33 > 0:09:36And it's all been built by this tortoise.
0:09:40 > 0:09:45Well... That is not a tortoise.
0:09:45 > 0:09:47That's a rattlesnake!
0:09:50 > 0:09:54Obviously taking shelter from the heat, just as the tortoise is.
0:09:58 > 0:10:02The tortoise is so well armoured, it's in no danger from the snake.
0:10:02 > 0:10:06And gopher tortoises don't seem to mind sharing their burrows.
0:10:09 > 0:10:11A-ha!
0:10:11 > 0:10:14It's changed its mind. There's its rattle.
0:10:17 > 0:10:20Over 100 different species of animal
0:10:20 > 0:10:25have been recorded taking shelter inside tortoises' tunnels.
0:10:25 > 0:10:28In fact, some can live nowhere else.
0:10:28 > 0:10:34But there's another reason why tortoises' homes are so popular.
0:10:35 > 0:10:40Tortoises are one of the few animals here that can actually dig.
0:10:42 > 0:10:47Each may have more than one burrow within its territory and that's very valuable,
0:10:47 > 0:10:51because they have to deal with an even greater danger than sunstroke.
0:10:56 > 0:10:57Bush fires.
0:11:03 > 0:11:06These are a major and recurrent threat
0:11:06 > 0:11:08to all the animals that live here.
0:11:22 > 0:11:28Tortoise tunnels are invaluable places in which to take refuge.
0:11:39 > 0:11:43Gopher tortoises may seem to be unassuming creatures,
0:11:43 > 0:11:46but their engineering skills are essential
0:11:46 > 0:11:49for the survival of the whole ecosystem.
0:11:55 > 0:11:57The ground is still smoking,
0:11:57 > 0:12:01but the flames have passed and the emergency is over.
0:12:06 > 0:12:11So by digging tunnels, tortoises save not only their lives,
0:12:11 > 0:12:14but the lives of hundreds of other animals.
0:12:14 > 0:12:20But tunnels aren't the only place where you can escape extremes of temperature.
0:12:20 > 0:12:24There's another environment that's cooler and even more stable...
0:12:24 > 0:12:25Water.
0:12:30 > 0:12:36Some of the ancestral tortoises started to spend all their time there
0:12:36 > 0:12:39and became turtles.
0:12:45 > 0:12:49Some still walk slowly along the bottom
0:12:49 > 0:12:52in much the same way as their ancestors walked on land.
0:12:54 > 0:12:58Water is a good place for a cold-blooded animal to live.
0:12:58 > 0:13:03It retains its warmth through the night and stays comfortably cool during the heat of the day.
0:13:03 > 0:13:09So turtles are able to keep their body temperature relatively constant without much difficulty.
0:13:23 > 0:13:27Many have developed webs between their toes
0:13:27 > 0:13:30and have become very efficient swimmers.
0:13:36 > 0:13:40The most aquatic of all freshwater turtles is found in New Guinea
0:13:40 > 0:13:44and a few rivers like this one in northern Australia.
0:13:46 > 0:13:49The pig-nosed turtle.
0:13:51 > 0:13:56Its feet have become completely transformed into flippers
0:13:56 > 0:13:58and are of little use on land.
0:14:00 > 0:14:04And indeed, the pig-nosed turtle rarely comes ashore.
0:14:07 > 0:14:11But turtles are descended from land-living ancestors
0:14:11 > 0:14:13and so they still need to breathe air.
0:14:23 > 0:14:30Females also have to return to land in order to lay their eggs.
0:14:30 > 0:14:35Pig-noses nest during the dry season high up on the river bank.
0:14:36 > 0:14:42If turtle eggs get wet, the babies inside them will drown.
0:14:42 > 0:14:47At least, that is the case with most turtles.
0:14:47 > 0:14:51An egg for a turtle represents a huge investment,
0:14:51 > 0:14:54as it does for any reptile.
0:14:54 > 0:14:57So turtles go to a great deal of trouble
0:14:57 > 0:15:03to make sure that they lay their eggs in safe, dry places.
0:15:03 > 0:15:07So you would think that dropping one into water would be a disaster.
0:15:07 > 0:15:10But watch.
0:15:35 > 0:15:38A fully-developed baby turtle.
