Family ties shape social lives of bull sharks

Bull sharks have a slight preference for spending time with their relatives, new research shows.

Scientists used genetic data and six years of observations at Shark Reef Marine Reserve in Fiji, where adult bull sharks regularly gather.

The results show most of the sharks are not closely related – but individuals are slightly more likely to associate and socialise with relatives than unrelated sharks.

Researchers – from the University of Exeter, Fiji Shark Lab and Lancaster University – coined the term “kinfluence” to describe the influence of kinship on social structure and social behaviour.

“Large sharks are often seen as solitary, but our previous research showed that bull sharks actively choose their social partners,” said lead author Natasha D. Marosi, from the University of Exeter and founder of Fiji Shark Lab.

“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 previous identified factors – is nonetheless extraordinary.

“To discover that some of these large, apex predators choose to socialise with their relatives is truly exciting, it’s something never found before in an adult population of sharks.”

Sharks’ social preferences were measured in two ways, by proximity based “associations” (being within one body length of each other) and directed social interactions such as parallel swimming, “lead-follow” swim configuration and “joining” (when one shark actively joins another to swim in synchronisation).

The results show family ties may contribute to both aspects, especially between pairs of male sharks.

Evolutionary theory suggests animals favour their relatives which benefits both parties and helps ensure shared genes are passed on to future generations.

Asked how sharks might identify relatives, Marosi said a variety of recognition mechanisms are at work during the early stages of life when full and half-siblings and cousins may spend their formative years in the same rivers and estuaries. They may then identify these relatives later in life.

Family ties could play an even greater role in bull shark social structure – but detecting these relationships is challenging. Young bull sharks experience high mortality rates in river and coastal habitats, largely due to illegal fishing, resulting in relatively few family members reaching adulthood. 

Dr Andrew Griffiths, from the University of Exeter, said: “The results are fascinating, especially as sharks are still widely perceived as fearsome and solitary animals.

“It suggests preferences for associating with kin are very subtle and vary according to the type of behaviour measured and operate alongside social preferences based on sex, size and age to shape a complex and dynamic social network. This paints these animals in a completely new light.”

The study was supported by Hai Stiftung Shark Foundation, the Rufford Foundation, the Waitt Foundation and Fiji Shark Lab.

The paper, published in the journal Ecology and Evolution, is entitled: “Is Blood Thicker Than Water? The Influence of Kin Within a Bull Shark Social Network.”