Proboscis monkeys in Danau Sentarum National Park.
Introduction--Ecology of Proboscis Monkeys
The proboscis monkey Nasalis larvatus belongs in a monotypic genus. Its closest relative is the pig-tailed langur Simias concolor, endemic to the Mentawai Islands off the west coast of Sumatra and classified by some authors as Nasalis concolor (Bennett and Gombek, 1993; Napier and Napier 1967). The proboscis monkey is a large colubine; adult males grow to 25 kg with females weighing between 7-11 kg (Schulz, 1942). They have bright, rufous-colored fur, fading to creamy, silvery white under parts and insides of limbs. Adult males have pure white rumps and tails (those of females are often gray) and the male develops a long pendulous nose, giving the species its name. Ankel-Simons (2000) explains that the nose of the male assists in enhancing vocalizations by acting as an organ of resonance. Proboscis monkeys also have partial webbing on the hind feet, a physical characteristic shared with no other primate, and may be an evolutionary adaptation for swimming and walking on mangrove mud (Bennett, 1986). This speci es is the most aquatic of all extant primates.
The local names for the proboscis monkey vary from different parts of Borneo, the most common being Bekantan, Beladan, Bangkatan, Pikas, Rasong and Orang Belanda. In DSNP, most of the locals use the name Pikas.
Distribution of Proboscis Monkeys on Borneo
The Proboscis monkey is endemic to Borneo and restricted to lowlands habitats. In Malaysian Borneo, the species is scattered and survives only in isolated pockets. The largest known populations in Sarawak are in the west, in the areas of Samunsam and the Bako-Sarawak Mangroves areas. The Samunsam Wildlife Sanctuary was established specifically to protect this population of proboscis monkeys (Basiuk, 1985). Recently gazetted extensions to this sanctuary in 2000 have brought the total area to 228 [km.sup.2], encompassing over 90% of the population's range. In Sabah, viable populations of proboscis monkeys occur in the freshwater wetlands of the Kinabatangan Floodplain (Boonratana 1993; Sharma 1992), while the mangroves and estuaries of Brunei Bay, Brunei Darussalam, support another population (Bennett and Gombek, 1993).
In Kalimantan, the strongholds for the species are Gunung Palung National Park, West Kalimantan (AWDB, 1993; Bennett and Gombek, 1993); Tanjung Puting National Park, Central Kalimantan, (AWDB, 1993; Bennett and Gombek, 1993); the Mahakam river basin (MacKinnon et. al., 1996; AWDB, 1993; Eve and Guige, 1989) and Kutai National Park (AWDB, 1993), both in eastern Kalimantan.
There are only four populations occurring inland on Borneo: Danau Sentarum (Sebastian, 1994; Giesen, 1987); upper Sungai Barito (MacKinnon et. al., 1996; Bennett and Gombek, 1993); Mahakam lakes (MacKinnon et. al., 1996) and the Kinabatangan wetlands (MacKinnon et. al., 1996; Bennett and Gombek, 1993). Of these four, only Danau Sentarum and the Mahakam lakes are wetland ecosystems discontinuous from the coast.
The proboscis monkey is described as being a folivore/frugivore (Yeager, 1989). Yeager (1989) found that in Tanjung Puting National Park in Indonesia, they utilized 47 different plant species, of which 17 plants were consumed for their fruit, seeds, or flowers and 30 species were consumed for their leaves. In the Kinabatangan floodplain in Sabah, Boonratana (1993) estimated that young leaves account for 72.7% of the diet of the harem groups at Sukau (riverine forest) and 49.7% at Abai (mangrove forest). Proboscis monkeys are very selective feeders and groups move directly between food sources, sometimes covering large distances (Yeager, 1989; Bennett and Sebastian, 1988).
Early work on this species suggested that they were dependent on mangrove forests for food and cover (Kawabe and Mano, 1972; Kern, 1964), but recent studies have shown them to be as commonly found in riverine forests, peat swamp and freshwater swamp forests (Salter et al., 1985). The reason proposed for the apparent preference of this colubine for coastal forests and areas along rivers is because the soils of the interior are low in minerals and salts, which are needed in the diet (Bennett and Sebastian, 1988).
