Salamanders, newts and frogs
The first forests that clothed the lands of the earth some 375 million years ago were austere, sombre places. The trees that stood ten metres or so (over 30 feet) tall carried not colourful blossoms but pods of spores. No flowers grew on the ground. There, only mosses and liverworts flourished among the rotting leaves that had fallen from above. In those places where rain or ground water were insufficient to sustain such plants, the earth was naked, except perhaps for a few lichens, for there were no grasses. And these forests were largely silent. No trilling songs rang through the branches, no roars and yelps sounded from the undergrowth, for neither birds nor mammals had yet appeared on earth.
Animals, however, there were. Giant millipedes, 2 metres (6 feet) or so long, munched their way through the leaf litter. Scorpions with their poison-loaded tails hoisted above their segmented bodies sought out primitive insects such as silverfish. Of four-limbed backboned animals, however, there was no sign-at least on the land. But in the pools and rivers, swirls in the water indicated the presence of much bigger creatures-fish. And some of them, before long, would venture up on to the land.
When exactly this crucial moment in the history of life took place and which kind of fish was the first to make the move we cannot be sure. The evidence provided by fossils is always, even at its best, very fragmentary. The chances of any individual animal leaving behind fossilised remains are infinitesimal. First, its dead body has to lie in a place where sediment accumulates. That is most commonly in a lake or the sea. Bones lying on the surface of the land are much more likely to be destroyed than preserved. Next, the sediment has to cover the bones before they disappear, preferably even before they are disarticulated. After that, the mud -and the bones within it-has to be compressed and turned into stone by the great, infinitely slow, movements that distort and crumple the earth's crust. That has to happen without the total obliteration of any sign of the bones. And finally, those bones have to be located in the tiny proportion of rocks which happen to be sufficiently close to the surface for them to be discovered by a prospecting palaeontologist. Thus not only have the vast majority of individual animals disappeared without trace but great numbers of species and families have doubtless existed of which we have no knowledge whatsoever.
It was only in the year 2004 that a fossil was found with all the anatomical characteristics that an animal needed to move from water to land. It was discovered by scientists working in the Canadian Arctic in rocks that had been laid down as sands in the bed of a stream meandering through one of the early forests. They called it Tiktaalik, a scientific name based on a word used by the local people for a large freshwater fish that today is frequently seen in the shallows there.
Tiktaalik was a large creature, up to two metres (6 feet) long. Although plainly a fish and covered in scales, it had a more mobile neck than any fish alive today and a huge mouth lined with very effective-looking teeth. But, most interestingly, the bones of its fore-limbs make it clear that it had elbows and that each forelimb ended in a fan of bony rays that could be folded outwards, like the front flippers of a sea-lion. Tiktaalik, it seems, was able to use its fore-limbs to support the front half of its body. But that support was probably insufficient to enable it to pull itself completely clear of the water. Maybe it found it profitable to lurk in the shallows along the water's edge and occasionally heave itself up on to the land to search for such invertebrates as could be collected there.
There were other fish living at this far distant time that had already developed another characteristic needed for any creature that was to spend time out of water-the ability to breathe air. Some were hefty animals, growing up to a metre (3 feet) in length. They too had paired fins with fleshy muscular bases though they lacked the line of bones needed to qualify them as legs. But they did have simple lungs. The evidence for this can, with care, be deduced from some of the better preserved fossils, but we can picture in detail the internal anatomy of these fish because they have living descendants that appear to have survived almost unchanged. They are known as lungfish.
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Lungfish have a pair of simple pouches opening from the throat that are lined with blood vessels. So when the fish fills them with air by taking a gulp at the surface, blood passing through their membranous lining is able to absorb oxygen. Three kinds of lungfish are alive today. One (Protopterus species) is found in tropical Africa and another (Lepidosiren paradoxa) in South America. Both these species, thanks to their air-breathing talents, are able to survive in swamps that dry up totally each year. In preparation for this, they dig down into the mud of their evaporating pools, wrap their tails around their heads and encase themselves in a slime that solidifies into a nearly water-tight parchment. Packaged in this way, a foot or so deep in the ground and connected to the surface by a thin air tube, they survive the dry season, breathing by means of their simple lungs. However, in contrast to those of Tiktaalik, their paired fins, fore and hind, are slender and whip-like and probably evolved rather later in their evolutionary history.
