title: Sharp acoustic boundaries across an altitudinal avian hybrid zone despite asymmetric introgression.
abstract: Birdsong is a sexually selected trait that could play an important evolutionary role when related taxa come into secondary contact. Many songbird species, however, learn their songs through copying one or more tutors, which complicates the evolutionary outcome of such contact. Two subspecies of a presumed vocal learner, the grey-breasted wood-wren (Henicorhina leucophrys), replace each other altitudinally across the western slope of the Ecuadorian Andes. These subspecies are morphologically very similar, but show striking differences in their song. We examined variation in acoustic traits and genetic composition across the altitudinal range covered by both subspecies and between two allopatric populations. The acoustic boundary between the subspecies was found to be highly abrupt across a narrow elevational range with virtually no evidence of song convergence. Mixed singing and use of hetero-subspecific song occurred in the contact zone and was biased towards the use of leucophrys song types. Hetero-subspecific song copying by hilaris and not by leucophrys reflected a previously found asymmetric pattern of response to song playback. Using amplified fragment length polymorphisms (AFLP) markers, we detected hybridization in the contact zone and asymmetric introgression in parapatric populations, with more leucophrys alleles present in hilaris populations than vice versa. This pattern may be a trail of introgression due to upslope displacement of leucophrys by hilaris. Our data suggest that song learning may impact speciation and hybridization in contrasting ways at different spatial scales: although learning may speed up population divergence in songs, thereby enhancing assortative mating and reducing gene flow, it may at a local level also lead to the copying of heterospecific songs, therefore allowing some level of hybridization and introgression.
authors.affiliation: Behavioural Biology, Institute of Biology (IBL), Leiden University, Leiden, the Netherlands.|Animal Ecology, Department of Ecological Science, VU University Amsterdam, Amsterdam, the Netherlands.
authors.forename: W
authors.identifier: 0000-0002-4111-0930
authors.initials: W
authors.lastname: Halfwerk
authors.affiliation: Behavioural Ecology Group, Zoology Department, Cambridge University, Cambridge, UK.|Department of Earth Sciences, The University of Hong Kong, Hong Kong, Hong Kong.
authors.forename: C
authors.initials: C
authors.lastname: Dingle
authors.affiliation: Behavioural Biology, Institute of Biology (IBL), Leiden University, Leiden, the Netherlands.|Animal Ecology Group, Centre for Ecological and Evolutionary Studies, Groningen University, Haren, the Netherlands.
authors.forename: D M
authors.initials: DM
authors.lastname: Brinkhuizen
authors.affiliation: Behavioural Biology, Institute of Biology (IBL), Leiden University, Leiden, the Netherlands.|Animal Ecology Group, Centre for Ecological and Evolutionary Studies, Groningen University, Haren, the Netherlands.
authors.forename: J W
authors.initials: JW
authors.lastname: Poelstra
authors.affiliation: Behavioural and Physiological Ecology Group, Groningen Institute for Evolutionary Life Sciences, Groningen University, Groningen, the Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Komdeur
authors.affiliation: Behavioural Biology, Institute of Biology (IBL), Leiden University, Leiden, the Netherlands.
authors.forename: H
authors.initials: H
authors.lastname: Slabbekoorn
mesh_terms: D000162:Acoustics; D054458:Amplified Fragment Length Polymorphism Analysis; D000818:Animals; D005075:Biological Evolution; D020308:Songbirds; D014828:Vocalization, Animal
keywords: Henicorhina leucophrys; acoustic variation; hybrid zone; introgression; vocal learning