Proboscis Monkey (Nasalis larvatus)

The average body mass for an adult male proboscis monkey is between 16 and 22 kilograms, and for the females it is between 7 and 12 kilograms. This is a sexually dimorphic species, especially in body size (Kern, 1964). The males of this species has a large pendulous nose which is smaller and more turned up in females. This nose of the male assists in enhancing vocalizations acting as an organ of resonance (Ankel-Simons, 2000). The proboscis monkey has interdigital webbing, and may be an adaptation for swimming. The pelage color is pink and brown with red around the crown, nape, and shoulders, and with gray on the arms, legs, and tail (Kern, 1964). There are cream-colored patches on the cheeks and throat. The males have a black colored scrotum and a penis which is red in color (Ankel-Simons, 2000). When the infants are born their faces are a vivid blue color (Pournelle, 1967). At age 2.5 months the facial color of the infant darkens to a sooty dark gray color, and then at 8.5 months the gray color starts to lighten to the flesh color of the adult (Pournelle, 1967).

The proboscis monkey is found on the island of Borneo. This species prefers to live in nipa-mangrove-mixed forest, mangrove forest, and lowland forest (Kawabe and Mano, 1972). In Sarawak it was found that this species prefers to live in riverain and mixed diterocarp/high kerangas forests (Salter et al., 1985). It has been suggested that this species is restricted to the coastal areas and areas nears rivers because the interior has soils which are low in minerals and salts, which are needed in the diet (Bennett and Sebastian, 1988). This species avoids areas with heavy deforestation, such as agricultural land (Salter et al., 1985).
Proboscis Monkey

The proboscis monkey is described as being a folivore/frugivore (Yeager, 1989). Yeager (1989) found that in Tanjung Puting National Park in Indonesia, that this species utilizes 47 different plant species, of which 17 plants were consumed for their fruit, seeds, or flowers and 30 species were consumed for their leaves. The most important species used in Tanjung Puting National Park were Eugenia sp., Ganua motleyana, and Lophopetalum javanicum (Yeager, 1989). At Gunung Palung Nature Reserve, the most important plant species consumed were Mesua lepidota, Palaquium sp., Baccaurea lanceolata, Barringtonia racemosa, and Salacia macrophylla (Ruhiyat, 1986). Also at Gunung Palung Nature Reserve the most frequent tree species visited for fruit was Syzigium borneense and Dialium indicum (Ruhiyat, 1986). At Samunsam Wildlife Sanctuary it was found the majority of food consumed was fruits or seeds and young leaves were next most important food item (Bennett and Sebastian, 1988). Fruits eaten tend to be of the dry and bitter-tasting type (Bennett and Sebastian, 1988). In Sarawak it was found that the food item consumed representing the most number of species were leaves (Salter et al., 1985). Leaves consumed tend to have high levels of protein and low levels of digestion inhibitors (fiber) (Yeager et al., 1997). The leaves consumed tend to be high in levels of phosphorus and potassium (Yeager et al., 1997). Seeds of fruits tend to more important than the flesh itself as a food source for the proboscis monkey (Yeager, 1989). At Samunsam Wildlife Sanctuary this species prefers the seeds of the families Leguminosae and Myristicaceae (Bennett and Sebastian, 1988). This species feeds predominately on the seeds of the following species: Eugenia sp., Ganua motleyana, Lophopetalum javanicum (Yeager, 1989). An effect of seed predation by this species is that it assists in maintaining vegetational diversity in the areas where it lives (Yeager, 1989). Invertebrates are also consumed, examples of which are mosquitos, caterpillars, and insect larvae (Yeager, 1989). In Tanjung Puting National Park it was found that this species tended to be more frugivorous from January to May and more folivorous from June through December (Yeager, 1989). This species also eats shoots of mangrove trees and flowers. The proboscis monkey prefers to consume immature leaves over older ones. It was found that in Tanjung Puting National Park that the diversity of plant species used for food increased when the trees that had young leaves decreased (Yeager, 1989). In the Lower Kinabatangan, Sabah, it was found that the food resources for this species are unevenly distributed and highly clumped, and it was suggested that this influences the low level of aggression between groups and group ranges that overlap (Boonratana, 1994). This species predominately feeds in the morning and in the evening (Macdonald, 1982). In Sarawak it was found that this species feeds from 1 meter above the ground to the mid to upper canopy trees (Salter et al., 1985). When feeding, lion-tailed macaques, Macaca silenus, and orangutans, Pongo pygmaeus, will displace the proboscis monkey at feeding sites (Yeager, 1989).

