Nature

DAVID ATTENBOROUGH

There are more than 5,000 species of frogs and toads, and they come in all colors, shapes, and sizes. Over the years, I've filmed many different kinds of them from across the world. This is the largest frog in the world, the goliath frog. And here, in the leaf litter in this Madagascan forest, the tiniest of them all. They can hop and climb. They can parachute from the treetops and burrow deep into the ground. Some seemingly can even walk on water. Frogs, like newts and salamanders, are amphibians, cold-blooded animals that need water to survive. But despite this, they have colonized some of the hottest and the coldest places on earth. Frogs are truly fascinating. You may not have thought much about them, and they don't necessarily grab the headlines, but there's more to frogs than you might suppose. Frogs were the first creatures that I kept when I was a boy, and I thought they were fascinating and beautiful. And I still think they are. They were, of course, the first creatures to move up onto land, because they were amphibians. Today's frogs are descended from a group of amphibians that lived around 300 million years ago. And they were very like the modern salamander. That is to say, they had a very, very long spine with about 30 vertebrae in it, ending in a tail. But then, about 250 million years ago, an intermediate form appeared, like this fossil from Madagascar. And already you can see a difference. Most importantly, the spine, instead of being 30 vertebrae long, is only about 15. The tail has almost been lost altogether. The hind legs are very much bigger. Compare that with today's amphibian. Most significantly, its spine is now, again, halved, but the pelvis has become greatly elongated. But that's nothing compared with what has happened to the legs. They are gigantic. So you can see that this animal is a leaper. And today, some exploit these spectacular legs very dramatically. This little frog is an amazing jumper. It can leap 30 times its own body length. And there are some that can go even farther. One can leap 55 times its body length -- the equivalent of me jumping the length of a football pitch. In the past, people couldn't understand how a frog could leap so far, but a little scientific research revealed how it does it. When a frog is in its sitting position, its leg muscles are contracted. This stretches the leg tendons. Tendons are elastic, like rubber bands. Stretching them stores energy within them. And then, when the frog jumps, the tendons release that energy like firing a catapult. And the frog is propelled into the air. When this skittering frog from India jumps, it simultaneously extends its webbed toes so that it virtually bounces across the surface of the water. The webs between the toes, so useful for swimming, also help when moving around on land. The gliding leaf frog has such extensive webbing that when it jumps, the outstretched toes of its feet act like parachutes and keep it in the air long enough to travel considerable distances. When the first amphibians emerged from the water, the only animal sounds to be heard on land were the whirs and hums of insects.

Croaking

DAVID ATTENBOROUGH

But the amphibians also needed to communicate with one another, and soon they added their own croaks and whistles.

Calling

DAVID ATTENBOROUGH

Frogs blow air from their lungs through vocal cords and so produce a croak. But the muscles that do that are comparatively weak. So many species amplify the sound with resonators -- cheek pouches or throat pouches.

Croaking

DAVID ATTENBOROUGH

One purpose for calling is to find a mate. A female gliding leaf frog in the jungles of South America is listening to all the males calling around her. Then she sets off in the direction of the loudest voice, because the owner is likely to be the strongest male, the best mate. It's a long haul. On the way, she has to avoid the weaker males. It's not all that easy. She has to fight off several at a time. Eventually the strongest male gets his reward. It's a case of "He who shouts loudest." This is called the splendid leaf frog, and quite right, too. Most frogs communicate with their voice by croaking or, indeed, squeaking. And this one does too, but it has rather a quiet voice, and it also communicates by using its legs. What he'll do is to use them to wave to other frogs. And its legs are specially adapted for the purpose. they have flaps on them so they appear to be specially wide and prominent. This waving technique is something I once filmed with another beautiful frog. We found it several years ago in the rain forests of Panama. This is a golden Panama male, and he is looking for a mate. But a rival stands in his way. Just in case his call can't be heard above the sound of water, he reinforces his message with a visual signal -- a wave. His rival waves back. He repeats his signal so there's no misunderstanding. Now another male arrives. He too is looking for a female, and he isn't going to let anyone stand in his way. This kind of argument has to be settled with a wrestling match. That should teach him. The loser submits, lowering his head. And the winner continues his hunt for a mate. Finally, a large golden female arrives -- just the ticket.

