Proboscis monkey

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Suborder: Haplorrhini
Infraorder: Simiiformes
Superfamily: Cercopithecoidea
Family: Cercopithecidae
Subfamily: Colobinae
Genus: Nasalis
Species: N. larvatus

Other names: long-nosed monkey; næseabe (Danish); neusaap (Dutch); Nenäapina (Finnish); nasique (French); bekantan, bangkatan, monyet belanda (Indonesian); mono narigudo, násico (Spanish); näsapa (Swedish).

Conservation status: please search the IUCN Red List.

Life span: >25 years (captive)
Total population: Unknown
Regions: Borneo
Gestation: 166-200 days
Height: 73 to 76 cm (M), 61 to 64 cm (F)
Weight: 20 kg (M), 10 kg (F)


Proboscis monkey vocalizing in a tree
Nasalis larvatus

Proboscis monkeys are among the larger colobines and are sexually dimorphic, with males’ head-body length measuring 75.5 cm (29.7 in) (range 73-76 cm (28.7-29.9 in)), and females measuring 62 cm (24.4 in)(range 61-64 cm (24.0-25.2 in)) (Napier & Napier 1985; Bennett & Sebastian 1988; Ankel-Simons 2007). Males weigh around 20kg (44.1 lb) while females weigh half of that (Napier & Napier 1985; Bennett & Gombek 1993). The face of adults is orange-pink. Females have a very large nose for a primate, but the noses of males dwarf even those of the females and often hang lower than the mouth (Payne et al. 1985; Ankel-Simons 2007). The nose of an adult male can exceed 10cm (3.9 in) in length (Ellis 1986). Overall, the pelage is somewhat long, and is bright orange, reddish brown, yellowish brown, or brick-red dorsally (Kern 1964; Payne et al. 1985; Groves 2001; Ankel-Simons 2007). The ventral surfaces are light-gray, yellowish, or grayish to light-orange (Kern 1964; Payne et al. 1985; Ankel-Simons 2007). The penis is red and the scrotum is black. Several of the toes are webbed (Kern 1964; Ankel-Simons 2007). Adult males have a dark brown mane on their back (Bennett & Gombek 1993). The species possesses large ischial callosities and both sexes look perpetually pregnant, due to oversized, protruding stomachs (Bennett & Gombek 1993; Groves 2001; Ankel-Simons 2007).

Proboscis monkeys are avid swimmers, capable of swimming up to 20m (65.6 ft) completely submerged, and may leap from high branches into water (Kern 1964; Bennett & Sebastian 1988; Boonratana 1993). In fact, among the primates they have the most aquatic lifestyle, and often swim across rivers (Yeager 1989b; Sebastian 2000). Depending on the habitat type, proboscis monkeys are predominantly arboreal and occasionally terrestrial; moving quadrupedally and through leaping (Kawabe & Mano 1972; Napier & Napier 1985). Modes of locomotion include semi-brachiation, climbing, leaping, quadrupedal and suspensory movement and rare bipedalism (Ramesh Boonratana pers comm.).

In captivity, proboscis monkeys have lived over 25 years (Weigl 2005).


Nasalis larvatus

Proboscis monkeys are only found on Borneo, and in parts of all three nations that divide the island; Brunei, Indonesia, and Malaysia (Bennett & Sebastian 1988; Brandon-Jones et al. 2004). They are not found throughout the island however, and are often concentrated in coastal regions. Inland, they are seen along rivers that are scattered throughout the Bornean interior, sometimes over 200 km (124.3 mi), from the coast (Bennett & Gombek 1993; Meijaard & Nijman 2000a). They are not present in much of the Bornean rainforest and much of their distribution within Borneo is unknown (Bennett & Gombek 1993).