0:15:41 > 0:15:44And it's come from an egg which as far as we know
0:15:44 > 0:15:46is unique in the reptile world.
0:15:46 > 0:15:49It can not only survive being flooded,
0:15:49 > 0:15:55it actually requires to be submerged in water in order to hatch.
0:15:55 > 0:15:59This enables the pig-nose to make the hatching of its eggs
0:15:59 > 0:16:04coincide with the onset of the rainy season.
0:16:04 > 0:16:09A view inside the egg would show the babies to be fully developed.
0:16:13 > 0:16:17They can remain there in a kind of suspended animation,
0:16:17 > 0:16:19if necessary, for weeks.
0:16:23 > 0:16:26THUNDER RUMBLES
0:16:29 > 0:16:32When the rains finally arrive, they are torrential.
0:16:34 > 0:16:39The river rises swiftly and soon, the nests are flooded.
0:16:51 > 0:16:54This would be a disaster for most turtles,
0:16:54 > 0:16:58but the unhatched pig-noses are ready for it.
0:17:00 > 0:17:02Indeed, its the moment they've been waiting for.
0:17:22 > 0:17:27Their unique waiting strategy ensures that no matter how late the rains are,
0:17:27 > 0:17:31the young turtles only emerge when the rivers are full
0:17:31 > 0:17:33and there's plenty to eat.
0:17:38 > 0:17:41They're able to swim immediately.
0:17:47 > 0:17:48In due course,
0:17:48 > 0:17:53the females among them will return here to lay eggs themselves.
0:17:53 > 0:17:59The males however will never set foot on dry land again.
0:18:01 > 0:18:06No turtles are better suited to life in freshwater than the pig-nose.
0:18:06 > 0:18:08But the most extreme adaptations for swimming
0:18:08 > 0:18:12are found in those turtles that went to sea.
0:18:20 > 0:18:24Marine turtles have altered their front legs really radically
0:18:24 > 0:18:27and turned them into oar-like flippers.
0:18:31 > 0:18:35They're so at home in the sea, they even mate while swimming.
0:18:40 > 0:18:43A male turtle has special hooks on his front flippers
0:18:43 > 0:18:47that enable him to cling on to the female's shell.
0:18:47 > 0:18:50And he has to have a firm grip,
0:18:50 > 0:18:53for she makes no allowance for him as she swims.
0:18:56 > 0:19:03But hanging on to his female is going to get much harder for this male.
0:19:03 > 0:19:05A rival has appeared.
0:19:08 > 0:19:12The male's armour protects most of his body,
0:19:12 > 0:19:14but his rear flippers are exposed
0:19:14 > 0:19:17and they are relatively soft and vulnerable.
0:19:33 > 0:19:36There can be little doubt that this hurts.
0:19:36 > 0:19:39But there's not much that the first male can do about it.
0:19:39 > 0:19:44If he lets go with even a single flipper, he will lose his grip
0:19:44 > 0:19:46and his female.
0:19:52 > 0:19:58The rival tries again and attacks the front flipper.
0:20:11 > 0:20:15And now the male's troubles are about to double.
0:20:17 > 0:20:19A second rival arrives.
0:20:28 > 0:20:33The two challengers join forces and attack the male from both sides.
0:20:43 > 0:20:47His only option is to grin and bear it.
0:20:50 > 0:20:54Now a third hopeful male joins in.
0:20:57 > 0:21:01The female tries to shake them off, but there's no shifting them.
0:21:12 > 0:21:14It's going from bad to worse.
0:21:14 > 0:21:17Even more males gang up on the hapless couple.
0:21:20 > 0:21:25Some of the gang try to force themselves between the mating pair.
0:21:34 > 0:21:37The pair have now been submerged for a long time
0:21:37 > 0:21:42and both of them are in desperate need of a breath.
0:21:42 > 0:21:46If the rivals can prevent the male from reaching the surface,
0:21:46 > 0:21:48he will have no choice but to let go.
0:21:48 > 0:21:50He's in real danger of drowning.
0:22:05 > 0:22:08At last, the determined couple break free
0:22:08 > 0:22:10and make a dash for the surface.
0:22:18 > 0:22:23With a welcome gasp of air, the pair escape.
0:22:35 > 0:22:39One by one, the gang give up.
0:22:50 > 0:22:54It was over 200 million years ago that the first turtles took to the water.
