Islands and island continents

Isolation is one of the most important mechanisms of evolution. When a group of creatures becomes separated from the main breeding population, the separated group evolves independently of the parent group because there is no longer any possibility of interbreeding. The new group interacts with its environment, changing into new forms and evolving along lines that would be totally closed to it if it were living among its original enemies and competitors. This phenomenon is particularly marked where animals become isolated in sparsely or hitherto unpopulated areas and is nowhere better seen than on the islands of the oceans.
There are two main varieties of isolation in this context, each producing its own environmental pressures and giving rise to its own forms of evolution.
The first takes place when one continental mass splits away from another. What then happens to their fauna is largely dependent on the subsequent movements of the two continents. One land mass may drift northwards or southwards with respect to the other, subjecting its fauna to new climatic and environmental conditions which could affect their evolution and ultimately lead to the production of totally new genera and species. Exactly this happened during the Age of Reptiles, when the South American continent, which had shared the same dinosaur fauna as Africa, split away, resulting in the evolution of totally different animals in each area.
When a drifting continent collides with another, very often a considerable interchange of faunas takes place between the two land areas. It may happen that the fauna from one continent completely replaces that of the other. This happened when the small continent that is now the Indian Peninsula collided with mainland Asia.
The second form of biological isolation occurs when a completely new group of volcanic islands is formed. In plate tectonics much of the activity between adjacent crustal plates takes place in the open ocean. New plates are created along the mid-oceanic ridges and are destroyed as they slide beneath one another in the deep oceanic trenches. Such violent activity produces earthquakes and volcanic eruptions, creating new islands from the ocean bed.
The volcanic islands, quite barren to begin with, are soon colonized by living organisms. Plants, germinated from windblown seeds, are usually the first to arrive and take hold followed by the insects. The first vertebrate inhabitants are usually flying creatures such as birds or bats. Only later do the other vertebrates, usually reptiles and small mammals, arrive, sometimes on floating branches and tree trunks - the result of some river flood hundreds of kilometres away. All these creatures then evolve independently of their ancestral continental stock to fill all the ecological niches of the island. The classic example of this sequence is the colonization of the Galapagos Islands off the west coast of South America during the early part of the Age of Mammals. These islands were initially populated by a small number of species, which gave rise eventually to a vast array of new creatures, including four-eyed fish, marine lizards and giant tortoises. The island's fauna, particularly the inter-island differences between related species, was thoroughly investigated and stimulated the development of evolutionary theory.

SOUTH AMERICAN FORESTS

The effect of continental drift on animal communities

The nightgligers erectile spines are formed from modified hairs that, in the course of evolution, have developed into stiff needle-like structures. The nightglider parachutes silently down on to its quarry, impaling it on its chest spines.

There are several species of nightglider.

Each is camouflaged against particular species of forest trees.

The tiny male matriarch tinamou spends entire life as a parasite on the back of a female.

The male has large claws on its feet and a single claw on each wing.

Although during the first half of the Age of Mammals South America did have a small population of primitive placental mammals, it was, like Australia, a bastion of the marsupials. However, just before the Age of Man, a land bridge was established between South America and North America which led to an exchange of faunas between the two areas. The result was that the placental mammals from the north, being more versatile, almost entirely replaced the marsupials and the primitive placentals of the south. The northern fauna were more versatile because they had been subjected to greater selective pressures in the preceding 50 million years; they had been compelled to adapt radically in the face of changing environmental conditions brought about by such factors as ice ages and faunal exchanges with Asia. The result at the time of the collision with South America was a very hardy and adaptable stock of animals. The mammals of South America, on the other hand, had experienced a stable unchanging environment during the same period and therefore lacked this essential adaptability. A similar fate did not befall the marsupials of Australia, since that continent, in drifting northwards, presented its fauna with constantly changing conditions, resulting in a population of hardy species that were able to survive the faunal exchanges that occurred during the period shortly after Australia impacted with Asia.
Twenty million years after the Age of Man the land connection with North America was again broken and South America became an island continent once more. Since the split, climatic conditions on the South American continent have remained unchanged and the fauna has therefore changed very little. This conservatism is well seen among the mammalian predators - a niche that has continued to be occupied by members of the order Carnivora despite the fact that this group has declined elsewhere.
The foremost predator of the South American tropical forest is the gurrath, Oncherpestes fodrhami, a giant hunting mongoose. Its ancestor, Herpestes, was introduced by man to the then offshore islands at the north of the continent, where it became a pest and overran them. When the islands became fused to the mainland the mongoose spread southwards and developed into its present jaguar-like form. Its chief prey is the tapimus, Tapimus maximus, a long-tusked rodent that feeds in open areas of the forest.
A much smaller carnivore, the nightglider, Hastatus volans, is derived from tree-dwelling mustellid stock. During the day it hangs on trees disguised against the bark, floating down to feed upon nocturnal insects, frogs and small mammals at night. Its method of hunting is to empale its prey on the spines that project from its chest. One of the strangest birds of these regions is the matriarch tinamou, Gynomorpha parasitica. The female of the species is a ground-living bird, very much larger than the male whom she carries around on her back. The male's wings and digestive system are degenerate and he is entirely parasitic on the female, sucking her blood through his needle-shaped beak. The male's only biological function is to provide sperm during mating. This relationship arises from the species' low population density, which makes it an advantage for each female to have a mate constantly available rather than to search for one each breeding season.

