Territorio y contexto geográfico de los guahibo

The intra-specific variation observed was lower than between ecotypes for both ecotypes, especially when whole call repertoires were compared. However, micro-geographic distinction in call features was still evident within the coastal bottlenose dolphin ecotype. Non-Bay of Islands areas were acoustically distinct from the Bay of Islands, although the difference was less distinct between Bay of Islands and Whangaroa Harbour than among all other areas. Coastal areas of Bay of Islands/Whangaroa Harbour are characterised by the frequent use of single elements and mixing in dendrogram analysis. Call duration was significantly longer in the Bay of Islands. Calls from the Bay of Islands also had significantly more contour inflections (particularly positive to negative), whilst Non-Bay of Islands calls had significantly more visible harmonics. These results suggest the calls recorded in the Bay of Islands contained more information than those recorded in non-Bay of Islands areas. Further investigation is therefore necessary (Chapter 4) to establish if similar forces are acting on the calls in the Bay of Islands and non-Bay of Islands areas, resulting in comparable features within repertoires.

Signals may be adapted or adjusted through vocal learning, which could result in the production of calls which are like or unlike one another (Janik & Slater, 2000). In birds, acoustic

Chapter 3 – Call differentiation of parapatrically occurring common bottlenose dolphin (Tursiops truncatus)ecotypes in Far North waters, New Zealand

106 convergence has been demonstrated in multiple species (Clement et al., 2000). Birds can learn and adapt their vocalisations like dolphins (Clement et al., 2000). In this study, possible convergence and exchange of similar call contours and features in coastal bottlenose dolphins are presented, despite some geographic variability. Connected multi-looped contours were rare or non-existent across all coastal bottlenose dolphin locations analysed, in contrast with results from oceanic bottlenose dolphins (Chapter 4). The pattern of call likeness may be the result of mitochondrial DNA relatedness. The areas within this study all form part of a large related population of coastal bottlenose dolphins (Tezanos-Pinto et al., 2009). If genetics alone underlie acoustic similarity, a greater similarity between the areas would be expected, which was not the case.

The significant differentiation between encounters (social group proxy) in the dendrogram for coastal bottlenose dolphins indicates that divergence between the two regions (Bay of Islands and non-Bay of Islands) could be affected as a result of differences amongst social groups. Call contours (particularly signature calls) are transferred through social learning, thus it is possible shared acoustic habitats have resulted in comparable call types in different areas (Janik & Slater, 1997). Yet, the habitat features of the areas are dissimilar. The Bay of Islands and Whangaroa Harbour are characterised by sheltered bays and reef, whereas Cavalli Islands and Doubtless Bay are characterised by sloping sea beds, sandy bottoms, and rocky shores (personal obs.). It is therefore clear that independent acoustic modification to specific habitat characteristics should not be dismissed. These two theories are not mutually exclusive. In this overlapping environment, social learning of call characteristics optimised for acoustic transmission may ensue, even if the adaptation does not persist for a long period (Wiszniewski et al., 2009). Additional research into call propagation in different habitats would aid in clarifying the likelihood of this possibility. Furthermore, consideration of social dynamics between areas could aid in the assessment of call convergence, i.e., is convergence universal across groups or socially motivated, e.g., male alliances (Connor et al. , 2001; Cook et al. , 2004; Watwood et al., 2005).

As in oceanic bottlenose dolphins, the call differences observed in coastal bottlenose dolphins between areas appear to fulfil the criterion of dialects, i.e., consistent differences in calls between neighbouring groupings or populations of potentially inter-breeding individuals (Conner, 1982; Marler & Tamura, 1962; Nottebohm, 1969). Evidence suggests dolphins in the different areas overlap to a degree, are part of a larger population, and are not genetically

Chapter 3 – Call differentiation of parapatrically occurring common bottlenose dolphin (Tursiops truncatus)ecotypes in Far North waters, New Zealand

distinct (Tezanos-Pinto et al., 2009). According to field efforts conducted on the local Bay of Islands population since 1993, transient bottlenose dolphins have periodically appeared and individual use of the area has altered (e.g., Constantine, 2000; Tezanos-Pinto et al., 2013). Outside of the Bay of Islands, in Far North waters, individuals have only been the focus of dedicated photo-identification for three years. This means it is currently not possible to confirm whether the transients individuals were from other parts of the survey area. Clearly marked individuals have been observed leaving the local Bay of Islands population for periods ranging from several months to indefinitely, with a consistent local Bay of Islands population decline (Tezanos-Pinto et al., 2013).

3.4.3 Study limitations

There was high temporal co-occurrence of contours, resulting in temporal patterning of calls. Biologically this might enhance information propagation by decreasing the chance of environmental conditions altering the signal between subsequent transmissions of that call type (Catchpole & Slater, 2008). Due to the fact calls may preserve group cohesion over extended distances (Janik, 2000b), the repetitive production observed in this study likely helps preserve contact of individuals in association (Quick, 2006; Smolker et al., 1993). Lengthier intervals result in a greater chance of the following call being missed. This could be due to the individual moving out of hydrophone range or a close noise masking the subsequent call. Utilising calls with longer inter-call intervals could, therefore, remain biologically useful for upholding group cohesion. Although this might be under represented by the methodology used herein.

The variations measured for call rate and repertoire may be partially explained by differences in the acoustic environments of recordings. Oceanic bottlenose dolphins were found in waters much less protected from the swell and wave action. Within coastal bottlenose dolphin locations, the Bay of Islands had a much greater degree of boat traffic, with four operational wharves for both tourist and fishing vessels (Peters & Stockin, 2016). In addition, the Bay of Islands and Whangaroa Harbour have many shallow and protected water areas. These factors may combine to produce higher ambient noise levels in the Bay of Islands and Whangaroa Harbour than Cavalli Islands, Doubtless Bay, and wider survey. Higher background noise levels may have affected some of the measurements made from individual calls. For example, spectrographic sidebands at higher frequencies may have become less apparent and peak frequency measurements more biased towards lower frequencies. Whilst only a careful

Chapter 3 – Call differentiation of parapatrically occurring common bottlenose dolphin (Tursiops truncatus)ecotypes in Far North waters, New Zealand

108 selection of calls with good signal-to-noise ratios were utilised, further exploration of this effect is required. Additionally, whilst attempts were made to randomise the selection of sample calls based on group size, behavioural state, season, and year, it is unlikely the capricious nature of these sounds were captured to their full extent. At present, the functional nature of the sequenced calls is unclear. However, clear differences existed between each ecotype in terms of their structure.

The use of loops by bottlenose dolphins was demonstrated, however, they were hard to identify and may have been under-represented due to less stereotyped contours within loops being missed (Buck & Tyack, 1993; Janik & Slater, 1998). The application of technologies such as acoustic tags (e.g., Johnson & Tyack, 2003) or array localisation (e.g., Quick et al., 2008) could aid in ascribing calls to a particular signaller and thus improve reliability.

Calls might be amplitude modulated, resulting in environmental prejudice dependent on the location in which the recording was taken (Esch et al., 2009b; Watwood et al., 2004). Call quality, in conjunction with equipment settings and analysis protocols (i.e., spectrogram dynamic range), would have influenced the perception of weaker calls (Watwood et al., 2004). Moreover, the frequency of a call may influence masking (e.g., low frequency masked by boat noise or attenuation of low amplitude high frequency calls). Subsequently, a greater proportion of calls may be missed than in a captive setting.