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This study indicates the use of vocal signals (calls) is influenced by the contact (tactile and posture) rate of individuals. The more the group engage in photic and mechanical tactile behaviour, the less the dolphins communicate through calls. As such it appears that oceanic bottlenose dolphins are using visual and tactile cues to locate each other and synchronise their behaviour, reducing their dependency on call signal exchange.

Signal exchange behaviour in oceanic bottlenose dolphin and pilot whale interactions showed a wide spread use yet subtle distinctions. This indicates the function of communication may vary as a result of group composition and behaviour. Communication behaviour serves numerous functions, e.g., during foraging events (coordinated hunting: Coscarella et al., 2015; Ridgway et al., 2015), parent-offspring interaction (synchronous swimming, surfacing,

breathing: Fellner et al., 2012; Mann & Smuts, 1999), play (Bel’kovich, 1991), and mate

competition (synchronous surfacing: Connor et al., 2006). In addition, the behavioural contexts associated with inter-specific interactions differ (e.g., aggressive/non-aggression, foraging/travel in any combination, Cusick, 2012), illustrating the complexity and underlying functions of signal exchange in these groupings. The current study quantifies the dynamics of signal exchange both intra- and inter-species. Results from oceanic bottlenose dolphin and pilot whale align with the current literature, especially findings that different surface behavioural states are correlated with call rate behaviour. A good example of this in the literature, is from Taruski (1979), where pilot whale mean call rate was significantly different during milling and transiting. Further to this, Taruski (1979) also linked vocal states to the level of arousal (low, moderate, and high), though this did not result in a viable explanation for call behaviour and thus system complexity was not captured by arousal alone.

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4.4.3 Inter-specific interactions

Within this dataset, which was primarily recorded when socialising, interactions between oceanic bottlenose dolphins and pilot whales were antagonistic. This aligns with long-term

inter-specific aggression noted in other delphinid populations (e.g., Atlantic bottlenose and

spotted dolphins, Cusick & Herzing, 2014). These interactions are generally described as not

including reversals of aggression, dynamic shifts, or bi-directionality (Acevedo-Gutiérrez et

al., 2005; Frantzis & Herzing, 2002; May-Collado, 2010; Psarakos et al., 2003; Quérouil et al.,

2008).

The ratio of pilot whale: oceanic bottlenose dolphin was key factor on the response of oceanic bottlenose dolphin vocal and tactile/postural behaviour. A higher ratio of pilot whale to oceanic bottlenose dolphin was the most important factor in call changes. The behaviour noted was similar to mobbing behaviour, observed in many species, which is defined as multiple individuals chasing after another individual (and/or species) as a cohesive unit (e.g., red-

winged blackbirds (Agelaius phoeniceus): Consla & Mumme, 2012; Olendorf et al., 2004). In

aggressive events, oceanic bottlenose dolphins, like the species targeted by mobbing behaviour (Consla & Mumme, 2012; Olendorf et al., 2004), adapted their behaviour in the presence of pilot whales. The effect of pilot whale groups on oceanic bottlenose dolphins was observed at multiple levels: call rate, call frequency, and tactile/posture rate.

Group synchrony is an example of cooperation (Drea & Carter, 2009; Noë, 2006). Cooperation during aggressive events can be especially important between unevenly sized individuals. This is particularly true for the smaller-sized cooperating individuals that can act together to counteract the inherent benefits of a larger individual (Cusick, 2012). Pilot whales, which are significantly larger than oceanic bottlenose dolphins, may use a combination of chase behaviour and physical size in order to physically dominate the oceanic bottlenose dolphins (as

observed in spotted dolphins (Stenella frontalis) and bottlenose dolphin in the Bahamas, Cusick

& Herzing, 2014). When synchronous and acting as a single unit, the likelihood of oceanic bottlenose dolphins initiating an interaction was increased when compared to individuals acting independently. Consequently, a single pilot whale would be affected by the summation of multiple individual oceanic bottlenose dolphins. This is similar to another form of mobbing reactionary behaviour which puts participants on a similar level and minimises aggression effects on each individual, ultimately maintaining overall group cohesion (Olendorf et al., 2004).

Chapter 4 – Signal exchange of oceanic common bottlenose dolphin (Tursiops truncatus) during intra- and inter-species associations in Far North waters, New Zealand

The current study additionally demonstrated that group size, ratios, and behaviour (both state and event) may change within and between encounters. This has been well-described during non-aggressive inter-specific encounters by Herzing & Johnson (1997). In Far North waters, inter-specific groups are larger on average than intra-specific groups (Chapter 1, Appendix 1.1). Furthermore, within inter-specific focal groups oceanic bottlenose dolphin individuals often outnumber pilot whales in both group size and in the number participating in interaction events (Chapter 2). Oceanic bottlenose dolphins in Far North waters live in fission - fusion societies (Tezanos-Pinto et al., 2009). This is unlike the matrilineal social structure of pilot whale noted as noted in other areas (e.g., Stephanis et al., 2008). This means there is a possibility that the full oceanic bottlenose dolphin population are not consistently together, unlike the stable social groups of pilot whales (e.g., Connor et al., 2000). Hence, frequent group size (and behaviour) changes within and between encounters are not surprising. In Far North waters, group size during inter-specific encounters altered relatively frequently. This is similar to observations in Shark Bay, Australia, where multiple group size changes can occur, especially in intra-specific aggression (mate access, Connor et al., 2011; Mann et al., 2008). The combination of context factors during an aggressive encounter, e.g., location or time, likely affected the aggression observed. The reason for the inter-specific aggression in Far North waters between oceanic bottlenose dolphins and pilot whales may result from the need to defend against inter-specific copulation (e.g., hybrid formation: Elliser, 2010; Cusick, 2012) or male mate defence (Connor et al., 1992, 2006). Aggression unlikely resulted principally from habitat and/or food competition, since pilot whales and oceanic bottlenose dolphins have morphological differences that allow foraging on different prey species. However, aggression as a driver of habitat selection, such as niche segregation (e.g., (Malinowski, 2011) cannot be ruled out. Alternatively, unlike in chimpanzees (Watts et al., 2006), territorial defence is unlikely, as in Far North waters mixed associations were not spatially correlated (Cusick, 2012). Subsequent research is required to identify the degree to which encounter context effects inter-specific aggression.

