Bull sharks, often portrayed as solitary predators that patrol rivers, coastlines and shallow tropical seas alone, may have a subtle social preference that changes how scientists understand their lives. New research suggests that these powerful apex predators are slightly more likely to associate and interact with their relatives than with unrelated sharks. The discovery, made at Fiji’s Shark Reef Marine Reserve, provides the first evidence that kinship can influence social behaviour in an adult population of sharks.
The study combined six years of behavioural observations with genetic analysis of bull sharks that regularly gather at the marine reserve. Researchers from the University of Exeter, Fiji Shark Lab and Lancaster University examined which sharks spent time near one another and which engaged in directed social behaviours. Although most of the animals were not closely related, the data revealed a small but consistent tendency for relatives to form associations and participate in social interactions.
The researchers describe this influence of family relationships as “kinfluence,” a term intended to capture the way kinship can shape the structure of an animal society. In the bull shark population, kinship was not the strongest factor determining social relationships. Sex, age and body size had more pronounced effects, but the contribution of genetic relatedness was nevertheless statistically and biologically notable. The finding suggests that shark social networks are shaped by several overlapping forces rather than by a single preference or rigid hierarchy.
Scientists measured social behaviour in two complementary ways. The first was proximity-based association, defined in the study as two sharks being within one body length of each other. The second involved directed interactions, in which one animal appeared to respond actively to another. These behaviours included parallel swimming, a “lead-follow” formation and “joining,” when one shark moved toward another and began swimming in synchrony. Together, the observations allowed researchers to distinguish simple co-occurrence from more coordinated forms of social contact.
Family ties appeared to influence both types of relationship, with the clearest pattern occurring among pairs of male bull sharks. This result is particularly significant because social preferences among adult sharks have been difficult to study in the wild. Bull sharks can travel across large areas, and their movements are affected by tides, food availability, reproduction and environmental conditions. A long-term observation programme at a predictable aggregation site provided researchers with an unusual opportunity to compare repeated interactions among known individuals.
The genetic component of the research was essential because visual observation alone cannot reveal whether two adult sharks are related. By analysing genetic data, scientists were able to estimate relationships among individuals and compare those relationships with patterns of association. The results did not indicate that bull sharks gather primarily in family groups. Instead, relatives appeared to be modestly overrepresented among social partners, suggesting that kinship operates as a subtle preference within a broader and dynamic network.
The evolutionary explanation for such behaviour is based on kin selection. When animals favour relatives, they may indirectly promote the survival of shared genes, even when the immediate benefit is not entirely personal. However, the mechanism by which adult bull sharks recognise one another remains unknown. Natasha D. Marosi, lead author of the study and founder of Fiji Shark Lab, said several recognition processes could be involved. Bull sharks are born in rivers and estuaries, where full siblings, half-siblings and cousins may spend their early years in the same environments. Familiarity acquired during these formative stages could potentially help them identify relatives later in life.
The apparent influence of kinship may be stronger than the adult population reveals. Young bull sharks face high mortality in rivers and coastal habitats, where illegal fishing and other human pressures remove many animals before they reach maturity. If only a small number of related individuals survive to adulthood, family groups would be difficult to detect even if kin recognition were important. This makes the observed pattern especially intriguing: a measurable preference emerged despite the limited number of surviving relatives available to interact.
The discovery also challenges the familiar image of sharks as fearsome, socially indifferent animals. Earlier research showed that bull sharks actively choose social partners, with sex, age and size influencing those decisions. The new study adds genetic relatedness to that picture, while emphasizing that its effect is weaker and more variable than the other factors. Andrew Griffiths of the University of Exeter said the findings suggest that kin preferences differ according to the behaviour being measured and operate alongside other social tendencies. Bull sharks, in other words, may not be solitary machines but participants in complex social networks whose structure changes with context.
Understanding these relationships could have implications for conservation. Social behaviour can affect how animals use habitats, respond to disturbance and encounter one another in areas exposed to fishing or tourism. If related sharks are more likely to associate, the loss of a few adults could disrupt connections that extend beyond individual animals. The study, published in Ecology and Evolution, demonstrates the value of combining field observation, behavioural analysis and genetics to investigate the hidden social lives of marine predators. At Shark Reef Marine Reserve, the evidence suggests that for bull sharks, blood may be only slightly thicker than water—but it is thick enough to leave a detectable mark on the social network.
Subject of Research: Bull shark social behaviour and the influence of genetic relatedness on associations and social interactions.
Article Title: “Is Blood Thicker Than Water? The Influence of Kin Within a Bull Shark Social Network.”
Web References: Fiji Shark Lab: https://www.fijisharklab.com/; University of Exeter previous research: https://news.exeter.ac.uk/faculty-of-health-and-life-sciences/bull-sharks-have-friends/
References: Ecology and Evolution; DOI: 10.1002/ece3.74050
Image Credits: Natasha D. Marosi
Keywords: Bull sharks, shark behaviour, kin recognition, kinfluence, social networks, marine biology, marine conservation, Fiji, Shark Reef Marine Reserve, animal behaviour, genetic relatedness, apex predators