Calling

DAVID ATTENBOROUGH

He waves to show he's interested. She waves back. It's a success. His waving courtship has worked. Calling and waving are comparatively subtle ways of attracting a female. Other frogs use a more macho approach. Rains on the African savanna. The African male bullfrog -- one of the biggest of all frogs. He fights for the right to mate. The males assemble in a newly filled pond and battle with each other to establish who is the strongest. The winner will mate with most of the females here. No such luck for the loser.

Rain falling

DAVID ATTENBOROUGH

Courtship techniques vary widely, but for one remarkable little brown frog in Madagascar, the key to mating rituals is skin color. Following a bout of heavy rain, the frogs all gather at a water hole. The females are still brown, but the males have turned a bright yellow. This enables males and females to tell each other apart in the multiple mating that is about to happen. They have to judge the moment very precisely. There must be enough water in which to lay their eggs, but not so much that the eggs are washed away. After a few hours, the orgy is over, and the males turn back to brown again. It'll happen all over again next year when the rains return and more eggs are laid. The life cycle of the frog involves a truly remarkable process. Let's see where it all begins. A frog's life starts here. These eggs were laid by a leaf frog, and inside the jelly of each one, you can see a little tiny dot, and if you look closely, you may even see it move. And over the next five or seven days, they will continue to develop until they're strong enough to emerge from the egg and drop into water beneath. And then, over the next four or five weeks, an enormous transformation takes place. The gills, which enable the tadpole to breathe in water, gradually disappear. The tadpole's intestines, accustomed to a vegetarian diet, have to be completely remodeled to allow them to digest animal tissues. Its skull, which is made of cartilage, turns to bone, and the backbone grows. The tadpole's legs get larger as the tail shrinks, absorbed by the body. It's an astonishing and radical transformation, which takes around six weeks.

Frog croaking

DAVID ATTENBOROUGH

This little frog is tiny, hardly bigger than my thumbnail. It's a strawberry poison-dart frog. It may be very small, but when it comes to caring for its young, it's a real champion. It lives in the wet lowland forests of Central America. This female is guarding a clump of newly fertilized eggs. She and her mate will keep watch, making sure that their offspring are safe from predators. But the leaf litter is drying out, and the tadpoles need water. She must move them, and fast. So she encourages one of them to jump onto her back. But it's not a pond on the forest floor that she's looking for now. She wants a bromeliad, a vase plant, and they grow up in the branches. Climbing a tree as tall as this is an immense journey for such a tiny creature. A bromeliad plant has a tiny permanent pool of water at its center -- an excellent nursery for a tadpole -- and there she delivers it. But she has several tadpoles, so now she must rush back to the others down on the ground. She collects them one by one and carries each to its own bromeliad. She continues to make her long, arduous journeys, while the male guards the remaining eggs on the forest floor. But the little bromeliad pools don't have any food in them, so she lays an unfertilized egg in each one for each tadpole to eat. A single egg won't sustain a developing tadpole for very long, so she has to return to each pool every few days and deliver another food parcel. This continues for the next two weeks, during which time she will have traveled over half a mile, an astonishing distance for such a tiny creature. Eventually, the tadpole develops into a froglet large enough to leave the pool and fend for itself. Mum has done her job well. But it isn't always the female who takes on the task of rearing the young. Sometimes, the male does. The African bullfrog is quite a character. It's the biggest frog in Africa, and it's very aggressive. It's got a very powerful bite, for one thing. But in spite of that, it's a devoted father. Bullfrogs spawn in little pools around the margins of a larger pond, and after mating is over, one male stays to watch over them all. But the water is evaporating, and the tadpoles are now crowded together in a single pool, and that too is now drying up. The tadpoles will be dead within an hour unless the male can do something to save them. And he starts doing just that -- he begins to dig a channel to connect the tadpoles' little pool to the main pond. He must be quick. It's a major task -- but he is determined. Down they swim. The tadpoles are saved! But perhaps the prize for fatherly care should go to a rare little frog that lives in the forests of Chile. This is Darwin's frog. After the female has laid her eggs on the moist ground and the male has fertilized them, he -- apparently -- eats them. But they go not into his stomach, but into his throat pouch. And there they develop -- and wriggle. And when they are ready... one jumps out. And another. All adult amphibians are hunters. To help them catch their prey, they have a secret weapon -- a special kind of tongue. The extendable tongue is an amphibian invention. No fish ever had one. And the tongue is not attached to the back of the mouth like ours, but to the front. It's recently been discovered that, way back in prehistory, there were some frogs big enough to catch mammals. A few years ago, fragments were found in Madagascar of a really giant frog dating back from about 65 million years ago. This is part of its skull. Here is the orbit of the eye. And this is where its spine, drawing, the spine would have run down here. And this is the right cheek -- so the animal's head, when complete, would have been about that wide. So this really was a monster. And here is a computer reconstruction of the complete skeleton. The scientists who worked on it referred to it among themselves as a devil frog, and when the time came to give it a scientific name, they called it Beelzebufo. 65 million years ago was just towards the end of the reign of the dinosaurs. So maybe this giant frog actually ate hatchling dinosaurs, as is shown in this artist's imaginative reconstruction. Some frogs today also have teeth, but they don't chew with them. The teeth are used either for defense or as a way of gripping prey. This large monkey frog is in the process of swallowing a cricket... And -- strangely, perhaps -- it uses its eyes to help it do so. I'm waiting for it to burp.

