Vocal conservatism in disjunct populations of the Marsh Tapaculo, Scytalopus iraiensis (Aves: Rhinocryptidae), in the Brazilian Atlantic Forest

Population fragmentation has the potential to cause divergences between individuals, which can result in allopatric speciation, with vocal manifestations standing out as a characteristic susceptible to differentiation. This evolutionary dynamic differs in birds of the suborder Tyranni, which are historically characterized by innate and genetically determined vocalizations; however, recent evidence points to exceptions, such as the plasticity of certain lineages. In this context, we investigated the song of Scytalopus iraiensis (Rhinocryptidae), an endangered species restricted to dense swamp vegetation and with a disjunct distribution between the South and Southeast macroregions of Brazil, to test the hypothesis of acoustic divergence driven by geographic isolation. For this, we used 80 recordings encompassing both subpopulations, from which spectral and temporal parameters were extracted and analyzed using Principal Component Analysis (PCA), K-means clustering, and Permutational Multivariate Analysis of Variance (PERMANOVA). The results revealed remarkable vocal conservatism, with K-means cluster analysis segregating vocalizations into two groups with a mixture of individuals from both macroregions. PERMANOVA confirmed the absence of significant differentiation between subpopulations, with geography explaining only 1.5% of the total acoustic variation. It is concluded, therefore, that the vocal conservatism found results from the synergy between the biomechanical constraints of the suborder Tyranni, strong habitat specialization, ancestral polymorphism, and probable recent historical connectivity through wetland corridors. This corroborates the recognition of the species as a single Evolutionarily Significant Unit (ESU) and the need for future phylogeographic, molecular, and in situ experimental investigations.