Abstract
A signature whistle type is a learned, individually distinctive whistle type in a dolphin's acoustic repertoire that broadcasts the identity of the whistle owner. The acquisition and use of signature whistles indicates complex cognitive functioning that requires wider investigation in wild dolphin populations. Here we identify signature whistle types from a population of approximately 100 wild common bottlenose dolphins (Tursiops truncatus) inhabiting Walvis Bay, and describe signature whistle occurrence, acoustic parameters and temporal production. A catalogue of 43 repeatedly emitted whistle types (REWTs) was generated by analysing 79 hrs of acoustic recordings. From this, 28 signature whistle types were identified using a method based on the temporal patterns in whistle sequences. A visual classification task conducted by 5 naïve judges showed high levels of agreement in classification of whistles (Fleiss-Kappa statistic, κ = 0.848, Z = 55.3, P<0.001) and supported our categorisation. Signature whistle structure remained stable over time and location, with most types (82%) recorded in 2 or more years, and 4 identified at Walvis Bay and a second field site approximately 450 km away. Whistle acoustic parameters were consistent with those of signature whistles documented in Sarasota Bay (Florida, USA). We provide evidence of possible two-voice signature whistle production by a common bottlenose dolphin. Although signature whistle types have potential use as a marker for studying individual habitat use, we only identified approximately 28% of those from the Walvis Bay population, despite considerable recording effort. We found that signature whistle type diversity was higher in larger dolphin groups and groups with calves present. This is the first study describing signature whistles in a wild free-ranging T. truncatus population inhabiting African waters and it provides a baseline on which more in depth behavioural studies can be based.
Highlights
Social animals need to keep in contact and contact vocalisations are fundamental to many animal communication systems [1,2,3,4]
We provide a baseline description of signature whistle occurrence, acoustic parameters and temporal production and compare signature whistle characteristics between Walvis Bay, Namibia and Sarasota Bay, Florida, USA
We investigated the relationship between the number of different signature whistle types recorded, i.e. the diversity of different types and group size, as well as the influence of calf presence using a generalised additive model (GAM)
Summary
Social animals need to keep in contact and contact vocalisations are fundamental to many animal communication systems [1,2,3,4]. Bottlenose dolphins (Tursiops spp.) use signature whistles as contact calls to maintain group cohesion [9,10]. They help facilitate reunions between familiar individuals including mother and calf pairs [11] and male alliances [9]. Vocal copying of signature whistles takes place between close associates [13] and may be used to address conspecifics [14]. Such vocal labelling is unusual in the animal kingdom and indicates complex cognitive functioning in bottlenose dolphins
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