Amazon river dolphins (Inia geoffrensis) use a high-frequency short-range biosonar

Abstract

Toothed whales produce echolocation clicks with source parameters related to body size; however, it may be equally important to consider the influence of habitat, as suggested by studies on echolocating bats. A few toothed whale species have fully adapted to river systems, where sonar operation is likely to result in higher clutter and reverberation levels than those experienced by most toothed whales at sea because of the shallow water and dense vegetation. To test the hypothesis that habitat shapes the evolution of toothed whale biosonar parameters by promoting simpler auditory scenes to interpret in acoustically complex habitats, echolocation clicks of wild Amazon river dolphins were recorded using a vertical seven-hydrophone array. We identified 404 on-axis biosonar clicks having a mean SL pp of 190.3±6.1 dB re. 1 µPa, mean SL EFD of 132.1±6.0 dB re. 1 µPa 2 s, mean F c of 101.2±10.5 kHz, mean BW RMS of 29.3±4.3 kHz and mean ICI of 35.1±17.9 ms. Piston fit modelling resulted in an estimated half-power beamwidth of 10.2 deg (95% CI: 9.6-10.5 deg) and directivity index of 25.2 dB (95% CI: 24.9-25.7 dB). These results support the hypothesis that river-dwelling toothed whales operate their biosonars at lower amplitude and higher sampling rates than similar-sized marine species without sacrificing high directivity, in order to provide high update rates in acoustically complex habitats and simplify auditory scenes through reduced clutter and reverberation levels. We conclude that habitat, along with body size, is an important evolutionary driver of source parameters in toothed whale biosonars.

Amazon river dolphins (Inia geoffrensis) modify biosonar output level and directivity during prey interception in the wild

“…All Cohen's d values were higher than 0.80, indicating large effect sizes, except for the peak frequency (F p ) difference (d=0.28), where effect size was small (Cohen, 1988). Of the observed differences, we wish to highlight that all mean SL measures were more than 10 dB higher in our previous study (Ladegaard et al, 2015). We also point out that caution is necessary when comparing data directly as a significant difference was found for mean localisation range (Table 1).…”

“…These lines represent conservative estimates of the dynamic recording range wherein clicks are likely to be adequately recorded and quantified. A higher click detector threshold was used in Ladegaard et al (2015) as a method for excluding low-amplitude clicks emitted primarily by animals at ranges exceeding the maximum range criteria, but this has probably resulted in some low-amplitude on-axis clicks produced at short range being missed. demonstrated (Norris et al, 1961;Penner and Murchison, 1970;Evans and Awbrey, 1988) and this form of active sensing has been extensively studied in even fewer (Au, 1993).…”

“…SL pp was found to decrease with a decreasing target range (Fig. 4) (Ladegaard et al, 2015). Effect size was quantified by calculating Cohen's d for all source parameter pairs.…”

“…Click duration, amplitude parameters, spectral parameters (using 256-point fast Fourier transform) and interclick interval (ICI) were quantified as previously described (Ladegaard et al, 2015) using the methods of Madsen and Wahlberg (2007) and Au (1993).…”

“…It has recently been demonstrated with array recordings that Amazon river dolphins [boto, Inia geoffrensis (Blainville 1817)] employ a low-power, high sampling rate biosonar as a likely adaptation to the often shallow and cluttered waters in which they hunt (Ladegaard et al, 2015). Here, we tested whether these wild toothed whales use different biosonar parameters and gaze changes when echolocating on actual prey rather than hydrophone arrays.…”