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| Acknowledgments | vii | ||||
| PART I EVOLUTION | |||||
|
3 | (14) | |||
|
17 | (15) | |||
|
32 | (17) | |||
| PART II BEHAVIOR | |||||
|
49 | (17) | |||
|
66 | (14) | |||
|
80 | (17) | |||
|
97 | (17) | |||
|
114 | (17) | |||
| PART III THE FUTURE OF THE SPECIES | |||||
|
131 | (16) | |||
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147 | (12) | |||
| Notes | 159 | (10) | |||
| References | 169 | (16) | |||
| Index | 185 |
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Chapter One
ARRIVING AT
AN ISLAND HOME
On a narrow road winding through the lower mountains and rain forest of Kalimantan, an orangutan exacts a toll from vehicle drivers who enter his territory. Without fail, he emerges from the thick jungle to stand in the middle of the track with palm outstretched. He is a lone and awesome figure, conspicuous now, but at other times completely hidden from view in the dense green foliage. A small piece of food or a trinket will satisfy him, and he will then disappear until the next intruder is heard in the distance. He is a wild orangutan and seeks no other human contact.
He symbolizes the pressing need to protect his habitat. The rain forests of Southeast Asia are the richest and most diverse in the world, teeming with the earth's greatest variety of plant and animal species. Wild orangutans are usually found in the uppermost treetops. They mingle with many other animals in the vaulted canopy high above ground level, feeding on leaves and fruits. Birds have no trouble flying from treetop to treetop, and some of the mammals have developed ways of gliding from tree to tree using sails made of skin flaps stretched between hindlimbs and forelimbs. The gliding squirrel moves about in this way, and there are even flying frogs and gliding snakes among these high-rise dwellers.
Orangutans are the heaviest tree-living animals, and they move from tree to tree by swinging on the branches. Often they climb clasping with hands and feet in all directions or they hang beneath the branches and move about by grasping with one hand after the other (known as brachiating ). Sometimes they propel themselves from one tree to another by standing more or less upright on a supple trunk or branch and setting it in motion like a swing. In this way they can make a large arching movement. After swaying back and forth several times, they reach out for a branch of another tree while letting go of the first one. Only rarely has an orangutan been seen to leap from one branch to another, as do so many smaller primates that live in the trees. Watching from below, we notice how agile they are.
Island Life
Orangutans live in the forests on just two islands of the Malay Archipelago--Sumatra and Borneo (Figure 1.1). Kalimantan is the Indonesian part of Borneo, and Sarawak and Sabah are the Malaysian parts of the same island. On these islands orangutans survive in small areas of rain forest that have escaped the bushfires and the logger's ax. In Sumatra, their range is restricted to the Gunung Leuser National Park and a few areas adjoining it, all in the north of the island. In Borneo, orangutans are more widely spread, but even so, their ever-diminishing range covers much less than one-third of the island.
As a result of the felling of the rain forests to make way for agriculture and to obtain timber for building, furniture, and even for disposable wooden crates, the number of orangutans is declining rapidly. The orangutan is now on the list of endangered species. Estimates indicate that the total population of orangutans in Sumatra and Borneo together declined from 80,000 in the 1980s to only 20,000 in the mid-1990s. Since the devastating bushfires of recent years, their number is probably much less than 20,000.
Today orangutans survive in the last outposts of their range, formerly much more extensive. They used to occur on mainland Asia, throughout all of Southeast Asia, including Myanmar, Laos, Thailand, Cambodia, and Peninsular Malaysia, as well as in Java and in their present homes in Sumatra and Kalimantan. Fossils of orangutans have been found in all these regions Their range also extended into the southern part of China as far north as Beijing, and even east to regions that are now the islands of Taiwan and Hainan. This was their distribution 2-3 million years ago, and it seems to have remained so until about 10,000 years ago. Only since that time have they become entirely island dwellers.
The First Orangutans
We can find out something about the first orangutans by unearthing their fossils and examining them in detail. Usually only fragments of fossils are found, and the most common fragments are teeth. These are made of hard materials that survive the passage of time, and orangutans' teeth are protected by an exceptionally thick layer of enamel. A tooth can be recognized as belonging to an orangutan by the presence of wrinkles on the grinding surface of each molar, a likely adaptation to eating fruit, although leaves and occasionally meat may also have been part of their diet.
