Proboscis monkey noses create vocal individuality with possibility of individual recognition

Male proboscis monkeys (Nasalis larvatus) living in Southeast Asian tropical rainforests develop enlarged external noses as they reach adulthood. These large noses have been thought to serve as visual status indicators for males and to make their voices lower to advertise their body size. In various animals, voice pitch correlates well with the body size of the individual producing it, so it is used as a clue to identify the body size of another individual. The large nose of proboscis monkeys has been thought to have evolved as an organ that emphasizes such functions.

A research group led by Professor Takeshi Nishimura from the Graduate School of Human Sciences at the University of Osaka, Professor Isao Tokuda from the Graduate School of Science and Engineering at Ritsumeikan University, and Professor Ikki Matsuda from the Wildlife Research Center at Kyoto University discovered that male proboscis monkeys may be using their voices, produced through their tengu-like large noses, for individual recognition. The findings were published in Interface of the Royal Society.

Through collaborative research with Yokohama Zoological Gardens 'ZOORASIA,' the research group conducted CT scanning of proboscis monkey specimens preserved at ZOORASIA. Based on the image data obtained, they created three-dimensional digital morphological models of the nasal passages. Using these digital models, they revealed through computational simulation which frequency bands of the voice are amplified. In acoustic science, this is called a transfer function, and it determines the timbre of the voice based on where the frequency bands that amplify voice components called formants are located—what corresponds to a "voiceprint." The group also created physical replicas and conducted acoustic measurement experiments to verify that the computational results were truly accurate. Through this series of experiments, they understood the acoustic effects of the nose in one adult individual and one juvenile individual respectively.

Next, they computationally varied nose size continuously from juvenile to adult and calculated the acoustic effects of how these changes alter the voice. They found that the effects accompanying changes from juvenile to adult differed from the effects of nose size variations among adults. Specifically, the former showed effects where formant frequencies that amplify the voice shifted to lower regions, making the voice sound deeper. On the other hand, the latter showed changes only in specific formant frequency components. Such differences in specific formant frequencies are thought to represent vocal individuality.

This finding suggests that proboscis monkeys may be identifying not only body size but also individual identity by hearing male voices. Proboscis monkeys live in units consisting of one male and multiple females. Furthermore, higher-level units where multiple units gather have also been confirmed. Both males and females disperse from their natal units, but more males tend to transfer to neighboring units compared with females. In such a unique multilevel society, being able to identify which adult male is vocalizing just by hearing their voice would not only help avoid accidental conflicts between males but also help young males and females take appropriate social actions. It is thought that the distinctive large nose developed and evolved due to such adaptive significance.

The three major elements of sound are "loudness," "pitch," and "timbre," and humans use these elements selectively for vocal communication. In humans, within complex multilevel societies, there are many situations where individuals recognize each other just by hearing voices and take social actions. The "timbre" analyzed in this study is determined by which frequency bands of the voice are strengthened and which are weakened, and it contains abundant individual characteristics. This result suggests that creating vocal individuality through timbre may not be unique to humans. It is expected that future research will clarify whether proboscis monkeys also utilize information about vocal individuality.

Nishimura commented: "This research was an unusual collaboration between a zoo working on the breeding and conservation of rare species, primatology researchers, and engineering researchers, and we were able to obtain unexpected results. We achieved results that provide insights into the evolution of vocal communication and society from the functions created by body form, and the entire team is delighted. Please look forward to future developments in this research."

Journal Information
Publication: Interface
Title: Individual vocal identity is enhanced by the enlarged external nose in male proboscis monkeys (Nasalis larvatus)
DOI: 10.1098/rsif.2025.0098