BORNEO'S ENDEMIC SPECIES
Endemism in Bornean Mammals
Dr Colin Groves
Department of Prehistory & Anthropology
Australian National University, Canberra, A.C.T.2601, Australia
Borneo, Sumatra, Java, and many smaller islands of the western Indo-Malayan archipelago rest on the Sunda Shelf, a shallow-water expanse which was exposed during periods of low sea-level (most lately, in the Upper Pleistocene, ending some 10,000 years ago). This does not mean that Sundaland, as the shelf and its islands are known, was a featureless plain across which mammals could move unhindered; on the contrary, there were broad, ramifying river systems running between the main land-masses (Ollier, 1985), and the forest belts were periodically reduced and reexpanded (Flenley, 1985). The expected picture is, therefore, that there will be a certain level of endemism on each of the main islands, but the degree of endemism, and the taxonomic level it reaches, cannot be predicted. By studying the phenomenon, in fact, we can generate hypotheses about the past distribution of land-bridges and vegetation zones, and these can be tested when geological and palaeobotanical data become available.
ENDEMISM IN SUNDALAND
Groves (1985) lists the numbers of species of indigenous terrestrial mammals on the main components of Sundaland. The listing excludes marine mammals and bats: although many bats are very restricted in their distribution, they have at least the capability of flying long distances, including over water barriers.
West Malaysia, as part of the mainland, has a richer mammalian fauna than the Sundaland islands: 112 species for a land area of only 50,000 km2. As we go further from the mainland, species richness declines: Sumatra, with a land area eight and half times that of Malaysia, has only slightly more terrestrial mammals (117); Borneo, half as large again as Sumatra, has 119; while Java, two and a half times as large as West Malaysia, has only 61.
The number of species that actually extend from the Malay Peninsula to any given island is even less than these diminished richness values would suggest. Of the Malay Peninsula's 112 species, 92 (that is, 82%) extend to Sumatra, 66 (that is, 59%) extend to Borneo, and only 38 (or 34%) extend to Java. (A few species, absent from the mainland, are common to two or more of the Sundaland islands). Subfossil discoveries may increase the number of shared species somewhat (the early Holocene of Borneo had tapir (Acrocodia indica),wild dog or ajag (Cuon javanicus) and Javan rhino (Rhinoceros sondaicus): see Cranbrook, 1986, 1988), but the fall-off remains. In the case of Borneo, the depauperisation has deprived it of Sundaland's largest carnivore, the tiger. The rest of the species are found on one of these islands alone: that is, they are endemic.
Whereas 4.5% of West Malaysia's species are endemic, 10% of Sumatra's, and 23% of Java's, a full 32% of Borneo's terrestrial mammals are endemic: that is, 38 species (Groves, 1985). This very high total goes some way towards "topping-up" the fauna in compensation for the fade-out effect mentioned above.
Borneo's endemic species can be divided into three groups. First are 10 species distributed over the entire island (Table 1a); most are found on a few offshore islands as well as the main island, and three extend onto the Karimata Islands. Next are 21 species whose distribution centres around the northeast: Sabah, eastern Sarawak, northeastern Kalimantan. Many of these are restricted to high altitudes, but this is by no means always the case: why an altitude-tolerant species like Presbytis hosei, for example, should be restricted to that region is mysterious. Finally, we have a group of "other distributions": four known only from the Baram River region, and three which seem to be spread along the periphery of the northeastern area inhabited by the second group. It is interesting that no species are yet known which are restricted to other parts of the Borneo such as the southeast or southwest.
It should also be mentioned that three genera are endemic to Borneo. There seems to be no doubt about the generic status of the two sciurids; that of Diplogale is perhaps marginal, but then others maintain that the Proboscis Monkey, Nasalis, represents a monotypic genus. The problem is whether these are relics of formerly wider distributions, or whether they have evolved to generic level on Borneo; this is of course the same problem as with the endemic species themselves, a problem to which we will now turn.
