Asymmetric acoustic divergence and microhabitat selection in two syntopic gladiator treefrogs

Abstract

Acoustic communication plays an important role in the behavior of anurans and is therefore susceptible to natural selection. Given that effective communication is vital for successful mating, the acoustic niche hypothesis postulates that vocalizing males of co-occurring species are naturally selected to produce distinct calls, ensuring they are recognized by the females of their own species. Geographically separate populations face different selection pressures over time and, when two species share the same environment, one or both may undergo reproductive character displacement (RCD) to ensure viable and effective communication. In this study, we analyzed two gladiator frogs, Bokermannohyla ibitipoca and B. feioi, co-occurring in the Serra do Ibitipoca State Park, and compared them to two allopatric populations. We aimed to assess whether the co-occurrence of these species caused divergence in their advertisement calls, by comparing the patterns of acoustic signal variation where they co-occur and in allopatric populations. We also analyzed divergence in microhabitat selection for vocalization and whether relative abundance varies, which could explain asymmetries in call differentiation between syntopic and allopatric populations. The results suggest an asymmetrical RCD on B. ibitipoca, since males emit lower pitched calls when co-occurring with their congener when compared with the allopatric population, while B. feioi does not show differentiation compared to the allopatric population. Males of B. ibitipoca vocalized more frequently from concealed locations compared to B. feioi and were less abundant when vocalizing alongside B. feioi, which likely intensified the selective pressure for efficient communication in B. ibitipoca. This is the first study to evaluate acoustic differentiation in sympatric anuran species, considering both the acoustic niche hypothesis, microhabitat choice and relative abundance. Our findings illustrate how evolutionary processes can distinctly influence reproductive traits, enabling the coexistence of both species within the same community. This underscores the importance of conducting further studies on similar dynamics in other Neotropical anurans.