Aliens with an extra-set of ears- any possible benefits?

Question

While doing some concept drawings of an alien species for a project of mine I somehow wound up drawing a head with two sets of ears, one pair on the top of the head and the other lower with one on each side. I considered erasing the drawing and starting over but part of me decided to keep it as is- should I write off one of the two sets as a vestigial pair that never got weeded out of the evolutionary process, or is there a possible advantage to four ears?

I ask because while we often see insects or arachnids with multiple sets of eyes, multiple sets of ears are, afaik, not really seen in nature and there's probably a good reason for that.

Answer 1

The species live in the trees in a very dense jungle

Four ears don't just give wider range of sensitivity, they also can act as a precise 3D locator, allowing their owner's brain to determine the exact source of any heard sound. This could be plain beneficial in an environment where both predators and prey could appear in the entire sphere around the specimen, and reacting properly would be the only way to survive. Vision, while helping, is mostly obscured by foliage and other stuff, thus the primary warning of "you're about to be eaten" would come from hearing.

Let's say the ancestors of those specimen are a kind of lemurs or monkeys, that left surface in order to save themselves from larger prey roaming the lower layer, then a mutation happened and a specimen that already relied on hearing got two pairs of ears, which allowed him to escape dangers due to acute hearing. So the mutation lingered, eventually a four-eared branch split off, their brains get accommodated to quadro sound and developed 3D hearing on par or better than sight (while retaining sight as a means of identifying stuff in direct vision, or silent things like trees), thus they have got two precise senses instead of our one. This in turn allowed them to spread and inhabit the entire jungle, and grow over time to expand to other areas.

Answer 2

The alien species spends or spent a lot of time with their body partially submerged in water.

Therefore

• the upper ears are used to listen to sounds coming from the emerged air side, while
• the lower ears are user to listen to sounds coming from the submerged water side.

Answer 3

One reason that hasn't been mentioned yet is is the ears could be optimised for different frequency ranges: soft cartilage-based ears are good at detecting lower frequency sounds while rigid bone ears will detect higher frequency sounds due to the differences resonant/natural frequency.

Answer 4

To add to L. Dutch's answer, there are fish with strange double eyes. The part of each eye which is usually above water seens well in air, while thepart of the eye which is usually below water sees well in water.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1771291/

Anableps anableps, the largescale four-eyes,2 is a species of four-eyed fish found in fresh and brackish waters of northern South America and Trinidad. This species grows to a length of 30 centimetres (12 in) TL. This fish can occasionally be found in the aquarium trade. The fish does not actually have four eyes, but instead each eye is split into two lobes by a horizontal band of tissue, each lobe with its own pupil and separate vision. This allows the fish to see above and below the surface of the water at the same time.1

https://en.wikipedia.org/wiki/Anableps_anableps

On Earth, some animals, like humans, rely mostly on their vision, while other animals rely mostly on hearing, or on smell, or taste, or touch.

If those aliens spend much time in low light environments, sound and hearing might be more important to them than vision. And if they spend a lot of time partially submerged, they might need to hear well in both water and air, possibly in both at the same time.

I note that sounds travel much better in solids, or liquids like water, than they do in air.

I hear my voice transmitted from my vocal cords thorugh my head to my ears, and it sounds good to me. But other people hear my voice through the air. And when I hear recordings of my voice, transmitted through the air, it sounds a lot different and much worse.

So one, presumably minor, side effect of having ears for water and ears for land, might be to help the aliens know the difference between how their voices sound in air and in water.

On Earth some animals which rely a lot on their hearing don't just wait to hear sounds. They generate sounds constantly, and by hearing those sounds reflected from objects in their environment they know where objects are. They echolocate.

So you might want to research how bat ears work well for echolocation in air and how dolphin and toothed whale ears work well for echolocation in water.

Answer 5

This could give significantly improved directional hearing aka figuring out what direction a sound came from.

Simple directional hearing works by measuring the difference in the time between when the sound arrives at the ears and so (knowing the speed of sound) what angle that implies for the source of the sound. However with only 2 ears this presents a problem; there are many directions the source of the sound could be in and arrive with that time difference. Take the simple case of the sound arriving at both ears at the same time; the source can't be to the left or right but could be directly in front or directly behind you, or above or below.

With the structures of the ear, resonances and a lot of signal processing the brain can actually do a fairly decent job of guessing the origin of a sound better than just 2 sensors would imply but a 3rd (or 4th) ear would make localising a sound in 3D much simpler and much more reliable.

Answer 6

We have 2 ears because we're essentially built to live on a 2D plane, and 2 ears is all you need to know which direction a sound came from in 2D. Our brains have enough other information available to extend our hearing ability to 3D for the fewer times that's really necessary (a predator in the trees above, a boulder bouncing down from a mountain, etc), but for "true" 3D hearing you'd need at least one more ear vertically offset from the other two. I assume that would give you better quality 3D hearing than ours, because our brains effectively "imagine" how far above/below a sound is, making usually-good guesses based on things like how your head is tilted and possibly variations in character of the sound as it bounces off different parts of your earlobe in different ways before reaching your eardrum.

So there's a bit of brainpower dedicated to this guesswork, and it's not always accurate. If we had 3 ears then no guessing would be needed, so I imagine it would require less brainpower and provide greater accuracy more consistently. If 3D hearing is more important in your story's world, multiple ears might therefore be more common, and assuming they require less brainpower it makes sense that intelligence could evolve more easily as more brainpower is available for other things like rational thought.

Four ears don't really add much benefit in these ways, but they do create symmetry that your intelligent species might find more attractive (so the creatures with 4 ears mated more often, until 3-eared creatures just disappeared). They also create a bit of redundancy, which would be valuable in a world where hearing is so important - especially if some environmental factor creates a higher risk of partial hearing loss, like super violent storms or an atmosphere that carries the percussive force of lightning strikes and such farther than ours does.

Answer 7

If you watch a cat or a dog (specially from breeds like shiba inu) for long enough, you will see that they can move their ears independently from each other, thus tracking two distinct sound sources at the same time.

A four eared creature would be able to track four sound sources at the same time.

Answer 8

If a species evolved in an environment like a cave with a lot of echoing, maybe an extra set of ears would enhance its ability to pinpoint exactly where a sound was coming from, accounting for reflections of sound off walls. I'm not sure if that makes perfect sense mathematically, but it certainly sounds plausible. In that case maybe it would make sense to give the alien an underdeveloped set of eyes.

Another potential factor, albeit maybe not so satisfying / not enough purely on its own, is mate selection. Species can develop all kinds of strange physiological characteristics if for some reason that makes them more desirable to prospective mates, even if those characteristics could impede basic survival tasks like hunting and evading predators.

So it is possible that while two ears would be sufficient for the environment, at some point a genetic mutation led to the development of an extra set of ears and that was appealing to mates for some reason.

While it may seem pretty inefficient for a species to develop an extra set of functioning ears (though maybe they're not functional) for something that's mostly aesthetic, evolution doesn't directly "care" about efficiency. As long as something has a significant enough impact on the likelihood of producing more viable offspring, it can develop into a widespread trait in a species.

Answer 9

While I don't know of any animals with more than a pair of ears, I do know of some animals that that have multiple paths for vibrations to reach the inner ears.

The hippo, for example, hears underwater sounds through their jaws, which are linked to the middle ear. Those vibrations bypass the outer ears entirely. They essentially do have more than a pair of ears.

As another example, the elephant essentially use the pads of their feet as additional ears for low-frequency sounds. By triangulating the two types of signals using both ears and feet, elephants can tune in to the direction, distance and content of a signal.