There is no doubt that some of Borneo's endemic mammal species are simply well-differentiated variants of species living elsewhere. The very rare Bornean Bay Cat, Profelis badia, is close to the Golden Cat, P.temminckii, which has a wide distribution from Sumatra north into southern China; it is, however, so different cranially that it has on occasion been placed in its own genus, Badiofelis. The same may be true of Presbytis hosei, Hylobates muelleri, Petaurillus hosei, Callosciurus adamsi, Lariscus hosei, Dremomys everetti, Maxomys alticola, M.ochraceiventer and Chiropodomys major: that they are Bornean sister-species of congenerics living elsewhere in Sundaland (or, in the case of Chiropodomys, in Palawan). [Rattus baluensis, however, is not one of these: it bears no close relationship to its reputed sister-species, Rattus korinchi, on Sumatra (Musser, 1986)]. This accounts for just 10 of the 38 endemics: the others are what we may call Supplementary Species, that is species which are in addition to the baseline Sundaic complement.
The distribution of these Supplementaries is often bizarre, and none more so than those belonging to the genus Presbytis, the Leaf-monkeys. The non-endemics all replace each other geographically: P.comata in Java, P.potenziani in the Mentawai Islands, P.thomasi in northern Sumatra, P.melalophos over most of the rest of Sumatra, and (the most widespread of them) P.femoralis in eastern coastal Sumatra, the Malay peninsula, the Natuna Islands, and extending into Borneo, mainly in Sarawak). But the Borneo endemics overlap both each other and P.femoralis: P.hosei in the northeast, P.frontata in the centre and southeast, and finally the widespread P.rubicunda, the Maroon Leaf-monkey, which overlaps all the other three; though whether there any any forests in which all four actually co-occur I do not know (possibly the Baram region?). Why this superfluity of potentially sympatric leaf-monkeys? The only Sundaland primate without a representative on Borneo is the Siamang, Hylobates syndactylus; field studies might reveal whether it is this species whose absence on Borneo vacates a niche for one or more Presbytis, or whether we must look further afield - hornbills, for example.
This whole problem is closely interwoven with the question of whether such Bornean Supplementaries are relict, having been at one time more widely distributed, or are locally evolved. In at least one case the Bornean endemic is the most primitive of a widespread genus, suggesting strongly that it is indeed a relict: this is the Yellow Muntjac, Muntiacus atherodes. But there are, certainly, some that are evidently locally evolved, being highland derivatives of widespread lowland species, such as Callosciurus baluensis (a local derivative of C.prevostii), Rattus baluensis (a derivative of R.tiomanicus), and probably Chiropodomys muroides (derived from C.gliroides).
In some species a single subspecies is known or thought to be distributed over the whole of Borneo. This may be very strongly differentiated from its conspecific(s) elsewhere, such that full species status has even been considered (Tupaia glis, Pongo pygmaeus); or it may be indistinguishable from representatives on Sumatra; or the question may be more complex. I will briefly consider a few of these cases.
The Sumatran Rhinoceros, Dicerorhinus sumatrensis, has a well-marked subspecies on Borneo (D.s.harrissoni), distinguished by its small size and other features, as well as by its lack of sexual size dimorphism. Interestingly, it can be shown that as far as the main distinguishing feature, the size difference, is concerned, the differentiation has occurred rather recently, within a short space of time. A rhinoceros radius, used as a pillow for a Neolithic-age burial in Niah Cave, Sarawak, is 350mm long, exceeding the mean value for the living Bornean subspecies by 19%, and even exceeding the mean of Sumatran and mainland subspecies by 13%. An early Holocene humerus from Sumatra similarly exceeds the mean for the living Sumatran/mainland Dicerorhinus by 13%, and living Bornean by 22%. A hypothesis to explain this might be that a single, large-sized chronosubspecies (for which the name D.s.eugenei has been proposed) was common to be both Sumatra and Borneo, and that this became reduced in size, apparently independently, and much more in Borneo than in Sumatra. Two rats, Leopoldamys sabanus and Sundamys muelleri, can also be shown to have decreased in size during the Holocene (Medway, 1964), although in this case there is no relationship with present-day subspecific differentiation.
The Sun Bear, Helarctos malayanus, also has clearly distinct subspecies in Borneo and elsewhere. In this case, there is no sexual size dimorphism in either subspecies.
Unexpectedly (because there is a persistent rumour that it is introduced, not indigenous), the Elephant, Elephas maximus, may also differ subspecifically on Borneo and Sumatra. Skulls with M6 in wear are much smaller, age for age, in Borneo than in Sumatra; but those with M5 in wear are not, nor are they different in size, though samples are very small. This implies that after M5 is in wear, Bornean females stop growing, Bornean males and Sumatran females grow just a little, and Sumatran males grow a great deal. The sizes of the teeth themselves do not differ between the two islands, however.
