The Acoustic Ecology of Coastal Dolphins by Assessing the Structural Variability of Sounds and the Influence of Contextual Factors

Abstract

The acoustic ecology of a previously unexamined dolphin population in the Mediterranean was assessed by investigating how sound emissions and acoustic features are influenced by concurrent conditions. Whistles and click-trains emission rates were compared among different environmental, social and behavioural conditions. Structural variability of 3928 good/high-quality vocalizations was analysed in relation to contexts through a two-stage approach. First, two multivariate MANOVA-models were fitted considering the entire set of acoustic parameters extracted from whistles and click trains, to investigate the effect of concomitant factors on the overall acoustic structure of each vocalization. Subsequently, GLMM models were applied to each acoustic feature individually to explore its response to different contextual factors. Emission rates increased significantly with calves and in larger groups, with also a positive effect of socialization on whistles and of muddy/sandy seabed and depth on impulsive sounds. The multivariate approach showed that all contextual factors influenced sounds' structure, with whistles being strongly affected by behaviour and calves' presence. The GLMM models highlighted that each acoustic parameter varied differently in response to specific factors, with (1) increasing trends in whistles' duration and inflection points during interaction with fishery and decreasing ones during socializing, and (2) decreasing inter-click-intervals and increasing click-repetition-rates in larger groups and during interactions with fishery. These results provide new findings on the acoustic plasticity of bottlenose dolphins and a more comprehensive view of the magnitude of the social, environmental and behavioural influence, highlighting how the complexity of the species' acoustic repertoire has yet to be unravelled at the local level.

Keywords: Mediterranean Sea; acoustic repertoire; behaviour; common bottlenose dolphin; environmental factors; sounds.

FIGURE 1

Map of the study area. The blue dots represent the initial location of each sighting. The red rectangle highlights the “Secche di Tor Paterno” MPA. The black diamonds indicate the two Single Point Moorings (SPMs) R1 and R2. Map created using the QGIS software.

FIGURE 2

Boxplot of whistles emission rates (ER) with respect to group sizes (left panel), calves’ presence (middle panel) and behavioural contexts (right panel).

FIGURE 3

Boxplot of click train emission rates (ER) with respect to seabed type (upper left), depth class (upper right), group sizes (lower left) and calves’ presence (lower right).