Seasonal fission and fusion of killer whale, Orcinus orca, social structure at sub-Antarctic Marion Island

Abstract

Variation in the distribution and abundance of food resources are key factors affecting animal sociality. In environments with variable resources, dynamic social organization, such as the fission and fusion of groups, is thought to increase the benefits of group living, while reducing the costs. We investigated the relationship between social organization and prey abundance in a highly social predator, the killer whale. This was achieved by analysing 12 years (2006–2018) of seasonally delineated (coinciding with high and low prey abundances) association data obtained from nearly 90 000 identification photographs of killer whales in-shore at Marion Island in the sub-Antarctic Indian Ocean. Association network measures were compared between periods using randomized association matrices. Half-weight association index, degree, number of modules and group size were all greater during periods of high prey abundance while mean distance, centrality and modularity were lower during this same period. Results suggest that killer whales at Marion Island were more social, formed larger groups and had more associations during periods of high prey abundance. During periods of lower prey abundance, fewer interactions, stronger clustering and more division in the association network were observed. These results indicate that the social organization of this population of killer whales is seasonally dynamic, with increased sociality measures coinciding with periods of higher prey abundance. These results are similar to those of other social species, emphasizing the importance of resource abundance as a driver of social structure in animal societies.