Signature whistle stereotypy and variability in the acoustic repertoire of Indo-Pacific bottlenose dolphins (Tursiops aduncus)

Abstract

Complex cognition and sociality in animals often demand elaborate communication systems to mediate social interactions. Bottlenose dolphins (Tursiops spp.) are an example of species with high cognitive abilities and a complex social system. Unlike terrestrial species that commonly use multimodal communication, the marine habitat limits dolphins largely to the acoustic domain. Within their vocal repertoire, bottlenose dolphins have individually distinctive ‘signature whistles’ which use different patterns of frequency modulation to encode identity. As these whistles constitute a large portion of the total whistle production, they likely contain information other than just identity.This thesis aimed to assess the potential for additional information content in the signature whistles of Indo-Pacific bottlenose dolphins (T. aduncus) by measuring their stability, variability, and social context of use. Data were collected between August 2017 and January 2018 from a group of habituated wild dolphins that are provisioned as a part of tourist activity at Tangalooma Island Resort on Moreton Island, Queensland, Australia. Candidate signature whistles were identified in the recordings using the ‘SIGID’ method (Janik et al. 2013). A hydrophone array allowed for whistle localisation and matching to individuals using accompanying photographs to confirm a subset of the whistles as signatures belonging to specific animals. Using a self-organising map (SOM), a tool not previously applied to dolphin whistles, 85% of the candidate and confirmed signature whistles were clustered separately from non-signature whistles (Chapter 2). The SOM proved to be a practical tool for the initial categorisation of a full whistle repertoire. It also demonstrated that signature whistles had significant structural differences to variant whistles, with greater bandwidth, longer duration, and higher levels of modulation.Several complexity metrics were applied to signature whistles to test if their structure was correlated with within-individual variability of whistles (Chapter 3). Whistles that had a higher level of frequency modulation demonstrated higher levels of variability than the less modulated ones. The variability also appeared different between males and females although the sample size was not sufficient for quantitative analysis. Potential differences between the sexes could indicate that signature whistles might be used in different social contexts and should be explored further. Quantitative measurements and visual assessment of frequency contours demonstrated that signature whistles remained remarkably stable over 15 years, with a slight decrease of minimum frequency possibly as an effect of age (Chapter 4). One highly stereotyped whistle type present in both periods could not be attributed to any single individual and might be an example of a shared whistle.The assessment of the social context of signature whistle production revealed that they were produced when animals were interacting in close proximity to each other as well as at a distance (Chapter 5). iThis result significantly expands on the work of previous studies which have considered signature whistles to be used primarily during separations of individuals. Short-distance use of signature whistles was associated with interactive behaviours, particularly in a female-calf pair, indicating an important role in maintaining interactions in close proximity.This thesis concludes that the signature whistles of T. aduncus are highly stable and stereotyped identity signals that at the same time can be highly variable, potentially to accommodate context- specific information. Expanding on the previous research, these results demonstrate that signature whistles are likely to be used to maintain ongoing interactions and not just in the context of separation. Additionally, signature whistles exhibit different degrees of variability between individuals, possibly related to different social contexts of their use. Taken together, this shows that dolphin signature whistles are likely to be more complex signals than previously thought. Their use is crucial in maintaining dolphin communication complexity in the absence of multiple sensory modalities.