0:22:54 > 0:22:56But they were not alone.
0:22:56 > 0:23:00Another group of reptiles were also making the same move.
0:23:00 > 0:23:03And they, too, were armoured giants.
0:23:12 > 0:23:15Crocodilians, like turtles and tortoises,
0:23:15 > 0:23:19have barely changed since the time of the dinosaurs.
0:23:27 > 0:23:33Today, crocodiles, cayman and alligators live in tropical waters throughout the world.
0:23:36 > 0:23:41Crocodiles and tortoises are obviously very different,
0:23:41 > 0:23:46but they do have one thing in common - armour.
0:23:46 > 0:23:50Their bodies are encased by tough, thick scales,
0:23:50 > 0:23:53particularly along the back.
0:23:53 > 0:23:58In tortoises, that armour is clearly defensive.
0:23:58 > 0:24:01But for crocodiles, it has an extra function.
0:24:02 > 0:24:08Just below each of these scales lies a network of blood vessels.
0:24:08 > 0:24:12A crocodile can control the flow of the blood within them.
0:24:12 > 0:24:15When basking, it allows it to circulate freely,
0:24:15 > 0:24:20so transferring the sun's warmth from these ridged scales
0:24:20 > 0:24:22to the rest of its body.
0:24:22 > 0:24:27The crocodile, in short, has rows of very effective solar panels
0:24:27 > 0:24:29all down its back.
0:24:32 > 0:24:36And a sophisticated solar heating system like that
0:24:36 > 0:24:40is a very valuable facility for a cold-blooded creature.
0:24:47 > 0:24:52Being cold-blooded brings considerable advantages to a crocodile.
0:24:52 > 0:24:56Whereas a warm-blooded predator like a lion would die
0:24:56 > 0:24:58if it didn't feed every few days,
0:24:58 > 0:25:03a crocodile, if necessary, can go without food for months on end.
0:25:03 > 0:25:06And that means that crocodiles can live in places
0:25:06 > 0:25:10where no warm-blooded predator could survive
0:25:10 > 0:25:16and wait for events that only happen two or three times each year.
0:25:16 > 0:25:21And one of those events will occur tonight, right here.
0:25:23 > 0:25:26This road in northern Australia is close to the coast
0:25:26 > 0:25:29and it floods at the highest tides.
0:25:36 > 0:25:40Night falls, and the scene changes dramatically.
0:25:45 > 0:25:49The road is now covered in water...and crocodiles.
0:25:55 > 0:25:59But this is no random gathering.
0:25:59 > 0:26:01The crocodiles are all here for a reason.
0:26:06 > 0:26:13We know that some of them have travelled over 60 miles - 100km - to get here.
0:26:24 > 0:26:27But how they know when to come here, we have little idea.
0:26:40 > 0:26:44There must be some 40 crocodiles
0:26:44 > 0:26:49assembled in the river behind me.
0:26:49 > 0:26:52And what makes this sight all the more remarkable
0:26:52 > 0:26:55is that these are saltwater crocodiles,
0:26:55 > 0:27:00which are normally very territorial and intolerant of one another.
0:27:00 > 0:27:05So there must be something pretty special happening in the river tonight.
0:27:05 > 0:27:08And indeed there is.
0:27:08 > 0:27:09This river is tidal.
0:27:09 > 0:27:14But it's been crossed by a barrage.
0:27:14 > 0:27:18However, at particularly high tides,
0:27:18 > 0:27:21the water flows over the barrage.
0:27:21 > 0:27:26And that is the moment that all these crocodiles are waiting for.
0:27:29 > 0:27:32With the saltwater come fish.
0:27:32 > 0:27:37Mullet have been waiting for weeks to migrate up the river to breed.
0:27:37 > 0:27:41This high tide is their first chance to cross the barrage.
0:27:45 > 0:27:49And the crocodiles are waiting for them.
0:27:56 > 0:28:00Remember, it's now pitch dark.
0:28:00 > 0:28:03Our infra-red cameras give us a clear view,
0:28:03 > 0:28:05but the crocodiles can see virtually nothing.
0:28:05 > 0:28:11So they wait with open jaws, ready to snap them shut at the first touch of a fish.
0:28:29 > 0:28:34Normally, saltwater crocodiles would not tolerate being so close to each other.
