As far as animals go, the proboscis monkey is one of the most unique–looking species on the planet.
They are instantly recognisable for their long, fleshy, nose – which is especially prominent in males.
It’s long been thought the bulbous appendage functions as a badge of social status and sexual quality.
But rather than just a visual signal of dominance, it also plays a crucial role in vocal communication, experts say.
Males often produce loud, long–distance calls filtered through the nasal passage, which can travel up to 7 kilometres (4.3 miles) through the jungles of Borneo where they live.
Researchers from the University of Osaka carried out CT scans on preserved proboscis monkey specimens, which allowed them to create detailed 3D digital models of their nasal passages.
Using computer simulation, they were able to work out that the larger the nose, the lower the frequency of the calls they make.
This acoustic signal could indicate a male is bigger and is sexually mature, they said – important details for potential mates to know.
The animals are instantly recognisable for their long, fleshy, nose – which is especially prominent in males
Lower–frequency signals could indicate a male is bigger and sexually mature, the researchers said
‘This research, a unique collaboration between a zoo dedicated to rare species conservation, primatologists, and engineers, yielded unexpected results,’ Professor Takeshi Nishimura, one of the study’s authors, said.
‘We are delighted to have uncovered findings that link physical characteristics to vocal communication and social evolution.’
Much like how humans have recognisable voices, the researchers said the different frequencies emitted by different males will be enough to identify individuals by their calls.
This could help females in the group keep track of where their dominant male is while allowing other males to monitor the location of potential competitors, they explained.
As part of the study, they compared the CT scans of an adult male proboscis monkey, named Jaka, to a juvenile called Niko.
Jaka’s nose was noticeably larger, and experiments revealed he made calls with lower–frequency vibrations.
‘Our findings support the view that adult males develop the external nose to exaggerate a signal of body size to female partners and immature males’, the scientists wrote in the Journal of the Royal Society Interface.
‘The enlarged external nose may be alternatively used to exaggerate vocal individual identification for adult male proboscis monkeys.’
This proboscis monkey (left) has a relatively small nose compared to other, older males (such as this one, pictured right)
As part of the study, the team compared the CT scans of an adult male proboscis monkey, named Jaka, to a juvenile called Niko
Scientists have previously suggested the pendulous nose helps to create an echo chamber that amplifies the monkey’s call.
Other studies have shown the nose can help produce louder, deeper honks, while others argue the exaggerated feature acts like an ‘advertisement’ to potential mates.
Despite the evolutionary benefits linked to a large nose, the pendulous snout has led to the species being branded one of the world’s ugliest animals.
The monkeys are endemic to the jungles of Borneo, never straying far from the island’s rivers, coastal mangroves and swamps.
They live in organised groups consisting of a dominant male and up to seven females and their offspring.
Male proboscis monkeys can reach a weight of 22.5kg (50lbs) although females only grow to around half that size.
Only males develop the namesake nose, with females retaining a more delicate snout.
WHY ARE NON-HUMAN PRIMATE NUMBERS DECLINING?
Behind the collapse in numbers is an increase in industrial agriculture, large-scale cattle ranching, logging, oil and gas drilling, mining, dam building and road construction.
The illegal trade in bushmeat – killing apes and monkeys for their flesh – is also decimating the animals, as is changing climates and diseases spread from humans to apes.
Growing trees to produce palm oil – used in many popular foods – is a particular threat to primates in Indonesia, as is mining for gold and sapphires in Madagascar.
With many species living in rainforests, the cutting down of millions of acres of forest to supply the increasing demand for timber or to clear land for agriculture is destroying their habitat and making populations more fragmented.