Proboscis monkeys are only found in lowland habitats including several types of coastal and riparian habitats, many that are tidally flooded (Kawabe & Mano 1972; Salter et al. 1985; Sebastian 2000). The species is generally not found above 200 m (656.2 ft) above sea level (Meijaard & Nijman 2000a). Habitats are usually near water or rivers, as proboscis monkeys rarely range more than a kilometer from water and sleep predominantly but not exclusively near it (Salter et al. 1985; Ruhiyat 1986; Yeager 1989a; Bennett & Gombek 1993; Boonratana 2000; review in Sebastian 2000; Onuma 2002). The species is also found in wetlands not directly connected with the coast (Sebastian 2000). Preferred habitats include certain dipterocarp forests, mangrove forest, or riverine forest (Salter & Aken 1983; Salter et al. 1985; Yeager 1989a; Benett & Gombek 1993; Sebastian 2000; Tuen & Pandong 2007). When both are available, tall forests are preferred over tidal or coastal habitats (Salter et al. 1985; Sebastian 2000). Availability of food is a strong factor in habitat choice, and in some cases, where high forest is unavailable, tidal forests are preferred (Salter et al. 1985). Several other types of proboscis habitat include dwarf swamp forest, stunted swamp forest, rubber forest, rubber plantations, limestone hill forest, nipa swamp, nibong swamp, and tall swamp forest as well as tropical heath forest and steep cliffs (Salter et al. 1985; see Sebastian 2000; Onuma 2002; Soendjoto 2004; Tuen & Pandong 2007). Occasionally, degraded tidal forests, logged peat swamps and logged high forest are utilized (Salter et al. 1985). More rarely, proboscis monkeys are also found in nipa vegetation or scrubby vegetation found on rocky headlands and exposed cliffs and have even been seen on agricultural land (Salter & Aken 1983; Salter et al. 1985). Proboscis monkeys may be precluded from living in many Bornean inland and hill forests by nutritional or food requirements that such habitat types cannot provide (Bennett & Sebastian 1988). In one study at Samunsam Wildlife Sanctuary in western Borneo, habitat use is seasonal, with a wider range of habitat types used during the wet season (November) as opposed to the dry season (August) (Tuen & Pandong 2007).

At the long-term study site at the Tanjung Putting National Park in southern Borneo, average temperatures remained relatively constant year-round, with highs averaging around 29°C (84.2°F) and lows averaging 22°C (71.6°F). Here, over 15 cm (5.9 in) of rain fell monthly, but less fell between July and October than during the rest of the year (Yeager 1989b). At a different study site in northern Borneo in eastern Sabah, temperatures were also relatively constant year-round, ranging from average lows of 23.7°C (74.7°F) to average highs around 33°C (91.4°F) (Boonratana 1993). Recorded annual rainfall totals at this site ranged from around 182 cm (71.7 in) to 298 cm (117.3 in) (Boonratana 1993).


Proboscis monkeys are seasonal folivore/frugivores, eating foods from over 90 species of plant including predominantly fruit and leaves (Salter et al. 1985; Boonratana 1993; Yeager 1989a). Annually, they consume fruits (40.33%), and flowers (2.97%) in addition to leaves (51.94%), insects and bark (<1%) and unknown foods (4.68%) (Yeager 1989a). Boonratana (1993; 2003) gives the following food data: mature leaves (0.3%), young leaves (72.7%), unripe fruits (6.6%), ripe fruits (1.7%), seeds (2.4%), flowers (7.8%), flower buds (0.5%), and unknown items (8.0%). Leaves that are eaten are usually young (although other parts are eaten as well), and of the fruit, seeds and flesh/seeds make up the majority of the consumed parts (Ruhiyat 1986; Yeager 1989a; Boonratana 1993). Ferns are also sometimes eaten and crops such as bananas are raided (Soendjoto 2004; Agoramoorthy 2007). Rarely, proboscis monkeys will drink directly from rivers (Boonratana 1993). Seasonally, there are shifts in food consumption; from mostly fruit (January-May) to mostly leaves (June-December) (Yeager 1989a).

