Common dolphin (D. delphis) bioacoustics project – a study about vocalization and sociality of the common dolphin population in the Aegean Sea, February-March 2018.
Figure 1 – Common dolphin (Delphinus delphis).
Common dolphins often live in pods consisting of around 12-20 individuals in the Mediterranean Sea (Murphy et al., 2008). The population of this particular species has been declining at an alarming rate in the Mediterranean Sea. Common dolphins get benefits from living in large groups including things such as aid in finding food, defense against predators and access to reproductive members (Evans, 1987). Because of this behavioural tendency towards group dependency and sociality, obtaining Common dolphin bioacoustics recordings is more feasible than when working with a more reclusive species (Roch et al., 2007). Common dolphins produce a number of vocalizations, including whistles, clicks, and burst pulse calls, some of which can be used to identify an individual (Soldevilla et al., 2008).
OBJECTIVES
The purpose of this study is to explore the vocal repertoire of the Common dolphin in the eastern Aegean Sea. The objectives set for this project are:
- record common dolphins vocalizations, including whistles, clicks, and burst pulse calls;
- measure certain parameters for each individual whistle recorded;
- classify recorded whistles according to shape and contour;
- compare whistle characteristics to the behaviour assumed by individuals during the sighting;
- use whistles to identify individuals and pairing with photo-ID for better identification
METHODS
For each recording, the Marine Mammal Bioacoustics Team will classify the types of vocalizations: whistles with and without harmonics, clicks and pulsed calls (Figure 2). In order to gather this data an AS-1 Aquarian hydrophone is used to record sound data. Raven software is then used to analyze the recorded data.
The whistle parameters that will be measured by the Team include: duration, start frequency, end frequency, minimum frequency, maximum frequency and more. Whistle shape and contour used can be found in the chart below (Figure 3).
Figure 3 Constant frequency (A); upsweep (B); downsweep (C); convex (D); concave (E); sine (F). Each general contour classification represents the overall whistle contour while the minor contour classifications, or subcategories, considered frequency modulations at the beginning (2), at the end (3), at both ends (4) of the whistle or no modulation at all (1) (Ansmann et al., 2007). F4-F8 are examples of signature whistles.
Figure 2 – Spectrograms of common dolphin vocalizations: distinct individual whistles with no harmonics (A); whistles with harmonics that are still individually distinct from each other (B); overlapped whistle bouts, with whistles that cannot to be uniquely identified (C and D). Clicks are also visible as vertical lines in (A), (C), and (D), and pulsed calls are visible in (D) (Henderson et al., 2012).
RESULTS
Whistle parameters change with the animal’s behaviour, an example of social situation adaptation in the vocal repertoire of common dolphins. Outside of behaviour, Recordings can also give insight to individual identifications, as signature whistles can be used alongside Photo-ID. The Marine Mammal Bioacoustics Team will compare the shape of the whistles with the behaviour of the dolphins at that time in order to understand if there is a relationship between shape and behaviour also in Aegean Common dolphin population as well as to identify individuals among the populations found in the Aegean Sea.
Chelsea Gragg
M.A. Anthrozoology
University of Exeter
England
Simone Antichi
Masters in Marine Biology,
Marche Polytechnic University
Italy
