Signature whistles of the demographic unit of bottlenose dolphins (<i>Tursiops truncatus</i>) inhabiting the Eastern Ligurian Sea: characterisation and comparison with the literature

Bottlenose dolphins (<i>Tursiops truncatus</i>) make extensive use of narrowband whistles with individually distinctive frequency modulations to broadcast their identity: the signature whistles. The acoustic structure of the signature whistles tends to be stable over decades. Therefore, these vocalisations have the potential to be used for monitoring individuals over time. Our study investigated the acoustic structure of the signature whistles of the demographic unit of bottlenose dolphins inhabiting the Eastern Ligurian Sea (Italy), within the Pelagos Sanctuary. Through visual categorisation of spectrograms, we identified 941 whistle contours, 469 of them were classified as signature whistles using the SIGnature IDentification (SIG-ID) method. We further grouped these into 19 different categories based on their stereotyped frequency modulation patterns. To support our categorisation, we used a combination of a visual classification task conducted by six naïve judges and an automated analysis, the principal component analysis. Overall, this study characterises the signature whistles of the demographic unit of bottlenose dolphins inhabiting the Ligurian Sea for the first time. Furthermore, when comparing the acoustic features of these whistles with those of other different demographic units available from the literature, we observed differences in the values of the minimum and maximum frequencies. Considering the critical vessel traffic of our study area, we interpreted these variations in light of the high levels of anthropogenic noise. We recommend further investigations to understand the mechanisms underpinning the acoustic variation of bottlenose dolphins’ signature whistles and to elaborate effective management and protection policies for this species.宽吻海豚（Tursiops truncatus）会广泛使用具有个体特异性频率调制特征的窄带哨声，以传递自身身份信息，即标志性哨声（signature whistles）。标志性哨声的声学结构可稳定维持数十年之久，因此这类发声具备长期追踪个体的应用潜力。本研究针对栖息于意大利东部利古里亚海佩拉戈斯保护区内的宽吻海豚种群单元的标志性哨声声学结构展开调查。研究人员通过对声谱图进行目视分类，共识别出941条哨声轮廓，并采用签名识别（SIGnature IDentification，简称SIG-ID）方法将其中469条归类为标志性哨声。随后，研究人员根据这些哨声定型化的频率调制模式，将其进一步划分为19个不同类别。为验证分类结果的可靠性，本研究结合了两项分析手段：一是由6名未受专业训练的评审人员开展的目视分类任务，二是自动化的主成分分析。总体而言，本研究首次对栖息于利古里亚海的宽吻海豚种群单元的标志性哨声进行了系统特征描述。此外，将本次研究中哨声的声学特征与文献记载的其他种群单元的哨声特征进行对比后，研究团队发现二者的最低与最高频率数值存在显著差异。考虑到研究区域内船舶通航密度极高，我们将这些声学特征差异归因于高水平的人为噪声干扰。最后，本研究建议开展进一步调查，以阐明宽吻海豚标志性哨声声学变异的内在机制，并为该物种制定行之有效的管理与保护政策。