SOUTH AMERICAN GRASSLANDS

Evolution on an island continent

The flower-faced potoo sits on the pampa with its mouth open during the middle part of the day when insects are flying.

The strick has a small head with long ears and wide nostrils.

The strick is bipedal, running on the tips of long two-toed feet.

The wakka is bipedal, but unlike the strick it has no front paws to help it keep its balance and instead relies on its tail.

Throughout its history, the movement of the crustal plate carrying the South American continent has been predominantly westwards, and hence the landmass has tended to remain within the same latitudes. This accounts for the constancy of the climatic regions and the conservatism of its fauna.
During the continent's early history the grasslands, or pampas, supported their own fauna of running ungulate animals, similar to, but totally isolated from, those in other parts of the world. These animals existed until the continent became joined by a land bridge to North America, when they and the native marsupial population were swept away completely by the influx of animals from the north. Strangely enough the northern ungulates did not find a permanent foothold on the pampas, but rather rodents such as the maras, Dolichotis, and the capybaras, Hydrochoerus, present at the time of man, were the more successful. In this respect the South American continent anticipated the rise of the advanced running rodents and lagomorphs in the rest of the world.
Once the continent separated from the supercontinent of the north the rodent fauna developed along its own lines. The running animals of the pampas are dominated by strange bipedal grazers, which are descended from the jumping rodents that evolved in the rain-shadow deserts along the western mountains. Although long hind legs evolved independently among desert rodents in other continents only those of South America changed from a jumping to a running mode in the course of their evolutionary history. Along with this change of gait went an increase in size and a change of dentition that effected the final transition from the jumping, gnawing rodent of the desert to the striding grazer of the plains.
The most generalized running rodent is the strick, Cursomys longipes, which looks very much like the grazing marsupial kangaroos that once existed in Australia. They graze among the long grasses in tightly knit groups that are large enough to ensure that there are always at least two or three individuals with their heads up, looking around for danger while the rest have their senses buried in the grass.
The most specialized creature in this family of animals, and perhaps the most highly adapted running animal in the world, is the wakka, Anabracchium struthioforme. Because of its bipedal stance its forelimbs have become less important and are now completely atrophied. Its globular body and long hind legs support an equally long neck and tail which balance one another, maintaining the animal's centre of gravity over its hips. These features give the creature a clear view of the surrounding countryside. Even when the wakka is grazing in long grass its eyes are placed high enough on its elongated head for it to see the approach of a predator.
The flower-faced potoo, Gryseonycta rostriflora, is the oddest bird found on the grasslands. The interior of its beak is coloured and patterned like the petals of a flower, so that when it has its mouth open it looks exactly like an open bloom. This elaborate mimicry is designed to deceive insects and provides the potoo with a meal by merely opening its mouth. Because tropical grassland flowers appear only when there is adequate moisture, the potoo migrates seasonally with the rains.

THE ISLAND OF LEMURIA

The bastion of the hoofed animals

The valuphant is found only on the plains of Lemuria.

Massive horns, growing to nearly a metre in lenght, are the valuphant's main means of defence. Its eyes and ears are small to keep out the dust.

The valuphant is a valuable element of the Lemurian ecology. In digging for the roots on which it feeds it disturbs the soil and stimulates the regrowth of vegetation.

The long-necked yippa, dependent on trees for food, migrates to the edge of the tropical forest during the dry season.

The snorke is a grass-grazer. As the herds move across the plains feeding, they expose the lower layers of vegetation, which provide food for the smaller herbivores.

