0:00:35 > 0:00:43Reptiles and amphibians are sometimes thought of as primitive, dull and dim-witted.
0:00:43 > 0:00:46In fact, of course, they can be lethally fast,
0:00:46 > 0:00:48spectacularly beautiful,
0:00:48 > 0:00:53surprisingly affectionate and very sophisticated.
0:00:59 > 0:01:02They have remarkably varied ways of catching their prey...
0:01:05 > 0:01:06and of defending themselves.
0:01:09 > 0:01:12They can produce a great turn of speed
0:01:12 > 0:01:17and fight with impressive zest.
0:01:27 > 0:01:33Some have spectacular colours and show off to one another.
0:01:35 > 0:01:37They communicate with calls.
0:01:37 > 0:01:40CROAKING
0:01:43 > 0:01:45And with gestures.
0:01:49 > 0:01:50And there! That's it!
0:01:52 > 0:01:55The full works!
0:01:55 > 0:01:59Reptiles have scaly skins
0:01:59 > 0:02:02and amphibians, soft moist ones.
0:02:06 > 0:02:08None of them live at a uniform pace,
0:02:08 > 0:02:13but switch from the fast to the slow lane within a year or an hour.
0:02:15 > 0:02:21Unlike us, they get their energy directly from the sun.
0:02:23 > 0:02:26And although being called "cold-blooded"
0:02:26 > 0:02:28might suggest they are unemotional,
0:02:28 > 0:02:32they can be touchingly warm-hearted as mates...
0:02:37 > 0:02:38..and as parents.
0:02:48 > 0:02:52And that's just the beginning.
0:02:52 > 0:02:57There are a whole lot of other warm-hearted truths to be discovered
0:02:57 > 0:03:01to give the phrase "Life in Cold Blood" a completely new meaning.
0:03:13 > 0:03:15The Galapagos Islands.
0:03:15 > 0:03:19Some of the reptiles that live here are particularly skilful
0:03:19 > 0:03:23at solving the problems of getting their energy directly from sunshine.
0:03:25 > 0:03:29Marine iguanas face a major thermal challenge
0:03:29 > 0:03:32every morning of their lives.
0:03:32 > 0:03:35During the night, their bodies cool.
0:03:35 > 0:03:38And now they must warm up quickly
0:03:38 > 0:03:42in order that they can become active and start feeding.
0:03:42 > 0:03:45Their bodies and skins are black,
0:03:45 > 0:03:48which is very efficient at absorbing heat
0:03:48 > 0:03:53and they bask with their black flanks broadside to the sun.
0:03:59 > 0:04:04The rate at which they absorb warmth is invisible to the naked eye,
0:04:04 > 0:04:08but very clear indeed to a thermal camera.
0:04:08 > 0:04:12First thing, they're cold and purplish blue.
0:04:12 > 0:04:14But slowly, as they warm up,
0:04:14 > 0:04:17a golden glow spreads through their bodies.
0:04:17 > 0:04:21And eventually, after half an hour or so,
0:04:21 > 0:04:24they become as hot as the rocks beneath them.
0:04:24 > 0:04:27Once they are thoroughly warmed up,
0:04:27 > 0:04:30marine iguanas can maintain their body temperature
0:04:30 > 0:04:33just about as constantly as I can, and what's more,
0:04:33 > 0:04:37at about the same level or indeed slightly higher,
0:04:37 > 0:04:39around 37 degrees Centigrade.
0:04:41 > 0:04:43Now they need to feed.
0:04:43 > 0:04:48There's nothing to eat on or around these barren rocks except seaweed,
0:04:48 > 0:04:50and to get that, they'll have to swim.
0:04:53 > 0:04:56But the sea around here is surprisingly cold,
0:04:56 > 0:04:59around 15 to 16 degrees C entigrade,
0:04:59 > 0:05:04and only the bigger iguanas can absorb enough heat
0:05:04 > 0:05:10to power the dives to enable them to go to the seaweed at any depth.
0:05:13 > 0:05:17However, their bodies are now thorough warmed up.
0:05:17 > 0:05:20The thermal camera shows them as golden yellow
0:05:20 > 0:05:22as they clamber down
0:05:22 > 0:05:26over the cold blue rocks and dive into the sea.
0:05:43 > 0:05:47Although their islands lie almost exactly on the equator,
0:05:47 > 0:05:50the sea here is permanently chilled
0:05:50 > 0:05:54by a cold current that sweeps up from the depths of the ocean.
0:05:54 > 0:05:57So they won't be able to stay in the water for very long.
0:05:57 > 0:06:00They have no time to waste.
0:06:01 > 0:06:03In the shallows close to the shore,
0:06:03 > 0:06:06the seaweed has been heavily cropped.
0:06:06 > 0:06:07To get a good meal,
0:06:07 > 0:06:11they may have to dive to at least 15 feet - five metres.
0:06:14 > 0:06:18They're able to reduce the chilling effect of the cold water by closing
0:06:18 > 0:06:22down the blood supply to their limbs and the outer part of their bodies.
0:06:22 > 0:06:28But even so, their body temperature may drop by ten degrees or so.
0:06:28 > 0:06:31A cooling like that would kill a human diver.
0:06:35 > 0:06:40After five to ten minutes on the sea floor, most iguanas have had enough
0:06:40 > 0:06:41and they return to the surface
0:06:41 > 0:06:44and the life-saving warmth of the rocky shore.
0:06:50 > 0:06:54A recently emerged iguana is black - it's chilled to the bone.
0:07:00 > 0:07:06Now they need heat in order to be able to digest that meal of seaweed
0:07:06 > 0:07:09and they get that by spread-eagling themselves
0:07:09 > 0:07:13on these black, hot, sun-baked rocks.
