SUMMARY
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.
FOOTNOTES
This work was supported by the Danish Ph.D. School of Aquatic Sciences (SOAS), Aarhus University, Denmark, WWF Verdensnaturfonden and Aase and Ejnar Danielsens Foundation, the Siemens Foundation, the Faculty of Science at the Aarhus University, Denmark and the Danish Natural Science Foundation via a Steno scholarship and a logistics grant to P.T.M. We thank M. Hansen, M. Wilson, H. Schack, H. Smith and J. Knust for assistance in the field as well as Murdoch University and Bunbury Dolphin Discovery for logistics support. We are grateful to Whitlow Au, Hugh Finn and two anonymous reviewers for constructive comments on this paper. Research in Australia was conducted under a permit to L.B. from the Department of Environment and Conservation and Ethics Approval from Murdoch University.
