Presence and drivers of geographic variation in delphinid vocal signals

Understanding the variation present in animal signals can provide insight into drivers behind it, such as the social structure, ecology, distribution and genetic diversity present in populations.

Geographic variation specifically has been observed in the vocalisations of several taxa and in some, particularly birds, has been used to evidence the presence of vocal 'dialects'.

In marine mammals, dialects have been reported for northern elephant seals (Mirounga angustirostris), killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus).

Though previous studies have been conducted to investigate geographic variation in delphinid vocalisations, little is known about such variation in UK waters.

Additionally, the vast majority of delphinid communication research focusses on their whistles alone, yet the animals also produce a variety of click sounds.

Several odontocete species do not produce whistles, suggesting that there are communicative aspects to these non-whistle vocalisations.

Methodological issues in previous studies have also led to unclear results since variation between groups may not represent geographic variation when sample sizes are low, particularly for animals that show large inter-individual and group diversity within geographic regions. Finally, the drivers of variation in delphinid vocalisations remain largely unknown.

In some species, such as the killer whales of the Pacific Northwest, dialects show a strong correlation with relatedness, with different familial units utilising distinct vocal repertoires from one another.

However, it could also be the case that differences in behaviour elicit vocal differences as certain populations utilise behaviourally specific calls more often than others.

Similarly, external environmental factors, such as ambient noise levels, may influence the structure and usage of different vocal signals.

Preliminary study indicates that bottlenose dolphins (Tursiops truncatus) on the east and west coasts of Scotland acoustically differ from one another substantially, providing some evidence for the presence of dialects given their geographical proximity and mixing potential (Hargrave, 2023 MSc. thesis).

Other species such as Risso's dolphins (Grampus griseus) and white-beaked dolphins (Lagenorhynchus albirostris) can also be found around the UK and their population structure has not been studied here before.

The proposed study will perform acoustic analyses on the vocal repertoires (whistles, clicks and burst-pulsed sounds) of these wild delphinid species in the waters of the Northeast Atlantic Ocean and the North Sea, evaluating potential similarities or discrepancies between the vocalisations of individuals, groups and geographic regions.

Historic recordings obtained from focal-follow and line transect acoustic surveys conducted in the North Sea and the Northeast Atlantic will be analysed using the Raven Pro software package to isolate delphinid vocalisations.

The acoustic parameters of these vocalisations will then be calculated using the ROCCA module for PAMguard (www.pamguard.org), producing a comprehensive suite of measures for both tonal and broadband signals (a total of 56 parameters).

This analysis will be conducted at the level of individuals, groups and broader geographic regions; investigated via the use of random forest classification modelling. Other machine learning classification methods may also be utilised depending on the task in question.

Additionally, the role of potential drivers of any variation, such as behavioural context or learning, environmental variables, and relatedness, will be investigated via behavioural observations (obtained via animal-mounted camera tags, drones and visual observation), photo identification, and available environmental (available from CEFAS, Scottish Coastal Observatory, British Oceanographic Data Centre) and genetic data (e.g.

Louis et al. 2021, Sci. Advan. 7: eabg1245).