Proboscis monkeyNasalis larvatusWhere and when to see them

The proboscis monkey is a Bornean primate closely associated with rivers, mangroves and swamp forests. Adult males have exceptionally enlarged noses, while females and younger animals have smaller, upturned noses. Its reddish-brown back, pale limbs and rounded abdomen give it an unmistakable appearance. The large abdomen houses a specialized digestive system for processing leafy food. Forest clearance, hunting and fragmentation have reduced populations, especially where riverbank forests have been converted or isolated.

Size

53–76 cm head-body length

Weight

7–22 kg

Lifespan

Not reliably established

Status

Endangered

Proboscis monkeys usually live in groups containing one adult male and several females with offspring, alongside separate all-male groups. Several groups may gather near evening sleeping trees beside rivers. They spend much of the day feeding, resting and moving through the canopy. Strong swimmers, they sometimes cross channels between feeding areas. Vocal calls, postures and conspicuous male noses contribute to social communication, while riverside sleeping sites influence their daily movements. Male proboscis monkeys often emit a distinctive 'honk' call when disturbed or as a warning to their group. They exhibit strong sexual dimorphism, with males being significantly larger than females and possessing the most prominent noses.

• The species occurs naturally only on Borneo.
• Males have much larger noses than females.
• Its chambered stomach supports microbial fermentation of leaves.
• Partially webbed feet help this unusually capable primate swimmer.

Proboscis monkeys can be observed throughout the year where suitable riverine forest remains. Relatively drier periods can simplify travel in parts of Indonesian Borneo, but rainfall varies considerably. Early morning and late afternoon usually suit observation better than midday heat. The dry season, typically from May to September in many parts of Indonesian Borneo, can offer slightly more predictable weather for river excursions. However, the monkeys are present year-round and their activity patterns are more influenced by daily cycles than seasonal changes.

This species inhabits lowland Borneo, especially mangrove forests, peat swamps, freshwater swamps and riverine forest. It depends on connected tree cover and is usually associated with waterways, although groups may travel inland through suitable forest while feeding and resting. While predominantly arboreal, they occasionally descend to the ground to forage or cross open areas. Their range is restricted to coastal and riverine regions of Borneo, emphasizing their reliance on specific wetland ecosystems.

Young leaves form a major part of the diet, accompanied by seeds, flowers and selected fruits, often unripe. Microbes in its chambered stomach help ferment fibrous vegetation. Feeding choices change seasonally as different trees produce fresh growth and reproductive material. Despite their preference for leaves, they selectively consume mature leaves and fruit only when young growth is scarce. Their specialized digestive system allows them to process toxins from certain plant species that other animals cannot eat.

Common name

Proboscis monkey

Scientific name

Nasalis larvatus

Animal group

Mammals

Size

53–76 cm head-body length

Weight

7–22 kg

Lifespan

Not reliably established

Status

Endangered

Identification

Unlike Borneo's macaques and leaf monkeys, proboscis monkeys combine a conspicuous projecting nose, rounded belly, reddish back and pale grey limbs. The long, pendulous nose is most prominent in adult males, used in social displays and to amplify their calls. Females and juveniles have smaller, upturned noses, making sex and age identification relatively straightforward.