Proboscis monkey in a tree
Nasalis larvatus

Home ranges overlap extensively with those of other groups, but territorial behavior is largely absent (Yeager 1989a; Bennett & Gombek 1993; Boonratana 1993; 2000). Recorded group home ranges usually vary between 1.3 and 9 km² (0.5 and 3.5 mi²) with similar estimates in other studies (Kawabe & Mano 1972; Salter et al. 1985; Yeager 1989a; Bennett & Gombek 1993; Boonratana 1993; see review in Boonratana 2000). Home range size variation can be attributed to population densities and habitat type (Sebastian 2000). Day ranges average 910 m-2000 m (2985.6-6561.7 ft) (Salter et al. 1985; Bennett & Gombek 1993; Boonratana 2000). In habitats associated with rivers, proboscis monkeys usually sleep by the river and move inland during the day, and then back to the river late in the day (Salter et al. 1985; Rajanthan & Bennett 1990). However, they never move more than a kilometer away from water (Salter et al. 1985; Ruhiyat 1986; review in Sebastian 2000).

Groups usually sleep in one or several nearby trees (Yeager 1990b).

Daily, predominant activities include resting, traveling, vigilance, and feeding (Salter & Aken 1983; Boonratana 1993). Upon awakening, proboscis monkeys will generally feed then move inland and rest. The majority of the day is spent feeding, resting, and traveling and at dusk, the animals will move back to the riverside and feed before dark (Bennett & Gombek 1993). On a daily basis, feeding peaks in the morning and at dusk (Boonratana 1993).

The false gavial (Tomistoma schlegeli), a crocodile-like reptile, is a major predator of proboscis monkeys and has been seen taking adult males and immature individuals from low branches over water (Galdikas 1985; Yeager 1989b; 1991). Another predator is the clouded leopard (Neofelis nebulosa) which has also been seen attacking the monkeys, especially immature individuals (Kern 1964; Matsuda et al. 2008a). In mid-1992, a clouded leopard with a freshly killed adult male proboscis monkey was seen and photographed (J. C. Prudente pers comm. cited in Boonratana 1993). Monitor lizards and pythons are potential predators, as well as estuarine crocodiles (Crocodylus porosus), and crested serpent eagles (Spilornis cheela) all of which can potentially take infants and young juveniles (Yeager 1989b; 1991; Sebastian 2000). When crossing rivers, the species will do so at narrows or areas where the crossing can be completed arboreally, possibly to avoid predation (Yeager 1991; Matsuda et al. 2008b).

Proboscis monkeys can be sympatric with a number of other primate species; as is the case in eastern Sabah, where the species is sympatric with Presbytis hosei, P. rubicunda, Hylobates muelleri, Pongo pygmaeus, Trachypithecus cristatus, Macaca nemestrina, M. fascicularis, Nycticebus coucang, and Tarsius bancanus. In fact, this is one of only two sites in the world that four species of colobine call home (Boonratana 1993). If a sympatric primate species is encountered, the other species is usually ignored, although both long-tailed macaques and orangutans displace proboscis monkeys at feeding sites (Yeager 1989a; Rajanthan & Bennett 1990). There is potential competition with long-tailed macaques (Macaca fascicularis) due to food resource overlap, especially between January-June and November-December (Yeager 1989a).

Content last modified: February 25, 2009

Written by Kurt Gron. Reviewed by Ramesh Boonratana.

Cite this page as:
Gron KJ. 2009 February 25. Primate Factsheets: Proboscis monkey (Nasalis larvatus) Taxonomy, Morphology, & Ecology . <>. Accessed 2020 July 14.


The social structure of the proboscis monkey is flexible but the most typical group is the one-male group consisting of an adult male, females and their offspring (Bennett & Sebastian 1988; Rajanthan & Bennett 1990; Yeager 1990b; 1992; Bennett & Gombek 1993; Boonratana 1993; Yeager 1995; Boonratana 2002; Murai 2004). However, other group types occur, most commonly all-male groups, but also less consistently recorded types (Bennett & Sebastian 1988; Yeager 1990b; 1992; Boonratana 1993; Yeager 1995; Boonratana 1999; 2002; Murai 2004). Solitary individuals of both sexes are seen, but solitary males are more common (Boonratana 1999). Groups are often found near one another, especially coming together at sleeping sites at the end of the day and there is a higher level social entity (the band) formed through the fission and fusion of groups with one another (Macdonald 1982; Bennett & Sebastian 1988; Rajanthan & Bennett 1990; Bennett & Gombek 1993; Boonratana 1993; Yeager 1995; Boonratana 2002). Such larger aggregations serve to help reduce predation or avoid displacement (Yeager 1992b). Also, groups meet during the day and travel together, but individuals will not groom or play with members of different groups (Yeager 1992b; Boonratana 2002). Average one-male group sizes from various study sites range between 9 and 19, but larger aggregations (the higher, band-level social grouping) have been 60 individuals or even more (Kawabe & Mano 1972; Macdonald 1982; Salter & Aken 1983; Ruhiyat 1986; Bennett & Sebastian 1988; Rajanthan & Bennett 1990; Yeager 1992b; Bennett & Gombek 1993; Boonratana 1999; Murai et al. 2007). All-male groups usually range between 3-12 individuals, but in fragmented habitats can be larger (reviewed in Murai 2004). Resident males in one-male groups have an estimated tenure of around 6-8 years and changes in the resident male appear to be without serious aggression (Murai 2004; Murai et al. 2007).

