They say size doesn't matter but for proboscis monkeys, bigger is better.
Known for their long droopy noses, proboscis monkeys have cruelly been described as one of the world's ugliest animals.
Yet females of the species have no complaints and the larger the nose, the more success the male has with the ladies.
Now scientists believe they can fully explain why.
In a world first, Australian researchers found the bony structure of the male's nasal cavity had evolved to let them make "honks and roars" that attract a mate and intimidate rivals.
The Australian National University team investigated 3D scans of skulls, discovering the nasal cavity was much larger than not only the female but other primate species.
"It was a bit of a long shot to look at the skull in a bid to see what we could find out about social behaviour but it was super surprising," the university's Katharine Balolia told AAP.
Previous research suggested the elongated nose had evolved to advertise status.
Indeed, dominant proboscis monkeys exhibit plenty of big-nose energy, living in harem groups as the sole male in a group of adult females.
But ANU research found the budding Romeos' unusual appendage was more than a visual sign to potential mates.
The team looked at the monkey's nasal cavity, the bony chamber of the skull behind their bulbous proboscis, and made an unexpected discovery.
First they compared the cavity to the female's skull along with three other species in the same "old world monkey" family - king colobus monkeys, blue monkeys and crab-eating macaques.
Then they found it was almost 30 per cent larger than the female.
For the other three species, the difference between sexes was between only seven and 17 per cent.
The big difference indicated the cavity had evolved to make distinctive sounds, helping the nose act like a trumpet to emit louder, deeper calls to solicit females and ward off rivals.
"We wanted to understand why male proboscis monkeys' noses are so big, and whether their nasal cavities have a distinctive shape," Dr Balolia said.
"We found that males have much bigger nasal cavities than female monkeys, and their nasal cavities also have a different shape compared to females.
"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."
Loud "nasal roars" would also help their hit rate as a Casanova in a jungle, Dr Balolia said.
Proboscis monkeys are native to the island of Borneo in Southeast Asia and typically found in mangroves, swamps and riverine forests.
"Proboscis monkeys ... often can't see each other through the trees. So loud, nasalised calls are important to communicate with each other," Dr Balolia said.
"The fleshy nose tissue likely got bigger over time as it enhanced their ability to let out nasal honks and roars.
"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."
Researchers also found the bony nasal cavity opening got bigger with age, which was consistent with older males having 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 said.
A proboscis monkey's nose can dangle past their mouths, and Dr Balolia would not be surprised if they only got bigger.
"The more that females prefer the trait, the better advantage it has for males to evolve that," she said.
"Basically the bigger noses get selected over time until it is so big that it is going to become cumbersome."
The research has been published in Scientific Reports.