0:28:48 > 0:28:52They do compete for the best fishing spots,
0:28:52 > 0:28:55but their disputes are settled with the minimum of fuss.
0:29:11 > 0:29:14The fish keep coming for over an hour.
0:29:14 > 0:29:19But as the tide starts to fall, so their numbers dwindle.
0:29:26 > 0:29:31With so many crocodiles competing, some inevitably go hungry.
0:29:32 > 0:29:39But for those that stay around, there will be a second bite to this particular cherry.
0:29:43 > 0:29:48The next high tide comes during the day, and brings yet more fish.
0:29:56 > 0:30:01The crocodiles can now see the fish, but that doesn't seem to make them any easier to catch.
0:30:21 > 0:30:24These skilful hunters are surely dramatic proof
0:30:24 > 0:30:27that reptiles are certainly not simple-minded creatures.
0:30:27 > 0:30:30They've predicted the time of the arrival of the fish
0:30:30 > 0:30:35with astonishing accuracy, and they have worked out just what they have to do to catch them.
0:30:57 > 0:31:00They have also managed to suppress their normal antagonism
0:31:00 > 0:31:05to one another so that they can all take advantage of this bonanza.
0:31:14 > 0:31:20The more we learn about crocodiles, the more we realise what complex creatures they are.
0:31:33 > 0:31:40Despite what you think, crocodilians are among the most talkative of reptiles and amphibians.
0:31:40 > 0:31:45Indeed, they are second only to the frogs in the variety of noises that they make.
0:31:46 > 0:31:52The most impressive of these sounds come from the American alligator.
0:31:52 > 0:31:58When the breeding season starts, the males begin to proclaim their ownership of territories.
0:32:13 > 0:32:15ALLIGATOR BELLOWS
0:32:17 > 0:32:22The vibrations in his body are so powerful, they make the water dance along his back.
0:32:22 > 0:32:25ALLIGATOR BELLOWS
0:32:32 > 0:32:34ALLIGATOR BELLOWS
0:32:40 > 0:32:42ALLIGATOR BELLOWS
0:32:44 > 0:32:48Sound travels through water even better than it does through air,
0:32:48 > 0:32:53and he can be heard by other alligators hundreds of metres away.
0:32:53 > 0:32:57This is a clear statement of ownership of territory.
0:33:01 > 0:33:03That's fine when he does it lying in his own patch,
0:33:03 > 0:33:08but watch what happens when he bellows close to another male.
0:33:16 > 0:33:19ALLIGATOR BELLOWS
0:33:40 > 0:33:43But the rivals don't come to blows.
0:33:43 > 0:33:47They sort out their differences with gestures.
0:33:50 > 0:33:53Head slaps and gaping jaws are very obvious signals,
0:33:53 > 0:33:58but alligators also send messages in less conspicuous ways.
0:33:58 > 0:34:03Raising their backs slightly above the surface of the water is a significant move.
0:34:03 > 0:34:05It's a claim to dominance.
0:34:09 > 0:34:14Using signals that are almost imperceptible to us, all these individuals are
0:34:14 > 0:34:18sending messages to each other, making claim and counter-claim.
0:34:23 > 0:34:27Communication between alligators can be very subtle,
0:34:27 > 0:34:31quiet, but there are some occasions
0:34:31 > 0:34:36when they really want to make their meaning very unambiguously clear.
0:34:36 > 0:34:40And one of those is when they're guarding their nests.
0:34:40 > 0:34:42As this one is.
0:34:54 > 0:34:56I think that was pretty clear!
0:34:59 > 0:35:01Anyway, I won't press the point.
0:35:08 > 0:35:13Communication between crocodiles starts even before they've hatched.
0:35:19 > 0:35:25A tranquil pool in Argentina and in it, a female broad-snouted cayman.
0:35:27 > 0:35:33She laid her eggs in a pile of vegetation close to the water almost three months ago.
0:35:33 > 0:35:36Now sounds are coming from it.
0:35:37 > 0:35:40The eggs are beginning to hatch.
0:35:43 > 0:35:45Even while the eggs are still
0:35:45 > 0:35:49within the nest, their mother can hear them from some way away.
0:36:10 > 0:36:13Back on the nest, she listens intently.
0:36:24 > 0:36:30Then, very gently, she starts to take it apart.
