Reinforced acoustic divergence in two syntopic neotropical treefrogs

Phylogenetic inertia often drives sister species to exhibit similar acoustic signals, compromising species recognition and increasing competition for acoustic space. Consequently, natural selection favours acoustic divergence in sympatry through behavioural plasticity or character displacement. Although well-documented in temperate regions, these phenomena have been less investigated in tropical communities, characterised by more complex interactions and saturated acoustic spaces. Here we examined acoustic divergence in the advertisement calls of two closely related neotropical treefrogs that share similar signals, habitat, and phenology, and that hybridise in nature. Our results revealed differences in call parameters between syntopy and allotopy, each species showing a specific response. While the smaller-sized species, Boana bischoffi, increased dominant frequency in syntopy, Boana prasina prolonged call duration, both increasing acoustic divergence between these sister species. In contrast, morphological and environmental factors had little influence on acoustic parameters, with only body size affecting dominant frequency in B. bischoffi. These findings suggest that vocal adjustment (acoustic plasticity) or character displacement (local adaptation) may enlarge acoustic divergence in advertisement calls, reinforcing reproductive isolation and reducing interspecific competition for acoustic space in sister taxa.