Laughter

DAVID ATTENBOROUGH

As a frog swallows, it pulls its bulging eyes downwards so they help to push the food down its throat. Most frogs have very big eyes. They have to be big, because frogs, since don't have necks, can't turn their heads to look to one side. Instead, their two eyes, between them, give a frog an almost 360-degree vision. They also have a kind of see-through third eyelid, which protects their eyes underwater without blocking their sight. The red-eyed tree frog's third eyelid has a distinctive green pattern which, when out of water, helps with camouflage. Frogs have managed to adapt to a surprising range of environments. The little red-eyed tree frog lives up in the forest canopy, catching insects. And to do that, of course, it has to be an expert climber. So -- not surprisingly -- they have very special hands and feet. Its toe-pads are highly complex structures. Each has a large surface area, which helps it get a good grip. Between the six-sided skin cells, there are small channels which fill with a sticky mucus. This acts as a continuously renewed glue, but one that allows the frog to peel off its foot and re-attach it as it climbs. Some frogs have evolved a way of using their hands and feet in a quite remarkable way. In the remote rain forests of South America, there is a small amphibian known as a waterfall toad. It, like so many frogs, is a favorite meal of snakes. Many frogs avoid their enemies by hopping, but this little toad can't hop more than an inch or so. Instead, when a swift retreat is needed, it has other techniques. Free-falling -- which is easy. And stopping -- which is more difficult. Above the rain forests, there are mountains so drenched with rain that the rocks are bare, except for a coat of slimy black algae. And here you can find the pebble toad. But there are predators here, too. This is a toad-eating tarantula -- an expert in ambushing its prey. The pebble toad can't hop very far either, but it has a different defense. It clenches its muscles so tightly that it becomes as bouncy as a rubber ball. Danger averted -- and no damage done. But hunters are everywhere, not only on the ground, but in the sky. Camouflage is an excellent defense. Blink and you could easily miss this Darwin's frog. Most frogs are beautifully camouflaged, so that it's very difficult sometimes to spot them. But this one, which is the tiger-striped monkey frog from South America, has tiger stripes, orange and black, on the inside of its legs. So when it's sitting, like that, it looks green. But if they're threatened by a predator, they can suddenly open their legs and reveal that orange and black underbelly, which, people think, puts off a predator. The great majority of frogs rely on camouflage. But a few take the other option -- conspicuousness. Bright coloring can be a warning that an animal is unpleasant to eat. But some are more than merely unpalatable. This is the golden poison-dart frog from Colombia, and its skin contains enough poison to kill ten human beings -- which is why I am taking no chances and wearing gloves. Traditionally, the people in Colombia used that poison to tip their blow-pipe darts. But, of course, for the frog, the poison serves as a defense against predators, and many poison-dart frogs are very brightly colored, just to warn predators of what would await them if they did take a mouthful. The toxicity of frog skin has traditionally been exploited by local people, but modern medicine has also found ways of using it. One chemical compound from the skin of the tri-colored poison-dart frog is being used in the development of a groundbreaking painkiller, several hundred times more powerful than morphine. A frog doesn't drink. It absorbs all the water it needs through its skin. It also gets most of its oxygen in the same way. But a permeable skin that allows water to flow in also allows it to flow out, and for some that can be a problem. If there's a dry spell, this giant monkey frog from South America produces a kind of ointment from glands in its skin which it uses as sun cream. At the beginning of the dry season, it takes steps to make sure that it doesn't get sun-burnt or dry out. And with their supple joints, frogs can manage to reach all those odd places that the rest of us find a little tricky. The dry season can last for weeks, so it is best to be really thorough. Some frogs, however, never leave water at all. This is the highest lake in the world, Lake Titicaca, in the Peruvian Andes. And the frog that lives here has very different problems. At four thousand meters above sea level, there is very little oxygen in the atmosphere and therefore in the water. So the frog has developed bizarre-looking flaps and folds that increase the skin's surface area and therefore its ability to absorb oxygen. The frog also increases the flow of water across its skin by doing what looks like press-ups. Because their skin is so sensitive to their surroundings, frogs are important biological indicators. If there are environmental problems, they are among the first creatures to be affected. But that permeability has also led, in recent years, to alarming declines. This, crouched on a leaf, is one of the rarest frogs in the world. It's called the lemur leaf frog, and it lives in Costa Rica. Once it was widespread there. But today, it's been reduced to a very small area. It is nocturnal and lives in the humid rainforest of the lowlands. Unusually for a leaf frog, it has no webbing between its toes, and its stick-like legs give it a very distinctive walk. What a lovely little creature. Here in the Manchester Museum, they're studying this species and doing their best to conserve it, both by breeding it in captivity and partly by going out to Costa Rica and studying it in the wild. The cause for its loss in numbers is threefold -- loss of habitat, pesticides, and a particular lethal kind of fungus called a chytrid. The fungus causes the cells in a frog's skin to suddenly multiply, so that the outer layers thicken. That blocks the flow of essential salts through the skin. The muscles then can't function properly. And eventually the heart simply stops beating. Almost a third of all amphibians are now threatened with extinction -- including the lemur leaf frog. The future of this little frog is still hanging in the balance, but hopefully the work that's being done here in Manchester will prevent it from becoming extinct. But other frogs have not been so lucky. Over the years, I have filmed a number of different species, some of which are now extinct in the wild. The waving golden frogs we filmed in Panama belonged to one of the last remaining populations. The chytrid fungus was already spreading up from South America. So when we had finished, scientists collected all they could find and took them to specially sterilized breeding centers to keep them in safety until such time -- if ever -- the fungus disappears and they can be reintroduced to their original home. But despite the decline, the fact that frogs are such adaptable creatures does offer some hope. Some species, after all, manage to survive in places that might seem to spell certain death for creatures with moist skins. This is part of the Australian desert, where several years can pass without rain.