0:36:35 > 0:36:40She can't know exactly where each of her babies is, and stops every few seconds to listen.
0:36:56 > 0:36:58At last, the young are free.
0:37:00 > 0:37:01But she doesn't abandon them.
0:37:10 > 0:37:14She is going to take them down to the pool that she's selected as their nursery.
0:37:19 > 0:37:23Some babies start to make the journey for themselves.
0:37:23 > 0:37:26But they continue to call, and that helps their mother locate them.
0:37:39 > 0:37:42Cayman jaws are among the strongest in the animal kingdom,
0:37:42 > 0:37:48but now she uses hers with the greatest delicacy and gentleness.
0:38:12 > 0:38:17So at last, her babies are brought together in the nursery pool.
0:38:25 > 0:38:28But their mother's job is still not finished.
0:38:31 > 0:38:33Several of the eggs have failed to hatch.
0:38:41 > 0:38:44One by one, she takes these in her mouth.
0:38:51 > 0:38:54The shell around the egg is quite strong.
0:38:54 > 0:38:57The babies must make a considerable effort to free themselves,
0:38:57 > 0:39:00and for some it's more than they can manage.
0:39:02 > 0:39:07So she starts to crack the unhatched egg with her teeth.
0:39:19 > 0:39:25Once the hard shell has been broken away, she has to pierce the leathery inner membrane.
0:39:46 > 0:39:50Without her help, this baby might not have hatched at all.
0:40:03 > 0:40:09But the female cayman's maternal duties are not over even now.
0:40:09 > 0:40:13She will stand guard over her babies for the next few months.
0:40:15 > 0:40:19All crocodilians take good care of their young.
0:40:19 > 0:40:24But one of them takes parental responsibilities to a really astonishing level.
0:40:28 > 0:40:34The Llanos of Venezuela - a lush wetland teeming with wildlife of all kinds.
0:40:42 > 0:40:48As well as birds, there are amphibious rodents, capybara and spectacled cayman.
0:40:56 > 0:41:02This female is looking after an extraordinarily large number of babies.
0:41:04 > 0:41:07But most of these are not, in fact, hers.
0:41:14 > 0:41:19This nursery pool is being used by several cayman families.
0:41:19 > 0:41:24But instead of all the mothers staying nearby, one of them has taken charge of the whole creche.
0:41:31 > 0:41:35Keeping an eye on all these energetic babies is not easy.
0:41:35 > 0:41:38There are lots of enemies around.
0:41:51 > 0:41:54When danger threatens, the babies all run for protection
0:41:54 > 0:41:57to the female, even though she may not be their mother.
0:42:02 > 0:42:07There are so many of them that there's not enough room for them all on her back.
0:42:16 > 0:42:23Soon these babies will face another hazard, one that's not quite so easy to escape from.
0:42:26 > 0:42:30The water that has kept them safe until now is beginning to dry up.
0:42:31 > 0:42:34Each year, in the space of a few short months,
0:42:34 > 0:42:40the Llanos is transformed from a flooded paradise to a baking oven.
0:42:43 > 0:42:48For some babies, the unrelenting heat and the lack of water has already been too much.
0:42:59 > 0:43:04Any babies that are left alive will certainly die if they stay here.
0:43:06 > 0:43:10So the mother decides to leave.
0:43:28 > 0:43:34Calling to her creche, she sets off across the parched land.
0:43:34 > 0:43:40In one long cavalcade, they march onwards in search of permanent water.
0:43:58 > 0:44:01For the mother, this is certainly exhausting.
0:44:01 > 0:44:07For her babies, with their tiny legs, it must be a real marathon.
0:44:16 > 0:44:19Some start to fall behind.
0:44:20 > 0:44:21But she stops.
0:44:26 > 0:44:29The babies call constantly.
0:44:29 > 0:44:34She knows exactly where they all are and waits until every single one of them has caught up.
0:44:56 > 0:45:00Only when all are with her will she set off again.
0:45:21 > 0:45:24At last - safety.
0:45:35 > 0:45:37The babies are close to exhaustion.
0:45:51 > 0:45:58Without such devotion from the female, few, if any, of these baby cayman would have survived.
0:45:58 > 0:46:02And remarkably, most of them aren't even her own.
