Bull sharks choose their social partners. That finding came from earlier research. Now a new study adds a layer: those partners are sometimes relatives.
The paper, published in the journal Ecology and Evolution, draws on genetic data and six years of observations at Shark Reef Marine Reserve in Fiji, where adult bull sharks gather regularly. Scientists from the University of Exeter, Fiji Shark Lab and Lancaster University tracked which sharks spent time near each other and which ones engaged in directed social behaviors such as parallel swimming or synchronized movement.
According to Phys.org, the researchers coined the term "kinfluence" to describe how kinship shapes social structure and behavior in the sharks. Most of the sharks in the study were not closely related. But individuals were slightly more likely to associate with relatives than with strangers, especially among pairs of male sharks.
Lead author Natasha D. Marosi, from the University of Exeter and founder of Fiji Shark Lab, described the findings as a new dimension in understanding these animals. "Large sharks are often seen as solitary, but our previous research showed that bull sharks actively choose their social partners," she said. "That study found that the attributes of sex, age and size influenced the social partner choices of individual sharks. This new study reveals another dimension of their social lives: The 'kinfluence' effect we have uncovered — while not as strong as the previously identified factors — is nonetheless extraordinary. To discover that some of these large, apex predators choose to socialize with their relatives is truly exciting; it's something never found before in an adult population of sharks."
The study measured social preferences in two ways. The first was proximity-based associations, defined as being within one body length of another shark. The second was directed interactions, including parallel swimming, a lead-follow swim configuration, and joining, which occurs when one shark actively joins another to swim in synchronization.
Evolutionary theory offers a possible explanation. Animals that favor relatives may benefit both themselves and those relatives, helping ensure shared genes are passed to future generations. When asked how sharks might recognize kin, Marosi pointed to early life. Full siblings, half siblings and cousins may spend their formative years in the same rivers and estuaries, and may identify those relatives again when they meet later as adults.
The researchers noted that family ties could play an even larger role than the data currently shows, but detecting those relationships is difficult. Young bull sharks face high mortality rates in river and coastal habitats, largely from illegal fishing. As a result, relatively few family members survive to adulthood, which limits the number of kin relationships that can even be observed.
Dr. Andrew Griffiths, also from the University of Exeter, said the results push back against a long-standing image of sharks as fearsome loners. "The results are fascinating, especially as sharks are still widely perceived as fearsome and solitary animals," he said. "It suggests preferences for associating with kin are very subtle, vary according to the type of" interaction observed.
The paper is titled "Is Blood Thicker Than Water? The Influence of Kin Within a Bull Shark Social Network."
