Supplementary data for World marine mammal conference 2019. Poster #57

Identification and characteristics of Signature whistles in wild bottlenose dolphins from the Ligurian Sea

Francesca Terranova1   (francesca.terrano@edu.unito.it) ,

Guido Gnone 2,3,Michela Bellingeri 3, Livio Favaro 1,4

1 Department of Life Sciences and Systems Biology, University of Turin, Italy

2 di Genova, Area Porto Antico, Ponte Spinola – 16128 Genova, Italy

4 Centro Interuniversitario Acquario di Genova, Costa Edutainment S.p.A., Ponte Spinola – 16128 Genova, Italy

3 Fondazione Acquario di Ricerca sui Cetacei (CIRCE) – Viale Benedetto XV, 5 – 16132 Genova, Italy

INTRODUCTION

What is the signature whistle? Bottlenose dolphins (Tursiops truncatus) make extensive use of narrowband whistles for intraspecific communication. Among the whistle types, each individual develops signature whistle: a learned, individually distinctive whistle type with unique frequency modulation pattern that broadcasts the identity of the whistle owner.A sort of name that dolphins use to address each other (Caldwell and Caldwell 1968).  

Rec 1. Example of typical dolphins vocalization with whistle and burst


 How study SW in wild dolphins? Using the SIGnature IDentification (SIGID) method, a bout analysis approach used to identify signature whistle in recordings of freely interacting animals. How does it work? At least 75% of all whistles of the same type must occur within 1–10 s of each other            (Janik et al. 2013).


SUBJECTS OF STUDY: METROPOLITAN DOLPHINS

Along the Ligurian coast there is a demographic unit of bottlenose dolphins composed today of nearly 250 individuals. That population  has been studied since 2001.

AIMS


To investigate the acoustic structure and contextual use of the signature whistles of bottlenose dolphins population that inhabiting the western Ligurian coast.

METHODS

Study area: Western Ligurian coast (Italy), within the Pelagos Sanctuary (NW Mediterranean Sea). (Fig 1.)

Fig 1. Study area. Green dots indicate the beginning of sighting, red dots for the end.
 

Vocal recordings: We collected acoustic recordings using an HTI-96-MIN hydrophone connected to a Zoom H5 Handy recorder (sampling rate 96 kHz) during 25 encounters in surveys that were conducted from July to October 2018.

Acoustic analysis: through visual and aural inspectionof spectrograms we selected audio segments containing whistles. Using SIGID method we discerned signature whistle and non signature whistle. Among signature whistle we found out 14 signature whistle type.

Fig 2. Specrogram of the signature whistle SW1.
Click here to hear the Signature whistle 1 !

Statistical analysis : For each whistle founded, we extracted the pitch contour of the fundamental frequency using the beluga toolbox and measured different acoustic parameters using automatized procedures in MATLAB.

Tab. 1. Mean ± SD for the acoustic parameters measured on the frequency contours (n = 504). Frequency is expressed in Hertz and Duration in milliseconds.
  1. Performed a component analysis (PCA) for validate statistically the 14 signature whistle types

RESULTS

  1. The analysis of the spectrograms allowed us to identify 936 whistles. Among these, 504 whistles passed the SIG-ID criteria and were further grouped into 14 different categories of frequency modulation. Overall, 53.8% of the entire whistle repertoire of the “Delfini Metropolitani” population was made of  signature whistles.
  2. The signature whistle emission rate recorded in this study was 1.43 whistles per minute.
  3. Fig. 5 shows an example of how signature whistles of the same cluster overlaps in the time/frequency domain.
  4. The PCA reduced the 12 acoustic parameters measured to three independent PCs. These three components explained 84.82 % of the total variance (PC1 = 40.66 %, PC2 = 24.21 %, PC3 = 19.94%). Each signature whistle type made a different cluster in the space defined by the first two PCs (Fig. 4). Fig. 5
Fig 4. Different cluster in the space defined by the first two PCs for different signature whistles

Fig 3. Density plots of three signature whistle types.

CONCLUSION & FUTURE AIMS

The values of the acoustic parameters measured from the signature whistles of the bottlenose dolphins of the Ligurian sea are comparable with those reported from dolphins recorded in Namibia (Kriesell et al. 2014) and Sarasota Bay (Esch et al. 2009). However, the average frequency values are slightly higher than Namibia dolphins but lower than Sarasota Bay. Further research effort matching of the signature whistles with the photo-identified individuals would be especially valuable for the long-term monitoring of this population.

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