By Hans Hazebroek
Data mainly provided by the Earl of Cranbrook.
Much of the southern portion of today’s South China Sea was dry land when the world experienced a particularly cold episode, about 21,000 years ago. This extreme lasted several thousand years. Map from Voris, 2000. Click to enlarge map.
Have you ever asked yourself what animals lived in Sarawak during the Ice Ages, when large parts of North America, Europe and Asia were buried underneath huge ice sheets? Did you know that during the coldest periods (such as the one that existed about 21,000 years ago) you could have walked overland from Kuching to Kuala Lumpur (even though you would have to make rafts to cross some big rivers that flowed where is today the floor of the South China Sea)!
A fascinating talk in October 2009 by the Earl of Cranbrook at the invitation of Dr. Charles Leh, zoologist and curator of natural history of the Sarawak Museum, shed much light on what is known about the mammals that lived in Sarawak during the Ice-Ages and what happened to them afterward. Below is a summary of his talk and paper (see reference at end of article *).
What are the Ice-Ages?
The term ‘Ice Ages’ refers to cold episodes during a period that lasted from about 2.6 million years to about 10,000 years ago. Colder periods alternated with warmer periods, and during the coldest periods a considerable percentage of the world’s water became locked-up in huge ice-sheets that extended from the North Pole far into North America, Europe and Asia. Resulting worldwide changes in sea level affected Borneo, which during colder periods was no longer an island. Due to dramatically lowered sea levels Borneo became connected to mainland Asia via dry portions of the floor of what is today the South China Sea.
These large changes in climate and the resulting effects on the environment had a tremendous influence on Borneo and on nearby islands. As sea levels fell and rose, shorelines advanced and retreated on the shallow floor of the South China Sea, so that the area of dry seafloor shrank or expanded, periodically forming a smaller or larger land bridge between Borneo, Sumatra, Java and mainland Asia. During cool periods such land bridges allowed animals to migrate from continental Asia to the islands and also between the islands themselves. Off Borneo’s east coast, however, the sea is much deeper, and continued to separate the island from Sulawesi and the other islands further east.
Primitive forms of elephant - such as this Mastodon - may have roamed in Sarawak during the Ice-Ages. Painting by Z. Burian.
Click on picture to enlarge.
The Early and Middle Ice-Ages
Two teeth of primitive forms of elephant, resembling fossils found in continental Asia, were claimed to originate from Borneo. One tooth, the type and only example of Stegolophodon lydekkeri was supposedly found in forest in Brunei. The second tooth was allegedly found near Samarinda, East Kalimantan, and identified as Elephas namadicus. If these teeth truly originate in Borneo, Stegolophodon, may have come to live in Borneo in the early Ice-Ages, and Elephas namadicus, in the middle Ice-Ages, via a land connection with mainland Asia along the northern part of the dry bed of the South China Sea.
Many fossils of mammals from the early- and middle Ice-Ages have been found in eastern Java, Indonesia. Large hoofed animals (related to those found on the Asian continent) dominate the numerous fossils found at Trinil (representing mammals that lived some 900,000 years ago) and Kedung Brubus (800,000 to 700,000 years ago). Some of these were of animals that no longer live today, such as an extinct hippopotamus, extinct relatives of the elephant, extinct cattle, an extinct buffalo, an extinct endemic antelope, an extinct giant pangolin and an extinct hyena. Other fossils represent animals that still live today, albeit in other places, such as the Malay tapir, Javan rhinoceros, barking deer or muntjac, leopard cat and tiger. Some of these animals require much land for grazing and browsing, others need woodland, while some, such as the hippopotamus, require lakes or large rivers. Thus it seems likely that the environment at that time included much grassy savanna with scattered woodland, watered by lakes or large rivers. Fossils from Peninsular Malaysia (collected in a cave at Tambun, near Ipoh) included hippopotamus and an antelope. Malay tapir and Javan rhinoceros, along with Sumatran rhinoceros were present in southern China from early through late Ice-Ages. Therefore it seems likely that at this time this group of animals and, therefore, the savanna-woodland habitat they required, extended from Java to the Malay Peninsula and further on to mainland Asia. Such a belt of a belt of savanna-woodland habitat may periodically have formed a separation between rain forest regions on either side in Borneo and Sumatra. Thus, it would have been an ecological barrier to animals adapted to live in a closed-forest habitat.
When the southern portion of today’s South China Sea was dry land, a belt of savanna-woodland probably formed a separation between the rain forests in Borneo and those in Sumatra.
Click to enlarge map.