Teeth of ancient orangutans have been found in the Yunnan and Guangxi provinces of Southern China and in Laos, Vietnam, Sumatra, Java, Kalimantan, and Sarawak. Most of the teeth found in these parts of Asia are about 2 million years old. None are older than this. This indicates that orangutans arrived in the southeastern region of Asia about 2 million years ago, during the geological epoch known as the Pleistocene.
Judging by the size of some of these ancient orangutan teeth, particularly those found by T. Harrisson in the Niah caves in Sarawak, they belonged to heftier specimens than their present-day relatives. These giant apes may have been up to two meters or more in height, and their weight may have forced them to spend a lot more time walking on the ground than climbing in the treetops. Richard Smith and David Pilbeam reason that ancestral orangutans spent as much time on the ground as present-day chimpanzees. Even today, heavier adult male orangutans (90-100 kilograms) probably spend much more time on the ground or closer to the ground than lighter females and juveniles.
From the eighteenth century on, there have been occasional reports of very large modern-day orangutans walking upright on the ground. The well-known orangutan researcher John MacKinnon was shocked when he almost bumped into a huge orangutan walking along a path. Likewise, Biruté Galdikas, who has spent most of her life studying orangutans, spoke of her encounter with an extremely large walking orangutan. He was ambling along a path, looking down, at first unaware of her presence. On catching sight of Biruté, some four meters away, he stared at her and then suddenly whirled around and was gone.
But where did the large ancient orangutans come from? We must go back to the very first apes. Fossil records suggest that the first apes appeared 17-23 million years ago, during the early geological epoch known as the Miocene. The tail is the most easily distinguishable feature separating apes and monkeys. Apes do not have tails, whereas monkeys do. Apart from humans, the apes alive today are the gorillas and chimpanzees, living in Africa, and the gibbons and orangutans, living in Asia. All these existing apes evolved from more ancient ancestral forms
The first primates without tails evolved in the warm rain forests of East Africa. The fossil of an ancestor of modern apes was found there, and it is called Proconsul . From its anatomy, Proconsul appears to have moved about mainly by walking on the ground using its feet and hands in quadrupedal style, although it may also have climbed trees. When Proconsul walked on the ground, it did so using the palms of its hands to give support, a method used by orangutans today. The weight is carried on the outside edges of the palms. This means of supporting the weight while walking contrasts with that used by gorillas and chimpanzees: They curl their fingers toward the palm and support their weight on their knuckles, not their palms.
About 17 million years ago, ancient apes like Proconsul gave rise to a branch of evolution that eventually led to the gibbons of today. This happened well before the ancestor of the orangutan appeared. Before the ancestral orangutan evolved, apes like Proconsul migrated from Africa into Europe. By dating the fossils of apes found in Europe, we know that this took place 14-16 million years ago. Somehow these ancient apes managed to cross the stretches of water that separated the European continent from North Africa (Figure 1.2). They may have reached Europe via the Gibraltar-to-Spain route or via the Middle East route. Although these are the shortest crossings, neither of these routes to the European continent would have been particularly easy. In fact, at that time, the continent of Africa was further from Europe and the Middle East than it is today. There were, however, times when the sea levels were lower and land bridges may have existed. Perhaps the ancestral apes crossed from Africa to Europe on these land bridges or perhaps they drifted on floating rafts of debris. Rafts could have formed from the dense plant growth occurring in the forests along the coastline of North Africa at that time. These are only ideas--no one knows how the apes managed to get to Europe.
In Europe, the ancestral apes evolved into many different forms and spread across the continent. Some of them migrated to Asia and may have gotten as far as Southeast China 5-10 million years ago. The first creatures that could be the ancestors of modern orangutans evolved a long way from their present home in Southeast Asia, in a region known as the Siwaliks, now in North Pakistan. Fragments of fossil skulls of apes called Sivapithecus have been found in this region. There were several species of Sivapithecus , which is the name of their common genus. Anthropologists have painstakingly pieced together Sivapithecus skulls to discover their similarities to, and differences from, present-day orangutans. Bone material taken from skull specimens of Sivapithecus has been used to determine the age of the skull, revealing that Sivapithecus lived 8-12 million years ago, in the late Miocene epoch.
