Justin PHD Dissertation

Citation:  Gregg, J. D. (2008). Joint Attention and Echoic Eavesdropping in Wild Bottlenose Dolphins (Tursiops aduncus). Unpublished doctoral dissertation, University of Dublin, Trinity College, Dublin, Ireland.

Experimental work has provided initial evidence that dolphins are able to use the information present in click echoes generated from the echolocation of conspecifics to succeed in object detection and discrimination tasks. The ‘echoic eavesdropping hypothesis’ suggests that dolphins in the wild may behave in ways that exploit this ability, and has been offered as a possible explanation as to why dolphins perform well on experiential tasks involving joint attention and point comprehension. In this thesis, I tested the echoic eavesdropping hypothesis using data collected from wild Indo-Pacific bottlenose dolphins (T. aduncus) inhabiting the waters around Mikura island, Japan. A method was developed to accurately measure distance and head angles between dolphin dyads during analysis of underwater video in conjunction with echolocation information collected on a 3-hydrophone system: the 3D MASC method. During investigative events (N=170) in which one dolphin initiated echolocation on a specific target (i.e., the camera system), the behavior of a potential eavesdropping dolphin was analyzed in order to determine if it: 1) remained silent, and 2) remained within an hypothesized ‘ideal eavesdropping position’ (i.e., measurements between rostrums of  ≤1 m, and ≤10°). It was found that the dolphins in the potential eavesdropping scenario were significantly more likely to remain silent than those than those in the baseline condition (p<0.001). However, the dolphins did not enter into or remain in an ideal eavesdropping position during eavesdropping events. A decrease in head angle during eavesdropping events was observed, although smaller head angles were correlated with more echolocation from the eavesdropper rather than more silence. It was found that dolphins move farther apart when positioned less than 1 m at the start of an echolocation/eavesdropping event, and closer together when positioned greater than 1 m at the start of the event. As a general observation, dolphins do not appear to enter into or remain in an ‘ideal eavesdropping position’ as has been defined for this study. These results suggest that dolphins may limit their own echolocation emissions when given the opportunity to eavesdrop, but that close swimming proximity and head alignment do not seem to be required for echoic eavesdropping to occur. Overall, these results find some support for the hypothesis that dolphins engage in eavesdropping behaviors in the wild, and bolster the idea that dolphins may have a special sensitivity to joint attention like scenarios, although modifications to the current hypothesis are required in order to address some of the observed behaviors.