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Yes, I know owls have feathery "ear tufts", but these are less suited for hearing and more for display.

And I find it hard to believe that animals like dinosaurs or other cursorial archosaurs would not rely on sound as much as a mammal would. It's just too useful of a sense.

One potential clue is the fact that most non-mammal tetrapods appear to use electrical tuning to discern different sounds, whereas mammals use the hair cells of their longer cochlea to mechanically discriminate sound. I feel like this is related but not completely connected to why a fleshy outer ear structure that can collect and focus in on the direction of a sound.

Why did the pinnae evolve in mammals and why are they the only ones with such a structure?

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  • $\begingroup$ If an organism does not have outer ear, it does not mean it does not rely on sounds. Birds heavily rely on sounds for so many aspect of their life. Owl is a great example, $\endgroup$
    – Remi.b
    Jun 28, 2019 at 19:55
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    $\begingroup$ Do you really think there can be an objective answer to a question of the form “why did this NOT evolve in any other animals? All you end up with is circular arguments of the type that are the very antithesis of science. The one answer you have so far just describes the thing and says how wonderful it is. $\endgroup$
    – David
    Jul 21, 2019 at 21:03
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    $\begingroup$ @David The question is literally "why did this evolve in only one group of animals", and I just got such an objective answer. It's always puzzled me why StackExchange users try to belittle the OP into thinking their question is stupid/too broad/impossible to answer while it clearly isn't that insurmountable. $\endgroup$
    – Thesaurus Rex
    Jul 22, 2019 at 8:38
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    $\begingroup$ I think it is worth noting that not all mammals have pinnae (the external component of the ear) – i.e. Australian monotremes and the marsupial mole. Also, props to T-Rex for calling out the online bullying, which is totally unnecessary. $\endgroup$
    – Poppins
    Sep 15, 2020 at 6:05
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    $\begingroup$ Ear pinnae in Katydid insects to detect bat ultrasounds: doi.org/10.7554/eLife.77628 $\endgroup$
    – Noil
    Dec 27, 2022 at 18:18

2 Answers 2

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Short answer Mammalian hearing is unique and amazing.

The mammalian ear is unique and highly sensitive with a built in amplification system that means even minute changes in sound can be detected. The bony amplification is also the reason the mammalian (or at least therian) cochlea is huge, every incremental increase yielded markedly more information, it has to get quite large before you hit the point of diminshing returns. This means mammals can get a lot out of even small investments into improving hearing. The vast majority of other groups just would not get any benefit from an external ear. their ears are not sensitive enough to gain anything the faint echoes external ears collect. The one group that would get some benefit (owls), has something functionally similar to an external ear.
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The mammalian ear evolved from the repurposing of vestigial jaw bones no other group had (or more correctly they did not become vestigial in other groups). Mammals are the only group with a single bone in the lower jaw. it is a fascinating story and well worth a look. It kinda makes up for how awful mammalian eyes are.

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A Lizard would need ears of 8cm to focus it's upper hearing range.

Lizards and birds can hear as high as 4kHz to 10 kHz, which corresponds to an energy wavelength of 8cm to 3cm, so a frog would need ears of about 8cm to collect and focus the sound, and a bird would need ears of about 3cm. Bird calls are also mostly limited to about 8khz, about 1 octave higher than note 88 on a piano.

Also, the distance in between the two ears of i.e. a bird is less than the wavelength of the sound that they detect, so they have less sense of which direction sounds come from.

Cats and mice can hear sound waves of about 0.5cm, so their ears can augment and focus the sound efficienty and can also shield the ears so they just hear one side each.

Soft materials absorb sound, that's why audio speakers use carbon fiber and other rigid diaphragms to transmit high frequencies, and that's why toast has higher frequencies than bread, so mammals had to use the stiffness of articulated bones to pick up high frequencies of sound. It's a complex adaptation step, a bit like the evolution of the eye.

Here is a vid about mammal ears the wiki.

Early mammals were nocturnal, it may be a contributing factor.

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  • $\begingroup$ The argument on body size / sound wavelength doesn't stand much with large dinosaures, does it? $\endgroup$
    – Noil
    Sep 3, 2022 at 16:49
  • $\begingroup$ Yes, it's more likely that a dinosaur could evolve giant ears to amplify and locate sound, especially if it's a nocturnal avian dinosaur. it's the high-frequency limit of a vertebrate's inner ear relative to it's size that influences if it would benefit from ears. Birds and lizards would have evolved ears if they didn't need giant ears relative to body size because their inner earbones are intrinsically limited to big wavelengths. Say a dinosaur has an upper hearing range for 5cm wavelengths, and it had 5cm ears. $\endgroup$
    – bandybabboon
    Sep 3, 2022 at 17:44

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