Dolphin echolocation strategies studied with the Biosonar Measurement Tool

Abstract

Two free-swimming dolphins (Tt722 and Tt673) were trained to carry the Biosonar Measurement Tool (BMT) during open water, proud target searches in order to explore echolocation behavior without the constraints of traditional experimental designs. The BMT recorded the angular motion, depth, and velocity of the dolphin as well as echolocation clicks and echoes returning from insonified targets. Mean search time for Tt722 was 24.6±7.3 s and 6.5±3.0 s for Tt673 on target present trials, the former strategy resulting in the lower false alarm rate. The majority of clicks exceeded 195 dB re: 1 μPa throughout all trials for both animals but each demonstrated preferences for particular frequency bands of echolocation. Considering all trials, only 3.6% of all clicks produced by Tt722 contained peak frequencies greater than 60 kHz whereas Tt673 produced clicks with peak frequencies above 60 kHz 20.4% of the time. Distinctive frequency bands in the distribution of clicks were notable: bands for Tt673 occurred at 38, 54, and 69 kHz with less defined higher order bands; bands for Tt722 occurred at 25, 35, and 40 kHz. Distinctive frequency bands suggest a preferential use or mechanical constraint on harmonically related click frequencies.