The movements of the earth's crustal plates that carry the continents and account for continental drift are the result of convection currents deep in the earth's mantle. The currents can build up stresses beneath the continents which eventually tear them apart.
Normally an elongated rift valley, associated with considerable volcanic activity, forms first of all along the line of the eventual split. The land on either side then separates and moves apart, an ocean area growing steadily to fill the gap. This happened when the small island continent of Malagasy split away from mainland Africa 100 million years ago, and again more recently when the whole of eastern Africa split away to form Lemuria.
In the case of Lemuria the separation occurred before the ungulate herds of Africa had been replaced by the rabbucks from temperate latitudes. As a result hooved animals are as plentiful on the grassy plains of Lemuria as they ever were in Africa before the Age of Man.
The valuphant, Valudorsum gravum, is the largest ungulate. It is a massive animal some 5 metres long with a squat, rounded body and massive legs, resembling those of the gigantelope to which it is distantly related. Its most distinctive feature is the tall ridge running down its back and neck. The ridge is supported by the neural spines of the vertebrae and may be of use in regulating the animal's temperature.
The valuphant feeds only on herbs and roots, which it gouges up with its horns. The grasses themselves are eaten by more lightly built fleet-footed ungulates such as the snorke, Lepidonasus lemurienses. The snorke has a very long head with its eyes placed near the top - an adaptation that enables it to keep a watch for predators while grazing. The upper layers of vegetation are exploited by the long-necked yippa, Altocephalus saddi, which can reach the leaves and young shoots of the savanna trees,
The cleft-back antelope, Castratragus grandiceros, a creature that is superficially similar to the ancestral antelope, has a curious symbiotic relationship with the tick bird, Invigilator commensalis. This relationship is really no more than a strengthening of the symbiosis that had developed between birds and grazing animals during the early part of the Age of Mammals. Birds on the grassy plains often accompanied the larger mammals, catching the insects disturbed by their hooves, or pecking ticks and mites from the hides of the animals themselves. The grazing animals tolerated this as the birds rid them of parasites and also gave warning of approaching danger. In the case of the cleft-back antelope the relationship has become more intimate and the animal's back has ceased to be a mere perch and has become a nesting site. Along the animal's back is a pair of ridges, supported by outgrowths from the vetebrae. Between the ridges is a deep cleft lined by stiff hairs that provide an ideal nesting medium for the tick bird. Several families may nest on its back at one time. Superficial warts on the animal's flanks produce pus at certain times of the year. The pus attracts flies, which lay eggs in the warts. The flies' maggots appear just as the young birds are hatching and provide them with a ready-made source of food. In return the antelope is supplied with both a constant grooming service and an early-warning system that alerts it to approaching predators.

THE ISLANDS OF BATAVIA

An island world of bats

The streamlined aquatic surfbat is descended from a conventional flying bat ancestor. Its flippers, formed from what were once wings, have become stubby and muscular.

On land the surfbat leaps along on ins tail and forelimbs. When resting its tail is curled under its body.

Excepting for the 'thumbs', the fingers of the shalloth's hand-like front feet are fused together.

Although volcanic mountains and islands usually form where two crustal plates meet and crush against one another they also form over 'hot spots' on the earth's crust - areas lying above intense activity deep in the earth's mantle. Directly over the hot spot a volcano is formed. When the crust passes away from the centre of activity the volcano becomes extinct and a fresh one then erupts alongside it, producing in time a chain of progressively older volcanic islands in the middle of the ocean. During the Age of Man, a hot spot was responsible for producing the Hawaiian island chain, and in the Pacific at the present time a hot spot is in the process of generating the Batavian Islands.
Birds are usually the first vertebrates to reach and settle on new islands, but in the case of Batavia the first vertebrates to arrive were their mammalian equivalents, the bats. By the time that the birds did arrive, the bats were so well established that there were few unoccupied evolutionary niches left and the birds have never colonized the islands to any extent. The presence of suitable food on the ground, and the absence of predators enabled many bats to take up a terrestrial existence and to fill a large number of ecological niches.
The flooer, Florifacies mirabila, has remained an insect-eater, but is now largely sedentary. Its brightly coloured ears and nose flaps mimic a species of flower found on the islands. It sits among them with its face turned upwards, snapping at any insect that attempts to land. Although it has arisen independently, the flooer's feeding mode is remarkably similar to that of the flower-face potoo, Griseonycta rostrifiora, of South America and is an interesting instance of convergent evolution.
The flightless shalloth, Arboverspertilio apteryx, is an omnivorous tree-dwelling bat which spends its life hanging upside down like the ancient sloth. It eats leaves and the occasional insect or small vertebrate caught by a swift jab of its single claw.
The beaches are home for the packs of surfbats, Remala madipella, which fish in the shallow waters around the coral reefs. Their hind legs, wings and tail flaps have developed into swimming and steering organs and their bodies have become sleek and streamlined. Their evolution from a flying, through a terrestrial form, into an aquatic creature is very similar to the evolutionary development of the penguin.
Once other vertebrates had established themselves on the islands, a family of ground-dwelling predator bats arose. These creatures walk on their front legs - on what would, in the case of a flying bat, be its wings, the site of most of its locomotor muscles. Their hind legs and feet are still used for grasping, but now fall forward to hang down below their chin. As the bats locate their prey purely by echolocation, their ears and nose flaps have developed at the expense of their eyes, which are now atrophied.
The largest and most fearsome of these creatures is the night stalker, Manambulus perhorridus. One and a half metres tall, it roams screeching and screaming through the Batavian forest at night in packs. They prey indiscriminately on mammals and reptiles, attacking them with their ferocious teeth and claws.

