The behavioural ecology of mixed-species groups of delphinids

Author: Jonathan Syme

Syme, Jonathan, 2023 The behavioural ecology of mixed-species groups of delphinids, Flinders University, College of Science and Engineering

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Many species form mixed-species groups to gain antipredator, foraging, and/or social benefits. Amongst delphinids, mixed-species groups appear to be widespread, however, in most cases their functions remain unknown. Moreover, research has been hindered by several shortcomings, notably inconsistent terminology, the lack of a conceptual framework for assessing their functions, and a limited number of targeted studies. In this thesis, I review the current knowledge on delphinid mixed-species groups (Chapter 2), address inconsistencies in how mixed-species groups are defined (Chapters 2 and 3), and develop a conceptual framework for their study (Chapter 2). I then apply this framework on a model system of Australian humpback, Sousa sahulensis (hereafter “humpback dolphin”), and Indo-Pacific bottlenose dolphins, Tursiops aduncus (hereafter “bottlenose dolphin”), around the North West Cape, Western Australia, to: assess habitat partitioning and co-occurrence patterns with a joint species distribution model and a temporal analysis (Chapter 4); compare the characteristics of single- and mixed-species sightings to determine the possible functions of these mixed-species groups (Chapter 5); and evaluate behavioural patterns using Markov chain analysis to better understand coexistence mechanisms and the functions of these mixed-species groups (Chapter 6). From the review in Chapter 2, I found reports of 54 cetacean species from five families apparently forming mixed-species groups. Most reports, however, were brief observational accounts and only 27 studies proposed and discussed potential functions. By combining the results of this review with broader theory, I constructed a framework for assessing the functions of cetacean mixed-species groups (Chapter 2). In Chapter 3, I used the results of a review and survey on how the terms group, school, party, and pod are defined in delphinid studies to make the recommendation that the term group, defined with spatial proximity criteria, be used exclusively for sampling units of dolphins observed in the field. Additionally, I propose a process for formulating biologically meaningful definitions by analysing interindividual distances (Chapter 3). From my study of the model system, I found that humpback and bottlenose dolphins around the North West Cape displayed habitat and temporal niche partitioning: humpback dolphins preferred shallower and more nearshore waters, bottlenose dolphins were sighted more often during the afternoon, and interspecific behavioural patterns indicated that the species differed in their use of the study area (Chapters 4 and 6). Despite this partitioning, the species co-occurred more often than expected by chance, indicating attraction between them and the active formation of mixed-species groups (Chapter 4). Both species displayed varied interspecific behavioural interactions, from aggressive and sexual behaviours to neutral and affiliative behaviours, suggesting that a variety of social benefits may be relevant, depending on the individuals involved and their traits (Chapter 5). Furthermore, when in mixed-species groups, humpback and bottlenose dolphins were more likely to transition to socialising from other behaviours and both species socialised more and travelled and foraged less, further indicating that the benefits that they gain are social (Chapter 6). This study advances our understanding of delphinid mixed-species groups and provides a solid platform for future studies of other populations and species.

Keywords: Cetacea, Delphinidae, Australian humpback dolphin, Indo-Pacific bottlenose dolphin, mixed-species groups, interspecific interactions, habitat partitioning, behaviour, social advantages

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Associate Professor Dr Guido J Parra