0:46:12 > 0:46:15When parental care was first described in crocodiles,
0:46:15 > 0:46:19the reports were dismissed as too extraordinary to be true.
0:46:24 > 0:46:29We may call reptiles cold-blooded, but they can show great tenderness.
0:46:33 > 0:46:37Reptiles and amphibians are full of surprises.
0:46:56 > 0:47:00They can look after their young with as much care as many a mammal.
0:47:08 > 0:47:12Their displays can be as colourful as that of any bird.
0:47:16 > 0:47:19And they can astonish and enthral us.
0:47:33 > 0:47:39Reptiles and amphibians are sometimes seen as simple primitive creatures.
0:47:39 > 0:47:42That's a long way from the truth.
0:47:42 > 0:47:44The fact that they are solar-powered
0:47:44 > 0:47:51means that their bodies require only 10% of the energy that mammals of a similar size require.
0:47:52 > 0:47:57At a time when we ourselves are becoming increasingly concerned
0:47:57 > 0:48:01about the way in which we get our energy from the environment
0:48:01 > 0:48:03and the wasteful way in which we use it,
0:48:03 > 0:48:09maybe there are things that we can learn from life in cold blood.
0:48:31 > 0:48:36We filmed a lot of different reptiles and amphibians during the making of this series.
0:48:45 > 0:48:51We were looking for extraordinary behaviour, preferably for things that had never been filmed before.
0:49:00 > 0:49:05To see such wonders, we needed the help of scientists who were working in the field.
0:49:11 > 0:49:15They passed onto us their insights and their discoveries,
0:49:15 > 0:49:21and then they helped us to interpret the footage that we'd shot. I learnt a lot and had a lot of fun.
0:49:22 > 0:49:28But I was also alarmed to discover just how rare some of the subjects of our series have now become.
0:49:32 > 0:49:35One of our key locations was the Galapagos Islands.
0:49:35 > 0:49:39Here, giant tortoises were going to be among our stars.
0:49:39 > 0:49:44The scientists working for the Galapagos National Parks care for the wild animal populations,
0:49:44 > 0:49:49but they also look after one extraordinary, unique individual,
0:49:49 > 0:49:52with whom I had a special appointment.
0:49:52 > 0:49:57This is the rarest living animal in all the world.
0:49:57 > 0:50:00There is none rarer.
0:50:00 > 0:50:02This is Lonesome George.
0:50:04 > 0:50:09He's about the same ages as I am, but his story starts a very long time ago.
0:50:11 > 0:50:16In the 17th century, when human beings first came to the Galapagos,
0:50:16 > 0:50:21there were about 15 different kinds of giant tortoise,
0:50:21 > 0:50:26each living on its own island or its own great volcano,
0:50:26 > 0:50:29isolated by impassable lava flows.
0:50:32 > 0:50:35There are 13 large islands in the Galapagos and many smaller ones,
0:50:35 > 0:50:39and they differ in both age and their vegetation.
0:50:40 > 0:50:43The tortoises differ too, because their shells have evolved into
0:50:43 > 0:50:49the different shapes best suited for eating the food available on their own particular island.
0:50:49 > 0:50:52On islands where there's abundant food on the ground,
0:50:52 > 0:50:57the tortoises have dome-shaped shells and short necks that only need reach downwards.
0:50:59 > 0:51:04But on islands where tortoises browse on higher bushes, their necks are longer and the shells
0:51:04 > 0:51:09are saddle-shaped at the front, so they can stretch their necks upwards.
0:51:10 > 0:51:14When the first ships arrived here, there were thousands of each kind of tortoise.
0:51:17 > 0:51:21But then people began to slaughter the tortoises for meat.
0:51:23 > 0:51:27They discovered the remarkable fact that these creatures could live
0:51:27 > 0:51:31for a year without water or food,
0:51:31 > 0:51:36so they took them on board their ships and slaughtered them at sea.
0:51:36 > 0:51:41The tortoises on Pinta Island were apparently exterminated.
0:51:43 > 0:51:46But then, in 1971,
0:51:46 > 0:51:53it was discovered that there was one lonely, single survivor.
0:51:53 > 0:51:57That was Lonesome George.
0:51:59 > 0:52:01This film was taken over 30 years ago by the team
0:52:01 > 0:52:05that brought George back to the Charles Darwin Research Station.
