Dear MARMAMers, For those of you interested in toothed whale echolocation, we wish to draw attention to a new publication. In a study of the echolocation of Amazon river dolphins in their natural habitat in Brazil, we show that in comparison with marine toothed whales of the same size, the Amazon river dolphins use a biosonar with low output levels and high click rates. We interpret this as a way for these highly adapted riverine animals to reduce the amount of echoes from distant objects while keeping a high update rate on nearby objects in their shallow and complex environment. The low amplitude clicks of these animals have much of the energy at higher frequencies which in similar sized toothed whales is generally a characteristic of high amplitude clicks. Higher frequencies are emitted in more directional beams than lower ones, so the high frequency clicks in combination with the short search ranges will reduce the amount of returning echoes. This in turn may help the Amazon river dolphins cope with the effects of clutter and reverberation that are likely to be highly disturbing in the acoustically complex shallow water environment of the Amazon. We therefore conclude that habitat is an important factor to consider when investigating and interpreting the biosonar characteristics of toothed whales. Amazon river dolphins (Inia geoffrensis) use a high-frequency short-range biosonar Michael Ladegaard<http://jeb.biologists.org/search?author1=Michael+Ladegaard&sortspec=date&submit=Submit>, Frants Havmand Jensen<http://jeb.biologists.org/search?author1=Frants+Havmand+Jensen&sortspec=date&submit=Submit>, Mafalda de Freitas<http://jeb.biologists.org/search?author1=Mafalda+de+Freitas&sortspec=date&submit=Submit>, Vera Maria Ferreira da Silva<http://jeb.biologists.org/search?author1=Vera+Maria+Ferreira+da+Silva&sortspec=date&submit=Submit> and Peter Teglberg Madsen<http://jeb.biologists.org/search?author1=Peter+Teglberg+Madsen&sortspec=date&submit=Submit> The Journal of Experimental Biology, October 1, 2015 Vol. 218, Num. 19, pp. 3091-3101 DOI: http://dx.doi.org/10.1242/jeb.120501 URL: http://jeb.biologists.org/content/218/19/3091.abstract?sid=3d98e50b-6057-4c4... INSIDE JEB link: http://jeb.biologists.org/content/218/19/2981.1 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 SLpp of 190.3±6.1 dB re. 1 µPa, mean SLEFD of 132.1±6.0 dB re. 1 µPa2s, mean Fc of 101.2±10.5 kHz, mean BWRMS 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. A pdf can also be provided by emailing michael.ladegaard@bios.au.dk<mailto:michael.ladegaard@bios.au.dk>. Best wishes on behalf of the authors, Michael Michael Ladegaard PhD student, Marine Bioacoustics Lab Zoophysiology - Department of Bioscience Aarhus University C.F. Møllers Allé 3 Building 1134, room 122 8000 Aarhus C Denmark