Proboscis Monkey

Borneo's Big-Nosed Mangrove Specialist, Squeezed by Palm Oil

The proboscis monkey is a large, distinctive primate endemic to the island of Borneo, instantly recognisable for the pendulous nose of adult males (which can exceed 10 cm and amplifies their vocalisations). It is a habitat specialist of coastal mangrove, riverine, and peat-swamp forests — exactly the lowland forests most rapidly being converted to oil palm plantation and aquaculture across Borneo. The IUCN lists Nasalis larvatus as Endangered, with a population that has declined by more than 50% over the past three to four decades and continues to fall [Boonratana et al. 2021].

Biology and identification

Nasalis larvatus is a large monkey: adult males reach 66–76 cm in head-body length and 16–22 kg, with females roughly half the male weight [Bennett & Gombek 1993]. The species is unmistakable — reddish-brown fur, a pale grey-and-cream underside and limbs, and, in males, the enormous pendulous nose that gives the species its name. The nose is thought to function as a resonating chamber amplifying the male's honking vocalisations and as a sexually-selected signal (larger noses correlate with larger body size and harem size). Local Indonesian and Malaysian names sometimes humorously reference the resemblance to Dutch colonists ("monyet belanda" — "Dutch monkey").

Proboscis monkeys are folivores and frugivores with a specialised, chambered, ruminant-like foregut that ferments leaves — which is why they have notably pot-bellied profiles. This digestive specialisation ties them to specific forest plant communities. They are also strong swimmers (with partially webbed feet) and routinely cross rivers, an adaptation to their riverine mangrove habitat — unusual among primates.

The species lives in harem groups (one male, several females and young) and larger aggregations that gather along riverbanks at night, which makes them relatively observable for population surveys and for river-based ecotourism.

Habitat and range

Endemic to Borneo — the Indonesian provinces of Kalimantan, the Malaysian states of Sabah and Sarawak, and Brunei. The species is restricted to lowland coastal and riverine habitats: mangrove forest, peat-swamp forest, and riverine forest, almost always near water and rarely more than a couple of kilometres from a river or coast [Boonratana et al. 2021].

This habitat specificity is the crux of the conservation problem: Borneo's coastal lowland and peat-swamp forests are precisely the areas being most rapidly converted to oil palm plantations, shrimp aquaculture, and coastal development. The proboscis monkey cannot use plantation or degraded habitat; its range is shrinking and fragmenting in lockstep with lowland-forest loss.

Conservation status

The proboscis monkey is listed as Endangered on the IUCN Red List [Boonratana et al. 2021]. It is on CITES Appendix I, prohibiting international commercial trade. It is legally protected in Indonesia, Malaysia, and Brunei. Key populations persist in protected areas including the Kinabatangan Wildlife Sanctuary (Sabah), Bako National Park (Sarawak), and various Kalimantan reserves and the Tanjung Puting National Park area.

The population trend is decreasing; the species has been extirpated from many parts of its former range and survives in fragmented subpopulations, many of them small and isolated by plantation and development.

Threats

Habitat loss to oil palm and aquaculture is the dominant threat. Borneo's lowland coastal and peat-swamp forests — the proboscis monkey's only habitat — have been converted to oil palm plantation at among the fastest rates of any tropical forest on Earth, alongside conversion to shrimp aquaculture ponds in coastal mangrove zones [Gaveau et al. 2016]. Because the monkey is a lowland-forest specialist that cannot use plantations, this conversion is a direct and continuing population driver.

Habitat fragmentation — even where forest patches remain, they are increasingly isolated by plantation, roads, and development, dividing the population into small groups with limited gene flow. The monkeys' river-crossing ability provides some connectivity but cannot compensate for landscape-scale fragmentation.

Hunting — for meat and, locally, for the bezoar stones (gastrointestinal concretions) used in some traditional medicine; a smaller threat than habitat loss but contributing in some areas.

Human-wildlife conflict and disturbance — as habitat shrinks, monkeys come into more frequent contact with humans; unregulated tourism at some riverbank sites can disturb groups.

What is being done

• Protected areas — the Lower Kinabatangan Wildlife Sanctuary (Sabah), Bako National Park (Sarawak), Tanjung Puting National Park (Central Kalimantan), and other reserves protect key populations. The Kinabatangan in particular is a stronghold and a major proboscis-monkey ecotourism site.
• HUTAN / Kinabatangan Orang-utan Conservation Programme and other Sabah NGOs work on habitat corridors and forest restoration in the fragmented Kinabatangan floodplain, benefiting proboscis monkeys alongside orangutans and pygmy elephants.
• RSPO and sustainable palm oil — the Roundtable on Sustainable Palm Oil certification is the principal market mechanism to slow lowland-forest conversion; its effectiveness is debated but it is the major industry-facing lever.
• WWF and Borneo-wide forest conservation — the broader "Heart of Borneo" tri-national conservation initiative (Indonesia, Malaysia, Brunei) protects forest landscapes that include proboscis monkey habitat.
• Ecotourism — well-managed river-based proboscis monkey viewing (notably along the Kinabatangan) provides an economic incentive for forest retention and a revenue stream for local communities.

How readers can help

• Choose RSPO-certified palm oil and reduce palm-oil consumption where practical. Oil palm expansion into Borneo's lowland forests is the single largest driver of proboscis monkey decline. Checking for RSPO certification on products containing palm oil (a very wide range of processed foods and cosmetics) is the most-direct consumer lever.
• Support Bornean forest-conservation NGOs — HUTAN, WWF's Heart of Borneo program, and Sabah/Sarawak/Kalimantan conservation organizations work on the habitat protection and corridor restoration the species needs.
• Travel responsibly to Borneo. River-based proboscis monkey ecotourism (especially the Kinabatangan in Sabah) funds conservation and local communities. Choose operators with documented conservation partnerships, maintain distance, and never feed wildlife.
• Support orangutan-focused conservation too. The proboscis monkey shares its lowland-forest habitat with the Critically Endangered Bornean orangutan; conservation organizations protecting orangutan habitat protect proboscis monkeys in the same landscapes.
• Engage on tropical-deforestation supply-chain policy — the EU Deforestation Regulation and similar measures aim to keep deforestation-linked commodities (including palm oil) out of major markets; supporting their implementation reduces the economic driver of Bornean lowland-forest conversion.

Last verified: 2026-05-24 Conservation status: Endangered (IUCN Red List 2021 assessment); CITES Appendix I; Borneo endemic.

References

• Bennett, E. L., & Gombek, F. (1993). Proboscis Monkeys of Borneo. Natural History Publications (Borneo).
• Boonratana, R., Cheyne, S. M., Traeholt, C., Nijman, V., & Supriatna, J. (2021). Nasalis larvatus. IUCN Red List of Threatened Species. e.T14352A195372486.
• Gaveau, D. L. A., Sheil, D., Husnayaen, et al. (2016). Rapid conversions and avoided deforestation: examining four decades of industrial plantation expansion in Borneo. Scientific Reports 6: 32017.