0:52:05 > 0:52:08The scientists hoped that another Pinta tortoise might be discovered
0:52:08 > 0:52:14in some corner of this island or even in a zoo somewhere in the world, but none has ever been found.
0:52:19 > 0:52:25So now George lives in his own enclosure, completely safe but entirely by himself.
0:52:25 > 0:52:27He's the last of his kind.
0:52:28 > 0:52:32It's better news for the other Galapagos tortoises.
0:52:32 > 0:52:37Felipe Cruz from the Research Station showed me some of the work being done there.
0:52:37 > 0:52:42They take eggs laid by wild tortoises and put them in incubators.
0:52:42 > 0:52:48The hatchlings are about the size of apples and have soft shells, so are vulnerable to predators,
0:52:48 > 0:52:52especially rats that were accidentally introduced to the Galapagos.
0:52:55 > 0:52:57The young ones I saw were only a few months old.
0:52:58 > 0:53:02They're kept in special enclosures and given all the foods
0:53:02 > 0:53:06they need to enable them to develop hard protective shells.
0:53:07 > 0:53:11It takes a few months for their shells to harden,
0:53:11 > 0:53:14and it's five years before they're totally predator-proof.
0:53:20 > 0:53:26So far, in total, we have repatriated over 3,000 tortoises.
0:53:26 > 0:53:273,000?!
0:53:29 > 0:53:33Scientists are also helping to solve another man-made problem.
0:53:35 > 0:53:41Domestic goats that have run wild are eating the tortoises' food and destroying the precious plant cover
0:53:41 > 0:53:46that they use for shade. So a systematic programme of eradication has started.
0:53:55 > 0:53:58I was able to see the effects of this programme for myself.
0:53:58 > 0:54:00We visited one island where two years earlier
0:54:00 > 0:54:07the goats had been eliminated, and the difference was dramatic. The lush vegetation had returned.
0:54:09 > 0:54:13Now the tortoises can find the shade that is so important for them,
0:54:13 > 0:54:15and there's plenty of grass for them to eat.
0:54:23 > 0:54:26Reptiles are not alone in being under threat.
0:54:26 > 0:54:31The amphibians if anything are in even greater danger.
0:54:31 > 0:54:35Not since the disappearance of the dinosaurs has a whole group
0:54:35 > 0:54:38of the animal kingdom been under such threat.
0:54:38 > 0:54:45In Japan, one of the most dramatic amphibians, the giant salamander, has fewer and fewer places to live.
0:54:51 > 0:54:54In Panama, we filmed the golden frog.
0:54:54 > 0:54:57Since we took this shot, the species has become so rare
0:54:57 > 0:55:04that the few survivors have been caught to be protected in zoos, so it's now extinct in the wild.
0:55:06 > 0:55:09The gharials that we filmed with their babies were nearly
0:55:09 > 0:55:15exterminated in the 1970s when they lost most of their natural habitat, and they're not safe yet.
0:55:16 > 0:55:22To try and halt their decline, their eggs are being collected, hatched in incubators
0:55:22 > 0:55:27and the babies reared in captivity until they can be released in the wild.
0:55:27 > 0:55:34So there may be hope for them yet. The gopher tortoise we filmed in Florida is also in trouble.
0:55:34 > 0:55:38The areas where it digs its burrows have become prime real estate
0:55:38 > 0:55:40and are now much sought after for building and farming.
0:55:43 > 0:55:46By explaining their problems to landowners and developers,
0:55:46 > 0:55:48they may yet have a future.
0:55:54 > 0:55:57In the great island of Madagascar, there are more species of chameleon
0:55:57 > 0:56:00than in all the rest of the world put together.
0:56:02 > 0:56:09But the destruction of the island's forests began centuries ago and only a few patches are left.
0:56:09 > 0:56:13They too are still being felled, and chameleon species may be lost
0:56:13 > 0:56:15even before they've been identified.
0:56:21 > 0:56:24We will need to act now if we're not to lose what remains to us
0:56:24 > 0:56:29of the reptiles and amphibians that have survived for 200 million years.
0:56:30 > 0:56:36Lonesome George, it seems, is doomed to be last of his kind.
0:56:37 > 0:56:44But at least he can be a living inspiration for us all to protect the remainder
0:56:44 > 0:56:48of the reptiles and amphibians of the world.