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00:00¶¶
00:30One of the most crucial steps in the story of life on Earth
00:55happened in a freshwater swamp about 350 million years ago.
01:02The fish began to haul themselves out onto the land.
01:06The land at the time was covered with the first plants,
01:10very different from these mangrove plants today, but nonetheless plants.
01:14And in order to get out among them, the fish had to solve two problems.
01:19First of all, they had the mechanical problem of hauling themselves onto land,
01:23and second, they had to be able to breathe once they got there.
01:28The way they solved the problem of hauling themselves up onto the land,
01:33we can see from a small little fish which lives in these mangrove swamps today.
01:38It's in no way closely related to those early fish,
01:41but it does give us an idea of what that sea must have been like, the mudskipper.
01:46They come up out of the water to browse on small creatures
01:56that swarm on the surface of the mud.
02:03Their front fins have jointed bones inside them
02:06so that the fish can use them as legs to lever itself along.
02:10The mudskipper is not by any means the only fish
02:22to have developed muscular fins like these.
02:24Fossils of one of the first have been found in rocks laid down
02:28just before the time that backboned animals ventured onto land,
02:32the coelacanth.
02:33Did this extremely ancient fish also use its fins as legs?
02:40Unfortunately, no fossils of them younger than 70 million years
02:44have ever been found,
02:46and up to 40 years ago, scientists concluded
02:48that they never would be able to answer that question for certain
02:51as the fish was obviously extinct.
02:54And then, in 1938,
02:57a living coelacanth was caught off the coast of South Africa.
03:00It was the scientific sensation of the century.
03:04But before scientists could get to it to examine its entrails
03:07and see how they confirmed or denied the deductions they'd made
03:12from the very ancient fossil coelacanths,
03:14the fish was already rotting,
03:16and its guts were thrown away unexamined.
03:20So a huge search was mounted to try and find another.
03:24Leaflets and posters were printed carrying pictures of the fish
03:27and offering a great reward
03:28and distributed among the countless fishing villages
03:31up the coast of Africa.
03:33But nothing.
03:35Until 14 years later,
03:38a second coelacanth was caught.
03:40And this time,
03:40it came from a place over 1,000 miles away
03:43from where the first one had been landed,
03:45here in the tiny Comoro Islands,
03:48a small group lying midway between Madagascar
03:51and the coast of East Africa.
03:53The first one, it seems, was astray.
03:57These waters out here are the true and only home
04:01of this extraordinary and rare fish,
04:03and the people who live in that tiny village down there
04:06are the world's greatest experts in catching coelacanths.
04:10A villager still had a dried and shriveled coelacanth,
04:14which he let me see.
04:17From what we know of the habits of the living coelacanth,
04:21which is not very much,
04:22it seems that these rear fins
04:25are the ones which are used for swimming,
04:27but the front ones are used for manoeuvring
04:30and for helping the fish to clamber about
04:33along the rocky bottom on which it lives.
04:35And all the fins have fleshy bases to them.
04:39The fishermen catch them at night
04:41from depths of 300 metres or so.
04:44Once hooked, the fish fight valiantly,
04:47and it may take all night to haul one to the surface,
04:50so it's usually dead on arrival.
04:52And scientists have still not been able to observe one alive.
04:56And then, while we were in the Comorros,
04:58one was caught,
05:00and although it was weak,
05:01it was still alive when the cameras arrived.
05:05350 million years ago,
05:10fish with fins like these
05:11were cruising the seas of the world.
05:14Some, living in shallow waters,
05:16produced descendants,
05:17which eventually clambered onto the land,
05:19while this creature's ancestors
05:21moved down to the unchanging depths,
05:23there to remain unchanged themselves.
05:30The Comorians catch one or two coelacanths every year.
05:34They use not to value them very much,
05:37for their flesh isn't particularly good to eat.
05:39Now, however,
05:40big rewards are offered by scientific institutions abroad,
05:43so the old man who caught this one
05:45will soon be rich.
05:46And some researcher,
05:47in a few weeks' time,
05:49will be absorbed in examining the structure of this fin,
05:52which scientists agree
05:53must resemble very closely those limbs
05:56that first took backboned animals onto the land.
05:59But how about that second problem,
06:04the problem of breathing up on land?
06:07The gills,
06:08which had served them very efficiently
06:10while they were swimming in water,
06:12extracting dissolved oxygen,
06:14wouldn't work in the air.
