Proboscis Monkey Profile

16. Proboscis Monkey Profile

Common Name: Proboscis Monkey
Scientific Name: Nasalis larvatus
Genus: Nasalis
Family: Cercopithecidae
Order: Primates
Class: Mammalia
Phylum: Chordata
Conservation Status: Endangered (IUCN)

Physical Description

The proboscis monkey is one of the most distinctive primates in the world, instantly recognized by the large, pendulous nose of adult males. While the exact purpose of this feature is debated, it is believed to play a role in attracting mates and amplifying vocalizations. Their reddish-brown coats fade to gray on the limbs and tail, with lighter underparts. Males are much larger than females and possess longer noses.

• Length: 60 – 76 cm (body)

• Tail: 55 – 75 cm

• Weight: Males 16 – 22 kg; Females 7 – 12 kg
  
  

They are semi-aquatic and excellent swimmers, often found near mangrove forests, riverbanks, and swamps in Borneo.

Diet

Proboscis monkeys are primarily folivores but have a varied diet depending on the season.

• Leaves: Their main food source, especially young leaves.

• Fruits: Particularly unripe fruits, which are easier for their complex stomachs to digest.

• Seeds & Flowers: Supplements to their leafy diet.
  
  

Their multi-chambered stomachs, similar to those of cows, help them ferment tough plant material. However, they are sensitive to changes in diet and cannot digest sugary or human-provided foods without risk.

Speed & Agility

Despite their bulky appearance, proboscis monkeys are agile in trees and skilled swimmers.

• Swimming: Strong, natural swimmers, often crossing rivers. They use a dog-paddle style and can even swim underwater.

• Leaping Distance: Up to 9 m (30 ft) in a single leap between trees.

• Agility: They move nimbly in mangrove forests and swampy habitats, adapting to both arboreal and aquatic environments.
  
  

Social Structure

Proboscis monkeys live in groups called harems, usually consisting of one dominant male, several females, and their offspring. Bachelor groups of males are also common. They communicate with loud honks, honks, and growls, with the large nose of males amplifying these sounds.

Fun Fact: The male’s “honking” calls not only deter rivals but also reassure the group while traveling.

Reproduction

Like many primates, proboscis monkeys reproduce slowly compared to smaller monkeys.

• Gestation Period: ~166 days (about 5.5 months)

• Offspring: Usually one infant

• Weaning: Around 1 year, though young may stay close to mothers for longer

• Maturity: Females ~5 years, Males ~6–7 years
  
  

Infants are born with dark fur and blue faces, which gradually lighten as they grow.

Did You Know?

• Natural Swimmers: They have partially webbed feet and hands, helping them swim efficiently.

• “Dutch Monkeys”: Locals once nicknamed them this, mocking Dutch colonists whose long noses and reddish hair resembled the monkeys.

• Nose Purpose: Bigger noses may help resonate calls through the dense forest, signaling strength to females and deterring rivals. This is also exactly why only the males have the ''really'' big noses.

• Protected Range: They are endemic to Borneo—found nowhere else on Earth.
  
  

Role in the Ecosystem

Proboscis monkeys are important seed dispersers, aiding in the regeneration of mangrove and swamp forests. By feeding on unripe fruits and seeds, they maintain plant diversity and support the health of fragile wetland ecosystems.

Why They’re At Risk

Endangered (IUCN Red List)
Their population has declined drastically due to:

• Habitat Loss: Large-scale destruction of mangroves and peat swamp forests for palm oil, logging, and human settlement.

• Hunting: Once hunted for meat and traditional medicine.

• Fragmentation: Small, isolated groups of individuals face extreme difficulties maintaining genetic diversity.
  
  

Conservation efforts focus on habitat protection, ecotourism initiatives, and awareness campaigns to prevent further decline of this uniquely charismatic primate.

How You Can Help: Support sustainable palm oil, back organizations protecting Borneo’s forests, and spread awareness about the fragile future of this species - even within the neighbourhood.