A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection.

@article{Mikula2020AGA,
  title={A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection.},
  author={Peter Mikula and Mihai Valcu and Henrik Brumm and Martin Bulla and Wolfgang Forstmeier and Tereza Petruskov{\'a} and Bart Kempenaers and Tom{\'a}{\vs} Albrecht},
  journal={Ecology letters},
  year={2020}
}
Animals use acoustic signals for communication, implying that the properties of these signals can be under strong selection. The acoustic adaptation hypothesis predicts that species in dense habitats emit lower-frequency sounds than those in open areas because low-frequency sounds propagate further in dense vegetation than high-frequency sounds. Signal frequency may also be under sexual selection because it correlates with body size and lower-frequency sounds are perceived as more intimidating… 
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