Very variable volume – Decoding song amplitude structure in flies

„Louder!“ The radio plays that film music that gives me the creeps: Lux Aeterna. It is an orchestra that starts off quiet and then grows into a loud storm, while in the film banality grows into horror. However it’s not the film, but the music that makes me feel uncanny. Now imagine the violins kicked in loudly and continued loud – that’d feel different, maybe even boring, wouldn’t it? Variations in volume are frequently used in music, which is why they have technical terms as ‘crescendo’ and ‘decrescendo’ describing increasing and decreasing volume respectively.
However I am not a musician, but a scientist and want to tell you about crescendos and decrescendos in the song of fruit flies. Yes, flies sing! Check out this video to see how. You can see increasing and decreasing volume in this image of a fly song recording:


Remarkably, other flies make sense of these variations in song volume: flies like volume variations typical for their species better than others. Finding what volume variations are typical for a fly species is not trivial, but doable. I illustrate it in this flowchart:


First I define song amplitude structure (SAS) as volume increase across a local volume peak (circled dots) followed by decrease to a local volume minimum. Then I detect pulse maxima and minima (dots) in courtship song recordings and arrange the pulse peaks relative to SAS peaks. Finally I present SAS data as a heat map. You can think of this procedure as decoding a temporal structure encoded in fly song. This uncovers a diamond-like structure that differs between species, as you can see here:


Flies imaged by Nicolas Gompel.

But how do flies produce these structures? And how can you test what structures flies like? This post is too long already, so I’ll try to answer these questions in later posts.