Thunder crashing

DAVID ATTENBOROUGH

It's a great relief when at last the drought breaks. And then -- amazingly -- little toads emerge from the sand. Numbers of them appear almost simultaneously. They have specially large legs to help them dig -- which gives them their name, "spadefoots." Now they must mate if possible before the sun rises. The desert dries very quickly, even after the heaviest of storms. Now any water on the surface will evaporate instantly. But the toads are already retreating and will soon be back underground. It really is a miracle that they are here at all. The spadefoot toad is not the only frog species to adapt to extreme environmental conditions. Of all the frogs in the world, this perhaps is the most extraordinary. It's called the wood frog. And it lives in America, north of the Arctic Circle, and it survives some of the coldest temperatures on earth. It does so by becoming frozen solid. Many animals can adapt to the harsh winter weather, but the wood frog has a unique method. As the winter frost starts to bite, ice begins to form on the frog's skin. The liver goes into overdrive, producing glucose which is pumped around the body by the heart. This glucose acts like an antifreeze within the cells, preventing them from freezing. Instead, ice forms around them. The blood, however, is frozen, and all the organs are encased in ice. And the heart stops. But then, months later, spring at last returns. And the ice around the wood frog begins to melt. The wood frog's ability to survive is truly extraordinary, and that does give us the hope that maybe in spite of the threats that face them today, frogs as a whole will continue to live on this planet. It would be truly sad if we lost them. That's it. To learn more about what you've seen on this "Nature" program, visit pbs.org.