Proboscis Monkey Group sizes range from 3 to 32 individuals. This is not a territorial species, with group ranges overlapping many groups (Kawabe and Mano, 1972; Boonratana, 2000). Groups of proboscis monkeys will usually never move father than 600 meters from a river or stream (Bennett and Sebastian, 1988). The greatest times of activity for this species is from late afternoon to dark (Kern, 1964; Ruhiyat, 1986). Groups will move away from the riverain forest in the morning and into the tree mangrove forest in the afternoon, and back to the riverain forest in the evening (Salter et al., 1985; Boonratana, 2000). This species has a high dependency on habitats that adjoin rivers (Boonratana, 2000). At Sukau, in Northern Borneo, was found to travel at a mean height of 12.25 meters, and at Abai the mean height for traveling was found to be 6.91 meters (Boonratana, 2000). This a diurnal and semi-terrestrial species.

It was found that in Brunei Bay that the favorite sleeping tree for the proboscis monkey is the pedada, Sonneratia alba (Kern, 1964). Kern (1964) suggests that this tree species is chosen because the upper branches protect the monkeys from predators during the night. This species is always found to sleep near rivers, 0 to 15 meters from the river's edge (Ruhiyat, 1986). At Gunung Palung Nature Reserve the favorite trees used for sleeping were the dungun, Heritiera sp. and the pisang-pisang, Mezzetia parviflora (Ruhiyat, 1986). This species will rarely sleeping in the same tree on consecutive nights (Ruhiyat, 1986). When leaving sleeping trees, males are usually the last individuals to leave (Yeager, 1990a). When sleeping the all-male groups tend to be more spread out in the trees than the unimale groups (Rajanathan and Bennett, 1990).

The proboscis monkey tends to cross rivers and streams at narrower points which could an anti-predator behavior (Yeager, 1991b); males tend to be the last individuals to cross amongst group members (Yeager, 1990a). Also this species will cross rivers in large numbers, use foliage to "spring-board" across rivers, and visually scan, as behavior to avoid predators (Yeager, 1991b). When individuals are alone when crossing a river they may enter the water quietly and swim to the other side, exiting quickly and hurrying up a tree (Yeager, 1991b). The false gavial, Tomistoma schegeli is a major predator of this species, and the river crossing behavior may be related to avoiding this and other riverine predators (Yeager, 1991b; Galdikas, 1985). The troop associations formed by the groups of this species may be a result of coordinated crossings at narrow points at rivers which would help to deter potential predators (Yeager, 1991b).

The proboscis monkey moves through the forest and on land quadrupedally (Fleagle, 1988). This species is also an excellent swimmer (Fleagle, 1988). Swimming generally occurs at the surface, but they will swim underwater for up to 20 meters if disturbed suddenly (Bennett and Sebastian, 1988). Swimming generally occurs in a single file line when the group crosses a river or a stream (Rajanathan and Bennett, 1990). Proboscis monkeys will leap out of the upper branches of trees overlooking rivers or streams and dive into the water (Kern, 1964).

