Dolphin echolocation behaviour during active long-range target approaches Michael Ladegaard, Jason Mulsow, Dorian S. Houser, Frants Havmand Jensen, Mark Johnson, Peter Teglberg Madsen, James J. Finneran Journal of Experimental Biology 2019 222: jeb189217 doi: 10.1242/jeb.189217 Published 25 January 2019 Dear colleagues, On behalf of my co-authors, I am pleased to present our publication titled Dolphin echolocation behaviour during active long-range target approaches. If this sounds remotely interesting then you might enjoy our movie of how the echolocation behaviour may change as a dolphin approaches a target (Movies 1 and 2, supplementary info: http://jeb.biologists.org/content/222/2/jeb189217.supplemental ). In our study, we show that long-range (>100 m) echolocation in free-swimming dolphins may involve the production of click packets and subsequent reception of echo streams. This markedly differs from the typical click-echo-click-echo mode of echolocation that is observed in most toothed whale echolocation studies. Our data also suggests that dolphins not always adjust their clicking rates so that a target echo is received before the emission of another outgoing click, which is also very different from how echolocation is generally understood and described. If you would like to read more about our study, you may access the full publication here: http://jeb.biologists.org/content/222/2/jeb189217 Alternatively, please send me an email if you would like a pdf. Best, Michael Michael Ladegaard Postdoc building 1130, room 110 Mobile: (+45) 4076 4035 Email: michael.ladegaard@bios.au.dk<mailto:mail@au.dk> Web: Michael Ladegaard<http://pure.au.dk/portal/en/persons/id(7365cdb9-ab50-4488-9839-034eb5c3cd5e).html> Department of Bioscience - Zoophysiology Aarhus University C. F. Møllers Allé 3, building 1131 8000 Aarhus C Email: mail@au.dk<mailto:mail@au.dk> Web: http://bios.au.dk/en/ Lab: http://marinebioacoustics.com/ ABSTRACT Echolocating toothed whales generally adjust click intensity and rate according to target range to ensure that echoes from targets of interest arrive before a subsequent click is produced, presumably facilitating range estimation from the delay between clicks and returning echoes. However, this click–echo–click paradigm for the dolphin biosonar is mostly based on experiments with stationary animals echolocating fixed targets at ranges below ∼120 m. Therefore, we trained two bottlenose dolphins instrumented with a sound recording tag to approach a target from ranges up to 400 m and either touch the target (subject TRO) or detect a target orientation change (subject SAY). We show that free-swimming dolphins dynamically increase interclick interval (ICI) out to target ranges of ∼100 m. TRO consistently kept ICIs above the two-way travel time (TWTT) for target ranges shorter than ∼100 m, whereas SAY switched between clicking at ICIs above and below the TWTT for target ranges down to ∼25 m. Source levels changed on average by 17log10(target range), but with considerable variation for individual slopes (4.1 standard deviations for by-trial random effects), demonstrating that dolphins do not adopt a fixed automatic gain control matched to target range. At target ranges exceeding ∼100 m, both dolphins frequently switched to click packet production in which interpacket intervals exceeded the TWTT, but ICIs were shorter than the TWTT. We conclude that the click–echo–click paradigm is not a fixed echolocation strategy in dolphins, and we demonstrate the first use of click packets for free-swimming dolphins when solving an echolocation task.