The third kind, the Queensland lungfish (Neoceratodus forsteri), is rather different. It occurs in three rivers in eastern Australia. Unlike its relatives, it cannot survive complete desiccation but its ability to breathe air by means of a lung enables it to live through the hot Australian summer, even if it has to do so in shallow semi-stagnant pools very poor in dissolved oxygen. It is a big fish, growing to a length of 1.75 metres (nearly 6 feet) and it more closely resembles its fossil ancestors than either of its living relatives. In particular, its paired fins have fleshy muscular bases very like those of the fossil form and from the way it uses them to push itself around in the shallowing pools, it is easy to imagine how valuable they must have been in the ancient swamps.
So the limbs of Tiktaalik and the lungs of living lungfish between them enable us to visualise the appearance of the animal, whatever it was, that pioneered the move out of water and on to land. The first creature to spend a significant amount of its life there was a monster about a metre long which has been named Ichthyostega, the fossils of which have been found in Greenland. Its snout was so long that its head looks more like that of a crocodile than a cod. It had a massive rib-cage that would have prevented its backbone from sagging when out of water and so allowed it to fill its lungs with air. Its tail, however, was still fish-like and fringed at the end by a fin, above and below. Such a tail looks as though it might well have been something of an impediment on land, so it is possible that this creature spent most of its life in water, sculling through the oxygen-poor swampy pools, making its way forward with prods of its fore-legs and undulations of its fin-fringed tail. But it might also, occasionally, have filled its simple lungs with air and made its pioneering advances up on to the mud to add insects and other invertebrates to its diet of fish.
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One creature alive today can give a faint impression of what those animals must have been like when alive. It is of a comparable size, growing to 1.5 metres (5 feet) in length. Swathed in a baggy black skin, with a paddle-like tail, four stumpy legs and tiny eyes on the sides of its huge head, it lurks beneath boulders in freshwater streams, coming to the surface every six to eight minutes to snatch a breath of air. This is the giant salamander of Japan (Andrias japonicus).
It is entirely aquatic, even though it has functional lungs. In spring the males fight among themselves for the possession of suitable places beneath boulders on the river-bed where they make their nests. Here the females come and each lays several hundred eggs. The male fertilises them and then drives the female away. Then for up to six months he crouches beside the eggs protecting them against predators until at last they hatch. The small creatures that emerge from the eggs are not miniature versions of their parents. Their hind limbs are mere stumps. Most importantly, they lack lungs. Instead each has a pair of fleshy feathery gills through which it absorbs oxygen from the water. It keeps this shape, slowly growing in size, until in its third year it develops internal lungs and loses its gills which simply wither away.
Such an intermediate form between egg and adult that is capable of independent life is known as a larva. All insects except the most simple and primitive pass through such a stage of development during their lives and often exploit it by drawing upon two different food sources during the course of their lives, one as a larva and the other as an adult. Amphibians are the only group of backboned animals to have such a stage in their life history. Interestingly, the newly hatched larvae of salamanders are indistinguishable to the naked eye from the hatchlings of the Queensland lungfish.
The length of time different species of salamanders spend in this condition varies considerably. Another species quite similar to the Japanese giant, the mud puppy (Necturus maculosus) that lives in North America, does not change into an adult for five years. In Mexico, in Lake Xochimilco, there is another species that may stay as a larva indefinitely and even reproduce in this form. The Aztecs called it 'axolotl' meaning water monster. In the wild, this remarkable creature is dark in colour. However it is so interesting zoologically that it has now been bred in captivity for many years and today an albino variety is better known than the wild, rarer and fully-pigmented form.
The axolotl is remarkable because it becomes sexually mature while still retaining the external gills of its larval form. It seems, however, that this is due to nutritional problems. The change from larva to adult is triggered by hormones, including thyroxine produced by the thyroid gland. Conditions in this one lake, both chemical and physical, are such that this gland does not develop properly. But that can be corrected. If an axolotl (Ambystoma mexicanum) is kept in a tank and a little thyroxine added to its water, the animal loses its external gills, climbs out of water and assumes a terrestrial life.
Another aquatic species, the olm (Proteus anguineus), however, is fixed unalterably in its larval form. It grows to about 30 centimetres (12 inches) in length and lives in cave pools and underground rivers in Italy and eastwards around the Adriatic. Like many creatures that live in permanent darkness, it is blind. Its eyes are physically present, but they are covered by skin. It has also usually lost all its pigmentation and is of an unearthly almost translucent whiteness. Only the plume-like gills on either side of its head have any colour. They are bright pink because of the blood that circulates through them.