There are two levels to the social system of the proboscis monkey. First there are unimale groups and all-male groups (Bennett and Sebastian, 1988;Yeager, 1990a). The next level consists of bands that are made up of many unimale groups (Bennett and Sebastian, 1988; Yeager, 1990a). All-male groups are made up of juvenile, adolescent, and adult males (Yeager, 1990a). Bennett and Sebastian (1988) found that in the Samunsam Wildlife Sanctuary in Sarawak that the average group size of the unimale groups was 9 individuals. Yeager (1990a) found that at Tanjung Puting National Park the sizes of unimale groups ranged from 3 to 23 individuals. Groups will come together during the evening near rivers to sleep in trees (Bennett and Sebastian, 1988). The same groups tend to associate with each other more than with other groups (Yeager, 1991a). These same groups have overlapping sleeping site areas (Yeager, 1991a). The majority of the time the different groups were within 50 meters of each other during the night (Bennett and Sebastian, 1988). Groups will also meet and travel together during the day (Bennett and Sebastian, 1988). The proboscis monkey social organization most resembles that of the gelada baboon, Theropithecus gelada (Yeager, 1991a). Adult females tend to coordinate group movements and lead groups when moving (Yeager, 1990a). Males tend to intervene during female-female agonistic interactions and also protect juveniles from female aggression (Yeager, 1990a). Adult females will sometimes leave their unimale group and join another unimale group, but adult females are never seen alone (Bennett and Sebastian, 1988). Females do most of the grooming in the group. Subadults and juveniles have also been found to engage in grooming bouts (Ruhiyat, 1986). Females will compete with each other to copulate with the male of the unimale group (Yeager, 1990b). Males leave their natal group, but some females also leave. At about 18 months juvenile males will leave their natal group and join all-male groups (Bennett and Sebastian, 1988). Little aggression occurs between males when the unimale groups come together to form troops (Kawabe and Mano, 1972). However, early morning displays by males of the unimale groups (vocalizations and branch shaking displays) may serve to mediate spacing of groups that share sleeping sites (Yeager, 1992). These unimale groups may come together to form troops as a protection against predators when moving or to prevent displacement by solitary groups at foraging sites (Yeager, 1992). Infants of this species will stay in close proximity to their mothers for about one year (Rajanathan and Bennett, 1990).

growls: These calls are made by adult males and this functions to calm another member of group down.

honks: These calls are made by adult males of the group. This functions as an aggressive call which threats other members of the group, and is also made in the presence of predators.

shrieks: These calls are given by juveniles of both sexes and adult females. This is emitted when the individual is agitated or excited.

scream: This vocalization is heard during agonistic interactions (Ruhiyat, 1986). This call is heard during feeding bouts and at night during sleep (Ruhiyat, 1986).


Proboscis Monkey

social grooming: This is when one individual grooms another and is used to reinforce the bonds between individuals. Ruhiyat (1986) found that grooming bouts last from 1 to 5 minutes. Usually this behavior is performed with both individuals in a sitting posture (Yeager, 1990a). The groomer will use the hands or the teeth when grooming (Yeager, 1990a).

The proboscis monkey gives birth to a single offspring. The gestation length for this species is about 166 days (Gorzitze, 1996). This species gives birth at night, and the female starts out the delivery by sitting at a branch point of 2 to 3 limbs up in a tree (Gorzitze, 1996). The female sits on one side of her ischial callosities, then the birth occurs, lasting about 3 minutes for the process to take place (Gorzitze, 1996). The female eats the placenta after delivery has taken place (Gorzitze, 1996).

Females initiate a majority of copulations in this species (Yeager, 1990b). They start by making a pout face and pursing the lips, then they perform head-shaking followed by presenting (Yeager, 1990b). The male mounts from the rear and they grab the females at the midsection with their hands and rear legs with their feet (Yeager, 1990b). Most copulatory bouts consist of single thrusts (Yeager, 1990b). During mating, juveniles will often harass the male while he is mounting the female (Rajanathan and Bennett, 1990). Same-sex mounting behavior has been observed, between adult females and between immature males (Yeager, 1990b). The females will solicit copulations from males postconception (Gorzitze, 1996). Mating was found to occur from February to November (Rajanathan and Bennett, 1990).

head-shaking: This is where the female will shake the head from side to side to solicit copulation from the male. Presenting occurs after this display.

presenting: This behavior is performed by the female to solicit a copulation from the male; this pattern tells the male that she is ready for copulation (Estes, 1991). Usually head-shaking occurs before this.

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Proboscis Monkey

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Last Updated: October 12, 2003.
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