Six decades of advances in studies on echolocation and social communication of Amazon river dolphins (Inia spp.)

Bioacoustics is a crucial component for understanding the sensory ecology of river dolphins
in complex fluvial environments, where sound plays a key role in both social communication
and echolocation. These habitats, characterized by high turbidity, structural heterogeneity,
and elevated levels of reverberation, impose sensory challenges that have shaped their acoustic
system. This systematic review, conducted under PRISMA guidelines, critically synthesizes the

available evidence on the bioacoustics of Inia spp., integrating information on vocal repertoire,
contextual functions of vocalizations, echolocation parameters, differences among taxa,
behavioral and physiological adaptations to the fluvial habitat, and potential effects of human
activities on acoustic behavior. The reviewed studies describe a highly specialized and flexible
biosonar system, with dynamic adjustments in the characteristics of its echolocation signals.
Consistent acoustic differences among species and populations are also reported. Relevant
knowledge gaps nevertheless remain, particularly regarding the functional interpretation of
many signals, the high methodological variability among studies, and the limited assessment
of the long-term effects of anthropogenic noise. Overall, this synthesis highlights the need to
strengthen comparability across studies through methodological standardization, the use of
broadband recording technologies, and the development of integrated approaches that allow more
robust inferences about the bioacoustics of the genus Inia, with direct implications for guiding
acoustic monitoring strategies and supporting the design of more effective conservation actions.