The earliest ancestors of the orangutan looked rather like present-day orangutans, although the skull was slightly broader and the limbs less mobile. They were larger than their modern descendants, although some of their relatives became even larger still, as the now-extinct Chinese orangutans show. The large size and reduced mobility of the limbs of Sivapithecus indicate that these apes spent more time on the ground than their lighter present-day relatives. Nevertheless, they still depended on the forest to survive. Sivapithecus met its ultimate fate and became extinct when the dense forests and woodlands of northern Pakistan and India disappeared. This came about as the world's climate became colder, drier, and more seasonal. Grasslands, deciduous forests, and scrub began to replace evergreen forests. By the end of the Miocene, all the ancient forms of apes in Europe and most parts of Asia were extinct, apart from some known to have been still surviving in China and a few other regions of East Asia. Monkeys in the same regions survived the colder drier climates because they had adapted to life in more open habitats. Apes require more specialized climatic conditions than monkeys, and so they survived only where the climate remained wetter and warmer than elsewhere.
Some people believe in the existence of the Yeti of the Himalayas and say that it might be a special, shy kind of orangutan that has adapted to living at high altitudes in freezing temperatures. This is almost certainly fantasy. As far as we know, orangutans need warm rain forests to survive. Today they do not even occur at the higher altitudes in the mountains of Borneo.
Fossil remains of orangutans that must have looked almost identical to present-day orangutans are about 2 million years old. Most scientists agree on this date for the first appearance of modern orangutans, but there are some who would like to set the date earlier, at 5 million years ago. This difference in timing is really not very large on an evolutionary timescale--whether 2 million or 5 million years ago, the date of the first appearance of orangutans that looked the same as we know them today was quite recent in geological time.
Reaching the Islands
It would be interesting to know when orangutans spread from mainland Southeast Malaysia to Borneo, Sumatra, and Java. Looking at a map of the present time, it seems they had to cross vast stretches of ocean between the mainland and these islands, Borneo in particular. But at the time the orangutans made their way to these regions, Southeast Asia looked very different from its appearance today. Two million years ago, at the very most 5 million years ago, the landmasses that were to become the islands of Borneo and Sumatra were still connected to mainland Asia, as shown in Figure 1.3. There was a large bridge of land stretching from mainland Malaysia to Borneo and another, smaller bridge across to the southern end of Sumatra.
Many maps of the land formations at that time show Java connected to the same landmass as Sumatra and Borneo in the Pleistocene epoch, but Robert Hall, of the South East Asia Research Group at London University thinks differently. According to Hall's recent geological information, the northern tip of Java was separated from the southern tip of Sumatra by a narrow strait of sea, just as it is today. Further geological research is needed before we can be sure of Java's position at the time orangutans reached it and left their fossils there. The matter is complicated by fluctuating sea levels caused by the freezing and melting of the polar ice caps (see below).
The early orangutans, together with many other species, could have migrated from the Asian mainland across the land bridges and into Borneo and Sumatra. The concept of migration, as it is used here, has nothing to do with migration in birds or humans. It refers to the colonization of new areas over a long period of time and many generations of orangutans--a gradual spreading of the population.
If there was no land bridge between Sumatra and Java at the time, perhaps the early orangutans crossed the short stretch of sea between the two islands by floating on branches or other debris. It probably happened accidentally, possibly by the orangutans being swept away in violent storms. Most people would assume that orangutans do not construct rafts to cross water, after deciding that they want to get to the other side--we have reserved such ways of thinking for humans. We assume humans are the only species that can plan ahead and wonder what is around the corner or on the other side of the ocean or river. But Biruté Galdikas has seen an orangutan at her rehabilitation center dragging logs to a stream and then placing them to make a bridge to allow him to cross to the other side. That orangutan had certainly formed the intention of crossing to the other side.
These observations make us wonder whether the ancient orangutans may have intended to cross the ocean between Sumatra and Java and even that they might have constructed some sort of raft to do so. Most anthropologists believe that orangutans learn such skills only when they have been in close contact with humans; they merely imitate the use of boats by the people living and working in their vicinity. Perhaps no wild orangutan would construct a raft or plan to cross a stretch of water, but we would like to keep the possibility open.
Of course, there may have been a time around 2 million years ago when the stretch of ocean between Sumatra and Java was narrower than it is today, and so crossing might not have been as daunting a task as we think. There were times when the levels of the ocean were much lower than today and more land was exposed; glacial periods dominated the earth's climate, locking up the water of the oceans in the polar ice caps and lowering the level of the seas as a result. It is even possible that for brief periods of geological time, the sea level was low enough to afford the orangutans a land crossing between Sumatra and Java or between Borneo and Java. Rafts or floating debris may not have been needed.
