Social and population structure of striped and Risso's dolphins in the Mediterranean Sea

Abstract

The aim of this research was to apply genetic methods to a better understanding of the evolution of population genetic structure in two dolphin species inhabiting a shared geographic range in the Mediterranean Sea and eastern North Atlantic (ENA). In support of this I analysed the pattern of population subdivision, genetic variability and group kin structure of striped (Stenella coeruleoalba) and Risso's dolphins (Grampus griseus) in the Mediterranean Sea, and compare theassessed the level of genetic differentiation between Mediterranean and ENA populations. The specific objectives were to: 1- study their distribution in the core study area (the Ligurian Sea) and group size variation as a possible behavioural responses to environmental features; 2- assess patterns of kinship and association within and between social groups (based on estimates of R); and 3- investigate patterns of genetic subdivision among the putative striped and Risso's dolphin populations within the Mediterranean Sea, and in comparison with the ENA. Risso's dolphins showed a clear preference for the steeper continental slope, while striped dolphins did not show this distribution preference. Samples from 165 striped dolphins were collected from the Mediterranean Sea (Adriatic, Tyrrhenian, and Spam) and ENA, and were analysed at 8 microsatellite DNA loci. The comparison between the Mediterranean and the ENA populations showed a clear separation (Fst=0.055). All loci were polymorphic (4-22 alleles), and mean Ho ranged from 0.68 in the Tyrrhenian to 0.8 in the ENA. All comparisons between putative populations showed significant differentiation, including the populations of the eastern (Adriatic) and western (Tyrrhenian) seas of Italy. Analysis of the Risso's dolphin samples (n=50) collected in the Mediterranean and the ENA showed that the two populations were significantly differentiated (Fst=0.0296). All loci were polymorphic (up to 17 alleles), and mean Ho was 0.467 m the Mediterranean, 0.548 in the ENA. Analysis of 619 bp of sequence of the mtDNA control region revealed 28 variable sites defining 16 unique haplotypes among the two populations. The degree of differentiation was highly significant for both FST (0.260) and ɸST (0.542).Individual relatedness within and between social groups of striped dolphins was investigated m the Ligurian population. On average the values were higher for intra- than inter-group comparisons, and females showed significantly higher relatedness within than between groups. The kin structure of the Risso's dolphins in the Ligurian population showed that females had a considerably higher relatedness within groups than among groups. For both species, the data suggest a relatively fluid model of kin structure with a trend for female philopatry, and male dispersal. My results indicate fine-scale population structure in both species, and significant differentiation between the Mediterranean and North Atlantic populations. A hierarchical pattern of kin structure is clearly influenced by social group structure and the dispersal behaviour of males and females, though there are some differences between the species, probably related to differences in social structure and habitat use.