This primate lives in harems centered around single, dominant males, though often group movements are led by females (Rajanathan and Bennett, 1990). Groups vary in size; in Samunsam Wildlife Sanctuary the average size of six identified groups was nine animals (Bennett and Sebastian, 1988), while the average size of ten identified groups in Tanjung Puting National Park was 12.1 animals (Yeager, 1993). Young and sub-adult males are excluded from the harems and form all-male "bachelor" groups which are flexible and non-permanent associations. These bachelor groups follow established harems closely.
Harems also frequently travel together, forming large associations of more than one harem but are always discernable as different groups by the dominant males. Two levels of social organization are suggested for this species: the group and the band (Yeager 1990, 1992). The primary social unit is the harem group while the band represents a secondary level of social organization with fission-fusion of stable harems within bands. These bands can comprise over sixty animals and may travel together for many days.
The proboscis monkey is found only along forested waterways. Home ranges can vary greatly depending on density of population and habitat types. Three major studies have found harem group home ranges to vary; 9 [km.sup.2] in Samunsam, Sarawak (Bennett and Sebastian, 1988); 2.21 [km.sup.2] in Sukau and 3.15 [km.sup.2] at Abai, Sabah (Boonratana, 1993); and 1.3 [km.sup.2] at Tanjung Puting, Indonesia (Yeager, 1989).
Although this species is known to have very large home ranges, groups never range very far from rivers. Bennett and Sebastian (1988) found that groups never ventured more than 600 meters from a river's edge. In Sukau, Boonratana (1993) and Sharma (1992) found this distance to be 1 km. This effectively restricts their range to long strips along rivers, but this is not the case in swamp forests and mangroves where extensive waterways are interlinked.
Home ranges of harem groups also overlap freely and no evidence of territorial behavior has been recorded to date. Yeager (1989) found the average spatial overlap of riverbank habitat used by four harems in Tanjung Puting to be between 92.0 and 97.7%.
The use of riverine habitat at night for sleeping serves an important need for groups to remain in contact. Bennett and Sebastian (1988) found that harem groups in Samunsam Wildlife Sanctuary spent the majority of nights within 50m of another and more than 70% of the nights within 100m. Groups frequently keep in contact with other groups by calls and the adult males make distinct loud, far-carrying resonant calls, intermittently throughout the night. Sleeping along the river's edge is thought to increase visibility and suggests that proboscis monkeys require both vocal and visual contact. This behavior is a characteristic of this species but has never been satisfactorily explained.
The affinity for water is thought to have an anti-predation function. Well-adapted for swimming, proboscis monkeys may use water as an escape from arboreal predators, although the Clouded Leopard Neofelis nebulosa is probably the only arboreal carnivore on Borneo capable of taking proboscis monkeys. Davis (1962) documents predation by N. nebulosa and more recently in 1992, this has been observed in Sukau, Sabah (Cede Prudente, pers. comm.). Galdikas and Yeager (1984) describe a crocodilian, False gavial Tomistoma schlegelii, preying on proboscis monkeys in Tanjung Puting. Although the False gavial is considered to be exclusively fish-eating, they grow to very large sizes and may occasionally take swimming proboscis monkeys. In 1994, a 4m long False gavial was found accidentally killed in a jermal net at DSNP. Its stomach contained distinct fur balls, suggesting that it preyed on mammals. (Giesen, pers. comm.) However, it is more likely that estuarine crocodiles Crocodylus porosus will take swimming proboscis monkeys given the opportunity. An observation by the author of a crested serpent eagle Spilornis cheela stooping into a tree of proboscis monkeys in Tanjung Puting National Park at dusk could also have been an attempt at predation. Boonratana (1993) recorded a similar attack by a serpent eagle in Sukau, Sabah. This may suggest that S. cheela may be a greater predator on proboscis monkeys that was previously thought, though this medium-sized eagle can only take young animals.
Proboscis monkeys probably occurred throughout the coastal lowlands of Borneo in the past. The increase in human population and urbanization over the last 100 years has resulted in large areas of the coastal lowlands being converted to cultivation and towns. This conversion has predominantly occurred close to rivers and estuaries, thus wiping out most of the habitat for this species. Today, proboscis monkeys are restricted to remaining undisturbed stretches of riverine forest, mangroves, swamp and floodplain forests and their numbers are greatly depleted. Isolation of remaining populations and extensive fragmentation of habitat has seriously threatened the survival and long-term viability of the species. The IUCN Red Data Book (IUCN, 1996) lists the proboscis monkey as Vulnerable and it is listed under Appendix I of CITES. It is also protected by law throughout its range.
