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Proboscis monkey
Nasalis larvatus


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).

Nasalis larvatus
Photo: Diana Mossman

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 2019 September 18.