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First published online March 27, 2009
Journal of Experimental Biology 212, 1078-1086 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.025619

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Biosonar adjustments to target range of echolocating bottlenose dolphins (Tursiops sp.) in the wild

F. H. Jensen^1,*, L. Bejder², M. Wahlberg^2,3 and P. T. Madsen^1,2,4

¹ Zoophysiology, Department of Biological Sciences, Aarhus University, 8000 Aarhus C, Denmark
² Murdoch University Cetacean Research Unit, Centre for Fish and Fisheries Research, Murdoch University, Perth, 6150 Western Australia
³ Fjord and Bælt and University of Southern Denmark, Margrethes Plads 1, 5300 Kerteminde, Denmark
⁴ Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

^* Author for correspondence (e-mail: Frants.Jensen{at}gmail.com)

Accepted 26 January 2009

Toothed whales use echolocation to locate and track prey. Most knowledge of toothed whale echolocation stems from studies on trained animals, and little is known about how toothed whales regulate and use their biosonar systems in the wild. Recent research suggests that an automatic gain control mechanism in delphinid biosonars adjusts the biosonar output to the one-way transmission loss to the target, possibly a consequence of pneumatic restrictions in how fast the sound generator can be actuated and still maintain high outputs. This study examines the relationships between target range (R), click intervals, and source levels of wild bottlenose dolphins (Tursiops sp.) by recording regular (non-buzz) echolocation clicks with a linear hydrophone array. Dolphins clicked faster with decreasing distance to the array, reflecting a decreasing delay between the outgoing echolocation click and the returning array echo. However, for interclick intervals longer than 30–40 ms, source levels were not limited by the repetition rate. Thus, pneumatic constraints in the sound-production apparatus cannot account for source level adjustments to range as a possible automatic gain control mechanism for target ranges longer than a few body lengths of the dolphin. Source level estimates drop with reducing range between the echolocating dolphins and the target as a function of 17 log(R). This may indicate either (1) an active form of time-varying gain in the biosonar independent of click intervals or (2) a bias in array recordings towards a 20 log(R) relationship for apparent source levels introduced by a threshold on received click levels included in the analysis.

Key words: Tursiops, dolphin, echolocation, biosonar, sound production, automatic gain control

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