Nasalis larvatusproboscis monkey

Geographic Range

Proboscis monkeys are confined to the island of Borneo; they prefer coastal regions to inland areas. (Medway, 1977)


Proboscis monkeys inhabit mangrove forest along rivers and estuaries, swamp-land, and lowland rainforest. (Kawabe and Mano, 1972; Kern, 1964)

Physical Description

Proboscis monkeys are sexually dimorphic. The males have a length of 70 cm and weight of between 16 and 22 kg. Females measure 60 cm and weigh between 7 and 12 kg.

Males have a large protruding nose, which enhances vocalizations through resonance. The nose of the female is smaller.

The fur of the adult proboscis monkey is pink and brown with red around the head and shoulders. The arms, legs, and tail are gray. Males have a black scrotum and a red penis. Infants are born with a blue colored face that at 2.5 months darkens to gray. By 8.5 months of age, the face has become cream colored as in the adults.

There is webbing between the digits to allow for swimming. (Kern, 1964; Wolfheim, 1983; Yeager, 1992)

  • Sexual Dimorphism
  • male larger
  • sexes colored or patterned differently
  • male more colorful
  • ornamentation
  • Range mass
    7 to 22 kg
    15.42 to 48.46 lb
  • Range length
    60 to 70 cm
    23.62 to 27.56 in


The basic social unit in proboscis monkeys is a single adult male with from 2 to 7 adult females. The males mate with females in their social group. (Bennett and Sebastian, 1988)

Proboscis monkeys give birth to a single offspring after a gestation of 166 days. Births usually occur at night. The female sits on a tree branch during the birth. After the infant is born, the mother consumes the placenta.

The breeding season is from February until November. Copulation is initiated by the female through pursing of the lips, shaking of the head from side to side, and presentation of the hindquarters to the male. Females will continue to initiate copulations even after they have conceived.

Infants stay close to their mothers for about one year. Males reach maturity at about 7 years. (Hayssen, et al., 1993; Wolfheim, 1983; Yeager, 1990)

  • Breeding interval
    Females can produce offspring each year.
  • Breeding season
    Proboscis monkeys breed from February until November
  • Average number of offspring
  • Average number of offspring
  • Average gestation period
    166 days
  • Average gestation period
    166 days
  • Range weaning age
    7 (high) months
  • Average time to independence
    12 months
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    1460 days
  • Average age at sexual or reproductive maturity (male)
    7 years

As is the case for most primates, newborn proboscis monkeys are fairly helpless. They must be carried by their mother until they are able to walk on their own. Mothers provide their offspring with milk, nursing them until they are about 7 months old. They also keep their infants clean through grooming. Infants stay close to their mothers for about one year. (Yeager, 1990)

The role of the male in parental care is less direct. Although males do not care for infants the way females do, it can be argued that they provide important protection for the young by excluding potentially infanticidal rival males from the group.

  • Parental Investment
  • no parental involvement
  • altricial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • male
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • male
      • female
  • extended period of juvenile learning


Only the lifespan in captivity is known; in most animals it is at least 23 years. (Wolfheim, 1983)


Proboscis monkeys are diurnal, preferring to be active from late afternoon until dark. They are primarily arboreal although they are never more than 600 m from a river. When moving through the trees, they are quadrupedal. These monkeys are good swimmers and will leap out of the trees into the water. They are capable of swimming 20 m underwater. They may cross rivers by swimming if alone or they may cross by jumping from a tree on one bank to one on the other side at narrow points if in a group.

There are two types of groups within the proboscis monkey society: unimale and all-male. These groups number 3 to 32 individuals. Several of the groups will come together in the evening to sleep. These multigroup gatherings are called bands. Proboscis monkeys sleep 0 to 15 m from the river's edge. They do not sleep in the same place on consecutive nights. The same groups associate regularly and there is little aggression between males in unimale groups.

Adult males coordinate the group's movements and lead the group. Females do not leave their natal group. Males disperse at 18 months of age. (Yeager, 1990; Yeager, 1991; Yeager, 1992)

Home Range

The home range size for these monkeys has not been reported.

