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I've designed a creature for my alien world that is a large apex predator known commonly as a wraith. It lives mostly in dense jungle and forest and uses stealth and ambush to catch its prey (it can take down creatures the size of a horse). Its senses include the contextually unimportant taste and touch, a sense of smell as strong as or slightly stronger than a dog and thermal sensors on its snout. The thermal is important, because it is completely blind (a result of its subterranean ancestry). Its hearing is also extremely sensitive, and can pick up high frequencies. The ears themselves are large and form a fixed tunnel that runs down the sides of the head (which is slightly elongated) from the brow.

I've contemplated adding echolocation to the wraith's arsenal, taking advantage of its sensitive hearing and reinforcing its spacial awareness, as well as giving it a powerful advantage against competition (as a disclaimer, it is part of group of blind predators, a majority of life isn't blind on this planet).

My question is, at which point does strong hearing stop being an advantage and start being a hindrance? (sudden loud noises damaging hearing, especially when human machinery comes close).

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First of all, note that echolocation may not be such a good idea for a predator, since whatever is the frequency a predator uses for it, its preys will ultimately evolve to ear it as well, and thus detect it. Some insect actually do it to avoid bats.

To go straight to the point, a more sensitive hearing may become a problem due to evolutionary factors. If your predator hear with high accuracy and sensitivity a wide range of frequencies, it must as well be very good at analysing the sounds it hears, otherwise it will have no advantages to hear that well.

Its prey may evolve strategies to confuse you predator, for example by emitting noise over a wide spectrum of frequencies or by living in (or even constructing) a place with complex acoustic (perhaps even symbiosis between an animal and a plant modelling the sounds, and in fact possibilities are endless).

To catch up with that evolution your wraith should get a better analysis of sounds, or in other words, a better brain. But better brain imply that you need more energy (thus food) to use it. At some point a better hearing may therefore become a problem : your improved brain cost you more food that it allows you to catch, and it would be more food-efficient to have a slightly worse brain with a smaller "maintenance" cost.

It is a bit counter-intuitive (for humans at least) to thing that being smarter may actually be an evolutionary disadvantage, but it makes sense (and it explains why there are still plenty of quite stupid animals out there, having sometimes no brain at all).

However the trade-off between brain improvement and energy needed is only one possible evolutionary trade-off. You may for example also argue that a better hearing need a more complex ear, and that a more complex ear is more fragile. So at some point the loss due to less reliability is bigger than the gain due to better sensitivity.

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Animals with good hearing may be more susceptible to hearing damage overall, but the exact amount may vary.

Hearing damage is caused when the vibrations from the sound waves damage the delicate equipment inside the ear. The reason we find loud noises painful and annoying is because our brain is telling us to get away from them before they damage our hearing. If the noise would not be damaging, it wouldn't bother us.

There are different ways of improving a creature's ability to hear, one of the simplest being improving the shape of the outer ear to funnel more sound into the ear and amplify it. Much like listening through a simple funnel, this by itself will make the creature more sensitive to noise. Cats and dogs may be more susceptible to loud noises than humans, for instance.

However, if the creature's hearing is so good at hearing that noises capable of damaging its hearing are common, it may evolve to have more durable structures inside the ear itself, without sacrificing sensitivity.

For example, bats have such good hearing that they would deafen themselves with their own echolocation calls, so they have a mechanism which automatically 'locks' their ear bones in place at the moment they send out a pulse. This means that a bat cannot hear its own cries, only the returning echo.

It is not unreasonable that a creature with particularly good hearing (and noisier prey) might have a reflex that automatically dulls its hearing when it detects a loud noise, functionally similar to clapping your hands over your ears, but using a mechanism that is in the ears themselves. It might also be able to voluntarily adjust its current noise sensitivity using muscles in the inner and outer ear, making itself more sensitive when it is hunting or alert.

If we're talking aliens, it might have a more efficient method. Maybe its eardrum is made out of something more durable than plain old skin, like a protein structurally similar to spider silk. This would allow it to remain sensitive all the time and still be less susceptible to hearing damage.

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  • $\begingroup$ Wow, first answer and it's already great quality. I forgot to explain the ear's structure, so I'll add a bit about that. $\endgroup$ – XenoDwarf May 11 '16 at 9:09
  • $\begingroup$ hearing damage sadly isn't limited to eardrum damage -- most hearing damage is caused by the fine sensor hairs in the cochlear breaking off / being damaged. You'd also need to find a way to fix those. Maybe better regenerative abilities for those things? Just like losing teeth is permanent for humans, but a shark just regrows them. $\endgroup$ – subrunner May 11 '16 at 11:16
  • $\begingroup$ Yeah, it seems clear that an echo-locating predator that has evolved to the point of dropping eyes entirely (nocturnal and underground hunting maybe?) would have extremely well developed and robust auditory systems $\endgroup$ – Tim B May 11 '16 at 11:22
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While I think there is a lot to be said for a predator having incredibly sensitive hearing being able to be traumatised or damaged by something like a much louder industrial noise created by humans, I can't see this working unless there was nothing that loud naturally occurring on this planet.

So this implies there is no thunder, no storms, no trees falling nearby etc.

That's a tricky one to argue. Anything that happens normally gets built into evolution.

So maybe look at the specific frequencies - could there be something new in the environment that now harms the creature. Again, this gives you the opportunity for human impact.

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Sensitivity, itself is never a hinderance. It is always useful to have more sensory input than less. However, what can be a hinderance is difficulties with powerful stimuli, like loud sounds.

The factor which is important here is dynamic range. What is the ratio between the loudest sound and the quietest sound a creature can hear. How can it adjust this balance? If a creature is capable of hearing a whisper at 100ft in the presence of a gunshot, because its eardrums are just that amazing, then there's no disadvantage to that hearing at all (other than perhaps the extra processing you'd have to do on the extra input).

This is often adjustable. Consider humans. We have three bones in our ears which transmit vibrations from the eardrum to the cochlea. The ear drum is fragile compared to, say, the rest of our skin, but it's actually pretty tough. It takes a pretty loud noise to rupture the ear drum. The cochlea, however, is a fine tuned instrument for teasing apart vibrations. It is not so tough, and easily saturated. Those three bones that transmit sound actually have muscles attached to them. Those muscles serve as an "automatic gain correction" system. In loud environments, they pull apart, decreasing their ability to transmit sound from the eardrum to the cochlea. In fact, they play a part in speech. Your vocal chords resonate your head quite a bit. They would be very loud if unchecked, so just before you start speaking, these muscles will engage to decrease the ear's sensitivity to your own voice!

A more advanced creature, dependent on sound, may instead have a cochlea that's got a larger dynamic range, permitting it to continue hearing without adjusting its own gain in more extreme situations.

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