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In a story that I'm writing, there is a giant waterfall which flows down a cliff face. The waterfall is extraordinarily large, and the torrents of water which flow over the side are deafening.

Behind this waterfall, within the rocky face, is a cave system in which a race of humanoid fae lives. While the caves allow protection from the elements and many predators, the echoing caverns provide no relief from the noise of the water.

Instead of relying on audio, they instead have evolved a strong sense of smell. Like some real-world species, the fae can detect direction and distance of their prey and predators based on stereo input into their sensitive nostrils. Additionally, they have developed a complex language involving a combination of pheromones and visual sign.

With all this said: I can justify their continuing to have ears to aid with balance and intracranial air pressure. But why would such a species have retained their ability to hear with these ears, in an environment where they are unable to hear anything besides the waterfall anyway?

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    $\begingroup$ They might not hear low frequencies, but they could still be sensitive to sound outside the range covered by the waterfall. Fae might have high, squeaky voices or even navigate by echolocation like bats in a cave. They may also be cranky. noiseandhealth.org/…. $\endgroup$ – DWKraus Dec 18 '20 at 2:38
  • $\begingroup$ Also, in an extremely noisy environment, there may be other uses for hearing variations in the noise. Maybe they can use the vibration like a low-frequency version of echolocation/sonar. Imagine being able to tell if a wall had a cave behind it based on how the sound was traveling through it. Touch can carry the vibrations and still be heard in the ears. $\endgroup$ – DWKraus Dec 18 '20 at 4:47
  • $\begingroup$ Have you ever been at a party with loud music and other conversations? And yet, people can talk without much problems because our brain is good at separating voice from background sound. $\endgroup$ – NomadMaker Dec 18 '20 at 12:35
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    $\begingroup$ @DWKraus Of all the answers in this thread I like your answers the best. If you post them as an answer I’ll mark it as accepted. $\endgroup$ – DonielF Dec 18 '20 at 21:17
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They can switch on and off their deafness

There are two ways you can do it : Biologically or with technology. The first way needs some adaptation time (like more than a few millenias), while the latter is better suited for short or medium term adaptation.

The idea is quite simple : Plug something in your ears, and you are mostly cut from the world's music. Well, more waterfall noise but you get the gist. And if you are out of the big noise, just unplug and you can hear everything clearly.

Using technology, you can inspire yourself with real world contraptions (earplugs, anti-noise headphones for construction sites), using materials you could find in your caverns or from trade with other people. Even if you don't want technology, just put a wriggling larva lying in the cave, it's nasty, but it could work.

Using biology, the feys would most probably have evolved to have some kind of sphincters in their ear, they can open and close it voluntarily or unvoluntarily like a door. This could lead to interesting arguments where everything go to a deaf ear, since the participants litterally closed the door to their ear to bear with the ruckus.

Note that closing the ear won't prevent all sounds to come in, but you can add it to the fact that constant high-volume noises tends to make one reduce one's sound sensibility, making them effectively "deaf". A bit like when you live on a house against a railroad; After some time, you care less and less about trains passing by.

Another solution using a lil' bit of frame challenge (and counter-arguments)

I am wondering a little why these fey people would have a lot of noise issues to the point of losing hearing capabilities, aside when they're near the waterfalls. Here's the main reason why, and a little bit of counter-arguments to help you fight it :) !

To be able to live correctly, you need a bit of space, especially if you're living in society where you're more likely to want some intimacy. Even if there are echoes, the sound will quickly drop of if your caverns are a hundred meter long, to a point it will be a background noise, or at least not enough to damage permanently one's ears. And there probably won't be since your feys would probably build some structure to lower it : Non-reverberating walls/floors (made from fur/leaves, cavern mystical ooze...?), sound-breakers like walls that send back the sound to where it comes...

All of this, unless? Unless waterfalls are also pouring inside the caves or your fey people are deaf by nature, meaning they don't need those structures or they can't stop the sound with it.

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  • $\begingroup$ I love the sphincters! $\endgroup$ – DonielF Dec 31 '20 at 2:50
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I don't think the fae can go deaf without some handwaving.