06:15How did the fish solve that problem?
06:18Well, this is East Africa,
06:20and it's the height of the dry season.
06:23There is not a drop of water
06:25to be found in this parched landscape.
06:28And yet,
06:29here,
06:29close by me,
06:30there are fish
06:31that are living and breathing in air.
06:35If only I can find them.
06:36six months ago,
06:57this was a pond
06:58several feet deep in water.
07:01But as the dry season progressed,
07:04the water evaporated,
07:05and the fish in it
07:07burrowed down
07:09into this,
07:11which was soft liquid mud
07:14and is now brick hard.
07:16And there,
07:17somewhere,
07:19they cocooned.
07:30And that,
07:32that looks like
07:35the nose of one
07:38poking out from the mud there.
07:42Now,
07:43if I take this
07:44and drop it in a tank of water,
07:46it should seem as though
07:47the rains have come a little early
07:49and the fish
07:50should come to life.
07:52as the water soaks in,
08:16the mud softens and falls away,
08:18exposing a papery cocoon
08:20to have dried mucus.
08:30And there is the throat
08:32of this extraordinary creature
08:33that can breathe in both air and water.
08:36It's a lungfish.
08:37in the air,
08:52it's a Jetzt
08:55Once
08:57you
08:58will
08:58you
08:59know,
08:59While its water-breathing apparatus, the gills, are getting into working order again,
09:17it snatches another gulp of air.
09:29It's able to breathe air because it has, opening from its gut, a long pouch lined with blood vessels,
09:38and they can absorb gaseous oxygen through its moist lining.
09:44The coenacanth has no lung, but it has got a simple leg, that fin with a fleshy base to it, supported by bones.
09:52Neither it, nor the lungfish, therefore, can be close to the ancestral creature that first made the move to land.
10:02But if those two crucial elements were to occur in one animal, then such a creature would certainly be a strong candidate.
10:10And indeed they do.
10:12This fossil fish, from rocks 450 million years old, has them both.
10:18It's called Eusthanopteron.
10:19And when the rock and scales around the base of its fin are carefully removed,
10:24you can see the bones, one close to the body, then two, then a group of small ones.
10:30Exactly the pattern found in the limb of all land vertebrates.
10:36And that adventurous ancestor may have been very like this.
10:40But why should it have climbed out onto the land?
10:43Perhaps it was forced out by droughts.
10:45Maybe it was tempted by food, the creatures that swarmed on the surface of the mud.
10:50Whatever the reason, its descendants came to spend more and more of their time out of water.
10:55And over millions of years, they evolved bodies that were more suited to life on land and became the first amphibians.
11:02The vegetation of the time was very different from that of today.
11:25There were no flowering plants or conifers, and one of the commonest plants was a kind of horsetail,
11:32rather like these growing in the north of England.
11:35Except that the horsetails of that time, 300 million years ago, grew to about 50 or 60 feet tall
11:41and formed dense forests growing in swamps.
11:45When they died, the horsetail trunks fell into the water and formed a kind of peat.
11:50And over the years, there were variations in the sea level which flooded these swamps
11:55and buried the peat beneath deposits of sand.
12:00Under the accumulating weight of these sediments, the peat then turned into coal.
12:06And in the mine, you can see the sand that's been turned to stone,
12:26and beneath it, the compressed remains of the plants.
12:30And in this particular scene have been found as well the bones of some of the animals that crawled in those ancient swamps.
12:39This is one of the most dramatic of them.
12:43It's a skull.
12:45Here are its huge teeth, which are simple teeth, rather like the peg-like teeth of the fish of that period.
12:52We know that this creature had a paddle-shaped tail and also four very good limbs,
13:01so it really was a true amphibian.
13:04And it must have been a very formidable creature, too,
13:07because it grew to a length of about 12 feet.
13:11There were many kinds of them, and they dominated the land for a hundred million years.
13:40The largest amphibian alive today, the giant salamander from Japan, grows to over a metre and a half, four feet or so.
13:55But even that is only a quarter as big as its ancestors.
13:59Most of its living relations, the rest of the salamanders and newts, are very much smaller, a few centimetres only from nose to tail.
14:14Though newts spend much of their time out of water, they don't go far from it.
14:20In early spring, after hibernating, they must move back into it.
14:27Their skin is permeable. It doesn't retain liquid very well.
14:31And if they dry out, they die.