0:07:16 > 0:07:18Their image warms from black...
0:07:18 > 0:07:21to purple...
0:07:23 > 0:07:25and then from red to orange.
0:07:29 > 0:07:31In the evening, the temperature falls,
0:07:31 > 0:07:35and they huddle together to retain their warmth as long as possible.
0:07:40 > 0:07:44They will have to wait until the following morning
0:07:44 > 0:07:47before they can re-warm themselves sufficiently to feed again.
0:07:51 > 0:07:54Most kinds of lizards have this daily schedule.
0:07:54 > 0:07:58Side-blotched lizards in California certainly do.
0:07:58 > 0:08:03You can see from the colour of my face that my body is warm.
0:08:03 > 0:08:06That's because I've got a central heating system
0:08:06 > 0:08:08which I have fuelled with my breakfast.
0:08:08 > 0:08:10In fact about 80% of what I eat
0:08:10 > 0:08:15is used in keeping my body temperature high and steady.
0:08:15 > 0:08:18These lizards, however, squander very little of the energy
0:08:18 > 0:08:22of the energy they get from their food on warming themselves.
0:08:22 > 0:08:24They, like the marine iguanas,
0:08:24 > 0:08:29get nearly all they need for that by basking on the warm rocks.
0:08:29 > 0:08:33And so important is the need for warmth,
0:08:33 > 0:08:37that the females actually choose their males
0:08:37 > 0:08:41on the basis of which has the best underfloor heating.
0:08:43 > 0:08:47Each male sits on his pile of boulders doing press-ups
0:08:47 > 0:08:50to signal his ownership and to warn off other males.
0:08:52 > 0:08:57Intruders are confronted immediately and, if necessary, attacked.
0:09:08 > 0:09:14And the victor returns to sit on his wonderfully warm throne.
0:09:20 > 0:09:23Look at his rocky kingdom with a thermal camera
0:09:23 > 0:09:26and it's immediately clear why it's so precious.
0:09:26 > 0:09:30The rocks are very much hotter than the surrounding grassland
0:09:30 > 0:09:34and big, tall ones catch the sun earlier and retain its heat longer.
0:09:34 > 0:09:38So not only does the sun warm him from above,
0:09:38 > 0:09:41his rocks do from beneath.
0:09:41 > 0:09:46The most powerful dominant male has, naturally, the best pile of rocks.
0:09:46 > 0:09:50And, not surprisingly, almost all the females.
0:09:57 > 0:09:59But is it the males themselves
0:09:59 > 0:10:02or their assets that the females are interested in?
0:10:02 > 0:10:06To find out, let's move their hot rocks
0:10:06 > 0:10:09and give them to a subordinate male.
0:10:22 > 0:10:26The females quickly recognise that a more desirable residence
0:10:26 > 0:10:31has appeared in the neighbourhood, and start to move across.
0:10:34 > 0:10:41And the sex-starved wimp suddenly finds himself amazingly popular!
0:10:43 > 0:10:49So, the females do indeed go for the males with the hottest rocks.
0:10:53 > 0:10:54These lizards on a small islet
0:10:54 > 0:10:58off the shores of Menorca in the Mediterranean
0:10:58 > 0:11:01get their heat from another, and very unusual, source.
0:11:03 > 0:11:04Ow!
0:11:06 > 0:11:09Sorry!
0:11:09 > 0:11:13They're very curious. I'm the new boy on the block.
0:11:13 > 0:11:16The new object in their environment.
0:11:16 > 0:11:19And that one just gave me a little nip.
0:11:22 > 0:11:25They investigate the world around them by tasting it,
0:11:25 > 0:11:27and they are still trying to work out what I am.
0:11:33 > 0:11:36Their island is rocky and not particularly rich in food.
0:11:36 > 0:11:38The lizards are primarily insect-eaters,
0:11:38 > 0:11:42but during the flowering season, they also take nectar.
0:11:42 > 0:11:46They collect it from plants, like spurge, which is very common.
0:11:46 > 0:11:51And they have a very special relationship with this flower.
0:11:51 > 0:11:54It's called the dead horse arum.
0:11:54 > 0:11:56It certainly looks like carrion.
0:11:56 > 0:11:58And...
0:11:58 > 0:12:02Ooh, dear me, it smells very strongly of carrion.
0:12:02 > 0:12:06As a consequence of both its looks and its smell,
0:12:06 > 0:12:08it attracts carrion flies.
0:12:08 > 0:12:11Of course, it's the flies that the lizards are after.
0:12:13 > 0:12:17But as well as providing food for the lizards,
0:12:17 > 0:12:21this extraordinary flower helps them in another way.
0:12:21 > 0:12:24This central part, which is called a spadix,
0:12:24 > 0:12:30is slightly warm, as you can see from a thermal camera.
0:12:32 > 0:12:35The chemical process that produces the disgusting smell
0:12:35 > 0:12:39also creates heat and raises the temperature of the flower
0:12:39 > 0:12:42by up to five degrees above the surroundings,
0:12:42 > 0:12:47sufficiently high for a lizard to warm itself on it on a cold morning.
0:12:47 > 0:12:49And in case you find that hard to believe,
0:12:49 > 0:12:52here is confirmation from the thermal camera.
0:12:52 > 0:12:55The purplish-blue lizard quickly takes on
0:12:55 > 0:12:58the same temperature and colour as the arum.
0:13:04 > 0:13:08And sitting on arums brings another benefit...
0:13:11 > 0:13:13Breakfast.
0:13:13 > 0:13:16A fly, lured by the smell, crawls inside.
0:13:20 > 0:13:23The lizard hears the fly buzzing within.
0:13:25 > 0:13:27The fly, of course,
0:13:27 > 0:13:29can't find anything it wants, but now it can't get out.
