In my story, set in an earth-like world, a marine species that had evolved a melon organ, much like that found in our dolphins, made its way back into the "land" (a large system of underground caves), re-evolving its limbs and making use of its echolocation to thrive, as the other predator species of the cave had basically gone extinct, leaving the niche open.

The re-evolution of the limbs and specialization for climbing apart, is the use of its melon organ for echolocation viable on land? To what extent does the change from water to the air alters the way this organ functions? The cave is mostly devoid of light, making this ability necessary.

  • $\begingroup$ I'd guess that the melon would gradually come to operate more and more like a bat's ears. There are many cases of parallel evolution. An octopus eye functions almost identically to a human one but they are about as far apart from us as can be for an animal. The Melon might still function normally in water, and caves can have LOTS of water, whereas a bat couldn't exploit this additional niche.Your dolphin would perhaps come to resemble a seal or walrus $\endgroup$
    – DWKraus
    Apr 8, 2020 at 23:09

1 Answer 1


No, the melon organ will be way less useful on land, due to the impedance mismatch.

Before you report me for insulting, let me explain what is it ;)

When an acoustic wave travels from a medium A (let's say water) to a medium B (let's say oil), the amount of transmitted and reflected power at the interface depends on a property called acoustic impedance, which you can see somehow equivalent to the electric resistance.

The maximum transmission of power from A to B, and conversely the minimum reflection, happens when the acoustic impedance of A matches the acoustic impedance of B.

If the impedance mismatch is too high, you end up reflecting most of the power back, instead of actually transmitting it. This is why when you get an echo scan you get smeared with that gel: to remove the layer of air between the transducer and your body, increasing the amount of ultrasound that goes into your body for the scan. And this is also why underwater explosions are way more destructive on a human body, for the same amount of explosive, than in air explosions.

Coming back to the melon, it works well underwater because its impedance matches well with the water. If you use it in air, you will have a poor matching, unless you spread gel from the melon to the target of the scan.

Bats solve the issue by using their vocal system and auditory system directly in air.

  • $\begingroup$ What if (by evolution through many generations) you drain the fluid from the melon & replace it with air? $\endgroup$
    – Pelinore
    Apr 9, 2020 at 14:01
  • $\begingroup$ @Pelinore, same story: there would be another interface between the cavity and the body, plus the one between the body and the outside $\endgroup$
    – L.Dutch
    Apr 9, 2020 at 14:06

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