Multiple Proboscis monkeys high in trees
Nasalis larvatus

Agonistic interactions within N. larvatus one-male groups (at the Sukau study site) occur at very low levels, suggesting that competition among members is low. This in turn reflects the abundance of resources. Conversely, competition may have been present at higher levels, but its manifestation is subtle, possibly because of an already established dominance hierarchy (Boonratana 1993). In general, agonism is not usually seen between groups and members of the same band are mostly accepting of one another (Yeager 1992b). Serious aggression is rare, while minor aggression, both within and without a band is common (Yeager 1992b). However, serious aggression does occur, and usually includes chases, slaps, and biting. Minor aggressive behaviors include vocalizations, facial threats, and postural threats (Yeager 1990b). Displays and vocalizations function in group spacing (Yeager 1992b). Within a one-male group, affiliation behaviors are seen between and among the females, but not usually with the male (Yeager 1990b). In one-male groups, males intervene most of the time in female-female agonism (Yeager 1990b). In one study, almost all female-female agonism was over preferred sleeping sites and there was evidence for a linear dominance hierarchy among group females (Boonratana 1993).

Allogrooming between individuals is done while seated (Yeager 1990b). Most grooming is performed by adult females, and it is often directed at infants. Adult males rarely receive or give allogrooming (Rajanthan & Bennett 1990; Yeager 1990b; Boonratana 1993).

Males leave their natal group before adulthood to enter all-male groups (Rajanathan & Bennett 1990; Yeager 1990b; Bennett & Gombek 1993; Boonratana 1999). Possible reasons for male transfer include inbreeding avoidance and sexual competition avoidance (Boonratana 1999). Both adult and sub-adult females also transfer between groups as well, although it seems more common among sub-adults (Bennett & Sebastian 1988; Boonratana 1993; 1999; Murai et al. 2007). Some females probably transfer to avoid inbreeding but other potential reasons for female transfer include infanticide avoidance, to increase their dominance status, and to reduce feeding competition (Boonratana 1999; Murai 2004).


Females reach sexual maturity in the wild at around 5 years of age (Murai 2004). Females show sexual swelling during which the genitals become pink or reddened (Gorzitze 1996; Murai 2004; 2006). There are some indications of reproductive seasonality in proboscis monkeys. In one population in western Borneo, there were indications of a mating peak at mid-year, but mating occurred between February and November. At this study site, most births were between March and May, near the very end of the rainy season (Rajanthan & Bennett 1990). Populations at other locations can have different birth seasons or peaks however, and captive populations also have bred seasonally (for example Griner 1980; Boonratana 1993; Gorzitze 1996).

Copulations are single mount and last on average around half a minute (Yeager 1990a; Boonratana 1993; Murai 2004; 2006). Males mount from the rear, grasping the female by the ankles or torso (Yeager 1990a; Boonratana 1993). Both sexes solicit mating, but males do so somewhat more often than females. However, solicitation does not always result in copulation (Murai 2006). Solicitation in both sexes is accompanied by a pouted face, while males sometimes vocalize and females will approach and turn away, presenting her backside and looking backward at the male (Hollihn 1973; Rajanthan & Bennett 1990; Yeager 1990a; Boonratana 1993; Murai 2006). Females will also sometimes shake their heads to solicit copulation (Yeager 1990a; Boonratana 1993). Sub-adults often harass the copulating pair (Rajanthan & Bennett 1990; Murai 2006). Nonsexual mounts are also seen, including playful and same-sex mounting (Yeager 1990a; Murai 2006).

