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