Land-bridges between Borneo, Sumatra, Java and continental Asia
The map shows how, about 21,000 years ago during a cool period, much of the floor of the South China Sea was dry land. Maps for later cool periods show comparable patterns, although land bridges were smaller. Alternating connection and isolation of Borneo, along with an ecological barrier of savanna-woodland during periods of low sea level, help explain why the mammals that we see today on Borneo occur here, while many others found on nearby islands or on the Malay Peninsula do not live in Borneo. This sheds some light on the mystery of why as many as 38% of Borneo’s land mammal species (50 species out of a total of 132, excluding the bats) are found nowhere else, and why some distinctive species or subspecies are only found in the northern part of the island!
Bones of the Giant Pangolin excavated at Niah Caves dwarf those of the pangolin that lives today. The larger ones are thigh bones and the smaller ones are middle-foot bones. Not long after the last Ice Ages the Giant Pangolin became extinct.
Click to enlarge.
The Late Ice-Ages
Caves in Sarawak (at Niah, Sireh and Jambusan) and Sabah (Madai), provide an amazing insight into the past 50,000 years (which included the last Ice-Ages). Bones excavated from the floor of these caves belonged to mammals of which ten species are also present among the savanna-adapted animals found in Java that lived during the middle Ice-Ages. Of these, five were large animals: a giant pangolin, Javan rhinoceros, Malay tapir, tiger and the Sumatran rhinoceros.
Information obtained from tiny plant remains (pollen) found together with the mammal fossils confirms that around 45,000 years ago the area around Niah was covered in closed forest. Around 35,000 years ago the global climate began to become cooler until the last major Ice Age culminated at about 23,000–18,000 years ago. However, continuous presence of a number of animals that live in the trees, including orang-utan, gibbon and leaf-monkeys, indicate that forest continued to clothe the Niah area through the last part of the Ice Ages. Because of the much cooler climate the forest at Niah was likely of the type that we find today in Sarawak on the middle to higher mountain slopes. The jaw of a ferret badger, an endemic species nowadays found only in the high mountain environment on Mt. Kinabalu at 1,070–3,000 m elevation, was found during excavations at Niah, which supports the inference of a much cooler climate in Borneo during the late Ice Ages.
There were local extinctions, and the giant pangolin apparently disappeared early in this period. But the tiger, Javan rhinoceros and tapir probably survived into the last one thousand years. Hunting of young animals by people represented by human remains found at Niah may explain the loss of the large hoofed animals, but the disappearance of the tiger is still a mystery. Despite hunting pressure throughout the latest Ice-Ages, which continued from then on, a population of orang-utan survived at Niah until perhaps the last one thousand years. Various large, medium and small mammal species became smaller in size over a period that began during the latest Ice-Ages and stretched towards the last few thousand years. This is thought to be the effect of warming of the climate with resulting changes in the vegetation, which made living conditions more suitable to their smaller-sized offspring.
A large number of mammal species that occur on continental Asia also live today on Sumatra, Borneo and Java. This has long been recognized as the outcome of migration via land connections during the Ice-Ages. The great majority of naturally occurring mammals of Borneo, including bats, live in woodland habitats and many have not been found outside closed tropical rainforest. Orang-utan and gibbons, for instance, strictly need rain forest for their living and they were present in continental Asia throughout the Ice-Ages. They could have reached Borneo during any cool period when forested land connected Borneo to the mainland. Perhaps they migrated along the northern shore of the dry bed of the South China Sea. This route is suggested by the presence in northern Borneo (but not in the Malay peninsula, Sumatra or Java) of two mammal species whose closest relatives are found in the Indochinese region: the smooth-tailed treeshrew and the ferret-badger.
The excavation and study of mammal bones show important differences in the development and demise of the late Ice-Ages mammal populations in northwest and north Borneo compared with Java. In Sarawak and Sabah, after the disappearance of the giant pangolin, the large savanna-adapted mammals that survived after the middle Ice-Ages lived in tropical rain forest together with typical forest-adapted mammals. Although various mammal species became smaller in size, ancient and ‘modern’ mammals continued to live together long beyond the Ice Ages. It appears that both human activity and changes in climate within the last 1000 years contributed to local disappearances or island-wide extinctions of three large mammals— tiger, tapir and Javan rhinoceros.
Coordinated efforts of state, provincial and national organizations of Malaysia, Brunei Darussalam and Indonesia are currently being undertaken to implement the 'Heart of Borneo' programme. This involves connecting a number of protected areas, which stretch mainly across the central part of Borneo. Together with other existing protected areas, including Sarawak’s beautiful National Parks and Wildlife Sanctuaries, the 'Heart of Borneo' programme now appears to offer the best hope for the survival of a reasonable proportion of the forest-adapted mammals and other wildlife of this great island of Borneo.
*) Earl of Cranbrook (2009) Late quaternary turnover of mammals in Borneo: the zooarchaeological record, Biodivers. Conserv., Published online 16 July 2009