Got a little carried away.
Corvidwish is actively trying to figure out the anatomical characteristic that is tied to language (unfortunately, she doesn’t have the internet or textbooks at her disposal)… however, some of you did figure it out! I loved reading your theories, guesses, and all of your reasonings for those theories.
It is based on heart anatomy!
Mammals and extant archosaurs (birds & crocodilians) have four chambered hearts, while reptiles (excluding crocodilians) & amphibians have three chambers and fish have two. Fish lack vocal cords so I think their languages would be built on sounds they produce with their muscles, swim bladders and bones… clicking, vibrations, etc… idk, fishspeak isn’t important to the plot at all but it’s fun to think about!
Four is a good number :o)