Unravelling the mystery of the 'world's ugliest animal': Scientists reveal why male proboscis monkeys have large, phallic noses - and say they're crucial for mating success

It's safe to say that proboscis monkeys are some of the strangest looking creatures in the animal kingdom.

While female monkeys have pointy noses, the males have large, rather phallic noses – earning them the title of the 'world's ugliest animals'.

Now, a study has finally got to the bottom of this unusual facial feature.

Scientists from the Australian National University say that their large noses are more than just an eye sore.

Instead, they offer several major benefits – especially when it comes to attracting a female partner.

It's safe to say that proboscis monkeys are some of the strangest looking creatures in the animal kingdom. While female monkeys have pointy noses, the males have large, rather phallic noses – earning them the title of the 'world's ugliest animals'

Scientists from the Australian National University say that their large noses are more than just an eye sore. Instead, they offer several major benefits – especially when it comes to attracting a female partner

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Proboscis monkeys are endemic to Borneo and have been branded one of the world's ugliest animals thanks to their unsightly noses.

In a new study, researchers set out to understand why this unusual feature evolved.

The team used 3D scans of monkeys' skulls to understand the size and shape measurements of the internal structure of their nasal cavity.

The scans revealed how the shape of the nasal cavity evolved to allow males to emit louder and deeper honks and roars.

'We wanted to understand why male proboscis monkeys' noses are so big, and whether their nasal cavities have a distinctive shape,' said Dr Katharine Balolia, who led the study.

'We found that males have much bigger nasal cavities than female monkeys, and their nasal cavities also have a different shape compared to females.

Proboscis monkeys are endemic to Borneo and have been branded one of the world's ugliest animals thanks to their unsightly noses

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'Being able to emit louder and deeper calls thanks to a longer and larger nasal cavity helps male monkeys to assert their health and dominance.

'This helps the male monkeys attract females and ward off other males.

'The more females a male can attract, the more it allows him to father more offspring. So having a large nose and being able to more easily emit honks and nasal roars due to a uniquely shaped nasal cavity really helps males show off their quality and status to prospective female partners.'

Borneo is a covered in a jungle-like environment, which could help to explain why the monkeys' noses evolved to become so big, according to Dr Balolia.

'Proboscis monkeys live in coastal mangroves and forested environments and often can't see each other through the trees,' she said.

'So loud, nasalised calls are important to communicate with each other, especially amongst males.'

'The fleshy nose tissue likely got bigger over time as it enhanced their ability to let out nasal honks and roars. Proboscis monkeys rigidly straighten their noses when giving these calls.

'Females may have then started to find large noses visually attractive for selecting a mate as it was an honest signal of health and dominance.'

The scans also revealed that the nasal cavity gets bigger with age, which explains why older males have larger noses.

'This coincides with when they achieve dominance status among other male monkeys, allowing them to scare off other males and attract a lot of females who are ready to breed,' Dr Balolia added.