The proboscis monkey, scientifically known as Nasalis larvatus, is an arboreal Old World monkey distinguished by its remarkably large, pendulous nose. These unique primates are endemic to the island of Borneo, where they inhabit mangrove forests, swamps, and lowland rainforests, typically staying near rivers and estuaries. Males possess a prominent nose that can extend over 10 centimeters, hanging down past their mouth, while females have a smaller, upturned nose. The striking appearance of the male’s nose has long fascinated researchers, prompting investigations into its evolutionary purpose.
Sexual Selection and Attraction
The exaggerated nose of the male proboscis monkey is primarily understood as a result of sexual selection, functioning as a significant visual and acoustic signal for attracting mates. Females show a strong preference for males with larger, more prominent noses, linking nose size to indicators of health, dominance, and genetic fitness, which drives the evolution of this exaggerated feature.
Studies have found a direct correlation between nose size and other traits considered desirable by females, such as body mass and testicular volume. A larger nose is believed to be an honest signal, indicating a male’s strength and overall quality, increasing their attractiveness to females and propagating this distinct trait.
Amplifying Communication
The large, fleshy nose of the male proboscis monkey also plays a functional role in vocal communication, acting as a resonance chamber that amplifies and deepens their calls. These loud, nasalized vocalizations, like honks or roars, are crucial for communication in their dense, forested habitat. The unique shape of the nasal cavity helps in producing deeper and louder calls, which can travel considerable distances through the rainforest environment.
These amplified calls serve multiple purposes, including asserting dominance among males, warning rivals, and communicating within social groups. Males use specific honks to signal their status to other group members, including reassurance calls directed towards infants. Louder, deeper calls signal a male’s quality and status to prospective female partners, complementing the visual appeal of a large nose.
Beyond Mating and Calls
While sexual selection and vocal amplification are the primary explanations for the male proboscis monkey’s large nose, other less prominent theories have been proposed. One such theory suggests a role in thermoregulation, where the nose’s vascular nature and surface area might help dissipate excess heat in Borneo’s warm, humid climate. However, this is not considered the main driver of its evolution, possibly being an exaptation—a trait that evolved for one purpose but later served another.
The nose has also been observed to swell and redden when a male is excited or agitated, suggesting a potential role in aggressive displays or signaling. Despite their aquatic nature and the nose’s prominent position, its primary evolutionary purpose is not linked to underwater navigation or breathing, though they can submerge for up to 20 meters.