Colour patterns and pigmentation variability on striped dolphin Stenella coeruleoalba in north-western Mediterranean Sea

Studies on differences in external morphology and pigmentation patterns were historically carried out using stranded individuals or opportunistic sightings; few studies have involved sampling systematically free-ranging individuals. In order to investigate and describe main pigmentation characteristics, outlining 'typical' regional pigmentations, this work analysed,systematic photographic information taken on free-ranging striped dolphins, Stenella coeruleoalba. Photographs of dolphins in the Ligurian Sea were collected between May 2004 and December 2006. All individuals were described by the presenc/absence of pigmentation variables and by differences in colour shades. The frequency of all the pigmentation variables analysed is stable in the population (10 'gene' variables, 19 'allele' variables), and remains similar between each different group of dolphins. But population presents widespread pigmentation variability between specimens, allowing identification even at single individual level. Cluster analysis also found that the majority of the pigmentations derive from two main colour patterns, called 'mat' and 'pale' patterns (f(mat) = 0.68; f(pale) = 0.12). The Bray-Curtis index showed a high variability of the intra-group pigmentation distance between groups. This resulted in a positive correlation between group size and 'intra-group' pigmentation distance: the distance increases rapidly up to a group size of 40 individuals. According to the results obtained, the striped dolphins seem to be concentrated in small groups in which there is a large phenotypic similarity among individuals. These small units could be associated between them to form temporary large groups observed only in pelagic waters.