14:34They also need to keep their skin moist, for like most amphibians, they breathe through it,
14:40supplementing oxygen from their lungs with more absorbed directly from the air.
14:45And one final shackle keeps them tied to water.
14:49They have to return to it to breed.
14:52Once in water, it sheds the thin outer skin it secreted to give it some degree of protection on land,
15:03and takes up an existence that is much more like that of a fish.
15:09It often seems that the newt is more at home here than on land,
15:12and indeed it retains many characteristics of its fish ancestors.
15:22The males become brightly coloured and develop flamboyant crests along their backs.
15:28Their courtship is in many ways reminiscent of that of fish.
15:32They flex the frills along their backs just as so many fish flex their fins.
15:37And they beat the water with their tails, sending powerful currents towards the female,
15:42which she detects with a line of sensors which resemble the lateral line system of the fish.
15:58Two males are courting one female. She's in the middle, without a crest.
16:07The female lays several hundred eggs, each individually stuck to a leaf.
16:15Development is swift. The tiny white sphere elongates.
16:23Pigment appears.
16:25And soon the young emerge.
16:28And they're even more fish-like than their parents.
16:31They have no legs, and they breathe not with lungs, but with feathery gills.
16:37But slowly their legs and lungs do develop,
16:39and the newt tadpole, for a short period, has the benefit of both ways of breathing.
16:44But there's one tadpole that remains in this condition all its life.
16:56Its external gills are large and feathery and permanent.
17:00It lives in one lake in Mexico, and the Aztecs called it the water monster.
17:05Axolotl.
17:06In the sea, we, you have the interest of the earth.
17:09Like one of those trees.
17:10Loving ae for the planet.
17:12The people in the sea.
17:18The fire retailers.
17:20The effort the air, the earth for the other.
17:23The feeling of the water.
17:25The way the lions.
17:27The sea.
17:29The ocean.
17:31The ocean.
17:32The ocean.
17:33But the most surprising thing about this overgrown, eternal tadpole is that it breeds in this condition.
18:03The eggs start developing immediately. The black part is the beginning of the tadpole's body, which will grow round and enclose the cream-coloured yolk, food supply for further development.
18:26Though the axolotl never changes into a landliving salamander in the wild, it has a close relative in Mexico, which retains its options.
18:36Sometimes it breeds like the axolotl, but if its lake dries, then it's able to turn into a normal landliving salamander.
18:43The tadpoles, still with their feathery gills, wriggle in the tepid, shallowing pools.
18:50But as time passes, the gills disappear, for now the animal has developed lungs.
19:00And eventually the little creature hauls itself up onto the mud.
19:06But many salamanders don't seem to be very enthusiastic walkers and show signs of abandoning the habit.
19:14This one from California has only tiny legs and spends most of its time burrowing under logs and stones.
19:21One entire group of amphibians has opted totally for this way of life and lost their legs altogether, the Sicilians.
19:30You might well confuse these creatures with large earthworms.
19:34This one comes from Southeast Asia.
19:37Its eyes are covered in skin and to replace them it has small white feelers just below its sightless eye.
19:43Their bodies have become greatly elongated and they've lost all traces of limbs.
19:51Most of them don't come up to the surface except at night.
20:04But then you really see that they're not earthworms solidly champing their way through soil.
20:09Blind though they are, they're hunters.
20:12This one comes from South America.
20:35Sicilians constitute the smallest amphibian group.
20:38Only about 160 species are known compared with over twice that number of salamanders and newts.
20:44But they're so unobtrusive and so easily mistaken for large worms and therefore ignored,
20:50that there may well be many more kinds still to be discovered burrowing their way through the soft damp soils of the tropics.
20:57But most of the amphibians living in the world today belong to a third group.
21:02A group that doesn't live below ground like the Sicilians but above it.
21:08And far from having lost their legs, they have developed their legs to an absolutely spectacular degree.
21:14The frogs and toads.
21:16And this is the king of them all.
21:19This is the largest frog in the world, the goliath frog.
21:24It's a very rare animal.
21:26It lives only in one small part of West Africa.
21:30In captivity, it lives on small birds, rats or mice and even fish.
21:37But in the wild it seems that its diet is not quite so ambitious.
21:42It takes dragonflies and other large insects and it also catches crabs from the bottom of the river
21:49because it's a very good swimmer with very large webs at the bottom of its feet.
21:55But these huge legs also enable it to jump very well.
