The proboscis monkey, Nasalis larvatus, stands out among primates due to its uniquely large, fleshy nose. This striking facial feature, particularly prominent in males, sparks curiosity about its purpose. Found exclusively on the island of Borneo, these arboreal Old World monkeys possess a distinctive appearance. Their noses, often hanging below their mouths, contribute to their captivating presence in the rainforest canopy.
A Symbol of Status and Attraction
The male proboscis monkey’s enlarged nose primarily serves a role in sexual selection. Females prefer males with larger noses, suggesting this trait advertises male quality and social status. Research indicates a correlation between nose size and the number of females a male attracts to his harem.
The nose size may signal a male’s underlying health, dominance, and even body and testis size. Males with larger noses might possess higher testosterone levels, indicating greater physical strength and reproductive capability. In the competitive social structure of proboscis monkeys, where males vie for access to female harems, a larger nose acts as a visual badge of status that can deter rivals and attract females.
The dense rainforest environment, where visibility is often limited, means visual cues are not always sufficient. A larger nose, by enhancing vocalizations, can effectively transmit signals over distances of 100 to 200 meters.
Beyond Mating: Other Hypotheses
While sexual selection is a primary driver, the proboscis monkey’s large nose also serves other functions. The fleshy nose tissue and uniquely shaped bony nasal cavity of males amplify and deepen their calls, such as honks and nasal roars. This acoustic enhancement allows males to assert their health and dominance effectively.
The amplified calls produced by larger noses can also warn other group members about predators, like crocodiles, or maintain group cohesion in dense vegetation. The nose acts as a resonating chamber, making the male’s vocalizations louder and more intimidating to other males. This dual function, both visual and acoustic, complements its role in attracting mates.
The Nose’s Growth and Development
Proboscis monkeys are not born with large noses; infants have small, upturned noses. The distinctive pendulous nose develops in males during puberty, typically around 4 years of age, and continues to grow throughout their lives, reaching full size in adulthood.
This growth coincides with the time when males achieve dominant social status and begin to form breeding groups. Physiological processes and hormonal influences, such as increased testosterone levels, contribute to this development. The nasal cartilages within the skull provide support for the increasing size of the fleshy nose.
Protecting Their Unique World
Proboscis monkeys are endemic to the island of Borneo, where they inhabit coastal mangrove forests, lowland rainforests, and swamp-lands, typically near rivers. They are skilled swimmers, aided by partially webbed feet, and often cross rivers or jump into the water when threatened. The species is classified as Endangered by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, reflecting a population decline of approximately 50% over the last 40 years.
The primary threats to proboscis monkeys include habitat loss and fragmentation, largely due to deforestation for logging and the expansion of palm oil plantations. Hunting for food and traditional medicine also contributes to their decline. Conservation efforts are underway, including habitat preservation and community engagement, but the continuous conversion of their natural habitats into agricultural lands and human settlements poses ongoing challenges to their survival.