Rolling in the deep : drivers of social preferences and social interactions within a bull shark aggregation in Fiji

Marosi, Natasha D. and Ellis, Samuel and Jacoby, David M.P. and Brunnschweiler, Juerg M. and Croft, Darren P. (2025) Rolling in the deep : drivers of social preferences and social interactions within a bull shark aggregation in Fiji. Animal Behaviour: 123511. ISSN 0003-3472 (In Press)

Abstract

Understanding the mechanisms and functions underpinning social behaviour in marine species is a fundamental aim in ecology and conservation. Many shark species have relatively complex social bonds, and anthropogenic effects may affect these bonds. However, studying these effects remains a challenge because of the difficulty observing social interactions in many shark species. This study utilized the Shark Reef Marine Reserve in Fiji, a site where a population of bull sharks, Carcharhinus leucas, are provisioned and interact socially, to record patterns of social structure at different social and spatial scales. Six years of observational data (473 dives, 8192 min) were used to examine broad-scale patterns of association and fine-scale social interactions. Using these data, the role that the phenotypic traits of age, sex and size play in structuring associations and social interactions were quantified in 184 individual bull sharks. Social network analysis indicated that association patterns are not solely artefacts of spatiotemporal overlap but rather attributed to active social preferences and the selective interactions by individuals. Assortative measures indicated that sex and age were positive predictors of broad-scale associations and fine-scale social interactions, whereas size appears to play a role in interaction choices only. Centrality measures revealed sociality differences between the sexes with males having more social connections on average than females. Differences in social connectivity were also found within age classes, where older and younger individuals are less socially connected than middle-aged bull sharks. Results indicate that older individuals trend towards asociality. These findings advance our understanding of the drivers of social preferences and interactions in an apex marine predator, while reinforcing the contribution of sociality on fitness at the individual level and on viability at the population level.