0:13:29 > 0:13:32The entrance to the flower is blocked by the lizard.
0:13:44 > 0:13:46And the lizard gets an easy meal.
0:13:53 > 0:13:55Two months later,
0:13:55 > 0:13:59the arum flowers have shrivelled and produced their fruits.
0:14:02 > 0:14:06Until 20 years ago, the lizards ignored these withered bundles.
0:14:06 > 0:14:09After all, they hardly look like food.
0:14:09 > 0:14:12But then a particularly inquisitive individual
0:14:12 > 0:14:17sampled a fruit and found the soft flesh around the seed rather good.
0:14:17 > 0:14:21The habit spread and now the whole lizard population,
0:14:21 > 0:14:25uniquely in the Mediterranean, have become arum fruit-eaters.
0:14:28 > 0:14:30They do take a bit of swallowing,
0:14:30 > 0:14:34but seeds passing through a lizard's gut not only survive
0:14:34 > 0:14:38but germinate even more easily.
0:14:38 > 0:14:42As a result, the arums, which were rather scarce here 20 years ago,
0:14:42 > 0:14:45have suddenly become abundant all over the island.
0:15:03 > 0:15:07A cold, windswept island off the coast of South Africa
0:15:07 > 0:15:12is not the first place you would go to if you were looking for reptiles.
0:15:12 > 0:15:16But here on Dassen Island, among penguins and seagulls,
0:15:16 > 0:15:19there's one of the greatest concentration of tortoises
0:15:19 > 0:15:21to be found anywhere on Earth.
0:15:24 > 0:15:28There are about 5,000 of them on this one tiny island.
0:15:33 > 0:15:36The penguins and other birds, thanks to their warm blood,
0:15:36 > 0:15:39are active, no matter how cold it is,
0:15:39 > 0:15:41but the tortoises have to wait for the day to warm up
0:15:41 > 0:15:43before they can get about their business.
0:15:49 > 0:15:53They bask in the sunshine, powering up their bodies
0:15:53 > 0:15:58to the optimum working temperature of 33 degrees Centigrade,
0:15:58 > 0:15:59and then they go off to feed.
0:16:15 > 0:16:18As the day progresses, the temperature rises quickly
0:16:18 > 0:16:22and even before noon it's too hot for comfort.
0:16:22 > 0:16:25The tortoises have to head for shade.
0:16:40 > 0:16:42In the late afternoon, it gets cooler
0:16:42 > 0:16:45and the tortoises venture out again.
0:16:45 > 0:16:47For them, this is the best time.
0:16:47 > 0:16:49They're thoroughly warmed up,
0:16:49 > 0:16:54they've digested their morning meal and they've got energy to spare.
0:17:01 > 0:17:04The males begin to fight,
0:17:04 > 0:17:07jousting like medieval knights,
0:17:07 > 0:17:11using a projection on the front of the shell like a lance.
0:17:16 > 0:17:19The technique is to get the spike under your opponent
0:17:19 > 0:17:22and then flick him over on to his back.
0:17:27 > 0:17:29Contests can last for half an hour.
0:18:05 > 0:18:07The loser tries to right himself,
0:18:07 > 0:18:10but the winner keeps biting his legs.
0:18:23 > 0:18:27At last, the victor loses interest and goes off to find the female
0:18:27 > 0:18:30who caused the argument in the first place.
0:18:30 > 0:18:35As for the loser, if he doesn't manage to right himself soon,
0:18:35 > 0:18:36he may cook in the sun.
0:18:45 > 0:18:48Tortoises are able to sunbathe out in the open,
0:18:48 > 0:18:49because their strong bony shell
0:18:49 > 0:18:53gives them almost complete protection from predators.
0:18:57 > 0:19:02Less well-armoured reptiles like lizards are vulnerable, of course,
0:19:02 > 0:19:07to hawks and coyotes and foxes and cats.
0:19:07 > 0:19:10And in the morning, when those warm-blooded animals
0:19:10 > 0:19:12are already active,
0:19:12 > 0:19:17the lizards are cold and can't move fast. So they have a problem.
0:19:17 > 0:19:22But they also have a solution...secret sunbathing.
0:19:24 > 0:19:27You really can't see them until you are right on top of them.
0:19:30 > 0:19:32And there's one there.
0:19:34 > 0:19:35I'm in Arizona,
0:19:35 > 0:19:41and that at my feet is a lizard buried in the sand up to its neck.
0:19:42 > 0:19:47Even while it's buried, it can use the sunshine to warm its whole body.
0:19:47 > 0:19:50It can control the supply of blood to its head
0:19:50 > 0:19:53so that it pools in a cavity behind the eye.
0:19:54 > 0:19:57Soon, the blood there is as much as five degrees
0:19:57 > 0:20:00above the temperature of the rest of its body.
0:20:00 > 0:20:04Then the animal opens the major blood vessels in its neck
0:20:04 > 0:20:06and the hot blood circulates
0:20:06 > 0:20:08so that its whole body is thoroughly warmed,
0:20:08 > 0:20:11even though it's still mostly below ground.
0:20:18 > 0:20:20This...
0:20:20 > 0:20:23is a horned lizard...
0:20:23 > 0:20:26and very beautiful, too.
0:20:26 > 0:20:30This particular species is called the regal horned lizard
0:20:30 > 0:20:36because it has this splendid crown of spikes at the back of his neck.
0:20:36 > 0:20:40When he's hidden, they break up the outline of his head
0:20:40 > 0:20:42and so you hardly see him at all.
0:20:42 > 0:20:45And now in the warmth of my hand and in the sunshine,
0:20:45 > 0:20:48I guess he has warmed up quite a lot and if I put him down,
0:20:48 > 0:20:53he now at last may be able to run for it.
