Mediterranean striped dolphins share echolocation information during traveling and foraging.
Marianna Anichini * 1,2, Cristina Giacoma2, Elena Papale1,2, Marta Azzolin1,2
1Gaia Research Institute Onlus. Italy2University of Torino, Dip. Biologia Animale e dell'Uomo, V. Accademia Albertina, 13, 10125 Torino, Italy
Discussion
References
IntroductionWho?
Free‐ranging Striped Dolphin (Stenella coeruleoalba), one of the most widespread cetacean species in the Mediterranean Sea, is an extremely vocal mammalian species but its visual and acoustic behaviour is almost unknown. (FIG 1).
Where?Gulf of Corinth (GOC), Greece. The study area is located between the Patra strait, opening on the Ionian sea, and the Corinth Canal
on the Alkyonides gulf. An area of about 700km2 has been covered (FIG 2).
When?During the months of July and August 2009 and 2010.
Why?In order to understand and analyze how Stenella coeruleoalba echolocation clicks rate changes in relation to focal group size and
behavioral context.
Results
Materials and methodsDuring this research, 63 vessel based surveys have been carried out in typical weather condition and ecologic and anthropogenic parameters were recorded every 15 minutes. During sightings, group size and interaction with boats were collected every 3 minutes. The animal surface behaviour was analyzed using the focal group sampling methodology (Altmann,1974), and following an ethogram created before and during the study: 41 behaviours and 4 different vocal type were recorded, thanks also to those already developed in previous studies on Atlantic spotted dolphins (Herzing,D.L.,1996; Dudzinski,K.M.,1996). Group dimension was divided in four categories: 1) 1‐5; 2) 6‐9; 3) 10‐15; 4) 20‐30 animals. Sounds were recorded in the proximity of the focal group (max distance 100m) with a single hydrophone at a sampling frequency of 96 kHz (16‐bit mono channel) connected to a digital audio M‐Audio microtrack recorder. In order to reduce the low frequency background noise a filter at 1‐kHz was used during recordings. Striped dolphin clicks have been classified in four qualities categories (0‐3): only signals clear and audible (2‐3 quality) were considered.In order to synchronize visual and acoustic behaviour, recordings have been subdivided in intervals of 3 minutes each. For each interval, only the clicks detected in the first 10 seconds were considered for analysis and only the most frequent (number of times in which appears the behavior / number of total behavioral sequences in intervals of 3 minutes) behavioral states (normal swim tail out, rafting and sharking) related to the main categories of Travelling and Foraging were included.
Statistical analysis (Kruscall‐Wallis Test) showed that the echolocation clicks rate was significantly affected by group size. The echolocation clicks production per dolphin (clicks/minute/animal) is minimum in the biggest group size . Jones and Sayigh (2002), have reported how the bottlenose dolphin in larger groups, emitted less of echolocation clicks per unit of time, sharing the echoes of the other animals (Tyack,1997). Statistical analysis (Rho Spearman’s Test) showed also, a positive and significant correlation of the click rate in relation to behavioral state linked to forage and travel activities, indicating that echolocation is used in foraging and travelling. We suggest that these results, support the idea that echolocation information can be shared by the animals in a larger group above all during specific behavioral activities as foraging and travelling.
•Altmann, J. (1974). Observational study of behavior: sampling methods. Behav.ior 49, 227‐267.•Dudzinski, K. M. (1996). Communication and behavior in the Atlantic spotted dolphins (Stenella frontalis): relationships between vocal and behavioral activities. Ph.D. Dissertation. Texas A&M University.•Herzing,D.L.(1996). ‘Vocalizations and associated underwater behavior of free‐ranging Atlantic spotted dolphins, Stenella frontalis and bottlenose dolphin, Tursiops truncatus’. Aqua.Mamm. 22,61–79. •Jones, G.J., and L.S. Sayigh. (2002). Geographic variation in rates of vocal production of free‐ranging bottlenose dolphins. Marine Mammal Science 18:374.393•Tyack, P. (1997). Studying how cetaceans use sound to explore their environment. Perspectives in Ethology 12:251–297.
Fig.3.Spectrogram OF cliks (quality 3) obtained by the software Adobe Audition 3.0 WITH a FFT‐size of 256 points IN Hamming window.
Fig.2.Study area: Gulf of Corinth (GOC), Greece.
Fig.1.Striped Dolphin (Stenella coeruleoalba)
Fig.5. Scatter plot showing the relation between the clicks/minute/A and the different group dimension.
Sightings 44Mean sightings frequency
FREQ=n°sightings/ hours survey0,5
Hours of behavioral observations and simultaneous recordings
3,5
Echolocation clicks totally detected in the first 10s for each interval (FIG 2)
5015
Fig.4. Scatter plot showing the relation between the clicks/10 and the different group dimension.
Both the relationships between group size category (number of animals) and the echolocation clicks (clicks/minute) and those divided for the number of animals (number of clicks/minute/animals), were examined.The non parametric Kruscall‐Wallis Test showed a significant difference (P< 0.05) between group sizes and the clicks/minute (X2= 11,268; Df= 3; P= 0,010) and clicks/minute/animals (X2= 15,743; Df= 3; P= 0,001), as clearly showed in the graphs (FIG 4 ‐ 5). The maximum value of clicks rate (number of clicks/minute/animals) produced is in the third group, while the minimum is in the fourth (20‐30 animals). Similar result was obtained from the non parametric Rho Spearman’s Test calculated to correlate the most frequent states (normal swim, rafting and sharking), with the clicks rate. The test highlighted that the clicks rate was positively correlated with Sharking (P.= 0,028; correlation coefficient 0,274), Rafting (P = 0,004; correlation coefficient = 0,357) and Normal swim tail out (P. = 0,000; correlation coefficient = 0,477 ).
Akwnoledgments• Thalassa, research and charter.• GAIA RESEARCH INSTITUTE ONLUS which have permitted me to develop my research. • All the people that have worked and studied with me in Lakazeza(Greece) and all the voulunteers that participated to surveys.
Check also: To be silent or to communicate?
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behaviour in order to improve passive acoustic monitoring.
Azzolin M., Anichini M., Giacoma C., Papale E.,.