The animal you are likely thinking of, with its large and distinctive nose, is the proboscis monkey. This unique primate, native to the island of Borneo in Southeast Asia, has an appearance that immediately sets it apart. Its prominent, fleshy nose is a striking feature, particularly in males.
Identifying the Proboscis Monkey
The proboscis monkey (Nasalis larvatus) primarily inhabits mangrove forests, swamp-lands, and lowland rainforests across Borneo, always remaining close to slow-moving water sources like rivers and estuaries.
Their specialized diet consists mainly of young leaves and unripe fruits (about 90% of intake), supplemented by flowers, bark, seeds, and insects. To digest this fibrous plant matter, they possess a complex, chambered stomach filled with bacteria that break down tough cellulose and toxins, allowing them to process foods other primates might find indigestible.
Physically, they are characterized by reddish-brown fur, long tails, and noticeable sexual dimorphism; adult males are significantly larger (16-22 kg) than females (7-12 kg).
The Purpose of the Large Nose
The large, pendulous nose, or proboscis, develops extensively in adult male proboscis monkeys, sometimes hanging over their mouths. Females and juveniles also have larger noses than most other primates, but not to the same exaggerated size as adult males. Scientific theories propose two primary functions for this prominent appendage, both related to sexual selection and communication.
One theory suggests the large nose acts as a visual signal for females, indicating male health and status. Females prefer males with larger noses, which may correlate with greater body mass and testis size, signaling reproductive fitness and social dominance. This preference drives the evolution of larger noses through female mate choice. The nose also plays a role in male-male competition, serving as a “badge of social status” to deter rivals.
The second function of the large nose is its role in vocalization, acting as a resonance chamber to amplify male calls. When a male vocalizes, the nose swells and turns red, enhancing the volume and depth of their loud honking sounds. These amplified calls attract females, communicate group status, and intimidate rival males, especially where visual cues are less effective over distance.
Unique Behaviors and Social Life
Proboscis monkeys live in distinct social structures, typically harem groups of one adult male with several females and their offspring. All-male “bachelor” groups also exist, composed of males who have left their natal groups upon reaching adulthood. While groups may occasionally congregate near abundant food sources or sleeping sites, individuals generally interact within their smaller units.
These monkeys are remarkably proficient swimmers, making them among the most aquatic primate species. They have partially webbed feet, aiding their movement in water. They swim across rivers, sometimes submerging for up to 20 meters, to forage or escape predators like crocodiles. They also frequently jump from trees directly into the water.
Conservation Status and Threats
The proboscis monkey is classified as Endangered on the IUCN Red List of Threatened Species. Their population has declined significantly, with a reduction of approximately 50% over the last 40 years. The primary threat to their survival is widespread habitat loss, driven largely by deforestation across Borneo.
Vast areas of their lowland swamp and mangrove forest habitats are being converted, particularly for palm oil plantations. This industry’s development, including plantations and processing facilities, directly destroys the specialized riverine forests proboscis monkeys depend on for food and shelter. Other threats include illegal logging, infrastructure development like roads, and hunting.