Humans (and many small animals) are able to shout over the noise of a real waterfall. Also, when you go deeper inside the rock behind the waterfall, the noise goes down very fast. This means that the species should retain their ability to hear and speak (although this may become not the preferred method of communication). Granted, their hearing would be very much impaired compared to the species from less noisy environments.

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Loud sounds are only deafening to humans because of how sensitive to sound we are.

If a species has hearing, but is far less sensitive to sound, then loud sounds like a waterfall would not cause any problems.

So, yes, your fae would not be deaf, but they would likely be what we would consider "hard of hearing" because they would only be able to hear sounds that are louder than waterfalls.

Being able to hear just very loud sounds is still a bit more beneficial to survival than pure deafness, so it is reasonable to believe that they evolved to retain some hearing.

Also, if they develop a strong diaphragm and tough vocal chords, they can still communicate via sound.

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Net-Casting Spiders have extremely sensitive eyes. This is a huge advantage for them when hunting, sense they are nocturnal. Unfortunately, this means that every day at dawn their eyes are burnt out and they go blind. They regrow the damaged portion every night. It might be possible for another creature to do something similar.

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    $\begingroup$ This is horrible. I love it. $\endgroup$ – DonielF Dec 31 '20 at 2:48
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They only hear high wavelengths.

The waterfall produces low pitched noise. The fairies' ears are designed not to register that noise, and so are not damaged by the low frequency vibrations. The noise just vibrates the whole ear without vibrating the sensitive hearing parts relative to each other.

A fairy standing beside the waterfall might feel the vibrations in their body similar to how I can feel when my phone vibrates on silent, but cannot hear the vibrations (unless it vibrates against a surface).

The fairies' ears are designed for high-pitched noise such as birdsong, their own voices, and things beyond human hearing range such as dog whistles and bat sonar.

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There is no survival advantage to deafness, even in this noisy environment. A fae who is born without hearing is no more able to survive and thrive than one who has full hearing that they have been conditioned to ignore because of the constant cacophony.

For a trait (or the loss of a trait) to become dominant in a gene pool, it has to offer something to those who have it.

Deafness doesn't.

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    $\begingroup$ I doubt it's entirely true :/. You'd lose less energy in growing/keeping the hearing trait, energy which could be used to develop other senses like smell. Look at all blind animals in underground : It's not "beneficial" to lose sight, yet they lost most if not all of it. In other words, gaining no advantage is a disadvantage of some sort because of the cost $\endgroup$ – Tortliena Dec 29 '20 at 9:42
  • $\begingroup$ @Tortliena deafness =/= absence of ears & associated structures. That sort of change take many thousands of generations to develop. $\endgroup$ – PcMan Dec 29 '20 at 10:06
  • $\begingroup$ @PcMan Yet people can use less brain "power" and especially training to develop further other skills, Daredevil like but with deafness ^^. The brain is surprisingly agile to do that kind of thing. $\endgroup$ – Tortliena Dec 29 '20 at 10:23
  • $\begingroup$ Anyhow, hearing is definitely a weakness in those caves since you won't sleep well due to a so high noise, so whether they change on the long term (ear-loss mutation) or short one (ear-plugs, brain habit/ear damages...), they would find a way to live in them. Or some other places, if they offer less inconveniences. $\endgroup$ – Tortliena Dec 29 '20 at 10:30
  • $\begingroup$ @PcMan I already considered in my question that they have a need for the ear canal and associated structures, if only for aid in balance and intracranial air pressure equilibrium. But that doesn't mean that they can hear with such structures. $\endgroup$ – DonielF Dec 31 '20 at 2:56
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Most likely, people living and reproducing in such a noisy environment would not evolutionarily de-emphasise hearing, but would instead emphasise improved sound discrimination ability.

This would mean that in a number of generations, these people would be able to discriminate quite well between the everpresent noise and meaningful sounds. Their speech would tend to use frequencies that are less affected by the noise.

Noise/signal discrimination may be achieved by a number of traits, all of which are likely to evolve. These include modifications of the external pinnae of the ears to improve directional discrimination, increased mobility of the pinnae (Even humans still have external ear muscles, and can learn to control them), and better noise-discriminating neurology in their brains.

Even growing up in such an environment would provide an advantage over parents who had grown up elsewhere, due to the neural plasticity of the developing brains of the infants.

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