0:20:53 > 0:20:56And indeed he does.
0:21:04 > 0:21:09South African armadillo lizards, which live on these rocky outcrops,
0:21:09 > 0:21:12have a different solution to the problem of safe sunbathing.
0:21:13 > 0:21:17They've turned it into a social activity.
0:21:17 > 0:21:22Whole families of them live together in the crevices among the rocks
0:21:22 > 0:21:27and in the morning, they all emerge to warm up in the sun.
0:21:28 > 0:21:31Of course, there is safety in numbers.
0:21:34 > 0:21:37There are lots of eyes to spot danger if it appears.
0:21:49 > 0:21:54And when one sunbather takes fright, they all dive for safety.
0:21:56 > 0:21:58If a predator is quick, it IS possible to grab one.
0:21:58 > 0:22:02But even then, an armadillo lizard is not going to be an easy meal.
0:22:02 > 0:22:03Ow!
0:22:04 > 0:22:08They have an additional form of defence.
0:22:08 > 0:22:11They bite their tails.
0:22:11 > 0:22:14The reason they do that is that it covers up
0:22:14 > 0:22:18their vulnerable underside
0:22:18 > 0:22:23and exposes only these very sharp spiny scales,
0:22:23 > 0:22:25which is a very good protection
0:22:25 > 0:22:29against predators like snakes or mongooses.
0:22:30 > 0:22:33They stay like this for quite a long time
0:22:33 > 0:22:36before they are confident enough to uncurl.
0:22:36 > 0:22:39I'll put him down and see how he does.
0:22:52 > 0:22:58Sunset, necessarily, brings an end to activity for most reptiles.
0:23:00 > 0:23:03But not for all.
0:23:04 > 0:23:05A leopard gecko.
0:23:05 > 0:23:07It, like most geckos, is nocturnal
0:23:07 > 0:23:10and it manages to get all the heat it needs from the rocks
0:23:10 > 0:23:12which retain something of their warmth
0:23:12 > 0:23:15for several hours after the sun has set.
0:23:17 > 0:23:20This male is in search of a mate.
0:23:20 > 0:23:23She is less brightly coloured.
0:23:26 > 0:23:30They inspect one another.
0:23:33 > 0:23:36He collects her scent with his tongue
0:23:36 > 0:23:40and discovers that not only is she female but she's sexually available.
0:23:47 > 0:23:50He's interested.
0:23:54 > 0:23:56He nibbles her neck and strokes her flanks,
0:23:56 > 0:23:59all part of his elaborate courtship routine.
0:24:07 > 0:24:10Copulation begins.
0:24:12 > 0:24:15This is the time in mammals and birds
0:24:15 > 0:24:17when the sex of the young is determined.
0:24:17 > 0:24:21But not in a number of reptiles, including geckos.
0:24:21 > 0:24:26Once again, it's temperature that profoundly influences their lives.
0:24:28 > 0:24:32The female goes away to lay her eggs.
0:24:33 > 0:24:37She has chosen a place where the temperature is about 31 degrees.
0:24:37 > 0:24:41As her body is the same temperature as her environment,
0:24:41 > 0:24:45she can't heat her eggs by sitting on them as warm-blooded birds do,
0:24:45 > 0:24:49so they're exactly the same temperature as the rocks beneath.
0:24:52 > 0:24:55After a couple of months, both eggs begin to hatch.
0:24:59 > 0:25:02The first to emerge is a male.
0:25:10 > 0:25:13And the second will be too.
0:25:13 > 0:25:16It's the temperature which has determined that.
0:25:16 > 0:25:18If it had been a few degrees lower,
0:25:18 > 0:25:22both eggs would have developed into females.
0:25:30 > 0:25:35Crocodiles have their sex determined by temperature in a similar way.
0:25:35 > 0:25:41This clutch belongs to the Indian fish-eating crocodile, the gharial.
0:25:48 > 0:25:53LOW SQUEAKING
0:25:53 > 0:25:56The female has heard the calls from below ground
0:25:56 > 0:25:59made by her hatching young
0:25:59 > 0:26:02and is helping them to dig their way out of the sand.
0:26:12 > 0:26:15They immediately make their way down to the water.
0:26:17 > 0:26:19And mother goes too.
0:26:34 > 0:26:38Here, of course, they are nice and warm.
0:26:38 > 0:26:42Water retains its daytime heat better and longer than rock,
0:26:42 > 0:26:46so unlike many other reptiles, gharials and other crocodilians
0:26:46 > 0:26:49have enough energy to feed actively all night.
0:27:18 > 0:27:23While being nocturnal is unusual among reptiles,
0:27:23 > 0:27:26it's the norm for amphibians.
0:27:26 > 0:27:29Their skin is not scaly and watertight like a reptile's.
0:27:29 > 0:27:31It's soft, moist and permeable.
0:27:31 > 0:27:36If they exposed themselves to sunlight for any length of time,
0:27:36 > 0:27:38they would dry out and die.
0:27:38 > 0:27:42So most frogs only leave their shelters at night.
0:27:46 > 0:27:49Since they can't absorb sunshine directly,
0:27:49 > 0:27:52they either get heat from their surroundings
0:27:52 > 0:27:54or draw their energy from the fat reserves
0:27:54 > 0:27:57that they built up when the feeding was good.
0:27:57 > 0:28:01But even so, they seldom hop unless they have very good reason to do so.
0:28:04 > 0:28:09This frog, however, the South American waxy monkey frog,
0:28:09 > 0:28:10is exceptional.
0:28:10 > 0:28:13It's one of the few that can tolerate
0:28:13 > 0:28:15direct sunshine for any length of time.
0:28:17 > 0:28:23And that is because it secretes a wax from glands on its neck.
