Bull sharks have long been viewed as solitary hunters that cross paths mostly by chance, but a six-year study in Fiji suggests otherwise. Researchers observed adult bull sharks gathering repeatedly at a single reef site and found that individuals showed a slight preference for spending time with their genetic relatives. The pattern was subtle rather than dominant, since most sharks in the group were not closely related to begin with, and sex, age, size and familiarity all played a larger role in shaping who associated with whom. Even so, scientists say the discovery marks the first time genetic kinship has been shown to influence social behaviour in an adult population of large sharks.
Why bull sharks were long considered solitary predators
Large sharks have traditionally been seen as animals that hunt alone and interact with others only briefly. Testing whether family relationships shape shark behaviour has been especially difficult because bull sharks travel widely as adults, juveniles face high mortality, and close relatives rarely survive long enough to encounter one another again once grown. Natasha D. Marosi, lead author of the research and a scientist at the University of Exeter, said earlier work by the same team had already shown that bull sharks actively choose their social partners based on traits like sex, age and size, which set the stage for asking whether family ties added another layer to those choices.
How researchers tracked six years of shark behaviour in Fiji
The team worked at Shark Reef Marine Reserve, a site off Fiji where adult bull sharks gather regularly during tourism dives that offer food. This setting allowed researchers to repeatedly identify individual sharks and observe their behaviour over time. Drawing on 473 dives conducted between 2018 and 2023, the team recorded moments when sharks came within one body length of each other, along with more active behaviours such as swimming side by side, joining another shark, or moving in a leader and follower pattern. These observations were used to build a social network showing which sharks spent time together and how strong each connection appeared.
What DNA analysis revealed about shark family ties
To measure genetic relatedness, researchers examined thousands of points in the DNA where individuals commonly differ, creating a detailed genetic profile for 129 sharks. This analysis identified 11 full sibling pairs and 46 half-sibling pairs within the group, though most pairs showed very low relatedness overall, meaning the adult gathering was not built primarily around family units. According to the study, titled Is Blood Thicker Than Water? The Influence of Kin Within a Bull Shark Social Network and published in the journal Ecology and Evolution, sharks with closer genetic ties tended to associate slightly more often than unrelated individuals, though the effect was modest and only became statistically significant under certain methods of measuring relatedness.
Why related male sharks showed stronger social bonds
The clearest pattern emerged when researchers looked specifically at active social interactions rather than simple proximity. Across a subset of 93 sharks, genetic similarity showed a small positive link with directed social behaviour, and this connection appeared strongest among male pairs specifically. All three genetic methods used in the analysis agreed that more closely related males interacted more than less related males, even though the sample of confirmed male sibling pairs was small. Researchers from the University of Exeter, in a statement carried on the institution’s official research news page, coined the term kinfluence to describe how kinship shapes shark social structure alongside other established factors like sex and body size.
How early life in rivers may explain the family bond?
Bull sharks spend their first years in rivers and estuaries before moving into coastal waters as adults, and researchers believe this early period may be where family bonds first form. Siblings and cousins are more likely to overlap repeatedly in these confined freshwater habitats, potentially creating familiarity that persists once the sharks disperse and meet again as adults at sites like Shark Reef. Scientists suspect sharks could recognise relatives through smell, sight or their electrical sensing ability, though the study did not directly test how recognition happens, leaving open whether sharks consciously identify kin or simply remember individuals from their early life stages. High mortality among juvenile bull sharks, driven largely by illegal gill net fishing, may also be limiting how many related sharks survive long enough to reunite as adults, which could mean the family influence documented here is only a partial picture of a stronger underlying pattern.