Although hideously ugly to humans, male proboscis monkeys (Nasalis larvatus) use their noses to woo their mates, both through the sounds they make and visual appeal, new evidence reveals.
Humans have a way of appreciating nature, even if we don’t always show it. We umm and ahh at the adorability of many species, but the proboscis monkey is more likely to be the subject of jokes, looking awfully like the character in a fairy story who got a sausage attached to his nose. When it comes to campaigns to save the rainforests of Borneo, it’s just as well the lungs of the planet have orangutans to act as their ambassadors, because these guys wouldn’t cut it.
It must be a challenge swinging through the trees with something like that attached to your face so the evolutionary benefits must outweigh some significant hindrances. Whereas the slightly similar appendage of the elephant seal is attributed to moisture preservation, that wouldn’t apply in a tropical rainforest.
From the start, it was obvious the feature that got the proboscis monkey its name was a form of sexual signaling like a peacock’s tail. The fact the nose is so much larger in males than females is a particularly large clue. A study six years ago found a relationship between nose size and the number of females a male could win to his harem.
This, however, raises the question of what is so attractive – to anyone – about such an appendage. One explanation is that it’s not about the looks, but the sounds the nose can make. Certainly, proboscis monkeys use their large nasal cavities to make honks and roars, which can be important for communicating in thick rainforest that often blocks the line of sight. Alternatively, maybe one set of females has evolved to find that thing beautiful.
It might seem hard to distinguish between these two explanations, but Dr Katharine Balolia of the Australian National University has found a way. Balolia and masters student Pippa Fitzgerald noted that the sound is dictated by the size of the nasal cavity, but the visual effect also takes into account the fleshy parts of the nose. If growth in the two is not correlated, it’s possible to test which matters.
“We found that males have much bigger nasal cavities than female monkeys, and their nasal cavities also have a different shape compared to females,” Balolia said in a statement. “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.”
A comparison of a male (left) and female proboscis monkey skull reveals the nasal cavities are much bigger on the male, even allowing for the larger skull.
Image credit: Katharine Balolia/Morphosource Media
Knowing the sound mattered didn’t prove the look was irrelevant, however.
Balolia had access to a collection of monkey skulls and told IFLScience, “We ranked them by toothwear, since the youngest have less.” The pair found that the nasal cavity reaches its final size in male proboscis monkeys about the time they reach sexual maturity, but the front part of the nose and the fleshy areas keep growing. In particular, the fleshy part is still growing at the point where a male is likely to establish dominance over other males and attract himself a harem.
This fleshy area appears to modify the sound in a potentially pleasing way – males have been reported to straighten the fleshy parts of their noses when honking. Nevertheless, its main role is apparently visual.
A young male is as loud as he will ever be, but an older one will have a more impressive-looking nose, which apparently helps him win more female admirers.
Dr Katharine Balolia examining one of a series of male proboscis monkey skulls to consider why anyone likes it.
Image credit: ANU
Balolia considers it likely the nose growth started as a way to emit more sound, but eventually, “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.” Consequently, even once the nasal cavities had reached their final size, the other parts kept expanding to offer a reminder of how well endowed an individual might be.
Apparently there’s one species in the world that finds an unsolicited dick pic appealing.
Then again, much as we may mock, someone else finds proboscis monkeys sexy, given the discovery of a hybrid two years ago.
The study is published in Scientific Reports.
Source Link: Proboscis Monkeys' Bulbous Noses Send Females Wild At The Sight – And Sound – Of Them