Only three populations are known to occur entirely within a protected area: Tanjung Puting National Park, Samunsam Wildlife Sanctuary and DSNP. Varying portions of populations elsewhere come under legal protection where their ranges extend into sanctuaries, national parks and wildlife reserves. It is a high conservation priority to ensure the long-term maintenance of genetic diversity of the species throughout Borneo and thus, more protection must be afforded to this species.
THE PROBOSCIS MONKEY POPULATION AT DSNP
Origins of the Danau Sentarum Population
There are very few significant populations of proboscis monkeys that occur away from the coast, and DSNP may support the largest inland population on Borneo. Though primarily a coastal species, inland ecosystems similar to those more commonly found on the coast, such as floodplain wetlands, have the potential to support proboscis monkey populations.
Proboscis monkeys have been recorded inland in the past, away from any river or waterway. These records have been attributed to natural, occasional dispersal of the population. Occasionally, an individual or a group may leave an area and turn nomadic, wandering until another suitable habitat is chanced upon (Bennett and Gombek, 1993).
The author postulates that this population in Danau Sentarum may have originated from the river basin of the Batang Lupar and Batang Saribas in Malaysian Sarawak, rather than the Kapuas River itself. The geographical location of the Kapuas lakes is much closer to the northern coast of Borneo than the western coast, and may have been part of the catchment of the northward flowing river basin in the geological past. Small populations of proboscis monkeys still occur in the delta of these two rivers in Sarawak.
The exact size of the proboscis monkey population of DSNP has not been determined. The only assessment of this population was conducted in 1993-1994, before gazettement of the park. Sebastian (1994) estimated the total population size by assigning an average group size based on known group sizes from other sites where long-term studies have been conducted. Bennett and Sebastian (1988) found average group sizes in Samunsam Wildlife Sanctuary to be nine, Sharma (1992) found the average to be 11 in the Kinabatangan Floodplain while in Tanjung Puting National Park, Yeager (1993) found the average to be 12.1 animals (refer Table II). This is based on the assumption that this population of proboscis monkeys does not differ greatly from others in terms of their group sizes.
Using a group size of 10.7, and working with a total area of 80,000ha, the estimated population permanently resident within the boundaries of DSNP was a minimum size of 600 (Sebastian, 1994). Given the available habitat surrounding the park, an estimate of an additional 30% would not be unreasonable, giving a possible population within the environs of DSNP as 800. This figure would make this the largest population of proboscis monkeys on Borneo, and therefore worldwide.
The total area of DSNP has been increased to 130,000ha in 1996, with the inclusion of proposed extensions. This is a 60% increase in the size of the protected area. DSNP needs to be re-surveyed to obtain a more accurate account of this globally significant population of proboscis monkeys.
The population within DSNP does not fluctuate significantly in response to wet and dry seasons. During the 1993-94 surveys, about 74% of the population is resident within the park throughout the year, and the remaining 26% uses habitats outside the park boundaries (Sebastian, 1994). Virtually all the habitats used by the population outside the reserve then are now within the park.
The main difference in the dry season is the increased area of habitat accessible to the population, resulting in slight dispersal. During the wet season, large areas of dwarf forest (Giesen, this volume) are almost completely submerged, with only their crowns above water. Proboscis monkeys will not use such areas. Although the population adapts its behavior (sleeping requirements) to use the dry areas, they basically remain in the same general areas. The rapidly rising water levels can concentrate the population into the areas of remaining habitat during the wet season.
The population within DSNP is unevenly distributed, and occurs mainly in the western half of the park. The distribution of habitat types has a major influence on the population distribution. Table I compares the details of the four main concentration areas with those of another outside the park boundary.
An estimated 475 [km.sup.2] of forest area remains above the floodwaters during the wet season. Forest cover significantly above the water line is vitally important to proboscis monkeys. Of this, it is estimated that the available habitat for proboscis monkeys within the reserve during the wet season does not exceed 270 [km.sup.2] (Sebastian, 1994).
