HABITAT-DEPENDENT SONG DIVERGENCE IN THE LITTLE GREENBUL: AN ANALYSIS OF ENVIRONMENTAL SELECTION PRESSURES ON ACOUSTIC SIGNALS

Authors

  • Hans Slabbekoorn,

    1. Center for Tropical Research and Department of Biology, San Francisco State University, San Francisco, California 94132
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      Present address: Institute of Evolutionary and Ecological Sciences, Leiden University, P.O. Box 9516, 2300 RA Leiden, The Netherlands; E-mail: Slabbekoorn@RULSFB.LeidenUniv.NL.

  • Thomas B. Smith

    1. Center for Tropical Research and Department of Biology, San Francisco State University, San Francisco, California 94132
    2. Center for Population Biology, University of California, Davis, California 95616
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      Present address: Center for Tropical Research, Institute of the Environment, UCLA, 1652 Hershey Hall, P.O. Box 951496, Los Angeles, California 90095–1496; E-mail: TBSmith@UCLA.EDU.


Abstract

Abstract.— Bird song is a sexual trait important in mate choice and known to be shaped by environmental selection. Here we investigate the ecological factors shaping song variation across a rainforest gradient in central Africa. We show that the little greenbul (Andropadus virens), previously shown to vary morphologically across the gradient in fitness-related characters, also varies with respect to song characteristics. Acoustic features, including minimum and maximum frequency, and delivery rate of song notes showed significant differences between habitats. In contrast, we found dialectal variation independent of habitat in population-typical songtype sequences. This pattern is consistent with ongoing gene flow across habitats and in line with the view that song variation in the order in which songtypes are produced is not dependent on habitat characteristics in the same way physical song characteristics are. Sound transmission characteristics of the two habitats did not vary significantly, but analyses of ambient noise spectra revealed dramatic and consistent habitat-dependent differences. Matching between low ambient noise levels for low frequencies in the rainforest and lower minimal frequencies in greenbul songs in this habitat suggests that part of the song divergence may be driven by habitat-dependent ambient noise patterns. These results suggest that habitat-dependent selection may act simultaneously on traits of ecological importance and those important in prezygotic isolation, leading to an association between morphological and acoustic divergence. Such an association may promote assortative mating and may be a mechanism driving reproductive divergence across ecological gradients.

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