Stereotypical rapid source level regulation in the harbour porpoise biosonar
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Some odontocetes and bats vary both click intensity and receiver sensitivity during echolocation, depending on target range. It is not known how this so-called automatic gain control is regulated by the animal. The source level of consecutive echolocation clicks from a harbour porpoise was measured with a hydrophone array while the animal detected an aluminium cylinder at 2, 4 or 8 m distance in a go/no-go paradigm. On-axis clicks had source levels of 145–174 dB re 1 μPa peak-to-peak. During target-present trials the click trains reached comparable source levels independent of the range to the target after three clicks. After an additional click, the source level was reduced for the 2 and 4 m trials until it equalled the one-way transmission loss. During target-absent trials, the source level remained high throughout the entire click train. Given typical values of harbour porpoise inter-click intervals, the source level reduction commenced within a few 100 ms from the first click in the click train. This may indicate a sub-cortically regulated source level regulation in the harbour porpoise.
KeywordsAutomatic gain control Target detection Hydrophone array Click train Biosonar
This study was funded by the Danish Council for Independent Research, the Faculty of Natural Science at the University of Southern Denmark, its Graduate School ‘SNAK’ and Fjord&Bælt. LNK was funded by ONR grant number #N00014-08-1-1160 issued to PEN and travel was sponsored by the Journal of Experimental Biology Student travel grant. The animals are kept under the permission from the Danish Forest and Nature Agency (J. nr. SN 343/FY-0014 and 1996-3446-0021).
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