The calculated overall population density within DSNP is 0.16 groups/[km.sup.2] or 1.70 individuals/[km.sup.2] (Sebastian, 1994). However, the population shows marked concentration in certain parts of the park (refer Table I). The highest densities were recorded in the Sg. Belitung - Sg. Punggau swamp forest (1.2 groups per [km.sup.2]) and the Sg. Tawang area (0.6 groups per [km.sup.2]).
The density of proboscis monkeys in DSNP appears to be considerably lower than other populations studied (refer Table II). There are two possible reasons for this. Firstly, DSNP is a very large area of dendritic waterways, making it impossible to survey all the rivers and waterways simultaneously. Thus prevents an accurate census. Furthermore, groups that do not sleep by the water's edge are impossible to census from a boat. This means that the actual densities at DSNP may be considerably higher.
The second possible reason is food. The floodplain ecosystem of DSNP is characterized by long periods of inundation alternating with drought-like conditions, and consequently has a reduced floral diversity, and a dominance of certain species. Added to the fact that the soils are nutrient poor, food may not be plentiful, nor of high nutritional value. If this is so, this might explain why the proboscis monkey is the only colubine to colonies this habitat. In areas where food is scarce, one would expect larger colubines to survive in favor of smaller species (Bennett, 1983). The proboscis monkey is the largest and heaviest of the Southeast Asian colubines, and thus is the most likely to be able to derive sustenance from the lower quality food source at DSNP.
37.5% of harem groups were recorded in vegetation with canopy height of 10in or lower and 56.3% were recorded in 11-20m tall vegetation.
The vegetation types within the park are not diverse. Single species dominance is common, especially in small tree / shrub vegetation (dwarf swamp forest; Giesen, this volume) and areas within the park differ significantly depending on flood levels and periods of inundation. Giesen (this volume) describes three main vegetation types; Dwarf swamp forest, 5-8(- 10)m tall, with Putat Barringtonia acutangula and a few other species; Stunted swamp forest, l2-20(-22)m, with two main types: one characterized by Kamsia (Mesua hexapetalum), Kenarin (Diospyros coriacea) and Menungau (Vatica cf. umbronata), the other by an absence of Kenarin and Menungau, and a presence of Kawi Shorea balangeran, which can be locally dominant; and Tall swamp forest: dominated by the occurrence of tall (25-30(-35)m) straight-stemmed trees, in areas that are flooded for 2-3 months annually by 1-2.5m of water. Two types recognized: (a) Kelansau-Emang-Melaban tall swamp forest, characterized by the occurrence of Kelansau Dryobalanops abnorm is, Emang Hopea mengerawan and Melaban Tristaniopsis obovata is the most common type of tall swamp forest; and (b) Ramin-Mentangur Kunyit tall swamp forest which may formerly have been more widespread, but as Ramin Gonystylus bancanus is much sought after by commercial timber companies, it is now uncommon, and now occurs only very locally.
On the drier levees of the bigger rivers and waterways, taller riparian forests of between 15-25m height occur. Riparian forest in much of the area appears to have many of the same species as the Kenarin-Menungau-Kamsia stunted swamp forest, but is characterized by the presence of typical riparian species such as Rengas Gluta renghas and Bungur Lagerstroemia speciosa. This vegetation type occurs on levees of the larger rivers in the Park (e.g. Sg. Tawang, Belitung and Empanang), and has a flooding regime similar to that of the stunted swamp forest.
Proboscis monkeys show a distinct preference for tall forest in DSNP. This is consistent with findings from other parts of Borneo where they prefer tall riverine forests (Salter et al. 1985). Therefore, during the wet season, the main concentrations of the population occur in the tall swamp forest and extend into the stunted swamp forest areas. Inundated dwarf swamp forest will be used to cross between taller forest areas, but are seldom used to sleep. During the dry season, the dwarf swamp forest areas become available to the population, which disperses more widely into these areas.
Proboscis monkeys in DSNP will readily come to the ground and are therefore often recorded in low vegetation along rivers during the day, but invariably seek taller trees to spend the night in. The lack of extensive and continuous tall habitat within the park appears to have resulted in the monkeys adapting to lower vegetation for sleeping.
