Forests echo with voices that carry both warnings and welcomes. Among these voices are the calls of the proboscis monkey, a primate best known for the exaggerated noses of its adult males. These pendulous features often puzzle visitors who see them for the first time.

What possible advantage could such a comical appearance bring? Scientists now reveal that these noses are not only ornaments for show but also resonant chambers that shape sound in remarkable ways.

A recent study published in the Journal of the Royal Society Interface shows how these noses alter vocal signals, strengthening individuality and broadcasting maturity. What once seemed like nature’s joke turns out to be a finely tuned instrument of communication.

Proboscis monkey nose shapes sound

The large external nose has long been linked to mate choice. Females may see it as a badge of health and strength, while males may use it as a sign of status to avoid unnecessary fights.

Yet vision alone cannot explain its full significance. Male proboscis monkeys produce loud calls known as brays, which travel through dense forests. These calls, filtered through their noses, gain acoustic qualities that deepen their meaning.

The nose exaggerates lower frequencies, making males sound larger than they are. This could discourage rivals while simultaneously attracting mates.

But the effect does not stop there. Differences in nose size among adults also shift the position of specific formants, turning each call into a kind of vocal fingerprint.

Juveniles to maturity

To uncover this transformation, researchers studied two males: Jaka, a 13-year-old adult, and Niko, a 3-year-old juvenile. CT scans of their nasal passages were used to create digital models. These models revealed how the nose grows not only longer but also more influential in shaping sound.

In juveniles, higher formants remain far apart. In adults, the third formant slides closer to the second, producing a more distinctive resonance. This subtle acoustic shift signals physical and sexual maturity. Replica models built from the scans confirmed the effect.

When sound was passed through these replicas, the results matched the simulations, proving that anatomy alone explains the change.

Proboscis monkey calls mark identity

The findings highlight that proboscis monkey noses are not only about body size. Variation in the third formant among adults suggests an added role in individual identification.

In human speech, individuality often comes from pitch, tone, and patterns of sound. Non-human primates, however, rarely produce long strings of varied vocalizations. Instead, individuality is encoded in specific acoustic features.

Because proboscis monkeys live in dense, complex forests, sound carries more reliably than sight. A unique call helps individuals stand out. Each male can be recognized by his resonance, making social interactions clearer.

In a community where males often overlap, avoiding confusion becomes vital for both survival and reproduction.

Monkeys that rely on individuality

Proboscis monkeys live in a society of layers. At the core are one-male, multi-female units, but these units cluster into larger communities that may include bachelor groups.

This multi-level society means males frequently encounter each other, while females may transfer between units.

In such a structure, individuality matters. Males must be able to recognize rivals and allies without constant confrontation.

Females, meanwhile, benefit from recognizing the calls of potential partners. The enlarged nose provides the acoustic distinction needed in this crowded setting.

Unlike geladas or mandrills, which manage massive groups with more complex visual signals, proboscis monkeys rely heavily on vocal individuality. Their smaller bands make recognition achievable and worthwhile.

Benefits of vocal recognition

The social system of proboscis monkeys is also patrilineal. Males remain within their natal communities, while females often disperse. This kin-based structure reduces the intensity of competition among related males and encourages cooperation.

In such settings, vocal recognition has additional benefits. Identifying the calls of relatives aids coordination, whether defending against predators or foraging in thick forest.

Females also take advantage of these signals. When moving between units, they can recognize individual males by voice, allowing them to monitor not only potential mates but also the social positions of others.

This interplay of biology, behavior, and community reveals why such an unusual feature evolved and persisted.

Proboscis monkey nose has dual roles

The enlarged nose operates on two levels. It acts as a visual ornament, signaling health, maturity, and status, while also serving as a vocal amplifier of individuality.

Together, these signals create a powerful package for communication. In environments where sight is limited and competition is high, such dual functions enhance survival and social cohesion.

“This research, a unique collaboration between a zoo dedicated to rare species conservation, primatologists, and engineers, yielded unexpected results,” noted Professor Takeshi Nishimura.

“We are delighted to have uncovered findings that link physical characteristics to vocal communication and social evolution. We are excited about the future prospects of this research.”

Shaping sound and perception

While the evidence is strong, questions remain. Do females actively choose mates based on vocal individuality? Do males respond differently to the calls of familiar rivals compared with strangers? Playback experiments and field observations could provide answers.

For now, the proboscis monkey’s nose stands as a striking example of nature’s creativity. It is both instrument and symbol, shaping sound as much as it shapes perception.

In the quiet forests of Borneo, the echo of a call carries not only across distance but also across the social bonds that define life in these remarkable primates.

The study is published in the Journal of The Royal Society Interface.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–