In male proboscis monkeys, their bulbous noses will often hang past their mouths.

But why evolve such a strange feature? Are they a visual sign of health and status to potential female mates, and to other males? Or did they evolve to help the monkeys make honks and other loud sounds?

A battle of noses
One of the largest monkey species in Asia, proboscis monkeys (Nasalis larvatus) are endemic to the island of Borneo. They live in coastal mangroves, peat swamps and riverine forests, and have an unusual diet made up mostly of leaves.

They can swim quite well and have webbed fingers and toes. They typically live in harem groups, made up of a single adult male (who tends to have a large, bulbous nose), some adult females and their offspring.

Males don’t often get the opportunity to attract a harem until they reach middle age. These older, dominant and large-nosed males don’t easily tolerate other large-nosed males, often trying to ward them off aggressively with deep honks and “nasal roars” – loud calls they make using their noses.

Young adult males with smaller noses often live in all-male bachelor groups, and don’t tend to fight aggressively with each other. When these bachelor males get older and become large (and large-nosed) enough to compete with males that are part of a breeding group, they are in a position to overthrow the tenured male. Females then often choose to form a harem group with this new, high-status male.

If male proboscis monkeys have a different nasal cavity shape to females, and a unique shape compared to the other monkey species, it would support the idea these enhanced nasal structures – both the fleshy nose and the cavity behind it – evolved to allow for more effective honks and nasal roars.

That was indeed what we found. The shape of the male nasal cavity was low and long compared to females. This allows males to build up resonance (sound vibration) in their nasal cavities, allowing them to emit deeper and louder calls through their noses.

The nasal aperture shape was also different between the sexes. In males, it looks a bit like an eggplant, while in females it looks more like an upside-down pear. This unique opening shape in males allows for higher intensity sounds to be emitted through the nose.