THE ISLANDS OF PACAUS

The evolution and versatility of the Pacauan whistlers

Nut-eater (Insulornis macrorhyncha) - heavy bill for cracking shells.

Insect-eater (Insulornis piciforma) - strong, pointed bill for penetrating the bark of trees.

Predator (Insulornis aviphaga) - powerful hooked bill for tearing flesh.

The terratail gains protection from the strong resemblance that the tip of its tail bears to the head of a bird snake.

The Pacaus bird snake is the Pacauan whistler's most important predator.

Several thousand kilometres east of the Australian sub-continent lies the island chain of Pacaus. It was formed during the last 40 million years by friction between the northward-moving Australian tectonic plate and the westward-moving Pacific plate. At the margin between the two plates, volcanic islands were thrown up which gradually acquired accretions of coral round their shores.
After the ash and lava slopes were covered with vegetation and an insect population had been established, the island began to be colonized by birds. The first birds to arrive were the golden whistlers, Pachycephala pectoralis, which were blown across the ocean from Australia. Originally a fairly unspecialized bird it had, during the Age of Man, begun to show some differentiation, with distinct beak shapes developing on the islands around the Australian coast. However, it was only on the Pacaus Archipelago, where all the ecological niches were thrown open to them, that the whistlers really developed spectacularly, producing both insectivorous and seed-eating as well as predatory forms.
The descendants of the particular group of golden whistlers that colonized these islands are now regarded as belonging to a single genus, Insulornis. All the species within this genus are now highly specialized and quite different from one another excepting I. harti, which is similar in form to the original ancestral bird.
I. piciforma has developed a strong, chisel-like bill with which it tears into the bark of trees to get at burrowing insects. Its feet are modified to allow it to cling to the vertical trunks and the bird closely resembles the extinct woodpeckers of the Northern Continent whose mode of life it closely follows in almost every respect.
Nuts and tough seeds are eaten by I. macrorhyncha, a parrot-like species which has developed a massive bill and the powerful musculature to operate it. This bird has retained the perching feet of its ancestor and has grown a long tail to balance the weight of its large head.
All the Pacauan whistlers are preyed upon by their hawk-like relative I. aviphaga, which shows the same adaptations that are found in birds of prey throughout the world, irrespective of their ancestry - a hooked beak, binocular vision through forward-facing eyes, and a high degree of manoeuvrability in pursuit.
Apart from the hawk whistler the only natural enemies faced by the Pacauan whistlers are the snakes, which have been rafted to Pacaus from Australia or the other islands in that corner of the Pacific at one time or another. The Pacauan whistlers' wariness of snakes is exploited by the terratail, Ophiocaudatus insulatus, a timid rodent and one of the few mammals living on the island chain. The markings on its tail mimic to a remarkable degree the markings on the head of the Pacaus bird snake, Avanguis pacausus, one of the most active and vicious snakes of the archipelago. When threatened by a bird, or indeed by any other creature, the terratail throws its tail into the typical snake-threat posture and utters a realistic hiss. It makes its escape rapidly into the undergrowth while its enemy is still recovering from the shock.

INTRODUCTION BY DESMOND MORRIS 9

AUTHOR'S INTRODUCTION 10

EVOLUTION 11

Cell Genetics : Natural Selection : Animal Behaviour : Form and Development :
Food Chains

HISTORY OF LIFE 22

The Origins of Life : Early Living Forms : The Age of Reptiles :
The Age of Mammals : The Age of Man

LIFE AFTER MAN 33

The World after Man

TEMPERATE WOODLANDS AND GRASSLANDS 36

The Rabbucks : The Predators : Creatures of the Undergrowth :
The Tree Dwellers : Nocturnal Animals : The Wetlands

CONIFEROUS FORESTS 50

The Browsing Mammals : The Hunters and the Hunted : Tree Life

TUNDRA AND THE POLAR REGIONS 58

The Migrants : The Meaching and its Enemies : The Polar Ocean :
The Southern Ocean : The Mountains

DESERTS : THE ARID LANDS 70

The Sand Dwellers : Large Desert Animals : The North American Deserts

TROPICAL GRASSLANDS 78

The Grass-eaters : Giants of the Plains : The Meat-eaters

TROPICAL FORESTS 86

The Tree-top Canopy : Living in the Trees : The Forest Floor :
Living with Water : Australian Forests : The Australian Forest Undergrowth

ISLANDS AND ISLAND CONTINENTS 100

South American Forests : South American Grasslands : The Island of Lemuria :
The Islands of Batavia : The Islands of Pacaus

FUTURE 113

The Destiny of Life

APPENDIX 117

Glossary : The Tree of Life : Index : Acknowledgements