The glacial periods were broken by times when the polar ice caps melted and the sea levels rose again. Geologists refer to these times as interglacial periods. The islands of Borneo, Sumatra, Java, and the thousands of smaller islands in the Malay Archipelago were formed during an interglacial period, and that is the state of the world today. In fact, at the present time the sea level is about as high as it has ever been and only about half of Southeast Asia is exposed. The last high point of a glacial period (i.e., a glacial maximum) occurred 21,000 years ago.
The polar ice caps have formed and melted about twenty times over the last 2 million years, and the sea levels have fluctuated by more than 200 meters. As a result, islands in the Malay Archipelago have come and gone. But the glacial periods were longer than the melting periods, and they provided the opportunity for orangutans and other animals to migrate from mainland Asia by a land route to Borneo, Sumatra, and maybe even Java.
Orangutans did not spread any further east than Borneo to the island of Sulawesi, probably because Sulawesi was separated from Borneo by a large stretch of very deep water, known as the Makassar Strait. A line can be drawn along the Makassar Strait separating the flora and fauna of Asia from the flora and fauna of the islands to the east of it (see Figure 1.1). This line is called the Wallace Line. To one side of it are the Asiatic plants and animals of mainland Asia, Borneo, Sumatra, Java, and Bali and to the other side of it are entirely different species on the islands of Sulawesi, Irian Jaya, Australia, and many smaller islands. Orangutans are on the Asian side of the line.
The presence of land bridges between the mainland and the islands of Borneo and Sumatra might explain how orangutans eventually reached their island homes, but their passage from the mainland would not have been without enormous obstacles. In the Pleistocene era when this migration occurred, major rivers flowed across the land bridges, especially across the bridge between the mainland and Borneo The huge North Sunda River flowed northward. Other rivers flowed across the trail between Sumatra and Borneo. They were all mighty, fast-flowing rivers, and migrating animals would have had to cross them. Somehow the orangutans did it, either by chance or design. If they had not done so, they would have become extinct. Only by reaching the areas that were to become their island homes did the orangutans survive.
The type of plant growth on the land bridges would also have created difficulties for the orangutans. The land bridges were unlikely to have supported the orangutans' preferred habitat of tropical rain forest because the rainfall would have been too low and seasonal during the glacial periods. The bridges are thought to have been corridors of savanna, covered in grasses and scrubby trees. This would have been hostile to orangutans because their diet probably consisted mainly of the leaves and fruits of the forest. On the other hand, orangutans today eat hundreds of different types of food, and their dietary adaptability may, perhaps, have allowed them to find sufficient food in the savanna.
The ancient orangutans may have had to climb trees to escape predators, particularly during the night--if that is why they sleep in nests above ground level. We should not forget that leopards and tigers were migrating over the same territories as the orangutans. Perhaps the migrating orangutans were able to find pockets of lowland forest on the savanna corridors and move between them. In fact, judging by the numbers of present-day orangutans living in different types of forest, it seems that they prefer lowland forest to higher mountain forest. Most of them live in the lowland forests, which have tall dipterocarp trees (endemic to Southeast Asia), at altitudes of less than 150 meters. These days, the logging and clearing of the lowland forests is driving them into the higher areas, but that is not where they choose to be. They did once live on the higher slopes of Mount Kinabalu, a mountain more than 4,000 meters high in Sabah, but they are no longer found at altitudes greater than 1,500 meters.
Seas and rivers were not the only obstacles for the migrating orangutans. High mountain ranges were another barrier. Orangutans would have met a major mountain range on the landmass that became Borneo. Today it is called the Crocker Range and runs through the middle of Borneo roughly parallel to the western coastline. Volcanoes within this range were active several million years ago. No orangutan would have braved the high altitudes and precipitous barriers.
Barriers and Genetic Variations
The presence of the high Crocker Range may explain the genetic differences that exist between at least two or three separate populations of orangutans in Borneo today (see Chapter 2). Genetic differences refer to differences in the inherited material (genes) inside the cells of the body. Genes are passed on from one generation to the next, carrying the information that influences the way individuals develop and function. Genetic variations, for example, make orangutans different from humans, and they are also the cause of differences between orangutans in different areas. Genetic differences begin to develop when populations of the same species are separated for long periods of time. If the two populations are separated for an extremely long time, they may become so different that they eventually form separate species. Of course, experience also causes differences to develop in different populations, and this needs to be taken into account too.
(Continues...)
Excerpted from THE ORANGUTANS by Gisela Kaplan Lesley J. Rogers. Copyright © 2000 by Gisela Kaplan and Lesley J. Rogers. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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