Animal vocalization in zones of contact and hybridization

Abstract Contact zones are geographic areas where the ranges of closely related taxa (species or subspecies) come into proximity or overlap in a partial way. In contact zones, hybridization often occurs, a process in which individuals that differ in phenotypic or genetic traits interbreed and produce offspring with mixed characteristics. Animals that use acoustic communication are especially attractive for studying such zones. In this review, we analyze vocalization in 22 contact zones in animals with vocal learning (songbirds and parrots) and in 31 contact zones in animals without vocal learning (birds, anurans, mammals, and insects). Hybridization has been described in 49 of the 53 zones (92.4%). In animals with vocal learning (songbirds and parrots), the intermediate (mixed) vocalization was not found in five (27.7%) zones. All the vocal patterns (song types) found in these zones belonged to the repertoires of the parental species, even despite widespread hybridization in all five cases. The vocal traits in 15 other songbirds and 2 parrot contact zones converged, primarily because of the mixing elements from the songs of parental species. Unilateral song divergence has been documented in only one (4.5%) case. Animals without vocal learning retain their parent species' vocalizations in three (9.7%) zones. Convergence of vocal characteristics among contacting forms was observed in 19 zones. In nine (29.0%) zones, there was evidence of mutual or unilateral divergence in vocal signal characteristics compared with allopatric populations. It seems that the acoustic behavior of species and subspecies that co-occur in contact zones is influenced by whether they have vocal learning abilities or not.