22:02This particular one can jump nine or ten feet, say about ten times its body length.
22:08But in the kingdom of the frogs, that's really nothing much.
22:12Some of the smaller frogs can do very much better than that.
22:15They are dazzling athletes.
22:18When each foot is webbed to form a parachute, then your leaps are spectacular indeed.
22:48This is the famous flying frog, though it would be more accurate to call it a glider than a flyer.
22:59Even so, in one leap and glide it can cover fifteen meters or so, say, a hundred times its body length.
23:06Several species have developed this talent.
23:09There's one in Japan, another in Malaya, and this one lives in Costa Rica.
23:18Its feet are not only webbed, but each toe ends in a sucker so that it can also cling to vertical leaves if it has a mind to.
23:31But its unique splendor is only revealed when it leaps and opens its four extraordinary parachutes.
23:39Another one has a crook, instead of the deserts.
23:55Or if it is possible for people to come to the sea.
24:00The flying frog seeks safety by launching itself into the air.
24:24However, this frog takes refuge underground, and its pointed nose gives it a very good start.
24:37And then its legs provide a pile-driving thrust.
24:45The Holy Cross Toad of Australia also buries itself, but it goes rear-end first and with
24:52a quite different kind of leg action.
25:04It's easy to understand why they hide.
25:07Frogs to a hungry hunter must appear to be both appetizing and vulnerable with their soft,
25:12defenseless bodies.
25:13And indeed many of them are.
25:15But some have developed defenses, and very surprising ones too.
25:21This grass snake is about to tackle an ordinary European toad.
25:30The combination of standing on tiptoe and inflating its body makes it look very much bigger than
25:49it really is.
25:56Whether this frightens the snake or merely baffles it, who can say?
26:00Whatever its effect, it works.
26:05The fire-bellied toad.
26:06Watch.
26:07This extraordinary posture deters predators by revealing the yellow and black pattern on
26:14the animal's stomach.
26:16A combination of colors that's widely recognized in the animal world as a warning.
26:20And it's not all bluff either.
26:22All amphibians have mucus glands in their skin which help in keeping them moist.
26:27And some of these glands in the fire-bellied toad have been adapted to produce a bitter-tasting
26:33poison.
26:34Skin has become very versatile in the amphibians for breathing, defense, and it comes in all sizes,
26:41and colors.
26:52In South America, some frogs have developed defense so far that they've become real killers.
26:57The poison they produce in their skins is so powerful that it can paralyze a monkey or
27:02a bird immediately.
27:04There are at least twenty different kinds of poison frogs in Central and South America,
27:09and conspicuousness is an important part of their defense strategy.
27:13They don't want to be eaten by mistake, for it's of no value to them as individuals if
27:18their attacker dies soon after they've provided it with a meal.
27:21So they're all dressed in spectacular colors.
27:24Color is of no use at night when it can't be seen, so unusually for frogs, these little
27:30creatures are active in the daytime, moving boldly around the forest, confident and secure
27:36in their brilliant livery.
27:46This particular species has good reason to be confident.
27:50It has the most poisonous skin secretion of all.
27:53It's only recently been discovered by science, and just a tiny smear from its skin could kill a man.
28:00The local Indians in Colombia use its poison on their blowgun darts by rubbing the tips
28:06on the backs of the living frogs.
28:13One frog in Argentina has developed a unique way of safeguarding against that, and at the same time
28:19keeping itself reasonably watertight when the weather gets too dry.
28:22It gives itself a varnish.
28:24There are many wax glands in its skin, and when it begins to feel that its body is drying out, it gives itself a good going over so as to produce a thin waterproof covering.
28:42The one — for you, that is the remold in the air flow, it has a good watertight tree.
28:45There are many more signs than bubbles, and when it comes to the waterized dry, it wouldn't look like that, and we just have to bring a deep water.
28:47Also, a few days and a few days, we can't reintroduce the water thinking of the oil.
28:48The earth is low, and we can see the water.
28:49The white energy will have those who don't buy it.
28:50It turns out once more.
28:51The moon can be done when it comes to the energy of the water.
28:52It turns out when the water is a green water.
28:53The water is big and the water will be very resistant with water.
28:54The water is a great way of the glass.
28:55The water is a great nesting water.
28:56It turns out the water into the water, and uses the water.
28:57But there is, of course, another opposite strategy.
