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First published online April 23, 2004
Journal of Experimental Biology 207, 1811-1823 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00966

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Echolocation clicks of two free-ranging, oceanic delphinids with different food preferences: false killer whales Pseudorca crassidens and Risso's dolphins Grampus griseus

P. T. Madsen^1,2,*, I. Kerr¹ and R. Payne¹

¹ Ocean Alliance, 191 Western Road, Lincoln, MA 01773, USA
² Department of Zoophysiology, Institute of Biological Sciences, University of Aarhus, Denmark

View larger version (12K): [in a new window]   Fig. 1. The experimental set up consists of a linear array of three hydrophones (A, B, C) suspended between a buoy and a lead weight. The distance between the hydrophones is 4 m, and the first hydrophone is at a depth of 4 m. Theclicking animal is localized from the time- of-arrival differences (t1, t2) of the same signal at the three receivers. The rotationally symmetric position of the sound source S(x,y) is given by the interception of the two hyperboloid surfaces (H1, H2). Analogue signals are amplified and band-pass filtered in the conditioning box before digitisation in the Wavebook 512, writing to the memory of a laptop.

View larger version (11K): [in a new window]   Fig. 2. Example of an ensonification event, during which an echolocating Grampus scans each of the three receivers (A,B,C). Note how the ensonification moves from hydrophone C to B to A. Full amplitude of the y-axes corresponds to an apparent source level (ASL) of 222 dB re. 1 µPa (pp). Local maxima on each of the three channels are likely to represent signals on or close to the acoustic axis of the sound beam. X marks a single click displayed in detail in Fig. 3.

View larger version (34K): [in a new window]   Fig. 3. Three versions of the same Grampus click X from Fig. 2 are displayed. (A) The presumed on-axis version of the click with a high apparent source level (ASL), short duration (left) and a smooth, broadband spectrum (right). (B) The same click recorded 5° off the recording aspect of A. The duration is longer, the ASL is lower, and the bandwidth and f0 are reduced compared to A. (C) The same click recorded in an aspect of 10° compared to A. The duration has increased, ASL, f0 and bandwidth have decreased compared to A and B, and a number of notches are seen in the spectrum.

View larger version (28K): [in a new window]   Fig. 4. Dynamics of the sound generator. Three examples (A–C) of on-axis clicks from Pseudorca are displayed (top) along with their spectra (bottom). All three clicks consist of one cycle with a short duration. While the fp values are rather constant, it is seen that the f0 values and the bandwidth are positively correlated with the source level.

View larger version (15K): [in a new window]   Fig. 5. Audiograms of Grampus (blue line) and Pseudorca (red line) along with representative spectra of on-axis clicks from each species on a relative dB scale. Note that high ambient noise levels masked the maximum sensitivity of the Grampus audiogram, which explains the large difference in threshold between the two species (Nachtigall et al., 1995). Coloured bars signify frequency range of the centroid frequencies of on-axis clicks from the two species recorded in the wild. The Grampus audiogram is from Nachtigall et al. (1995) and the Pseudorca audiogram from Thomas et al. (1988a).