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

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 to the acoustic variability in the whistles of common
bottlenose dolphin (Tursiops truncatus), inhabiting two Mediterranean
areas (Sardinia and Croatia). We explored which factors, among 1)
geographical isolation of populations, 2) different environments in terms
of noise and boat presence and 3) social factors (including group size,
behaviour and presence of calves), were associated to whistle
characteristics. We first applied a principal component analysis to reduce
the number of collinear whistle frequency and temporal characteristics,
and then generalised linear mixed models on the first two principal
components. The study revealed that both geographic distance/isolation and
local environment are associated to 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 behaviour were also found to be associated with dolphin whistle
variation. The second principal component (PC2) was associated only to
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 behavioural context influenced significantly the structure
of whistles, and they should be considered when investigating acoustic
variability among distant dolphin populations to avoid confounding
factors.