Communication and Perception

The proboscis monkey has several sounds for communication. Growls are made by males and are used to calm the group members. Honks are made by males as a threat or to warn of predators. Shrieks are made by females and both sexes of juveniles to show aggitation or excitement, and screams are given during agonistic encounters. Social grooming is performed, usually between females. The grooming usually last 1 to 5 minutes and is performed by both individuals. (Kawabe and Mano, 1972; Kern, 1964; Yeager, 1992)

Food Habits

Proboscis monkeys are folivores and frugivores. They prefer fruits, seeds, young leaves, and shoots of mangrove. They may also eat some invertebrates such as caterpillars and larvae. They are more frugivorous from January through May and more folivorous from June through December. (Yeager, 1990; Kern, 1964; Wolfheim, 1983)

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • fruit


The anti-predator behavior of these monkeys has not been described in detail. Leopards are known to prey upon them, as are crocodiles. Adult males sometimes vocalize, apparently to scare off potential predators.

Ecosystem Roles

The role of N. larvatus in the ecosystem is not well understood. As herbivores, they probably have some affect on plant populations. To the extent that predators rely on these animals for food, proboscis monkey populations may affect predators.

Economic Importance for Humans: Positive

Proboscis monkeys are considered a delicacy although they are not heavily hunted. They are also desired for zoos because of their unique appearance. (Medway, 1977)

  • Positive Impacts
  • food
  • research and education

Economic Importance for Humans: Negative

There are no known adverse affects of N. larvatus on humans.

Conservation Status

Proboscis monkeys are protected from hunting and capture in Borneo but the destruction of the mangrove forest has limited the population. They are listed as Appendix I by the Convention on International Trade in Endangered Species (CITES). (Appendix I is defined as a species threatened with extinction with trade allowed only in extreme circumstances.) They are listed as endangered by the International Trade in Endangered Species of Wild Fauna and Flora (IUCN). ('Endangered' is defined as an estimated 50% reduction in the population in the next 10 years.) ("Convention on International Trade in Endangered Species of Wild Flora and Fauna", 2003; "International Union for Conservation of Nature and Natural Resources", 1997)


Nancy Shefferly (editor), Animal Diversity Web.

Amy Woltanski (author), Michigan State University, Barbara Lundrigan (editor), Michigan State University.



uses sound to communicate


young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.


Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate

  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


union of egg and spermatozoan


an animal that mainly eats leaves.


A substance that provides both nutrients and energy to a living thing.


an animal that mainly eats fruit


An animal that eats mainly plants or parts of plants.

island endemic

animals that live only on an island or set of islands.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.


found in the oriental region of the world. In other words, India and southeast Asia.

World Map


having more than one female as a mate at one time


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

seasonal breeding

breeding is confined to a particular season


remains in the same area


reproduction that includes combining the genetic contribution of two individuals, a male and a female

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.


associates with others of its species; forms social groups.


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


CITES Secretariat. 2003. "Convention on International Trade in Endangered Species of Wild Flora and Fauna" (On-line ). Accessed 02/02/03 at

IUCN. 1997. "International Union for Conservation of Nature and Natural Resources" (On-line). Accessed February 02, 2003 at

Bennett, N., A. Sebastian. 1988. Social organization and ecology of proboscis monkeys (Nasalis larvatus) in mixed coastal forest in Sarawak. International Journal of Primatology, 9: 233-255.

Hayssen, V., A. Van Tienhoven, A. Van Tienhoven. 1993. Asdell's patterns of mammalian reproduction: a compendium of species-specific data. Ithaca, NY: Comstock/Cornell University Press.

Kawabe, M., T. Mano. 1972. Ecology and Behavior of the Wild Proboscis Monkey in Sabah, Malaysia. Primates, 13: 213-228.

Kern, J. 1964. Observations on the Habits of the Proboscis Monkey made in the Brunei Bay Area, Borneo. Zoologica, 49: 183-192.

Medway, L. 1977. Mammals of Borneo.

Wolfheim, J. 1983. Primates of the World. Seattle and London: University of Washington Press.

Yeager, C. 1990. Proboscis Monkey Social Organization: Group Structure. American Journal of Primatology, 20: 95-106.

Yeager, C. 1991. Proboscis Monkey Social Organization: Intergroup Patterns of Association. American Journal of Primatology, 23: 73-86.

Yeager, C. 1992. Proboscis Monkey Social Organization: Nature and Possible Functions of Intergroup Patterns of Association. American Journal of Primatology, 26: 133-137.