The most significant behavioral observation recorded for this population is the instances where harem groups were seen not to sleep by rivers at night. Three instances were recorded where harem groups spent the night away from the river's edge (Sebastian, 1994). The furthest distance from a river was 770m.
This behavior of sleeping beside a river or water body remains one of the most enigmatic characteristics of this species. Proboscis monkeys are known to invariably sleep by rivers, and the main reasons suggested for this are social factors such as visibility and vocalizations and to a lesser extent, displacement and predation avoidance.
The records of harem groups not sleeping by a river in DSNP is of considerable interest to this debate. The question raised by this finding suggests that the forest edge may be of greater importance than the presence of water. However, another explanation may be valid given the habitat of DSNP. During the dry season, the water body shrinks significantly, leaving only small channels and eventually, only the main river still flowing. Foraging groups have to constantly navigate the maze of dry channels, necessitating ground crossings to get from one forest area to the next. The demand that these movements place on energy expenditure may be high. Therefore, remaining where they are, despite the lack of water, may be more energy efficient than traveling the large distances back to the river every night.
Observations in November 1999 at Samunsam Wildlife Sanctuary recorded at least four harem groups traveling away from the river's edge at dusk (unpublished data, Sebastian and Bennett). This was the first record of this behavior at Samunsam, and the reasons for this remain unknown.
FACTORS AFFECTING THE POPULATION AT DSNP
In DSNP, areas of suitable habitat for proboscis monkeys are fragmented, and the distances between these remaining habitat pockets place a constraint on their usage by the population. Therefore, only a certain portion of the park can support a viable population in the long term.
In addition to loss of habitat, availability of favorable habitat varies during the dry and wet seasons. The wet season inundation of large areas of potential habitat causes a further reduction of available habitat within the park. This results in a concentration of the population in remaining suitable areas and the movement of some of the population to the periphery of the park along rivers.
Fragmentation and Isolation
The continuing conversion of riverine forests outside the park to agriculture effectively restricts this population to within the park boundaries. This loss of habitat is exacerbated by indiscriminate burning, which occurs within DSNP as well (Dennis, this volume). The resulting fragmentation of habitat causes isolation of small populations. Within the park itself, there are already at least three distinct populations: Sg. Seputung/Sg.Sengirin, Sg. Batang Ketam/Sg. Belitung and Sg. Tawang/Hutan Nung (Sebastian, 1994).
Fragmentation of the population is not caused only by human activity. The characteristics of the floodplain vegetation itself contributes to isolation. The width of most of the rivers which remain navigable during the dry season is significant. This requires groups to come to the ground, walk considerable distances to the water's edge and then swim a distance of at least 20m in most cases, to the other bank.
There is a high level of human activity along rivers in DSNP during the dry season. This presents a significant problem for a riverine species such as the proboscis monkey. During the dry season, activities such as fishing, fish-salting and smoking, trapping and also temporary encampments all take place along riverbanks. This may be an important factor keeping harem groups away from the water's edge (Sebastian, 1994).
There is also human activity within the forest, such as collection of rattans, bee-honey (though mostly during the wet season), and firewood.
No hunting of proboscis monkeys is known within the park. Observations however, suggest that disturbance occurs. Groups that move out of the park may be subject to hunting pressure there.
Food availability during the dry season is an important limiting factor for the population. This would be particularly significant if they are unable to move elsewhere when food supplies are low. Furthermore, the availability of food within the park may be seasonal, as is often the case in areas of monotypic vegetation. A preliminary study of plant phenology, based on monitoring a limited number of species (30) for a year, has shown that about 1/4 tend to flower and fruit, at low densities, throughout the year, about 1/8 flower and fruit in the dry season and that the majority (about 5/8th flower/fruit in the wet season (Giesen, this volume).
The quality of food may also be an influencing factor. A large animal such as the proboscis monkey can subsist on a lower quality diet, i.e. it can subsist on foods with a lower protein to fiber ratio than smaller colubines (Bennett, 1983). Therefore, if the food quality is indeed low in DSNP, the larger proboscis monkey may be the only colubine that is able to survive there.
MANAGING THE POPULATION AT DSNP
Management of the proboscis monkey population at DSNP will not require any specific actions to be taken involving the monkeys directly. The key to ensuring the long-term viability of this population is to address all the current direct threats and to apply management to the sources of indirect threats.