Gestation has been estimated at 166 days, but also at slightly more than 200 days (Schultz 1942; Rajanthan & Bennett 1990).


In general, little is known about infant development in proboscis monkeys and what is known comes from only a couple of observations both in the wild and in captivity. Births occur during the night or early morning. The placenta is consumed, and the infant is licked clean by the mother (Gorzitze 1996). Infanticide by adult males after a group takeover is assumed to occur in wild proboscis monkeys (Agoramoorthy & Hsu 2004).

The birth weight of proboscis monkeys is around 450 g (15.9 oz) (Schultz 1942). At birth in captivity, the face is blue, but fades to gray by 2.5 months old and later to the color of adults. The nose of the infant male is upturned in contrast to the pendulous nose of the adult and grows slowly until reaching adult size at maturity. Solid food is first eaten at 6 weeks old and the infant is weaned by about 7 months old. Mating resumes in captivity 6 months postpartum (Pournelle 1967). However, for over a year after birth in the wild, an infant will stay close to its mother (Rajanthan & Bennett 1990).

In the wild, other females, including juveniles carry the infants of adult females and the mother allows other group members to hold a newborn (Boonratana 1993; Gorzitze 1996; Rajanathan & Bennett 1990). It is most usually infants and juveniles that indulge in play behavior (Boonratana 1993).


Proboscis monkeys vocalize frequently and have at least six types of call; the honk, honk to infant, alarm call, threat call, infant call, and female call (Kawabe & Mano 1972; Messeri & Trombi 2000). There are differences by sex in vocal behavior (Messeri & Trombi 2000). Honks are heard from males and communicates territorial information or information about group cohesion. Honk to infant calls are also given by males and communicates information about group cohesion or reassurance. Males emit alarm calls in situations of danger. Both males and females give threats, but each sex has different threat calls. Finally, females or immature individuals will emit female calls, normally when agitated (Messeri & Trombi 2000).

Proboscis monkeys use displays and vocalizations in low intensity agonism. Vocalizations in such situations include honks, roars and snarls, while displays include leaping-branch shaking, a bared-teeth open-mouth display, and the erect penis display in males (Yeager 1992b; Boonratana 1993). The erect penis display is usually seen during and before any agonistic interactions (Yeager 1992b). Leaping-branch shaking displays between more than one adult male are often seen in the morning (around a third of the time) (Yeager 1992b). Such displays, often accompanied by vocalizations, help to maintain proper spacing between groups at nighttime sleeping locations, both in the morning and the evening (Yeager 1991; 1992).

Content last modified: February 25, 2009

Written by Kurt Gron. Reviewed by Ramesh Boonratana.

Cite this page as:
Gron KJ. 2009 February 25. Primate Factsheets: Proboscis monkey (Nasalis larvatus) Behavior . <>. Accessed 2020 July 14.


For individual primate species conservation status, please search the IUCN Red List.
Also search the current scientific literature for primate conservation status (overall as well as for individual species), and visit CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora).

Conservation information last updated in 2009 follows, for comparison:

Proboscis monkey sitting in a tree nest
Nasalis larvatus

It is very difficult to keep proboscis monkeys in captivity and few zoos have been successful. However, with very careful husbandry, it is possible keep proboscis monkeys in captivity and thus there is potential for ex situ conservation (Agoramoorthy et al. 2004). Proboscis monkeys are particularly threatened because one of their preferred habitats is coastal lowland forests, locations which are also often preferentially developed or cultivated by people. In sum, most of their habitat is threatened or already disturbed (Sebastian 2000). Compounding the problem, most proboscis monkeys are found outside of protected reserves (Boonratana & Sharma 1992).


