Is Blood Thicker Than Water? The Influence of Kin Within a Bull Shark Social Network

The influence of kinship on social structure (what we novelly refer to as "kinfluence") has been studied extensively within terrestrial taxa yet remains poorly understood in marine systems. While kin-based associations have been explored in teleost fish and cetaceans, few have investigated this within elasmobranchs, and none have combined fine-scale social behavior with both genomic relatedness and categorical kinship inference. This study utilized the Shark Reef Marine Reserve in Fiji, a site where a population of adult bull sharks (Carcharhinus leucas) aggregate in response to provisioning, and socially interact, to examine the role of genetic relatedness and categorical kinship in shaping social behavior. We integrated DArTseq SNP genotyping (3919 SNPs across 129 individuals) with behavioral data on active social preferences (join, lead, follow, and parallel swim events) collected over 473 observation dives conducted across 6 years. Social network analyses revealed a structured network driven by active preferences with genomic relatedness exerting a small yet significant influence on social behavior at both dyadic and community levels, particularly within male-male dyads. Pedigree reconstruction identified full- and half-sibling dyads, allowing direct examination of kin-category effects. At the community level, analysis utilizing a weighted kinship matrix revealed associations between full and half-sibling dyads were slightly stronger than between non-kin dyads, whereas no detectable relationship was uncovered for social interactions at the dyadic level. Our mixed findings may reflect differences in the datasets or be partially explained by sibship reconstruction omitting third order relationships (first cousin avuncular) in defining kin categories. Our study provides rare empirical support for kin-selection in a large marine predator, underscoring the need to further investigate the role of kin in shaping social systems within elasmobranchs.