Larval competition with invasive Aquarana catesbeiana induces differential effects on brain morphometry of Rhinella arenarum metamorphs

Anuran larvae show remarkable sensitivity to environmental stressors, displaying phenotypic plasticity in developmental rates, body morphology, and neural organization. In Rhinella arenarum, larval competition with the invasive American bullfrog (Aquarana catesbeiana) delays larval development and reduces body size at metamorphosis. Here, we tested whether competition with A. catesbeiana tadpoles of different size classes induces differential effects on R. arenarum brain morphometry, specifically analyzing the telencephalon, diencephalon, optic tectum, and rhombencephalon. While competition with small bullfrog larvae did not alter the relative size of any brain region, exposure to large A. catesbeiana larvae significantly modified brain proportions at metamorphosis, leading to a relative enlargement of the diencephalon (p = 0.0034) and a reduction of the optic tectum (p = 0.012). These results demonstrate that interspecific competition with A. catesbeiana does not merely constrain somatic growth, but triggers a size-dependent developmental response in the central nervous system of the native species. Future studies should determine whether these altered proportions affect specific neuronal populations or post-metamorphic behaviors, highlighting how pressure from invasive competitors shapes neural plasticity during early development.