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Interactions between heterospecific species with similar sexual signals can lead to errors in mate attraction and species recognition. As a consequence sexual signals might diverge to reduce communication errors. Since high-density populations create more noise than low-density populations we hypothesize that individuals should suffer less acoustic interference from sympatric congeners with low-density populations than higher density populations of congeners. Even though species might be sympatric at multiple sites, only populations at sites with a relatively high abundance of both species should require one species to make vocal adjustments that minimize acoustic interference. To test this hypothesis we studied Eleutherodactylus coqui populations in the presence of relatively high numbers of E. portoricensis and in the presence of relatively low abundance of E. portoricensis. We found subtle differences in advertisement calls of E. coqui at locations where E. portoricensis had relatively high a...