BOTTLENOSE DOLPHINS VOCAL COMMUNICATION
O Grove, Spain ©Ylenia Vimercati
Tursiops truncatus whistles research
From January to June 2016 I worked as an intern at the Bottlenose Dolphin Research Institute - BDRI (Spain) with director and chief biologist Dr. Bruno Díaz López to pursue my Specialization's degree dissertation: A comparative study of whistle characteristics between Atlantic and Mediterranean populations of wild bottlenose dolphins (Tursiops truncatus).
The research involved boat-based and land-based data collection, data transcription, use of SPECTROGRAM® to analyze vocal recordings, photo identification, and use of GIMP® to create silhouettes of dorsal fins.
Communication is a fundamental aspect of all social behaviors. It involves the exchange of a signal from a signaler to a receiver. Signals have a wide variety of functions that help to increase the fitness of individuals. Communication in cetaceans can be displayed by adopting three modalities: acoustic, tactile, and visual. Studying the acoustic communication of these animals can be very challenging, not only because of the marine environment they inhabit, but also because we can rarely establish which individual emitted the sound. When speaking of auditory capabilities, sound production, and communication, the proper word used to describe this field is bioacoustics. Dealing with cetacean communication means to get to know sound properties and how the sound propagates in the water medium.
The aim of this study was to gain knowledge about the characteristics of the vocal repertoire emitted by two non-adjacent populations of wild bottlenose dolphins (Tursiops truncatus), one from Galicia (Spain) and one from Sardinia (Italy). The bottlenose dolphin is an extremely vocal mammalian species usually living in groups whose size and composition can be highly variable. Among the three types of vocalizations adopted by T. truncatus, which are whistles, burst-pulsed sounds, and clicks, I investigated the first one by determining the characteristics, the presence of common features shared by the two populations, and making hypothesis upon the factors shaping whistle production.
Data used for this study were collected from Golfo Aranci in Sardinia Island (Italy) during year 2013 and from the Ría de Arousa (Galicia, Spain) in 2015 as acoustic recordings. These data came from two resident populations of bottlenose dolphin which have been extensively studied. The study areas are currently characterized by the presence of individuals all year round and are well exploited by anthropogenic activities such as fishing and aquaculture. Sounds were registered from boat-trips using a hydrophone and digitally recorded. Data collection also included the evaluation of the behavioral activity displayed by the dolphins in order to include this information when comparing the vocal repertoire of the two populations and the possible effects it has on sound production.
Recordings were played back and analyzed through a visual and aural analysis using SPECTROGRAM® 16.0 software. Whistles were included within three quality categories depending on contour parameters: high, average, and low. Only high quality whistles were used for qualitative and quantitative analyses. Whistle categorization (or qualitative analysis) involved the classification of the sounds into several type contours, which were: rise, fall, sine, flat, wave, U-shape, and multiloop, according to definitions provided by previous studies. Quantitative analysis consisted in measuring the acoustic features of the high quality sounds for subsequent statistical analyses.
Statistical procedures were performed using PAST® software. I provided a basic descriptive statistics of the two populations’ vocal repertoire for several variables selected. I focused on the type characteristics and investigated on the existence of a possible association between types and locations. The Shapiro-Wilk test was used to establish whether the variables selected followed a normal distribution. Then the Mann-Whitney or the t-test for equal medians and means were adopted to determine whether a same variable was statistically different or not in the two populations. To test for correlations between variables within each bottlenose dolphin population I selected Spearman’s rs as coefficient. At last I focused on behavior to investigate about a possible link between locations and behavioral display (feeding, socializing, and travelling).
The results of this study clearly showed that same whistle types were detected within each population, but the percentages of each type category differed locally. I found a high statistically significant relationship between whistle types and locations. According to the descriptive statistics for whistle characteristics, type multiloop was the one with the longest mean duration in both populations. Variables comparison between populations revealed that all the variables except for finish frequency and frequency modulation statistically differed (p-value>0,05). Correlations between whistle characteristics clearly showed the existence of strong correlations (p-value<0,01) in the two sites. As for behavior, feeding activity was the predominant activity while whistling in Sardinia and Galicia. I found an association between certain behavioral displays and locations, and observed that socializing and travelling were responsible for the discrepancy observed in the populations.
The vocal repertoire consisted of 7 whistle types, with type multiloop being the most frequent in Sardinia and type rise in Galicia. The discrepancy observed between the frequency of these two types in the two locations explained most of the difference in their vocal repertoire. The longer mean duration registered in Sardinia could be explained by the higher occurrence of type multiloop in that area having this whistle the longest mean duration. Testing for correlations showed that some variables were linked to each other. The same strongest correlations were observed in the two populations.
Reasons to geographic variations in the vocal repertoire of non-adjacent populations can be various. Natural and anthropogenic characteristics could account for the dissimilarities observed. Many studies have shown how strong is the impact of human activities on dolphin’s vocal production, and how these animals modify their whistles according to anthropogenic noise levels.
Comparisons between other populations from different parts of the world allowed me to investigate whistle parameters of separated locations with specific environmental backgrounds. Studies have reported longer whistle duration when in presence of high levels of noise pollution. A longer duration was interpreted to be a strategy to avoid signal interference. Feeding activity accounted for most of the vocalizations in Spain and Galicia and it has been linked to longer duration. Studies have also reported an association between feeding behavior and type multiloop. My results supported these findings.
Future studies on whistle types respect to behavioral activities would improve our knowledge on the effect of behavior on sound production. Furthermore a deeper investigation on the anthropogenic impact on whistle characteristics, by measuring noise disturbance, would provide me a better understanding upon the effect of human activities on bottlenose dolphins communication.