0:28:36 > 0:28:40No human sunbather goes to more trouble than they do
0:28:40 > 0:28:45to make quite sure that every part of their skin is properly anointed.
0:29:10 > 0:29:14The sunshine may also bring them an extra benefit.
0:29:14 > 0:29:17It probably protects them from the fungal infections
0:29:17 > 0:29:21to which many moist-skinned amphibians are prone.
0:29:28 > 0:29:31In the rainforests of Central America,
0:29:31 > 0:29:33the air is heavy with moisture.
0:29:33 > 0:29:35So the poison arrow frogs
0:29:35 > 0:29:38can risk basking in the little patches of sunshine
0:29:38 > 0:29:41that dapple the forest floor and if they begin to dry out,
0:29:41 > 0:29:44they can retreat into the leaf-litter.
0:29:45 > 0:29:47RAPID CHIRRUPING
0:29:47 > 0:29:51The sunshine gives them sufficient energy to permit the extravagance of
0:29:51 > 0:29:57calling almost continuously in defence of their territories.
0:30:00 > 0:30:02They even have enough spare energy
0:30:02 > 0:30:06to indulge in long battles with their neighbours.
0:30:52 > 0:30:56These fights can go on for well over half an hour at a time
0:30:56 > 0:31:00until both contestants are completely exhausted.
0:31:07 > 0:31:12So a moist skin limits not only where amphibians can live,
0:31:12 > 0:31:15but how energetic they can be.
0:31:15 > 0:31:21Out in the sunshine, dry-skinned reptiles have more options.
0:31:21 > 0:31:25By collecting solar power so efficiently,
0:31:25 > 0:31:30reptiles need to use very little of the energy they generate themselves
0:31:30 > 0:31:31to warm their bodies.
0:31:31 > 0:31:33In fact, they use around a tenth
0:31:33 > 0:31:36compared with a mammal of a similar size.
0:31:36 > 0:31:40That means they don't have to eat very often.
0:31:40 > 0:31:42A puff adder like this one
0:31:42 > 0:31:46can wait almost indefinitely for its next meal.
0:31:46 > 0:31:51Amongst predators, patience really is a virtue.
0:31:55 > 0:31:59Whilst waiting for a meal to wander within striking distance,
0:31:59 > 0:32:03a snake shuts down its body processes
0:32:03 > 0:32:06so that it uses the minimum amount of energy.
0:32:06 > 0:32:09Only the equivalent of a pilot light is left on.
0:32:09 > 0:32:12And it can remain like this for weeks.
0:32:15 > 0:32:19All around it, mammals are expending their energy in a way that,
0:32:19 > 0:32:23compared with the snake, seems extraordinarily extravagant.
0:32:37 > 0:32:42But when a snake needs to move fast, it can do so with lightning speed.
0:32:52 > 0:32:57Once its prey is secured, a snake can take its time over its meal.
0:32:57 > 0:33:03This gigantic python is feeding on a deer.
0:33:03 > 0:33:07A python kills its prey by wrapping its coils around it and squeezing
0:33:07 > 0:33:13its victim so tightly and for long that it can no longer breathe.
0:33:13 > 0:33:16But swallowing its meal takes time.
0:33:16 > 0:33:20The deer will go down head first.
0:33:20 > 0:33:22It's much easier that way.
0:33:31 > 0:33:35The ligaments connecting the snake's upper and lower jaw are elastic
0:33:35 > 0:33:38so that it can engulf the deer's head,
0:33:38 > 0:33:42even though it is much bigger than its own.
0:33:42 > 0:33:45With its mouth stretched tightly around its meal,
0:33:45 > 0:33:48the snake can't breathe in a normal way.
0:33:48 > 0:33:52But it's able to push the top of its windpipe right out of its mouth
0:33:52 > 0:33:55and so continue to take in air.
0:34:15 > 0:34:21After some hours, all that can be seen of the deer are its hind legs.
0:34:32 > 0:34:35Once the meal has been completely swallowed,
0:34:35 > 0:34:38the inner workings of the snake's body change greatly.
0:34:40 > 0:34:44Its digestive processes switch to full power
0:34:44 > 0:34:47and increase their activity 40 times.
0:34:47 > 0:34:51There is an explosion of biochemical activity.
0:34:51 > 0:34:55The liver, the secretions of which power digestion,
0:34:55 > 0:34:57doubles in size within two days.
0:34:59 > 0:35:02The heart grows by some 40%.
0:35:05 > 0:35:11It will take the python at least a week to completely digest this enormous meal.
0:35:11 > 0:35:16But then it will not need to feed again for months, or even a year.
0:35:19 > 0:35:21This ability to switch off
0:35:21 > 0:35:24helps reptiles and amphibians in another way.
0:35:26 > 0:35:30A baby North American painted turtle.
0:35:30 > 0:35:34It and the rest of its clutch have only just hatched.
0:35:34 > 0:35:39But it's late in the year, and the chill of winter has already begun.
0:35:40 > 0:35:43If the hatchlings clambered out of their hole now,
0:35:43 > 0:35:46they would find nothing to eat, so they stay where they are.
0:35:53 > 0:35:57The temperature will fall to -10 degrees.
0:35:59 > 0:36:04Ice crystals grow around the babies and even inside their bodies.
0:36:04 > 0:36:08But their tissues are protected by a kind of antifreeze.
0:36:12 > 0:36:14This would kill any mammal or bird.
0:36:18 > 0:36:22They remain in this deep freeze for up to six months.
0:36:28 > 0:36:32But spring comes at last.
0:36:37 > 0:36:42The ice melts around them and eventually within them.
0:36:50 > 0:36:54Slowly they begin to come to life.
0:37:11 > 0:37:15It takes quite a time for them to become fully functional,
0:37:15 > 0:37:18but eventually they are ready to face the outside world.