All the issues relating to the conservation status of proboscis monkeys at DSNP can be placed into two main categories: addressing loss of habitat and controlling human activities that adversely impact upon the population.
Protection of Core Proboscis Monkey Habitat
Five areas hold virtually the entire population of proboscis monkeys in Danau Sentarum (Sebastian, 1994). They also represent the only substantial areas of tall forest, comprising Shorea balangeran forests, freshwater, peat swamp and flooded forest. These habitat types are crucial to the long-term viability of the population. The five core protection areas proposed by Sebastian (1994) are:
* Sg. Tawang Forest--Hutan Nung
* Sg. Batang Ketam
* Sg. Seputung Forest
* Sg. Embaluh Leboyan
* Sg. Belitung--Sg. Punggau
Sg. Belitung is used as an entry route to the park. Potential development of naturetourism activities within DSNP, where proboscis monkeys would feature prominently, would benefit greatly from the easily accessible population along this river.
In addition, Sebastian (1994) outlines a proposal for a proboscis monkey sanctuary within DSNP. The area identified is along the Sg. Sianda, covering the area of best habitat in the western part of the reserve. The proposed proboscis sanctuary covers an area of 32 [km.sup.2].
Extending Legislative Protection
DSNP currently protects more than 90% of the population. Ideally, the entire population's range should be included within the boundaries of DSNP. Sebastian (1994) identified four critical areas to be afforded legislative protection, three of which have already been included within the park. These areas represent refuges for animals during periods of high water levels. The rivers also retain some flowing water throughout the dry season, providing access to freshwater. These are:
* the upper reaches of the Sg. Leboyan and its adjacent lakes;
* the lakes of Danau Bekuan and Danau Sekawi, through which the rivers Sg. Belitung and Sg. Punggau flow;
* the area of freshwater and peat swamp forest between them Lakes of Bekuan and Sekawi; and
* the Sg. Batang Putus which flows from Danau Bekuan to the Kapuas river and the adjacent ox-bow lakes.
Only the Sg. Batang Putus area remains unprotected at present, and should be proposed as an extension to the park.
Reducing Habitat Fragmentation
Management with the integral involvement of the resident communities will be required to reduce habitat fragmentation within DSNP. Any top-down approach towards management at DSNP, without the participation of local communities will inevitably fail. For the areas currently outside the park (which are used by the population), extending the boundaries of DSNP will bring these into the management coverage. The critical areas requiring management are:
* Prevent further removal / degradation of tall forest on levees
* Ensure maximum habitat contiguity within the park, taking into account dry and wet season range and movements of the population
* Rehabilitation of key corridors which have been degraded
* Eliminate indiscriminate burning within the park
Control of Human Activity
The various activities carried out by villagers within the park have been in existence for decades. Leaving matters as they are would, under ordinary circumstances, not be detrimental to the proboscis monkey population as a whole. However, things have changed over the past decade, mainly as a result of the increased movement of people into the park.
Conflicts have arisen between the traditional villages within the park and new immigrants. New areas are being cleared for settlements and agriculture, and there is competition for fishing zones. These factors need addressing to ensure the proboscis monkey population does not suffer.
Fishing and collection of forest produce are an integral part of the existence and culture of the local communities. The approach should be to obtain recognition, through a participatory effort, for the core protection zones (and the sanctuary) for the population. Through this recognition, certain self-control mechanisms can then be put into place with regards to these specific areas. Proposed controls are:
* Reduced human presence in designated core protection areas
* Minimize ground travel in these areas, allowing boat traffic
* Relocate fishing activities (traps, nets, fish-salting, encampments on banks, etc) to the periphery of these areas.
Tourism in the Future
The proboscis monkey has become one of the main attractions for visitors to Borneo. The Kinabatangan River in Sabah is one of the main nature tourism destinations in Malaysia, focused primarily on proboscis monkey viewing along the Menanggul River. No statistics are available, but the seven lodges operating from the village of Sukau attract an estimated 10,000 foreign visitors a year. Similarly, Bako National Park in Sarawak, also offering excellent views of proboscis monkeys, attracts over 50,000 visitors a year.
