Three-dimensional dive trajectories of three sperm whales in the Gulf of Mexico have been obtained by measuring the relative arrival times and bearings of the animals' acoustic multipath reflections, using two elements of a towed hydrophone array deployed at an unknown depth and orientation. Within the first 6-12 min of the start of a dive, the intervals between successive "clicks" of all three whales corresponded closely with the two-way travel time of an acoustic pulse traveling vertically between the animals' position and the ocean bottom. The click spectra contained multiple peaks, including a faint band of energy originally centered near 10 kHz. As the animals descended over 500 m in depth, the center frequency of this band shifted to nearly 15 kHz, but subsequently remained near this value during the rest of the dive. This frequency shift is consistent with that expected from energy scattering from an ensemble of incompressible small-scale air-filled resonators, with diameters on the order of 4 mm. One possible candidate for such an ensemble is proposed to reside in the collapsed frontal sac of the animal. A comparison of the received levels for the bottom and direct multipath arrivals indicates that the whales' acoustic directivity must range between 10-30 dB in the 5-20-kHz region.
We examined the hypothesis that dolphins increase their rate of sound production during feeding events to recruit new individuals. We recorded 135.5 min of underwater sounds from bottlenose dolphins (Tursiops truncatus) near Isla del Coco, Costa Rica. Data were collected from eight feeding groups and three nonfeeding groups. We classified sounds as whistles, click trains, or pulse bursts. The number of whistles per min per dolphin was higher in feeding groups than in nonfeeding groups. More whistles than click trains or pulse bursts were produced when dolphins were feeding. On the other hand, there was no difference in the proportion of each sound type produced when dolphins were not feeding. New dolphins joined the feeding events for which we recorded dolphin sounds. Results supported the hypothesis that dolphin group size increases in response to an increase in the number of whistles by conspecifics; however, confounding factors, such as the use of specific feeding calls, need to be accounted for to support the increased sound-rate hypothesis.
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