0:37:31 > 0:37:37So by allowing their bodies to cool, they have avoided the hard times.
0:37:48 > 0:37:50With the arrival of spring,
0:37:50 > 0:37:53their parents are now preparing to breed again.
0:38:01 > 0:38:03The male courts the female
0:38:03 > 0:38:07by gently strumming her cheeks with his long claws.
0:38:11 > 0:38:13And she responds.
0:38:19 > 0:38:23Cold blood is clearly no barrier to affection.
0:38:23 > 0:38:25In fact, reptiles can conduct
0:38:25 > 0:38:29as complex and as sensitive a courtship as many a mammal.
0:38:37 > 0:38:41This is the biggest of all living reptiles and one of the most feared.
0:38:41 > 0:38:46If one creature were to be labelled a cold-blooded killer,
0:38:46 > 0:38:47it would be this.
0:38:47 > 0:38:49A saltwater crocodile,
0:38:49 > 0:38:54a monster that can grow to a length of 20 feet - 6 metres -
0:38:54 > 0:38:56and weigh a tonne.
0:39:03 > 0:39:06But male and female, when they court,
0:39:06 > 0:39:08blow bubbles at one another.
0:39:15 > 0:39:19He is three times her size and could easily crush her,
0:39:19 > 0:39:23yet he treats her with great gentleness.
0:39:28 > 0:39:29He strokes her back.
0:39:42 > 0:39:46Slowly, he aligns his body with hers.
0:40:13 > 0:40:16So union is achieved.
0:40:26 > 0:40:30Crocodiles are among the most ancient of reptiles.
0:40:30 > 0:40:35Their ancestors appeared at about the same time as the dinosaurs.
0:40:35 > 0:40:40But what about them? Were dinosaurs similarly cold-blooded?
0:40:42 > 0:40:44The rocks of the North American West
0:40:44 > 0:40:47are particularly rich in dinosaur fossils.
0:40:49 > 0:40:51A hundred million years ago
0:40:51 > 0:40:56this was a horizontal mud-flat at the edge of the sea.
0:40:56 > 0:40:59And across it came an adult dinosaur
0:40:59 > 0:41:03with a smaller, younger one trotting alongside,
0:41:03 > 0:41:08leaving their footprints behind to be fossilised.
0:41:09 > 0:41:13They were iguanadons, a herd of them,
0:41:13 > 0:41:17together with some bird-footed dinosaurs.
0:41:17 > 0:41:19Were these all solar-powered?
0:41:21 > 0:41:25Some of the ancient reptiles had specific adaptations
0:41:25 > 0:41:27to help them collect heat.
0:41:27 > 0:41:34This is a plate from the back of a stegosaurus,
0:41:34 > 0:41:38and you can still see the lines where the blood vessels ran
0:41:38 > 0:41:42which collected the heat and carried it to the rest of the body.
0:41:42 > 0:41:46So, for the stegosaurus at least, the need to collect heat
0:41:46 > 0:41:49seems to have been just as important
0:41:49 > 0:41:52as it is for its relatives alive today.
0:41:52 > 0:41:58But there are clues that suggest that ancient reptiles were better at
0:41:58 > 0:42:02maintaining their temperature than their modern counterparts.
0:42:02 > 0:42:07This is the jaw-bone of a very large and very famous dinosaur.
0:42:10 > 0:42:16In life, its head would have been 18 feet - six metres - above ground.
0:42:19 > 0:42:23This is the jaw of Tyrannosaurus rex.
0:42:31 > 0:42:34An animal as big as this has a very large body mass
0:42:34 > 0:42:37which retains heat very well,
0:42:37 > 0:42:42so perhaps these huge dinosaurs were, in fact, warm all the time,
0:42:42 > 0:42:45simply because they were too big to lose all their heat overnight,
0:42:45 > 0:42:47as a smaller reptile would.
0:42:50 > 0:42:52But what about when they were small?
0:42:52 > 0:42:57Were adolescent tyrannosaurs able to maintain a steady body temperature?
0:42:57 > 0:43:00Were they, in short, warm-blooded?
0:43:00 > 0:43:02Evidence on that can be found
0:43:02 > 0:43:05in the microscopic structure of their bones.
0:43:12 > 0:43:14This...
0:43:16 > 0:43:19is the leg bone of a young Tyrannosaurus,
0:43:19 > 0:43:22and it has bands in it.
0:43:22 > 0:43:26The inner section, formed when the animal was young,
0:43:26 > 0:43:27has an open structure,
0:43:27 > 0:43:30like the bone of a fast-growing warm-blooded mammal.
0:43:32 > 0:43:37The outer part is more dense, more like that of today's reptiles.
0:43:37 > 0:43:42But whether the dinosaurs were really truly warm-blooded,
0:43:42 > 0:43:44we may never know.
0:43:44 > 0:43:48What we do know, however, is that dinosaurs were extraordinarily
0:43:48 > 0:43:53successful and dominated the earth for 150 million years.
0:43:55 > 0:43:59But there are some reptiles today that can keep their body temperature
0:43:59 > 0:44:02well above that of their surroundings.
0:44:02 > 0:44:07And these are the tracks of one of them.
0:44:07 > 0:44:12These giants haul themselves up out of the sea along beaches like this
0:44:12 > 0:44:15in many parts of the tropics.
0:44:15 > 0:44:17But in order not to disturb them,
0:44:17 > 0:44:21I'll turn this light out and we'll look for them with infrared cameras.
0:44:28 > 0:44:30Leatherback turtles.
0:44:30 > 0:44:33Like crocodiles, turtles are very ancient creatures,
0:44:33 > 0:44:38having first appeared at about the same time as the early dinosaurs.
