Insights on common dolphin (<Emphasis Type="Italic">Delphinus delphis</Emphasis>) social organization from genetic analysis of a mass-stranded pod

Compared to terrestrial mammals, little is known of cetacean social systems as they are generally less accessible to behavioral investigations due to their aquatic environment. The present study investigates group structure of the pelagic common dolphin, Delphinus delphis, using genetic markers. Tissue samples from 52 individuals representing a recent live mass-stranding event were compared to 42 single strandings taken from presumably different groups. The mass-stranding event occurred in 2002 on the French coast of the English Channel, whereas the single strandings were collected between 1993 and 2003 along the western coast of France (Bay of Biscay and English Channel). Analysis of mitochondrial DNA control region sequences indicated that genetic variability within the mass-stranded pod was similar to variability observed in single strandings. The mass-stranded group was composed of 41 different mitochondrial haplotypes or matrilines while the single strandings revealed 29 different haplotypes. Analysis of 11 microsatellite loci revealed that average relatedness of the mass-stranded pod was not different from average relatedness among all single strandings suggesting that individuals within the group had no closer kin relationships than animals taken from presumably different groups. These results do not support a matriarchal system and suggest that common dolphins constituting a pod are not necessarily genetically related.