29:22If you're without defences of any kind,
29:24then it may be much more effective to spend the day concealed,
29:27camouflaged as part of the leaf.
29:33Some conceal themselves not to escape but to lurk in ambush.
29:37This big toad will pounce on mice and fledglings as well as worms.
29:46Cleaning bits of earth and twigs from the worm is important
29:49for the toad has no teeth
29:51and so simply swallows whole whatever gets into its mouth
29:54and it doesn't want any hard or spiky inedible bits.
30:00A tongue that can be stuck out is an amphibian invention.
30:04No fish ever had one and very effective it is too.
30:13The blink is an essential part of swallowing.
30:16Frogs and toads have no bony base to their eye sockets
30:19so their eyeballs bulge down into their mouths.
30:22When they blink, the underside of the eyes helps to squeeze the food in its mouth
30:27back towards its throat.
30:29Their tongue is not attached to the back of the mouth like ours but to the front
30:33so they can stick it out very much farther than we can
30:36which is very useful for a somewhat ungainly hunter without a neck like a toad.
30:41Its end is sticky and muscular and it grabs the worm with the underside.
30:46And then the tongue has one final function.
30:49It lubricates the food so that it can be swallowed easily
30:52without scratching the delicate membranes of the throat.
30:59The eyes of the amphibians are fundamentally the same in structure
31:02as those of their fish ancestors.
31:04There was no need to change them for they work as well in air as they did in water.
31:08But they have to be kept moist and clean so the amphibians have developed an ability to blink
31:13and a membrane to wipe the surface.
31:28Protection from strong light by closing the iris
31:33or by using a membrane which still lets light in.
31:38In air however you do need quite a different hearing apparatus than you need in water.
31:43Eardrums.
31:44And with them came a voice.
31:47Some frogs call during the day like these edible frogs.
31:52Eardrums.
31:57Eardrums.
31:58Eardrums.
31:59Eardrums.
32:00Eardrums.
32:01Eardrums.
32:02Eardrums.
32:03Eardrums.
32:04Eardrums.
32:05Eardrums.
32:06Eardrums.
32:07Eardrums.
32:08Eardrums.
32:09Eardrums.
32:10But most sing at night.
32:12Before the amphibians had crawled out of the water 300 million years ago
32:16the only animal sounds on earth had been whirs and chirps of insects.
32:21So the first animal chorus to break the silence of the land
32:25may well have been something like this.
32:30The frog's lungs, which blow air through its tiny vocal cords,
32:34are relatively feeble, but resonating sax
32:37bulging from the ankle of the jaws or the throat
32:39amplify it many times so that some calls can be heard for over a mile away.
32:51BeEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
33:07Go, go.
33:10Camry!
33:16RMINUTH?
33:19REECE?
33:21I don't know.
33:51I don't know.
34:21The cue for these choruses is usually a change in the weather, for these songs are the prelude
34:31to mating.
34:32In the tropics, the trigger is usually the onset of the rainy season.
34:51As the forest is drenched, so the moisture-loving amphibians can get out and about seeking mates
35:13and laying eggs.
35:14In this very rare species from Costa Rica, the male is yellow, the female red and brown.
35:37They abandon their eggs, after they've laid them, to return to land.
35:42Streams and ponds and other such obvious spawning sites often swarm with fish that will eat
35:48any eggs or young that they can find, so hundreds must be laid if just one or two are to survive.
35:54In temperate areas, breeding begins when the weather warms in spring.
35:59European toads migrate for miles around to a single favoured pond and assemble there in
36:05great numbers, all within a few days.
36:07The breeding period may only last a week or so, and towards the end of it, females with eggs
36:12still to lay become very rare, and the males, in their frenzy to couple with them, clasp anything
36:18in the neighbourhood that moves, male or female, and so form tangled writhing groups.
36:37The colocrupt that rocks will always daties into areas, moderately, with a lot of
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37:02These toads rely for breeding success on numbers.
37:18A single female may deposit 6,000 eggs.
37:23Mass production, be it in temperate or tropical places, seems to be a very effective strategy.
37:32These tadpoles, for example, develop from eggs that were laid in enormous numbers in pools beside this South American river.
37:43But amphibians have another option.
37:47They, after all, can climb up onto land, and so they can lay their eggs in places that no fish could possibly reach.
37:55When they first evolved and had the land to themselves, that must have been a particularly effective strategy.
