Whistle variation in Mediterranean common bottlenose dolphin: The role of geographical, anthropogenic, social, and behavioral factors

Abstract

The studies on the variation of acoustic communication in different species have provided insight that genetics, geographic isolation, and adaptation to ecological and social conditions play important roles in the variability of acoustic signals. The dolphin whistles are communication signals that can vary significantly among and within populations. Although it is known that they are influenced by different environmental and social variables, the factors influencing the variation between populations have received scant attention. In the present study, we investigated the factors associated with the acoustic variability in the whistles of common bottlenose dolphin (Tursiops truncatus), inhabiting two Mediterranean areas (Sardinia and Croatia). We explored which factors, among (a) geographical isolation of populations, (b) different environments in terms of noise and boat presence, and (c) social factors (including group size, behavior, and presence of calves), were associated with whistle characteristics. We first applied a principal component analysis to reduce the number of collinear whistle frequency and temporal characteristics and then generalized linear mixed models on the first two principal components. The study revealed that both geographic distance/isolation and local environment are associated with whistle variations between localities. The prominent differences in the acoustic environments between the two areas, which contributed to the acoustic variability in the first principal component (PC1), were found. The calf's presence and foraging and social behavior were also found to be associated with dolphin whistle variation. The second principal component (PC2) was associated only with locality and group size, showing that longer and more complex tonal sound may facilitate individual recognition and cohesion in social groups. Thus, both social and behavioral context influenced significantly the structure of whistles, and they should be considered when investigating acoustic variability among distant dolphin populations to avoid confounding factors.

Dolphin communication during widespread systematic noise reduction-a natural experiment amid COVID-19 lockdowns

“…Previous studies have associated whistle duration with context and group size (Janik et al, 1994;Wang et al, 1995;Morisaka et al, 2005b). For example, in Croatia and Sardinia bottlenose dolphins increased whistle duration with increasing group size regardless of boat traffic (La Manna et al, 2020). Unfortunately, we do not have data on the number of dolphins present at Dolphin Bay during the lockdown.…”

First Report of Epimeletic and Acoustic Behavior in Mediterranean Common Bottlenose Dolphins (Tursiops truncatus) Carrying Dead Calves

“…The detection of whistles, in particular, could be expected since they are known to play a fundamental role in bottlenose dolphins’ intraspecific communication and social activity [ 49 , 50 ]. However, only a couple of studies have characterized whistles emitted in epimeletic contexts, and they reported high emission rates [ 9 , 17 ], and increased duration and number of inflection points [ 9 ].…”

“…Indeed bottlenose dolphins are known to change the acoustic parameters of their whistles depending on the contexts [ 51 ] and these variations could potentially act as stress indicators [ 16 ]. In the first event, a majority of modulated whistles (11/12 OWs) were recorded, that differed in duration and frequency parameters from the mean values observed in the Mediterranean sea [ 50 , 52 ]. All frequency parameters were higher in the present study with respect to the mean Mediterranean values, while duration and number of inflection points were slightly lower.…”

Birds of a feather speak together

“…The dolphin habitat in the oceans is totally different, and individuals within groups are often dispersed over wide distances. They communicate in the form of whistles that transmit efficiently under water and differ in character chiefly by geographic distance and local environmental factors (La Manna et al , 2020). This study analyzed whistle variation in two geographically separate groups of the Mediterranean common bottlenose dolphin ( Tursiops truncatus ), one around Sardinia and the other near Croatia on the opposite side of the Italian peninsula.…”