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So I’m trying to design my alien life around the pressures they might face just like earth’s wildlife, and most things I’ve made plausible life that is similar to ear but not giant dinosaur cats. The problem I’ve come to now is hearing.

There are many, many examples across the animal kingdom of different life exploiting sight, smell and touch as ways to serve their environment, but as I see it the only life that has taken full advantage of sound is mammals (not including monotremes). Nothing else has the characteristic pinna that capture (and amplify) sound and direct it to the inner ear, and even without that mammals internal ear anatomy is more complex than most other life.

In a sci-fi perspective, I have not found non-humanoid alien designs with outer ears. It seems that universally people steer clear of adding ears, as they are a very familiar trait and a design may seem less alien with them. I’m inclined to follow suit for the same reasons as well as simply trying to find an alien alternative.

So far I have designed a rough concept of a creature with closable, fixed intakes (kinda similar to air intakes on a car or jet) to funnel air into the ear canal. My problem with this design is that it misses out on the swivelling advantage of mammals, having the creature need to turn their head to catch the sound.

TLDR, outer ears (pinna) seem to be exclusive to mammals, and so the excuse of convergent evolution is harder to grasp. Is it too earth-like to give aliens ‘ears’ and if not, is there another plausible structure that serves a similar purpose?

Edit: I’ll specify a land-based creature. Aquatic hearing performs under different constraints, pinna are massive sources of drag in that circumstance anyway.

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    $\begingroup$ You might look into the hearing of birds (ornithology.com/the-hearing-of-birds ), or consider that there's at least one large group of mammals -cetaceans - that make considerable use of sound but have no external ears. $\endgroup$ – jamesqf Aug 13 at 5:22
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    $\begingroup$ just a nitpick: "outer ears (pinna) seem to be exclusive to placental mammals", have you ever seen a kangaroo? $\endgroup$ – Mori Aug 13 at 6:51
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    $\begingroup$ Someone on the biology stack exchange says here that the evolution of external ears is prob. tied to mammal's evolution of middle ear bones from what were originally reptilian jaw bones, which makes hearing more sensitive. That seems to be supported by the mini-article on p. 57 of this issue of Scientific American, which says jaws and ears develop in tandem in embryos, and ear bones specifically help with high-freq. sounds and external ears also help with that. $\endgroup$ – Hypnosifl Aug 13 at 6:59
  • $\begingroup$ @Mori heck you’re right I twisted it around. I meant to exclude monotremes but didn’t think to mention marsupials (I’m from Aus so I have no excuse lol). $\endgroup$ – XenoDwarf Aug 13 at 7:24
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    $\begingroup$ "but as I see it the only life that has taken full advantage of sound is mammals ...Nothing else has the characteristic pinna that capture (and amplify) sound and direct it to the inner ear" - I would suggest that the owls would have a serious issue with that assertion. $\endgroup$ – Keith Morrison Aug 13 at 16:20
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Mammals have unusually well-developed hearing compared to most animals because they evolved from nocturnal species, and most nocturnal vertebrate niches have been dominated by mammals since that happened. So the fact that it only evolved once on Earth might not imply that external ears are an unlikely feature (unlikely to evolve on another planet) - there hasn't been a strong reason for another group of animals to evolve them.

Nevertheless, there are other methods for enhancing hearing without external ears. The other vertebrate that dominates the night, the owl, might not have external ears (they do have ear-like feather tufts, but these do not aid hearing and appear to be a coincidence) but they instead have a face with irregularly-shaped indentations that funnels sound toward their ear-holes and serves a similar purpose. The irregular shape is important, by the way - for both owls and mammals, it helps determine which direction the sound is coming from.

Insects can have tympanic membranes pretty much anywhere (most insects are deaf or nearly so, and the handful of groups that can hear well appear to have evolved that capability independently) and despite their small size they can pinpoint sounds based on which membranes detect the sound first. I wouldn't expect to see leg-ears or torso-ears on a larger creature though; insects are the way they are because their small size requires them to have comparatively larger sensory organs that probably wouldn't fit in their head. In general concentrating the sensory organs in one place (the head) is the best way to give you the most information about the world.

So it's not especially unlikely for an alien to have external ears, but it isn't the only way to go about evolving sound detection. It's up to you, really.

