interesting! i think i remember reading somewhere (or maybe it was a david attenborough documentary 
that rainforest jungle fauna that rely on sound for mating, marking territory etc, have evolved in such a way that each species takes up its own frequency spectrum so as not top disrupt other species in their day to day lives.
kind of like a nature's FM broadcast licensing system.
I remember something like this too. The only reference I can find is Schafer's
Soundscape (Chapter 16 / Rhythms in the Natural Soundscape, pp. 229-231), where he discusses the acoustic activity of birds and two species of frogs. The lower-frequency bullfrogs show a pattern that overlaps the other two, but the activity of the higher-frequency chirping frogs never overlaps with the similar-frequency bird calls. Schafer says:
"...as the voices of both sets of performers occupy a similar high-pitched range, they would, if sounded together, tend to mask one another, thus reducing the clarity obtained by vocalizing in rotation. The bullfrogs, on the other hand, with deep voices, offer no competition for either performer. They continued to croack intermittently through both day and night."
But I also vaguely recall some more general theory (maybe via Attenborough) which suggested that in densely-populated ecologies each species occupies a niche in the audio spectrum. Would love to track it down again - anyone else remember this?
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