Tuesday, March 19

Ice Age mammals of Borneo


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?

ONCE JOINED: 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)

ONCE JOINED: 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)

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 had to make rafts to cross some big rivers that flowed where today is the floor of the South China Sea)!

A fascinating talk last year by the Earl of Cranbrook, at the invitation of Sarawak Museum zoologist and natural curator Dr Charles Leh, shed much light on what is known about the mammals that lived in Sarawak during the Ice Ages and what happened to them afterwards. The following is a summary of his talk and paper.

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 sea floor 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 was much deeper, and continued to separate the island from Sulawesi and the other islands further east.

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 was 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 required much land for grazing and browsing, others needed woodland, while some, such as the hippopotamus, required lakes or large rivers. Thus it seems likely that the environment at that time included much grassy savannah 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 the early through late Ice Ages.

Therefore it seems likely that at this time this group of animals and, therefore, the savannah-woodland habitat they required, extended from Java to the Malay peninsula and further on to mainland Asia. Such a belt of savannah woodland habitat may periodically have formed a separation between rainforest 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.

Land-bridges between Borneo, Sumatra, Java and continental Asia

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 savannah 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 per cent 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!— Data mainly provided by the Earl of Cranbrook.