ABSTRACT
For the few toothed whale species fully adapted to life in river systems the task of echolocation may be more challenging than for many toothed whales at sea due to problems of clutter and reverberation in shallow waters. In general, the source parameters of echolocation clicks scale with body size so that larger toothed whales emit clicks at higher output levels, lower sampling rates, and lower frequencies compared to smaller species. However, within the same size ranges, toothed whales inhabit vastly different aquatic niches from shallow rivers to the deep sea raising the question of whether habitat is also an important driver in the evolution of biosonar parameters. To address that question we recorded wild Amazon river dolphins (Inia geoffrensis Blainville 1817) in three different areas of the Amazon using a vertical seven-hydrophone array to allow for acoustic animal localization and source parameter estimation of echolocation clicks fulfilling strict on-axis criteria. Specifically, we hypothesized that river dolphins emit low intensity clicks at high frequencies to produce a short range biosonar with high directivity that they operate at high clicking rates to provide high update rates in an acoustically complex habitat. We identified 404 on-axis clicks having a mean source level of 190.3 ± 6.1 dB re μPa (peak-peak), mean centroid frequency of 101.2 ± 10.5 kHz, mean directivity index of 25.2 dB, and mean interclick interval of 35.1 ± 17.9 ms. These data show that Amazon river dolphins click at faster sampling rates, lower output levels, and higher frequency than similar sized marine dolphins, but with equally high directionality, showing that habitat is an important co-driver of biosonar evolution in echolocating toothed whales.