Any place where proboscis monkeys can be viewed in the wild is a potential tourism destination. Danau Sentarum qualifies as a high quality destination with its combination of stunning landscapes, traditional fishing villages, longhouses and rich wildlife including proboscis monkeys.
Following the successes of eco-tourism initiatives elsewhere on Borneo, Indonesia will eventually follow suit. DSNP will undoubtedly be eventually developed as such a destination. The proximity of Sarawak, with its growing nature tourism focused around Batang Ai National Park (located just across the border) will make access easier. Improving road networks and more efficient border crossing facilities all contribute to the potential for nature tourism to be a viable economic venture for DSNP, and its local populace.
A management regime designed specifically at preserving the remaining core habitat areas for proboscis monkeys will be crucial to ensure the sustainability of this resource.
TABLE I THE DISTRIBUTION OF THE PROBOSCIS MONKEY POPULATION AT DANAU SENTARUM FROM WET SEASON SURVEYS (From Sebastian, 1994). SITE LOCALITY AND EST. AREA NO. OF EST. NO. DESCRIPTION OF GROUPS OF HABITAT ANIMALS ([km.sup.2]) Sg. Tawang Forest, an area of 53.0 34 364 levee, flooded and swamp forest along Sg. Tawang. Includes fringe of Hutan Nung along Danau Pengembung and the lakes of D. Pemerak and D. Sepatas. Sg. Batang Ketam, the stretch of 12.0 4 43 river from Kpg. Sekolat, separted from the lakes only by a broad levee, dividing Danau Belida from Danau Genali. Sg. Seputung Forest, an area of 18.0 6 64 freshwater swamp forest between D. Sumbai and D. Luar, includes Sg. Sengirin and a tributary of Sg. Putang west of D. Sumbai. Lower reaches of Sg. Embaluh 20.0 2 21 Leboyan within DSNP, including the upper reaches outside the reserve. Sg. Belitung, joining D. Sekawi to 9.0 11 117 Sg. Kapuas, rejoining the lake via Sg. Punggau (9.0 km surveyed) SITE LOCALITY AND DENSITY DESCRIPTION (grp per [km.sup.2] / ind. per [km.sup.2]) Sg. Tawang Forest, an area of 0.6 / 6.9 levee, flooded and swamp forest along Sg. Tawang. Includes fringe of Hutan Nung along Danau Pengembung and the lakes of D. Pemerak and D. Sepatas. Sg. Batang Ketam, the stretch of 0.3 / 3.6 river from Kpg. Sekolat, separted from the lakes only by a broad levee, dividing Danau Belida from Danau Genali. Sg. Seputung Forest, an area of 0.3 / 3.6 freshwater swamp forest between D. Sumbai and D. Luar, includes Sg. Sengirin and a tributary of Sg. Putang west of D. Sumbai. Lower reaches of Sg. Embaluh 0.1 / 1.1 Leboyan within DSNP, including the upper reaches outside the reserve. Sg. Belitung, joining D. Sekawi to 1.2 / 13.0 Sg. Kapuas, rejoining the lake via Sg. Punggau (9.0 km surveyed) NOTES * Area of available habitat is calculated as the length of river/ waterway/lake edge surveyed, extending away from the water for 500m. * Surveys were conducted when total area of reserve was 80,000ha. TABLE II COMPARISON WITH OTHER LONG TERM STUDIES ON PROBOSCIS MONKEYS (From Sebastian, 1994). AREA AVAIL- TOTAL SUR- ABLE DEN- SITE AREA VEYED HABITAT SITY ([km.sup.2]) ([km.sup.2]) ([km.sup.2]) (ind./km) Samunsam Wildlife 60 (a) 27 27 5.93 Sanctuary Kinabatangan 2,800 ? ? 20.46 Floodplain Tanjung Putting 3,700 2 ? 62.6 National Park Danau Sentarum 829 109 475 1.47 National Park AVER- AGE SITE GROUP SOURCE SIZE Samunsam Wildlife 9 Bennett and Sanctuary Sebastian (1988) Kinabatangan 11 Sharma Floodplain (1992) Tanjung Putting 12.1 Yeager National Park (1993) Danau Sentarum 10.7 (b) Sebastian National Park (1994) NOTES (a) Size prior to extensions. Current size: 228 [km.sup.2]. (b) Figure derived from average of previous three group size estimates
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