Nasalis larvatus
proboscis monkey

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)

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)

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)

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.

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)

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

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)

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)

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.

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.

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)

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

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)