Bull Sharks in Fiji Show Subtle Family Ties in Social Networks

Large sharks are widely thought of as lone hunters that encounter one another only accidentally.

However, six years of research in Fiji indicates that adult bull sharks make deliberate social decisions, with family relationships perhaps having a modest influence on which individuals associate.

This does not suggest that bull shark groups are organised around relatives.

Instead, it indicates a slight tendency to favour kin within a social structure also influenced by sex, age, size, familiarity and individual behaviour.

Family ties shape behaviour

Animals may gain advantages by assisting or remaining close to relatives, since family members share some genes.

Biologists describe this as kin selection, although animals do not need to understand genetics or deliberately safeguard a bloodline for it to occur.

Testing family-related behaviour in bull sharks has proved challenging. Adult sharks range over large areas, juvenile death rates are high, and close relatives may seldom live long enough to encounter one another again in adulthood.

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

“That study found that the attributes of sex, age and size influenced the social partner choices of individual sharks.”

Watching sharks across years

The researchers studied Shark Reef Marine Reserve in Fiji, where adult bull sharks congregate at a location providing food during tourism dives.

This environment enabled observers to repeatedly recognise individual sharks and document their behaviour.

The team drew on observations from 473 dives between 2018 and 2023.

They noted whenever sharks were within one body length of one another, then recorded active actions including swimming alongside another shark, joining one, or travelling in leader-and-follower formations.

Bull sharks actively choose social partners

The observations created a social network: a map of which sharks spent time together and the apparent strength of each relationship.

The researchers distinguished simple proximity from purposeful interactions, as sharks can occupy the same area without actively engaging with each other.

“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.” Marosi said.

DNA maps shark relationships

Researchers analysed 3,919 DNA locations at which individuals commonly vary, producing detailed genetic profiles for 129 sharks. These variations served as markers of shared ancestry.

They calculated the relatedness of every pair and reconstructed probable family connections. Across the full genetic sample, the work identified 11 full sibling pairs and 46 half sibling pairs.

Relatedness was very low for most pairs, meaning the adult aggregation was not principally made up of family groups. Just a small proportion of sharks had a close relative within the sample.

“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,” Marosi said.

Kinship leaves subtle traces

Of the 99 sharks with both genetic and proximity data, more closely related individuals appeared to associate marginally more frequently.

The effect was weak, with only one of the three approaches used to assess relatedness generating a result that passed the conventional threshold for excluding chance.

A separate analysis classified sharks as full siblings, half siblings or unrelated.

When full siblings were weighted more heavily than half siblings, relatives displayed slightly stronger associations, although the finding only narrowly met the usual statistical threshold.

Male relatives show stronger interactions

Interaction data produced a different pattern. Among 93 sharks, genetic similarity had a small positive association with active social behaviour, and the relationship seemed most pronounced in male pairs.

Each of the three genetic methods showed that related males interacted more strongly than males with lower relatedness.

However, analysis by family category found no distinct effect, partly because it involved only two male half sibling pairs and no male full sibling pairs.

Early life may matter

Bull sharks live in rivers and estuaries during their early years, where siblings and cousins may encounter each other repeatedly within limited spaces.

These meetings might build familiarity that persists after the sharks enter coastal waters.

The sharks could potentially recognise familiar relatives through scent, sight or an electrical sense capable of detecting faint signals.

Nonetheless, the team did not investigate recognition, meaning the findings cannot establish whether sharks directly identify relatives or simply remember individuals encountered early in life.

High mortality among juveniles may further reduce family patterns in later life. Illegal gill-net fishing and natural predators lower the number of young relatives surviving to join adult aggregations.

Evidence remains carefully limited

Routine feeding at Shark Reef Marine Reserve brings sharks together more frequently than they might otherwise meet.

Although this site allowed detailed observations, social patterns could differ in areas where food is not provided.

The genetic sample covered only around one quarter of the sharks known to visit the reserve.

Fewer animals were included in behavioural analyses, while females greatly outnumbered males in the aggregation, making conclusions about male family connections particularly uncertain.

The two genetic methods also assessed family relationships differently.

One measured a wide spectrum of shared ancestry, whereas the other classified only full and half siblings, treating cousins and more distant relatives as unrelated.

Sharks have complex social lives

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

Dr. Andrew Griffiths said that bull sharks show a slight preference for spending time with relatives.

He said that sex, size, age and behaviour also influence their social relationships, providing a new perspective on these animals.

The findings do not demonstrate that bull sharks create enduring family groups, cooperate with relatives or recognise kin through a particular sense.

They indicate that genetic relatedness may affect certain social choices, particularly some interactions between males, while more powerful factors determine most relationships.

Long-term observations alongside DNA evidence exposed patterns that casual sightings would overlook. Bull sharks may pass through shifting groups, but their social choices are not wholly random.

Family relationships seem to form one small strand within that social network.