0:44:38 > 0:44:42Today leatherbacks are the biggest of all reptiles
0:44:42 > 0:44:44and the most widely distributed,
0:44:44 > 0:44:48for they are found all the way from these warm tropical waters
0:44:48 > 0:44:50to the freezing seas of the Arctic.
0:44:57 > 0:45:00These have come ashore on a beach in Trinidad,
0:45:00 > 0:45:03where almost certainly they were hatched.
0:45:03 > 0:45:06Now they, in their turn, are laying their eggs here.
0:45:13 > 0:45:19Leatherbacks, we know, can generate heat internally,
0:45:19 > 0:45:23and there is proof of that if you have a look at her eggs
0:45:23 > 0:45:28that she is laying right now on that thermal camera.
0:45:28 > 0:45:31They are emerging from her body
0:45:31 > 0:45:37and lo and behold, they are bright yellow, verging on white,
0:45:37 > 0:45:41proving that they are warmer than their surroundings.
0:45:42 > 0:45:48She generates that heat within her body from special deposits of fat
0:45:48 > 0:45:52so that she can maintain her internal body temperature
0:45:52 > 0:45:56up to 8C above that of the water through which she swims.
0:45:59 > 0:46:01As she sweeps away the surface sand,
0:46:01 > 0:46:05you can see that the sand too is yellower, warmer, than the outside
0:46:05 > 0:46:09of her shell, for it still retains the heat it acquired during the day.
0:46:16 > 0:46:19So how do leatherbacks retain
0:46:19 > 0:46:24that precious and expensive internally generated heat?
0:46:24 > 0:46:29Well, to start with, they have their huge size to help them.
0:46:29 > 0:46:32They really are massive animals.
0:46:32 > 0:46:35This one is getting on for two metres - six feet long,
0:46:35 > 0:46:38and they can grow to weigh a tonne and a half.
0:46:38 > 0:46:42And, of course, big objects retain their heat
0:46:42 > 0:46:45very much more readily than small ones do.
0:46:45 > 0:46:47And there's another reason.
0:46:47 > 0:46:52Now, I am bright yellow going into white,
0:46:52 > 0:46:56which shows that I'm losing a great deal of my heat.
0:46:56 > 0:47:00But she, on the other hand, is very much darker,
0:47:00 > 0:47:05and that is because she has an internal layer of fat,
0:47:05 > 0:47:10an insulating layer just beneath the shell, which wraps round her body.
0:47:12 > 0:47:16The leatherbacks are the only reptiles in the world
0:47:16 > 0:47:19to have this kind of insulation.
0:47:28 > 0:47:33Her eggs laid, she fills in the hole with sand.
0:47:35 > 0:47:38And now she's on her way back to the sea.
0:47:40 > 0:47:43Life in cold blood has been a great success.
0:47:43 > 0:47:48It has, after all, endured for some 350 million years.
0:47:50 > 0:47:51But how did it all begin?
0:47:51 > 0:47:55To find the answer to that, we have to go back in time
0:47:55 > 0:47:56and back to the water,
0:47:56 > 0:48:02to the age when strange fish were hauling themselves up onto the land,
0:48:02 > 0:48:05fish that were the ancestors of the amphibians.
0:48:19 > 0:48:24Amphibians and reptiles are not easy creatures to film.
0:48:24 > 0:48:27They certainly do interesting things,
0:48:27 > 0:48:32but they also spend a great deal of time doing nothing much.
0:48:33 > 0:48:39We needed the help of scientists who really understood these creatures.
0:48:39 > 0:48:44Some workers have spent over 20 years studying their animals
0:48:44 > 0:48:47both in the lab and in the field.
0:48:47 > 0:48:49They investigate the lives of their chosen species
0:48:49 > 0:48:54using all kinds of gear, some sophisticated,
0:48:54 > 0:48:57some perhaps less so.
0:49:06 > 0:49:08With their help we had a rare chance
0:49:08 > 0:49:12to get under the skin of some of our subjects.
0:49:20 > 0:49:26Madagascar was going to be a very important location for us.
0:49:26 > 0:49:28It's a huge island, 1,000 miles long,
0:49:28 > 0:49:31with a great variety of habitats,
0:49:31 > 0:49:33and it's extraordinarily rich in reptiles.
0:49:38 > 0:49:42I first went to Madagascar back in 1960
0:49:42 > 0:49:46filming for a series called Zoo Quest.
0:49:48 > 0:49:51Back then I was trying to film all kinds of creatures,
0:49:51 > 0:49:56including the monkey-like lemurs and many rare birds.
0:49:56 > 0:50:00But I was particularly fascinated by the island's chameleons.
0:50:03 > 0:50:07There are, in fact, more species of chameleons in Madagascar
0:50:07 > 0:50:11than in all the rest of the world put together.
0:50:14 > 0:50:17There is one, the pygmy leaf chameleon,
0:50:17 > 0:50:20which was said to be only an inch or so long.
0:50:20 > 0:50:23I yearned to see it, but I never found it.
0:50:26 > 0:50:32Now I was back, and this time reptiles were our sole subject.
0:50:34 > 0:50:37Although Madagascar is only separated
0:50:37 > 0:50:40from the east coast of Africa by 300 miles of sea,
0:50:40 > 0:50:43its people, and particularly its animals,
0:50:43 > 0:50:46are very different indeed from those on the continent,
0:50:46 > 0:50:50with hundred of species that are found nowhere else in the world.
0:50:51 > 0:50:54Once again, I was in search of chameleons.
0:50:57 > 0:51:00Then, all television was black and white,
0:51:00 > 0:51:04but now I could film and record chameleons in colour,
0:51:04 > 0:51:06and what colours they have!
0:51:24 > 0:51:27We had come in the rainy season,
0:51:27 > 0:51:30when most creatures, including reptiles, tend to breed
0:51:30 > 0:51:34and are therefore particularly active and interesting.
