Bull shark mating system

Bull sharks (Carcharhinus leucas) are wide-ranging coastal predators whose mating and reproductive structure are rarely documented directly in the wild. Yet the distribution of reproductive success among individuals is relevant for understanding patterns of genetic diversity and effective population size. Using genome-wide kinship reconstruction, we re-analysed 4,146 SNPs from 408 bull sharks sampled across the Indo-Pacific (South Africa, Indonesia, Papua New Guinea, Australia and Fiji), including juveniles collected across three consecutive parturition cohorts in Fiji. We used sibship inference (COLONY) to quantify multiple paternity and multiple maternity across regions and breeding seasons, and to estimate individual breeder contributions and breeding sex ratios from juvenile-derived pedigrees. Recurrent polygamy was detected across sites and years: multiple paternity occurred in 19 sires out of 191 sires (9.9%) while multiple maternity (polygyny) was identified in 13 dams out of 205 (6.3%). In several seasons, individual breeders contributed disproportionately to reproduction, indicating uneven reproductive success among adults. In Fiji, the presence of related juveniles both within and across cohorts indicated that consecutive parturition cohorts contained sufficient kin structure to reconstruct breeder contributions across seasons using juvenile sampling alone. Across the full dataset, the inferred breeding sex ratio was close to parity, with only minor spatial variation among regions. Together, these results provide basin-scale genomic evidence that bull shark reproduction commonly involves mating with multiple partners, and further demonstrate that SNP-based kinship reconstruction can effectively quantify mating-system variation, breeder turnover and sex-specific breeding participation without the need for destructive sampling, parameters that are directly relevant for demographic inference and conservation planning in exploited and spatially structured shark populations.