Dolphin echolocation behaviour during active long-range target approaches

• Published in: Journal of experimental biology Vol. 222; no. Pt 2
• Main Authors: Ladegaard, Michael, Mulsow, Jason, Houser, Dorian S, Jensen, Frants Havmand, Johnson, Mark, Madsen, Peter Teglberg, Finneran, James J
• Format: Journal Article
• Language: English
• Published: England 25.01.2019
• Subjects: Animals; Bottle-Nosed Dolphin - physiology; Echolocation; Male; Orientation, Spatial; Rabbits; Swimming; Interclick interval; Toothed whale; Click packet; Biosonar; Source level; Dtag
• ISSN: 0022-0949; 1477-9145
• DOI: 10.1242/jeb.189217

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 17log (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.