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  • $\begingroup$ I think it was first mentioned by @Keith Morrison, but the mention of owls has got me kicking myself. Great answer. $\endgroup$ – XenoDwarf Aug 14 at 10:23
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You ask for another plausible structure that serves a similar purpose... I think you should reconsider non-mammalian options.

For example, some insects have Tympanal organs on various body parts that detect the pressure component of sound. They look like blisters. They are usually symmetrically paired in insects but you could add multiple tympanal organs to various places on your alien to give it great directional awareness.

Also consider antennae and the Johnston organ in many insects. They can have hundreds of sensory neurons detecting sound, as well as gravitational and mechanical stimuli.

Or consider giving your alien lateral lines, like a fish, with tiny cilia helping identify sound and direction.

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  • $\begingroup$ Yeah, what Tig said. Also frogs! They are vertebrates and count on hearing to find mates, but have a big tympanic membrane outermost - which one might argue is a better method than hiding a tiny tm deep at the bottom of a hole. $\endgroup$ – Willk Aug 13 at 14:55
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Convergent Evolution

Convergent evolution is an important factor in any creature design. The idea is that if you're doing very much the same task under very much the same circumstances then it's reasonable to come up with much the same solution. Sea birds tend to be black and white. Sharks, dolphins and whales have come up with very similar body plans and tend to be dark on top and lighter underneath. The same environment, the same requirements, the same solutions.

If you consider the solutions used by fish and insects, they differ massively either in terms of scale or environment. A macro creature listening to vibrations in the air will likely come up with something resembling one of the many variants on ears used by mammals on earth. Moveable or not, scaled for temperature, but still an external structure designed to received and focus sound to a point.

Ultimately it's entirely reasonable to give your bipedal intelligent, obligate tool using aliens very familiar body plans to ours. Along with equivalently similar body plans to other alien creatures such as quadruped grazers and predators.

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    $\begingroup$ Convergence is believable and understandable in the case of fish/bird colouring and hand-like grasping because there are multiple examples on earth - pinna are particularly perplexing because they are one of the few adaptions that don't have equivelants on earth. While I apreciate the example of intellegent life believably being humanoid, that is based partly on factors of convergence (four limbs, standing up, grasping 'hands') and there are many alternative body plans that are equally feasable. $\endgroup$ – XenoDwarf Aug 13 at 10:43
  • $\begingroup$ @XenoDwarf, I counter your equally feasible by saying that if all things were equal they would be represented on Earth. Given enough time, even the tiniest advantage becomes a major imbalance. $\endgroup$ – Separatrix Aug 13 at 10:58
  • $\begingroup$ Maybe I'm misunderstanding but it seems like those statements contradict eachother? At the least they're unrelated. Again, could be my mistake. $\endgroup$ – XenoDwarf Aug 13 at 11:07
  • $\begingroup$ @XenoDwarf, it's easier to see if we look at the vast variety of insects, all those different body plans were equally feasible so they all exist, but for large animals (apart from kangaroos) it gets comparatively boring and samey with a small range of very similar options. So I'd argue that whatever distinctly different body plan you had in mind, it's not actually equally feasible for some reason that we may not fully understand. $\endgroup$ – Separatrix Aug 13 at 11:21
  • $\begingroup$ I think lineage is a factor here, if tetrapod vertebrates weren't the first to claim the land terrestrial life may have been very different. Our only large terrestrial animals are mammals and archosaurs, so we dont have a broad range of animals to compare (unlike fish vs birds, mammals vs insects etc). $\endgroup$ – XenoDwarf Aug 13 at 11:40
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Convergent evolution works fine for external ears, you just need the right preadaptations.

external ears are unique to mammals becasue ears with built in amplifiers are unique to mammals. the two extra mammalian ear bones at play make for a powerful sound amplifier. mammals are the only vertebrates that get a benefit from external ears. This is why external ears are believed to have evolved twice independently in mammals and not anything else.

Owls for instance have highly sensitive ears but not an amplifier so an external ear so they have no distortion to account for external ears add distortion in return for better collection, mammalian ears already encourages the evolution of neurological correction for distortion because of the amplification so external ears do not add any problems they have not already solved. In early mammals amplification was more important than clarity in hearing, later mammals developed more active neurological compensation for the distortion. Whereas an owl would have to evolve both systems (external ears and neurological correction) at the exact same time for it to be useful which is very unlikely.

As long as your aliens have an internal amplifier like mammals do evolving external ears is easy.

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