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.

Keywords: Acoustic divergence; Cetaceans; Communication signals; Geographic variability; Phenotypic diversity.

Fig. 1

Map of the ocean basins included in the study. Two sub-areas were investigated per basin. In the Atlantic Ocean: the Azores islands and the Canary Islands. In the Mediterranean Sea: the Alboran Sea, and the Western Mediterranean (A Ligurian Sea, B Tyrrhenian Sea, C Sardinian waters, D Provençal Sea, E Spanish waters, F Balearic waters). Dots represent striped dolphin sightings, stars bottlenose dolphin sightings and triangles common dolphin sightings. Map was generated by using QGis 2.2.0 (http://qgis.org/it/site)

Fig. 2

Variable importance for the correct classifications of the groups as estimated by the Random Forest Analysis performed in order to determine the effect of genetics on whistle structure regardless isolation (i.e. using samples from the Mediterranean Sea, the Alboran Sea and the Atlantic Ocean)