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Olm, axolotl and giant salamander are all primarily if not entirely aquatic. Other tailed amphibians, however, divide their time between living in and out of water. Newts during half of the year lie curled up in cool damp chambers beneath stones or clambering about in the undergrowth feeding on such slugs, worms and insects as they may encounter. When spring arrives they migrate to ponds in order to breed, usually the ones where they themselves hatched. Now they become truly aquatic. The male great crested newt (Triturus cristatus), soon after he arrives in a pond, develops the finery that makes him one of the most spectacular and dramatic animals of the British countryside, even though he is only about 14 centimetres (5 inches) long. His body is black with a somewhat granular surface, but now, in the water, he develops a jagged crest along his back which continues, with a slightly smoother edge, along his tail. His flanks are spotted with silvery white and an almost iridescent stripe of the same colour runs from his pelvis to the tip of his tail. Most splendid of all, he has a bright orange belly, patterned dramatically with black.
This magnificent creature sits among the water plants, sometimes with a shaft of sunlight illuminating the silver streak on his tail. But he does not start his courtship in earnest until nightfall. Then he moves to an open space among the plants and awaits a female. She is somewhat larger than he is and plumper too, for her abdomen is packed with eggs. He moves alongside her and begins to vibrate his handsome tail. The silver stripe down its side, which reflects ultraviolet light exceptionally well, glints particularly vividly in the dim light of evening. The beats of his tail also create a current that drives pheromones, chemical messages released from his genital pore, towards the female. Occasionally, he waves his tail so vigorously that he may flick the female's head with its tip. Dramatic though this display is, it doesn't always convince the female and frequently, with a beat of her own tail, she will glide away to another part of the pond. But if she is ready to lay, she may well stay.
All amphibians have just a single aperture at the rear end of the body, the cloaca, from which comes both digestive waste and reproductive cells. From this the male now extrudes a small capsule containing sperm which sticks to a leaf or a rock. He moves forward and the female follows him until her own cloaca is directly above the sperm bundle and she then takes it in. Mating is complete.
Within her body, the male's sperm fertilises her eggs. Then she lays them, carefully planting them one at a time on a submerged leaf and folding it with her hind legs so that it forms a neat parcel. Many amphibian eggs are black with the pigment melanin that protects their delicate cells from damage by ultra-violet light, Newt eggs, however, are white and lack this pigment so they need the protection of leaves.
The female produces two or three every twenty four hours between March and mid July until she has laid up to three hundred of them. The larvae, when they hatch, have external gills but they are otherwise very similar to their parents. Almost immediately they start to feed on small crustaceans such as water 'fleas' (Daphnia) and other near-microscopic creatures. They feed and grow throughout the summer but by September their external gills have withered and disappeared. They then creep out of the pond and spend the autumn and winter months on land.
Although salamanders are closely related to newts, many of them spend very little of their lives in open water. The European fire salamander (Salamandra salamandra) is one such. It is a magnificent creature, its moist skin a glossy black marked with bold sulphur-yellow blotches. These vary, both in size and distribution. Some individuals seem to have yellow bodies blotched with black. Mating time might be thought to be the one period in their lives when they would return to water, but in fact fire salamanders mate on land. The male chases the female. When at last he catches up with her, he crawls beneath her and entwines his arms with hers so that he is carrying her, piggy-back. He then deposits his sperm bundle on the moist ground and twists his body to one side so that the female is able to pick it up in her cloaca in much the same way as newts do in water.
Her independence from water continues. Instead of laying eggs in a pond which hatch into gill-equipped water-breathing larvae, she retains her eggs within her body. There, nourished by generous packets of yolk that she attached to each one of them, the eggs develop into larvae. The female now searches for open water. Even a troy puddle will do. And there she ejects her babies, one at a time. The stage in the larvae's life at which this happens varies. In northern Europe, the newly emerged young have external gills. In the Pyrenees and north Spain, on the other hand, some female salamanders retain their young for so long that the larvae pass through the aquatic phase and shed their external gills while they are still within her so that by the time they emerge they are miniature versions of their parents.
Excerpted from Life in Cold Bloodby David Attenborough Copyright © 2008 by David Attenborough Productions Ltd. . Excerpted by permission.
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