Habitat-dependent ambient noise: consistent spectral profiles in two African forest types

Many animal species use acoustic signals to attract mates, to defend territories, or to convey information that may contribute to their fitness in other ways. However, the natural environment is usually filled with competing sounds. Therefore, if ambient noise conditions are relatively constant, acoustic interference can drive evolutionary changes in animal signals. Furthermore, masking noise may cause acoustic divergence between populations of the same species if noise conditions differ consistently among habitats. In this study, ambient noise was sampled in a replicate set of sites in two habitat types in Cameroon: contiguous rainforest and ecotone forest patches north of the rainforest. The noise characteristics of the two forest types show significant and consistent differences. Multiple samples taken at two rainforest sites in different seasons vary little and remain distinct from those in ecotone forest. The rainforest recordings show many distinctive frequency bands, with a general increase in amplitude from low to high frequencies. Ecotone forest only shows a distinctive high-frequency band at some parts of the day. Habitat-dependent abiotic and biotic sound sources and to some extent habitat-dependent sound transmission are the likely causes of these habitat-dependent noise spectra.