Date Published: June 6, 2019
Publisher: Public Library of Science
Author(s): Thiago Orion Simões Amorim, Franciele Rezende de Castro, Juliana Rodrigues Moron, Bruna Ribeiro Duque, Juliana Couto Di Tullio, Eduardo Resende Secchi, Artur Andriolo, Songhai Li.
This study presents an integrative bioacoustics approach to discriminate eight species of odontocetes found on the outer continental shelf and slope of the western South Atlantic Ocean. Spinner, Atlantic spotted, rough-toothed, Risso’s, bottlenose, short-beaked common dolphins, killer and long-finned pilot whales were visually confirmed during recordings with a 3-element omnidirectional hydrophone array. Spectral and time parameters of whistles and echolocation clicks were used in a discriminant function analysis and a classification tree model. As a first step, whistles and clicks were analysed separately; a further analysis consisted of both vocalisations jointly classified. All species showed species-specific properties in their vocalisations. Whistles had greater misclassification rates when compared to clicks. The correct classification was enhanced by the joint step, given the 5.8% error in the discriminant function analysis and a misclassification rate of 18.8% in the tree model. In addition, Receiver Operating Characteristic curves resulting from the tree algorithm analysis exhibited better model efficiency for all species in the joint classification. These findings on acoustical discrimination of such abundant and cosmopolitan species contribute to delphinid classification systems.
Odontocetes species commonly emit tonal frequency-modulated whistles and broadband pulsed clicks and burst sounds , and the production patterns of these acoustic signals vary with geographic location, behavioural state, and geometric spacing of conspecifics [2–4].
Whistle and click parameters were significantly different among species considering the Kruskal-Wallis test, which was important to determine if such parameters were eligible for the further discriminant analysis (e.g. ), then used all parameters. Unlike Kruskal-Wallis test that compare parameter-by-parameter, the discriminant analysis distinguishes the species from the result of an interaction among the acoustical parameters. Therefore, the observed significance for Kruskal-Wallis is not necessarily associated to a clear separation among species, since the discrimination is given by an interaction of parameters and not an individual analysis of each one.
The eight delphinid species, recorded on the outer continental shelf and slope of the western South Atlantic Ocean, showed overall species-specific qualities in their whistles and clicks. When taken individually, echolocation clicks were more efficient in distinguishing species; this may be related to the behavioural context encoded in whistles, the relationship between echolocation signal features, and an animal’s head morphology, particularly the organs associated with the production of sound, which make it feasible to accurately determine a species by its clicks. However, analysing both signals in combination enhanced the correct classification scores. An integrative approach potentially improves the classification process, once it considers the different signals produced by the species as part of a whole bioacoustics system employed in different ecological contexts. However, it is important to highlight that our findings were resulted from a limited database, especially for whistles. Future work should further investigate the effect of behaviour on whistle classification systems by including more data, which would allow accessing the variability of species instead of individual variability. Additionally, future comparisons among acoustic recordings of the same species in different geographic regions may provide information that could be used to elucidate phylogenetic and evolutionary patterns , particularly when associated to morphological and genetic aspects, and to investigate inter- and intra-population variations based on differences observed in their acoustic parameters.