38:02And even today, there are many frogs that go to quite extraordinary lengths in order to lay their eggs away from ponds and rivers.
38:11Of course, they have to keep their eggs moist, otherwise they would dry out.
38:16But around this river, at any rate, that's not too difficult.
38:28These are the Kaitur Falls on the Potaro River in Guyana.
38:32The forest round here must be sheer paradise for frogs, for here, in effect, there is permanent rain, and quite warm rain at that.
38:46The Potaro above the falls is 100 or so metres across, and its waters fall sheer for over 200 metres.
38:56Much of these great masses of falling water turn to spray and drenching mists.
39:01And as the mist comes swirling up, it condenses into drops which fill the centre of such plants as this,
39:18and turn them into miniature ponds, ideal for the frog's purposes.
39:23And inside this particular one, lives a tiny, beautiful, golden frog.
39:35It shares its minute pool with a few insect larvae, but with nothing it does it or it takes any harm.
39:41There are many such plants with water-filled chalices in their centres in the South American rainforest,
39:56and many of them grow not on the ground, but high up, perched on branches, with their roots dangling in the moist air.
40:03So they are, in effect, ponds up trees, and they provide the frogs with little oases,
40:09where they can live and spawn away from predators, generation after generation, without ever coming down to the ground.
40:16In the African savannahs, where there's much less rain, there are no such plants,
40:26but there is nonetheless a frog that manages to breed up in the trees.
40:31Instead of water, it uses foam.
40:34The trick is done at night.
40:37The female excretes a special liquid which she beats into a lather with her hind legs.
40:48The male joins her and fertilises the 150 or so eggs which she deposits in the foam.
40:54The sun will bake the outside into a hard crust,
40:58but inside it remains liquid, and there the eggs develop.
41:02The nests are always made above water,
41:05so in due time when the crust cracks and the young ooze out,
41:08they drop straight down into a river or a swamp.
41:35This frog from South America also has a way of keeping its eggs away from the dangers of the river.
41:45Here, however, where the air is more humid, it doesn't need foam,
41:49because the jelly surrounding its eggs doesn't dry out.
41:52The young tadpoles develop inside the jelly as the young of many other species do,
41:57but these stay there while they go through most of their larval development.
42:12They even develop gills inside the egg.
42:16And their hearts begin to beat.
42:28Eventually they too will emerge and drop down into water.
42:33The tadpoles of one Caribbean frog have managed, astonishingly,
42:37to dispense with water altogether for their development.
42:40The whistling frog lays a cluster of eggs on the ground.
42:44They're only small, but inside each there is liquid,
42:48and in it the young develop not only to the tadpole stage, but beyond.
42:53Their tiny stomachs are full of yolk that must fuel their entire development.
43:11The front legs are formed.
43:15And so are the back legs.
43:25And at last it becomes virtually a tiny, fully developed replica of its parent.
43:32On its nose it has a tiny spike.
43:49And with that it punctures the egg membrane.
43:52And after about 18 days it hatches,
43:55having eliminated altogether the tadpole's normal need for open water.
43:59Laying your eggs away from water and all its dangers
44:03is one very successful breeding strategy.
44:06But yet other frogs have taken a different line.
44:09Instead of abandoning their eggs in as safe a place as they can find,
44:13they stay with them and look after them.
44:16The midwife toad lives in Europe.
44:18Its name is not very accurate because it's not the female,
44:21but the male that carries the eggs entangled round his legs.
44:25There may be 60 or so of them and he carries them for six or seven weeks.
44:30When the moment comes for hatching,
44:32he takes them down to water and the tadpoles simply swim away.
44:39Some toads spend all their time in water.
44:42This peeper from Brazil,
44:44instead of laying and abandoning 6,000 eggs like the European toad,
44:48lays a mere hundred or so.
44:50But they look after them in the most extraordinary manner.
44:57The male with these elegant and careful movements of its hind feet
45:02takes care that as many eggs as possible
45:04are gathered together on the female's back.
45:07And they stick.
45:12And they stick.
45:37Then the skin on the female's back begins to swell.
45:42The eggs rapidly become embedded in it.
45:52Soon a membrane grows over them to enclose them completely.
46:00After only 30 hours, almost all the eggs have disappeared entirely
46:04and the skin is complete again.
46:07After nearly three weeks, it's moving.
46:13And then, after another three weeks or so,
46:15the young begin to emerge.
46:17.
