Résumé
In social species, associating with peers can provide fitness benefits, particularly through associations between related individuals. Killer whales (
Orcinus orca
) are a highly social species with a matrilineal social organization that varies across populations with different feeding ecologies. Around the Crozet archipelago (French subantarctic islands), killer whales feed on marine mammals, seabirds and fish, but also depredate fish caught by local fisheries. Using both photo-identification data and tissue samples, this study examined social structure in relation to kinship of Crozet killer whales between 2005 and 2024. Social analyses showed that the 113 individuals in our study belonged to 21 social units with strong, preferred and stable associations over time. The number of associates and the strength of associations increased over the study period (from < 10 to > 40 and from < 3 to > 7, respectively) but average size of social units (5.4 ± 2.7 individuals) remained constant. From genetic analyses conducted on 31 biopsy-sampled individuals using 19 microsatellite markers, a low correlation was found between their association and relatedness indices, with close kin relationships occurring both within and between social units. Together, our findings suggest that the social organization of the Crozet killer whales is similar to other mixed-diet killer whale populations, but shows patterns that may be attributed to new associations formed by individuals that have survived the additive mortality event the population has undergone in the late 1990s. Therefore, this study highlights that sociality in killer whales can be driven by both their feeding ecology and the demographic history of populations.