Dolphin whistles can be useful tools in identifying units of conservation

Abstract

Background Prioritizing groupings of organisms or ‘units’ below the species level is a critical issue for conservation purposes. Several techniques encompassing different time-frames, from genetics to ecological markers, have been considered to evaluate existing biological diversity at a sufficient temporal resolution to define conservation units. Given that acoustic signals are expressions of phenotypic diversity, their analysis may provide crucial information on current differentiation patterns within species. Here, we tested whether differences previously delineated within dolphin species based on i) geographic isolation, ii) genetics regardless isolation, and iii) habitat, regardless isolation and genetics, can be detected through acoustic monitoring. Recordings collected from 104 acoustic encounters of Stenella coeruleoalba, Delphinus delphis and Tursiops truncatus in the Azores, Canary Islands, the Alboran Sea and the Western Mediterranean basin between 1996 and 2012 were analyzed. The acoustic structure of communication signals was evaluated by analyzing parameters of whistles in relation to the known genetic and habitat-driven population structure. Results Recordings from the Atlantic and Mediterranean were accurately assigned to their respective basins of origin through Discriminant Function Analysis, with a minimum 83.8% and a maximum 93.8% classification rate. A parallel pattern between divergence in acoustic features and in the genetic and ecological traits within the basins was highlighted through Random Forest analysis. Although it is not yet possible to establish a causal link between each driver and acoustic differences between basins, we showed that signal variation reflects fine-scale diversity and may be used as a proxy for recognizing discrete units. Conclusion We recommend that acoustic analysis be included in assessments of delphinid population structure, together with genetics and ecological tracer analysis. This cost-efficient non-invasive method can be applied to uncover distinctiveness and local adaptation in other wide-ranging marine species.

Changes in whistle parameters of two common bottlenose dolphin ecotypes as a result of the physical presence of the research vessel

“…This result is in contrast with the hypothesis that the level of perceived risk may be higher for small groups of dolphins ( Frid & Dill, 2002 ), as the coastal ecotype ones were compared to the oceanic in this study. Bottlenose dolphins already showed whistle variation between populations ( La Manna et al, 2020 ; Luís et al, 2021 ; May-Collado & Wartzok, 2008 ; Papale et al, 2021 ) and ecotypes ( Hoffmann et al, 2012 ; Peters, 2018 ; Rio et al, 2022 ), possibly due to geographical distance, group size and composition, behavioral activities, genetic isolation, morphology, and habitat acoustic characteristics. The peculiarity of the results found in the present study is the different reaction to the physical presence of the research vessel between the coastal and oceanic ecotypes.…”

“…In addition, the stereotyped proportion was calculated as the ratio between the stereotyped whistles and the total whistles. Two or more whistles were defined as stereotyped when the signals had identical time-frequency contours visually matched by a trained observer ( Papale et al, 2021 ). The normality and homoscedasticity of the data were checked by significance tests (Shapiro–Wilk and Levene’s test, respectively).…”

Whistle characterization of long-beaked common dolphin (Delphinus delphis bairdii) in La Paz Bay, Gulf of California

“…Specifically, the whistles of the long-beaked common dolphins from La Paz Bay showed lower ending frequency, maximum frequency, duration and inflection points compared to the ones recorded by Oswald et al (2007) . Whistle variation has been previously reported at interspecific ( Oswald et al, 2007 ; Papale et al, 2021 ; Steiner, 1981 ) and intraspecific level, for common dolphins ( Ansmann et al, 2007 ; Azzolin et al, 2021 ; Gannier et al, 2010 ; Papale et al, 2014 ; Petrella et al, 2012 ) and other dolphin species ( Akkaya et al, 2023 ; Azzolin et al, 2013 ; Luís et al, 2021 ; May-Collado & Wartzok, 2008 ; Yuan et al, 2021 ). The high plasticity of dolphin whistles can be attributed to many factors, including geographical variability, behavioral state, general environment, group size, social context and individual variability ( Bazúa-Durán & Au, 2002 ; Bazúa-Durán & Au, 2004 ; Camargo Jr et al, 2006 ; La Manna et al, 2020 ; May-Collado & Wartzok, 2008 ; May-Collado, 2010 ).…”

Initial results on the variation of whistle characteristics of bottlenose dolphins from two neighbouring regions of the Mediterranean Sea: Northern Ionian and Southern Adriatic Sea

“…The whistle characteristics of bottlenose dolphins reported here are consistent with those reported in the Mediterranean Sea (Rendell et al, 1999;Gannier et al, 2010;Dı ́az Lo ́pez, 2011;Papale et al, 2014a;Bolger et al, 2017;La Manna et al, FIGURE 5 PCA correlation biplot of the whistle parameters grouped by the study location (IT = Taranto Gulf, MNE = Boka Bay) with 95% confidence interval ellipses. , 2020;Papale et al, 2021) and elsewhere (Conner, 1982;Boisseau, 2005;May-Collado and Wartzok, 2008;Baron et al, 2008;Hawkins, 2010). The mean peak frequency was reported at 10kHz, with a frequency ranging between a mean low frequency of 7kHz and a mean maximum frequency of 14 kHz, with a mean duration of 500 ms in each of the locations.…”