46:37.
46:41Now the parent leaves the young to fend for themselves,
46:50but at least they're now independent swimmers,
46:52able to find hiding places for themselves.
46:55So a much higher proportion is likely to survive
46:58than would have happened if they had been abandoned as eggs.
47:03This little South American frog also keeps her eggs and young on her back
47:07in a pouch with an opening just above the base of her spine.
47:11Her developing young remain inside it for three months or more
47:22until at last she releases them into a pool as tadpoles.
47:33They're already on their way towards their final change into the adult form
47:37for they have their back legs.
47:41These many differing ways of carrying their developing young
47:48may seem extraordinary enough,
47:50but some other frogs in South America
47:52actually retain their tadpoles inside their bodies
47:55and in the most unlikely parts.
47:59There is one which calls in the beech forests of southern Chile.
48:03It's Darwin's frog.
48:04These little creatures, only a few centimetres long,
48:26are all males, even though they vary in colour.
48:28They're still calling, but the actual breeding is over.
48:32The females have laid their eggs in groups of 20 or 30 on the forest floor.
48:37As soon as the males see a movement in the eggs,
48:40they will apparently eat them.
48:42Each male may take a dozen or so, but he doesn't swallow them.
48:46Instead, they go into his vocal sac that runs down the front of his throat,
48:50and there they develop and wriggle.
48:53The males sit about, struck dumb by their own offspring.
48:57But eventually, after some weeks,
48:59their extraordinary vocal pregnancy comes to an end.
49:02And here is that amazing berth once again, in slow motion.
49:26The male's vocal sac is now ready again for singing,
49:40before it's turned next season, once more, into a nursery.
49:46But the prize for the care of the young must surely go to this frog
49:50that lives only on a remote mountain in West Africa.
49:53The female, only about a centimetre long,
49:59keeps the eggs inside her distended oviducts
50:02and holds them there throughout the nine months of the dry season.
50:06As they grow, she secretes internally some white flakes.
50:11The tadpoles, moving around freely inside the oviduct,
50:15eat the flakes and digest them in their gut,
50:17just as they would do if they were outside in the pond.
50:20When the rain comes, she gives birth.
50:23Her stomach and oviduct don't have muscles
50:26which can contract and expel the young,
50:28so she does it by bracing her body against the ground with her forelegs
50:32and inflating her lungs so that they bulge down into her abdomen
50:36and squeeze the young out.
50:39And they're born fully formed froglets,
50:42a real triumph of parental care.
50:44By providing their young with moisture of some kind
50:48and using all these varying and astonishing techniques,
50:51frogs and toads have managed to colonise almost all the world.
50:56Even so, you would think that with their thin, permeable, moist skins,
51:00they would never be able to survive in the parched desert of Australia.
51:03But one or two species manage to live even here.
51:07They spend nearly all their lives buried deep in the ground away from the murdering sun.
51:12They may lie here for years, waiting,
51:15but eventually the rains do come.
51:17When the frogs burrowed down here during the last rains,
51:33they were bloated with water,
51:35and they've conserved it in their underground chambers
51:37by sealing themselves inside a membrane secreted from their skins.
51:41But when the rain arrives again,
51:44they must quickly get rid of their watertight packaging
51:46to be ready for breeding.
52:05In the brief period when the desert is wet,
52:08these frogs will dig themselves out
52:11and mate,
52:12and their tadpoles will develop
52:14in the few days that there's water in the temporary pools.
52:17And then, as the desert dries out,
52:19the young frogs will bury themselves
52:21and remain underground for perhaps five years or more.
52:32In times of drought,
52:34the desert aborigines search eagerly for frogs like this,
52:39and this is why.
52:39If you squeeze one,
52:41you can get quite a reasonable drink of water from it.
52:44It's tasteless and really quite drinkable.
53:04And now I'm going to have to find a pond for this little creature
53:09where it will survive until the next rains come.
53:13For the fact of the matter is that its success as a desert liver
53:16is very limited.
53:18It can only be active and breed during that very short period
53:21when there's rain.
53:22In order to survive in a desert and breed there,
53:26if necessary without any rain at all,
53:28you need a device that no frog or amphibian has got.
53:34An egg with a waterproof shell.
53:39That was the next great evolutionary breakthrough,
53:43and it was achieved by the reptiles.
53:46The astonishment is that without it,
53:50the amphibians managed to colonise so much of the world.
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