Study Finds Large Noses Help Male Proboscis Monkeys Amplify Long‑Distance Calls

Scientists have identified a functional reason for the distinctive oversized noses of male proboscis monkeys, concluding that the bulbous appendage helps lower the pitch of long‑distance calls and may serve as an acoustic signal of male size and sexual maturity.

Researchers at the University of Osaka conducted CT scans on preserved proboscis monkey specimens to create detailed three‑dimensional digital models of their nasal passages. Using computer simulations based on those models, the team found that larger noses produce lower‑frequency vocalizations when calls are filtered through the nasal cavities. Lower frequencies are commonly associated with larger body size in animals and can convey information about an individual’s physical attributes to rivals and potential mates.

Proboscis monkeys (Nasalis larvatus) are endemic to the island of Borneo and are easily recognized by the exaggerated nasal growth that is markedly larger in males. Scientists had previously proposed that the nose functions primarily as a visual badge of social status and sexual quality. The new study adds an acoustic dimension to that explanation by showing how nasal morphology alters the spectral properties of calls.

Male proboscis monkeys are known to produce loud calls that can travel through dense jungle terrain. The research notes that these calls can be propagated across distances of up to about 7 kilometres (4.3 miles) in Borneo’s forests, enabling long‑range communication among widely spaced groups. By lowering the fundamental frequency of calls, a larger nose could make a male sound larger and more mature to listeners at a distance.

The team’s approach—combining high‑resolution imaging of internal nasal anatomy with computational acoustic modelling—allowed them to isolate how changes in nasal geometry affect sound filtering without relying on invasive procedures in live animals. The use of preserved specimens provided consistent anatomical data for constructing the virtual models and testing how variations in nose size shift acoustic output.

The findings refine understanding of how sexual selection and social signaling can shape extreme morphological traits. In many vertebrate species, acoustic cues such as call frequency correlate with body size and competitive ability; the proboscis monkey provides a clear example in which exaggerated soft‑tissue anatomy modifies vocal signals as well as visual appearance.

Researchers say the results illuminate the multifunctional nature of sexually dimorphic traits and suggest that mate choice and male–male competition may both have favored enlargement of the male nose. The study also underscores the role of acoustic ecology—how sound propagates through particular habitats—in the evolution of animal communication.

Further research will be needed to link anatomical and acoustic measurements directly with behavioral responses in wild populations, but the new evidence establishes a plausible mechanism by which the famously large nose of the male proboscis monkey contributes to long‑range vocal signalling in Borneo’s forests.