0:51:35 > 0:51:39And this time I had the help of Bertrand Razafimahatratra,
0:51:39 > 0:51:44a Malagasy naturalist who's made a particular study of chameleons.
0:51:44 > 0:51:46He's worked on them for over ten years
0:51:46 > 0:51:48and knows most kinds very well.
0:51:48 > 0:51:53I asked him about the pygmy species that had fascinated me for so long.
0:51:53 > 0:51:56So, that really is full grown?
0:51:56 > 0:51:58Yes, full grown.
0:51:58 > 0:52:00But it's only that big.
0:52:00 > 0:52:02Yes, it's very small.
0:52:04 > 0:52:07He suggested that although chameleons
0:52:07 > 0:52:11are mostly active during the day, we should look for them at night,
0:52:11 > 0:52:14because most of them turn pale in the dark
0:52:14 > 0:52:18and are therefore easily picked out in the light of our torches.
0:52:23 > 0:52:24Ah!
0:52:28 > 0:52:30What is that?
0:52:30 > 0:52:32What species?
0:52:32 > 0:52:33This is oustelets.
0:52:33 > 0:52:35Oustelets...
0:52:35 > 0:52:38And male or female?
0:52:38 > 0:52:40- Female.- How do you know?
0:52:40 > 0:52:42The colour.
0:52:42 > 0:52:45Oh, there's another!
0:52:45 > 0:52:48This one was far from upset at being woken up.
0:52:50 > 0:52:51Ah!
0:52:53 > 0:52:56It fed!
0:52:56 > 0:52:58That's absolutely extraordinary.
0:52:58 > 0:53:01It can't possibly feed normally in the darkness.
0:53:02 > 0:53:07It just takes advantage of our light and finds an insect. Bravo.
0:53:07 > 0:53:10Let's go and see if we can find more.
0:53:12 > 0:53:15Bertrand explained that there was another reason
0:53:15 > 0:53:18why night was the best time to look for chameleons.
0:53:18 > 0:53:22When they go to sleep, they climb to the very far end of branches,
0:53:22 > 0:53:26where they're out of the way of predators such as snakes.
0:53:26 > 0:53:31And of course that was where we found them, just as he said.
0:53:43 > 0:53:45Oh, that's a big one.
0:53:47 > 0:53:48Beautiful.
0:53:55 > 0:53:58This one is just a baby.
0:53:58 > 0:54:00And how old do you think that is?
0:54:00 > 0:54:02I think just a few days.
0:54:02 > 0:54:05A few days?
0:54:06 > 0:54:08So even when it's newly hatched,
0:54:08 > 0:54:12it knows to come to the end of the branch?
0:54:12 > 0:54:14Yes, yeah.
0:54:14 > 0:54:18Look, they chew the tip of the branch.
0:54:18 > 0:54:21Yes... They're very difficult to get.
0:54:21 > 0:54:25Of course, if it was in the day, a bird could get it, but at night...
0:54:25 > 0:54:27At night, safe.
0:54:28 > 0:54:35Back in 1960, my chameleon-hunting techniques weren't quite so expert.
0:54:43 > 0:54:48However, I did discover that if you put a stick in front of a chameleon,
0:54:48 > 0:54:51it will usually obligingly walk onto it.
0:55:00 > 0:55:02But now, with Bertrand as my guide,
0:55:02 > 0:55:07we could search for the wonderful species I'd failed to find before.
0:55:07 > 0:55:08Would they be down here?
0:55:11 > 0:55:16'It lives on the ground, almost invisible among the leaf-litter.'
0:55:16 > 0:55:19That?
0:55:19 > 0:55:21'But Bertrand spotted it.'
0:55:28 > 0:55:30How extraordinary!
0:55:32 > 0:55:37This is the pygmy leaf chameleon,
0:55:37 > 0:55:41the smallest chameleon in the world.
0:55:41 > 0:55:45- In the world!- And probably the smallest reptiles in the world.
0:55:45 > 0:55:47Of any kind.
0:55:49 > 0:55:53You know...I'd heard about these,
0:55:53 > 0:55:57and I was here in Madagascar
0:55:57 > 0:56:0047 years ago,
0:56:00 > 0:56:06and I read about these, and I never saw one,
0:56:06 > 0:56:10and I think it was because...
0:56:10 > 0:56:12I never knew they were as small as this.
0:56:14 > 0:56:18That is absolutely extraordinary.
0:56:18 > 0:56:22It's about the size of a bluebottle,
0:56:22 > 0:56:24a blow-fly.
0:56:24 > 0:56:27And what does it feed on?
0:56:27 > 0:56:30- Small fly.- Small flies?
0:56:31 > 0:56:34How absolutely wonderful!
0:56:37 > 0:56:39I am astonished.
0:56:40 > 0:56:44That is the most marvellous thing I have seen
0:56:44 > 0:56:47for a very, very long time.
0:56:47 > 0:56:51Finding the pygmy chameleon would not have been possible
0:56:51 > 0:56:54without Bertrand's expertise and sharp eyes.
0:56:54 > 0:56:57He's just one of the scientists who has helped reveal to us
0:56:57 > 0:57:01the secret lives of reptiles and amphibians.
0:57:09 > 0:57:12To find out more about Open University programmes,
0:57:12 > 0:57:16call 0870 787 4455
0:57:16 > 0:57:21or go to bbc.co.uk/nature.
0:57:26 > 0:57:30Next week, join me for the story of the amorous amphibians,
0:57:30 > 0:57:32the land invaders.
0:57:59 > 0:58:02Subtitles by Red Bee Media Ltd
0:58:03 > 0:58:05E-mail subtitling@bbc.co.uk