Abstract
Passive acoustic monitoring methods have enabled the study of cetaceans sounds in both captive and free-ranging environments. Audio recordings often replace visual observations, so it is uncommon to have access at the same time to data related to cetaceans movements. To accurately study the whistles of free-ranging individuals, it is essential to first ensure that their recorded vocalisations are not varying based on their positioning. We investigate the variations of whistles emitted by free-ranging individuals by using estimations of their orientations deducted from their echolocation clicks. We recorded wild short-beaked common dolphins (Delphinus delphis) off the coast of Brittany, France, using a small 4-hydrophone antenna (~1m wide) attached to a buoy and placed 3m below the sea surface. One-minute recordings with a sampling rate of 256kHz were made, 20 minutes of recordings containing common dolphin vocalisations were obtained. To determine bearings, we extracted dolphin echolocation clicks using a custom detector, then we determined the time delay of arrival (TDoA) of each click using a cross-correlation method. Finally, we estimated the direction of arrival (DoA) of each click knowing its TDoAs and the hydrophone’s distances. In parallel, we also manually extracted the frequency contours of the whistles emitted during the experiments using a custom annotation tool. We used the angular speed of dolphin echolocation click trains to determine their orientation towards the antenna. However, echolocation clicks are often inaudible, so dolphin orientation is sometimes unknown. We investigated the relation between the orientation of the dolphins and the features of the whistles recorded. Our results confirm studies showing that the fundamental frequencies of dolphin whistles (10-15 kHz) are omnidirectional. Whistles emitted at frequencies >20 kHz should be studied with knowledge of the orientation and/or distance of the animal relative to the recording device. Detailed datasets with information on dolphin orientation and/or distance are needed to improve our understanding of dolphin communication in groups.