Bull sharks pick their friends - and avoid their rivals - just like humans do

Somewhere off the coast of Fiji, a bull shark glides past a dozen of its kind, ignores most of them, and settles into a parallel swimming pattern alongside one specific individual. Not a random encounter. Not a feeding frenzy bringing strangers together. A choice.

That is the picture emerging from a six-year study of 184 bull sharks at the Shark Reef Marine Reserve in Fiji, published in the journal Animal Behaviour. Researchers from the University of Exeter, Lancaster University, Fiji Shark Lab, and Beqa Adventure Divers found that bull sharks do not simply tolerate each other when resources bring them together. They form selective, lasting social relationships - complete with preferred companions and individuals they actively avoid.

Choosing swimming partners, not just sharing space

The research team, led by Natasha D. Marosi, an Exeter researcher and founder of Fiji Shark Lab, tracked sharks across three age categories: sub-adult (not yet sexually mature), adult, and advanced-adult (post-reproductive). They measured two scales of social behavior. Broad associations were defined as individuals remaining within one body length of each other. Fine-scale interactions included specific behaviors like lead-follow dynamics and parallel swimming.

"As humans we cultivate a range of social relationships - from casual acquaintances to our best friends, but we also actively avoid certain people - and these bull sharks are doing similar things," Marosi said.

The results were clear. Social ties were most common among adults, and sharks consistently preferred partners of similar size. Both sexes showed a preference for socializing with females. But males maintained more social connections on average than females did - a pattern the researchers attribute to the size difference between the sexes.

Why smaller males network harder

Female bull sharks are physically larger than males. In a species where size determines dominance, being the smaller sex carries risks. The researchers suggest that males compensate by being more socially integrated. A well-connected male is buffered from aggressive encounters with larger individuals. It is, in effect, a social strategy for physical survival.

"Contrary to commonly held perceptions of sharks, our study shows they have relatively rich and complex social lives," said Professor Darren Croft from Exeter's Centre for Research in Animal Behaviour. "We are only just beginning to really understand the social lives of many shark species. Just like other animals, they likely gain benefits from being social - this may include learning new skills, finding food and potential mates while avoiding confrontations."

The social dropout effect of aging

One of the study's more intriguing findings involves age. Adult sharks in their prime formed the core of the social network. Advanced-adult sharks - those past reproductive age - were notably less connected. Sub-adults rarely appeared at the reserve at all, likely because younger sharks occupy nearshore and estuarine habitats to avoid predation, including from adult bull sharks.

Marosi offered an explanation for the older sharks' social withdrawal. "These older individuals have many years of experience honing their skill sets, hunting and mating, and sociality may not be as integral to their survival as it is for an individual in their prime."

The few bold sub-adults that did visit the reserve had established social ties with some adults. Marosi suggested these older sharks may serve as facilitators, helping younger individuals integrate into the broader social network and potentially providing pathways for social learning.

What six years of ecotourism data made possible

The study capitalized on a rare resource: one of the longest-running shark ecotourism operations in the world. The Shark Reef Marine Reserve is a protected area where large numbers of bull sharks gather year-round, giving researchers repeated access to the same individuals over many years.

"This offered a unique opportunity to observe the detailed behaviour of these individuals over many years, as they grow, develop and manage their social relationships," said Dr. David Jacoby from Lancaster University's Lancaster Environment Centre.

That longitudinal dataset is what separates this work from shorter studies that capture snapshots of shark aggregations. Six years of repeated observation allowed the team to distinguish genuine social preferences from coincidental proximity - the difference between choosing a friend and simply being in the same place at the same time.

From behavior to conservation policy

The findings carry practical weight. If sharks are not interchangeable loners but socially structured populations with meaningful individual relationships, then conservation strategies need to account for that complexity. Removing key individuals from a population - through fishing, finning, or habitat disruption - could fragment social networks in ways that affect survival, reproduction, and learned behaviors across the group.

Marosi stressed this point directly. Fiji Shark Lab is currently working alongside Fiji's Ministry of Fisheries to incorporate the study's findings into conservation frameworks. Understanding which individuals anchor social networks, and how those networks function, could shape decisions about marine protected areas, fishing regulations, and ecotourism management.

The research was funded by Fiji Shark Lab, Hai Stiftung Shark Foundation, and the Waitt Foundation.

So the next time someone describes sharks as mindless, solitary predators, consider this: somewhere in Fiji, a bull shark just swam past a dozen others to find its friend.