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First published online June 29, 2006
Journal of Experimental Biology 209, 2726-2733 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02306

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Acoustic radiation from the head of echolocating harbor porpoises (Phocoena phocoena)

Whitlow W. L. Au^1,*, Ronald A. Kastelein², Kelly J. Benoit-Bird³, Ted W. Cranford⁴ and Megan F. McKenna⁴

¹ Marine Mammal Research Program, Hawaii Institute of Marine Biology, P.O. Box 1106, Kailua, HI 96734, USA
² Sea Mammal Research Co., Harderwijk, The Netherlands
³ College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA
⁴ Biology Department, San Diego State University, San Diego CA 92182, USA

^* Author for correspondence (e-mail: wau{at}hawaii.edu)

Accepted 3 May 2006

An experiment was conducted to investigate the sound pressure patterns on the melon of odontocetes by using four broadband hydrophones embedded in suction cups to measure echolocation signals on the surface of the forehead of two harbor porpoises (Phocoena phocoena). It has long been hypothesized that the special lipids found in the melon of odontocetes, and not in any other mammals, focus sounds produced in the nasal region that then propagate through the melon, producing a beam that is directional in both the horizontal and vertical planes. The results of our measurements supported the melon-focusing hypothesis, with the maximum click amplitude, representing the axis of the echolocation beam, located approximately 5.6-6.1 cm from the edge of the animal's upper lip along the midline of the melon. The focusing is not sharp but is sufficient to produce a transmission beam of about 16°. Click amplitude dropped off rapidly at locations away from the location of site of maximum amplitude. Based on comparisons of forehead anatomy from similar sized porpoises, the beam axis coincided with a pathway extending from the phonic lips through the axis of the low-density/low sound velocity lipid core of the melon. The significant interaction between click number and hydrophone position suggests that the echolocation signals can take slightly different pathways through the melon, probably as a result of how the signals are launched by the production mechanism and the position of the acoustically reflective air sacs.

Key words: harbor porpoise, Phocoena phocoena, echolocation signals, sound pressure level, melon, lipid, contact hydrophone

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