Threat: Human-Induced Habitat Loss and Degradation

As with many primate species, habitat loss is a major threat to proboscis monkeys and much of their former habitat has already been lost (Bennett 1988; Boonratana & Sharma 1992; Sebastian 2000). In addition, extensive proboscis habitats are being targeted for development, as is the case in Central Kalimantan, Indonesia (Meijaard & Nijman 2000a). Human land use, including clearance for agriculture, plantation development (including for coca and oil palm production), mining, for human resettlement and for industrial sites, as well as burning, swamp reclamation, commercial aquaculture (fish and shrimp farming), and logging destroys habitats and also isolates populations from one another (Bennett 1988; Boonratana & Sharma 1992; Yeager 1992a; Sebastian 2000; Meijaard & Nijman 2000a; Soendjoto 2004; Agoramoorthy 2007). These problems are also made worse by access roads, which fragment populations and increases inbreeding (Soendjoto 2004). Also, many plantations have electric fences around them, which may prevent normal proboscis monkey ranging (Boonratana & Sharma 1992). Selective logging sometimes destroys preferred sleeping and feeding trees (Yeager 1992a). After logging, forests are sometimes poisoned to prevent undesirable vegetation from regrowing with the goal of increasing timber productivity as the forest regenerates. This likely affects proboscis monkeys adversely and prevents them from living in those habitats (Bennett 1988). In addition, human immigration from other areas is increasing land use and other sources of disturbance (Sebastian 2000).

Threat: Harvesting (hunting/gathering)

In some areas, proboscis monkeys are persecuted as crop raiders and are poisoned. In some regions, they are hunted for food, and in others, they are hunted recreationally, and as bait or feed for lizards, snakes and crocodiles (Bennett et al. 1987; Bennett 1988; Boonratana & Sharma 1992; Meijaard & Nijman 2000a; Soendjoto 2004). Hunting has already significantly reduced proboscis monkey numbers at some localities (Meijaard & Nijman 2000a). Also, even though they might not be the target species, firearm hunting of other game has the potential to disturb proboscis monkeys and alter their behavior (Tuen & Pandong 2007). However, hunting for food and persecution as crop pests is not always the case, as local Muslim inhabitants will not eat monkey meat for religious reasons (Boonratana & Sharma 1992). Proboscis monkeys are also kept illegally as pets (Meijaard & Nijman 2000b). Their sleeping habits near rivers also make them especially susceptible to hunting from boats (Tuen & Pandong 2007).

Threat: Pollution

In riverine areas of increased human activity, pollution may increase, especially due to spilled oil and fuel from boats, but also from human septic waste (Yeager 1992a). All of these types of pollution have the potential to alter proboscis monkey habitats.

Threat: Natural Disasters

Natural droughts increase the susceptibility of proboscis monkey habitats to destructive fires (Yeager 1992a).

Threat: Human Disturbance

Summertime activities of humans, such as fishing, fish-salting and smoking, trapping, and encampments, all disturb proboscis monkeys, keeping them away from preferred areas along rivers. Other activities that also disturb proboscis monkeys include rattan collection, the collection of honey from bees, and firewood gathering (Sebastian 1994 cited in Sebastian 2000). Further, increasing boat traffic, sometimes as a result of gold extraction along rivers, disturbs proboscis river crossing behavior and causes declines in population density and group size. Also, increased boat traffic threatens proboscis monkeys due to the potential for collisions while they are crossing rivers (Yeager 1992a). In Sabah, some proboscis monkey populations are threatened by unmanaged tourism (Ramesh Boonratana pers comm.).





Content last modified: February 25, 2009

Written by Kurt Gron. Reviewed by Ramesh Boonratana.

Cite this page as:
Gron KJ. 2009 February 25. Primate Factsheets: Proboscis monkey (Nasalis larvatus) Conservation . <>. Accessed 2020 July 14.

The following references were used in the writing of this factsheet. To find current references for Nasalis larvatus, search PrimateLit.


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Proboscis artwork
Nasallis larvatus

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Content last modified: February 25, 2009


Nasalis larvatus
Photo: A. S. Clarke
Nasalis larvatus
Photo: Anne Savage
Nasalis larvatus
Photo: Bruce Panton Wheatley
Nasalis larvatus
Photo: Diana Mossman
Nasalis larvatus
Photo: Kevin Schafer
Nasalis larvatus
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