Another case of clear size differentiation, associated with sexual dimorphism, between Sumatra and Borneo, is the Yellow-throated Marten Charronia flavigula. Less clear-cut is the case of the Leopard-cat, Prionailurus bengalensis; in this case there is also a slight colour-pattern difference between the two subspecies.
The Marbled cat, Pardofelis marmorata, shows no noteworthy size differences, and Bornean and Sumatran representatives differ only slightly, on average. On such evidence no subspecies can be recognised.
The Flat-headed Cat, Ictailurus planiceps, is more curious. Only the female seems to differ: females are smaller than males on Borneo, but apparently not on Sumatra. This case clearly needs further study.
Finally Borneo's largest carnivore, the Clouded Leopard, Neofelis nebulosa, shows no signs of being in any way different from the Sumatran representative, let alone subspecifically.
As well as species which seem not to vary between Borneo and Sumatra, there are several which vary within Borneo itself and show mosaic affinities between Borneo and Sumatra. One of these is the gibbon, Hylobates muelleri. There has in the past been some doubt whether all Bornean gibbons actually belong in this species, as the southwestern form, albibarbis, more closely resembles the Sumatran/Malayan H.agilis in vocalisations, if not in colour: a study on the hybrid zone between these two taxa by Robert Mather is in process of elucidating details on this remarkable case of speciation in progress. Another such case is Prevost's Squirrel, Callosciurus prevostii, which divides into two distinctive subspecies-groups: those in which the limbs are black, hardly (or not at all) different from the body colour; and those in which the forearms are red and the hindlimbs grey, sharply contrasting with the black body tones. The black-armed subspecies are confined to Borneo; the red-armed ones occur in the Malay Peninsula and Sumatra, and in western Borneo: this is not quite the same division as within the gibbons, but recalls it very closely. It is perhaps also appropriate to recall that Courtenay et al. (1988) found that craniometrically the Orang Utans, Pongo pygmaeus, of western Borneo are also closer to the Sumatran subspecies than are those of the rest of Borneo.
Borneo has evidently proved relatively easy to get into, but difficult to get out of: a good number of species have differentiated there, as well as some distinctive subspecies. There are also some species that are evidently primitive relicts, and some that have evolved in particular locales from more widespread congeners. Other species treat the seaway between Borneo and Sumatra as if it did not exist. For all these reasons, Borneo may be regarded as a natural laboratory of mammalian evolution. There is a pressing need for further study of its indigenous mammals: their biodiversity, distribution patterns, ecological constraints. If the habitats remain fairly intact, this can be done; to destroy them with biological studies still in their infancy is not only an international scandal but a scientific crime.
Courtenay, J., C.Groves & P.Andrews. 1988. Inter- or intra-island variation? An assessment of the differences between Bornean and Sumatran Orang-utans. In J.H.Schwartz, ed., Orang-utan Biology, 19-29.
Cranbrook, Earl of. 1986. A review of fossil and prehistoric remains of rhinoceroses in Borneo. Sabah Mus.& Archive J., 1:50-111.
Cranbrook, Earl of. 1988. The contribution of archaeology to the zoogeography of Borneo, with the first record of a wild canid of Early Holocene age. Fieldiana, N.S.42:1-7.
Flenley, J.R. 1985. Quaternary vegetational and climatic history of Island Southeast Asia. Mod.Quaternay Res.SE Asia, Mod.Quaternary Res.SE Asia, 9:55-63.
Groves, C.P. 1985. Plio-Pleistocene Mammals in Island Southeast Asia. Mod.Quaternary Res.SE Asia, 9:43-54.
Medway, Lord. 1964. Niah Cave bone. VII. Size change in the teeth of two rats, Rattus sabanus Thomas and R.muelleri Jentink. Sarawak Mus.J.11:616-623.
Musser, G.G. 1979. Results of the Archbold Expeditions. No.102. The species of Chiropodomys, arboreal mice of Indochina and the Malay Archipelago. Bull.Amer.Mus.N.H.162:377-445.
Musser, G.G. 1986. Sundaic Rattus: definitions of Rattus baluensis and Rattus korinchi. Amer.Mus.Novit.2862:1-24.
Musser, G.G., J.T.Marshall & Boeadi. 1979. Definition and contents of the Sundaic genus Maxomys (Rodentia, Muridae). J.Mamm.60:592-606.
Ollier, C.D. 1985. The geological background to prehistory in Island Southeast Asia. Mod.Quaternary Res.SE Asia, 9:25-42.