Inter-specific encounters can add stress to participating individuals. Additional mechanical signal exchange during social events was assessed in this study, adding insight to the behaviour events (tactile and posture). Calls emitted had a higher mean end frequency, suggesting an upsweep in contour (particularly in low ratio groups). Thus, inter-specific call modifications may have resulted from a skew towards the ‘minority’ oceanic bottlenose dolphins. Stress-

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related signal modification has been described in cetaceans. During Guiana (Sotalia

guianensis) and bottlenose dolphin inter-specific social-reproductive interactions (formation

of hybrids), aggression was the predominant event type (Acevedo-Gutiérrez et al., 2005). When the same species were in intra-specific social states, more events were described as ‘play’ (touch, tail slap, leap, body roll, and spy hop). The utilisation of upsweep contours by oceanic bottlenose dolphins in the presence of pilot whales may occur to: 1) convey stress and/or communicate with conspecifics; and 2) emit a threat(s) “in the language” of the other species (Gorissen et al., 2006, p. 267). The need to convey context with conspecifics when isolated and/or distressed could necessitate signal modification (Watts et al., 2001). Call duration was found to be longer in this study in inter-specific than intra-specific groups. By utilising an increased call duration, oceanic bottlenose dolphins may be utilising context-specific signal exchange to convey specific stress signals to their conspecifics who may not be adjacent to the signaller. As occasions were observed when no other oceanic bottlenose dolphins remained in field of view during an aggressive pilot whale dominated event, it could be suggested that conspecifics move away from the central area of the event. Further to this, during travelling events, oceanic bottlenose dolphin only group calls and inter-specific group calls were not significantly different, indicating modification primarily occurs during other behaviours, such as socialising. This may indicate that the predominantly calling individual in aggressive

(social) events was oceanic bottlenose dolphins (May-Collado, 2010).

4.4.4 Study limitations

One potential source of bias is vocal masking, resulting in an underestimation of large group call rate. Call rate and frequency were still effectively measured, throughout their range. It is

therefore reasonable to suggest signal masking in the spectrograms was minimal. Other data

collection considerations include limited water clarity and sea conditions for subsurface behaviour. This can limit the length of follows and the ability to extend analysis to the individual level. Additionally, the stationary research vessel effect was not assessed. Whilst efforts to minimise disturbance were made (best practice manoeuvring and a quiet four-stroke engine to reduce the impact on signal exchange), it remains unquantified.

A further potential source of bias is the limited field of view of camera equipment as a result of the angle of view. Some tactile behaviour may be missed, resulting in an underestimation of tactile and posture rate. Given that a broad range of tactile/posture contact rates and types were measured for a representative range of group sizes, this suggests that visibility and field of view

Chapter 4 – Signal exchange of oceanic common bottlenose dolphin (Tursiops truncatus) during intra- and inter-species associations in Far North waters, New Zealand

error were minimal and that this error would have underestimated the effect of inter-specific groups on the tactile behaviour. However, this source of bias is noted and could be improved in further study using animal borne systems and/or mobile camera systems (Pearson et al., 2017) able to remain close to the animals. However, these methodologies also have their own limitations that must be considered, particularly for gregarious oceanic species.

The contexts and behaviours considered in this study were not exhaustive. Throughout the literature variables not considered here are documented to affect the vocalisation of delphinids. An example of this is travelling speed, as discussed by Henderson et al. (2012). The inclusion of such variables should be considered in further research. Unfortunately, hypotheses relating to signal convergence and signal stress could not be assessed fully. Future research could include integrating data from acoustic tag and directional recording systems. This would allow individual level rather than group level assessments. Overall, larger datasets that can accommodate multiple covariates in hidden Markov models could add insights to the importance of TTCh. However, the results of this work indicate that call rates alter with subsurface context and the time of shifting of those contexts. Extending the behavioural comparison of coastal bottlenose dolphins and oceanic bottlenose dolphins beyond aggressive encounters was outside the scope of this dataset (due to non-aggressive sample size). Additionally, the degree to which inter-specific mate avoidance, food separation, etc. lead to inter-specific interactions was beyond this study’s scope.

Despite the identified limitations, this study is the first to quantify the dynamic of inter- and intra-specific interactions for this population. Results establish the complexity of multi-species groupings, the need to holistically examine context, and the importance of examining how contextual factors interact and change.