Bull sharks: surprising social networks off Fiji

A species many people consider among the most frightening sharks in the ocean was at the centre of attention. Rather than being nothing but ruthless predators, these animals display something rarely associated with them: long-lasting relationships, preferences for particular individuals - and even fixed “acquaintances” they clearly prefer to avoid.

Bull sharks under continuous observation

A British–Swiss research team tracked 184 bull sharks off the island of Fiji over a period of six years. The sharks live there inside a marine protected area, which made it possible to film their behaviour systematically and analyse it in detail.

The researchers documented which sharks appeared together and when, who swam close to whom, and which individuals consistently steered clear of each other. The point was not simply to map where the animals moved, but above all to understand who they chose to spend time with.

“The data show: bull sharks do not drift around in a group at random; they follow clear social patterns - with favourite partners and ‘no-go’ contacts.”

That finding challenges the old stereotype of a solitary, unfeeling hunter driven only by feeding and reproduction.

Sharks with a circle of friends: like people, they choose their contacts

When the team evaluated countless video recordings, a relationship network emerged. Certain sharks repeatedly showed up together, changed direction at the same time, and maintained a consistent distance of just a few metres from one another. Other animals avoided each other in ways that were easy to see.

What particularly intrigued the researchers was the stability of these contacts: they formed enduring structures that persisted over long stretches of time. In other words, these were not chance meetings around a feeding spot.

Who likes whom? Clear patterns in the shark network

Within this social network, adult animals occupy a key position. They form something like a core area in which many strong connections converge. Younger sharks and very old individuals sit more on the edges: they do appear, but they are less tightly integrated.

• Adult sharks: central nodes in the network, many contacts
• Juveniles: more often on the periphery, less connected
• Very old sharks: also largely peripheral, lower social activity

Another detail: bull sharks tend to select their “acquaintances” by age and body size. Individuals of similar size appear more comfortable together - much like people, who often get on best with peers of a similar age.

One more striking point: both males and females spend especially large amounts of time with females. Female bull sharks therefore function as something like the backbone of the entire social system.

From beach terror to an underestimated social animal

Bull sharks are regarded as one of the most dangerous shark species for humans. In statistics on unprovoked attacks, they regularly rank near the top, behind well-known species such as the great white shark. They can enter river mouths and even move into freshwater, which has helped cement their aggressive reputation.

Precisely because of that image, their social complexity comes as a surprise to many. The study indicates the animals do not merely turn up near food sources by chance and then disappear again. Instead, they return time and again to the same groups and the same partners.

“The image of the dull loner is crumbling. In its place is a predatory fish that lives in stable networks and maintains clear relationship structures.”

For the researchers, this is more than an appealing anecdote. Anyone aiming to protect sharks needs to understand how their social bonds work - and what happens when those bonds are broken.

Why social bonds pay off in the sea

At first glance it may seem illogical that an apex predator would bother with a social life at all. Yet the study offers several plausible explanations for the advantages these bonds can bring.

Benefits of a shark friendship circle

Social relationships can help sharks in several ways:

• More efficient foraging: if a shark knows which partners it hunts successfully with, it wastes less energy.
• Learning strategies: younger animals may watch experienced sharks locate prey - or avoid certain situations.
• Fewer conflicts: stable hierarchies and familiar partners reduce the risk of clashes with larger, dominant individuals.
• Safety in proximity: within a group, threats can be spotted more quickly - including threats from even larger predators.

Much of this echoes mechanisms seen in mammals: individuals that are well embedded in social networks tend to have better access to food, fewer conflicts, and improved chances of reproducing successfully.

How researchers make sharks’ social lives visible

To draw clear conclusions from thousands of hours of video, the scientists used network-analysis methods also known from sociology. In that field, such models show how closely people are linked through friendship circles, clubs, or online communities.

Aspect	People	Bull sharks
Contacts	Friends, colleagues, family	Preferred partners within the range
Central figures	People with many connections	Adult sharks at the network core
Peripheral groups	Outsiders, loose acquaintances	Juveniles and very old individuals
Benefits	Information, help, status	Tips on food, fewer conflicts, greater safety

By recording which animals appeared together - and how often - the researchers could build a comparable network for the sharks. The result was a clear picture: some individuals form stable clusters, others act as “bridges” between groups, and others remain loners.

What this means for shark conservation

If sharks live in social structures, it is not enough to monitor only the total size of a population. What also matters is how robust their network remains. If central individuals disappear, the consequences could be far-reaching.

It is conceivable, for example, that knowledge about productive hunting grounds or safe routes through the sea is lost when certain experienced sharks vanish. The ability to avoid conflict could also deteriorate if established groups are split apart.

“If you lose a ‘key shark’, in a worst-case scenario you endanger the balance of an entire community - even if the total number of animals initially appears stable.”

For marine protected areas, this implies that conservation plans should consider where adult animals are concentrated, how groups overlap, and which zones are important for social interaction - not only for foraging.

How human are these parallels really?

When researchers talk about “friends” among sharks, they do not mean the animals experience human-like emotions. The term is used as a metaphor to describe a network of preferred contacts. The patterns are what matter, not our interpretation.

Biologists often refer to “social groups” or “associations”. What they mean are stable, repeated contacts that go beyond coincidence. Whether a shark “likes” its favourite partner is an open question - what can be measured is simply that certain individuals stay close to each other more often and over longer periods than they do with others.

For non-specialists, it can help to picture something like a sports club: you recognise many faces, but with some people you train far more often - and more willingly. In much the same way, bull sharks appear to repeatedly “choose” particular individuals.

What we can learn from sharks

The study highlights how easily people underestimate predators when they attribute everything to instinct and “blind aggression”. Even species with a dangerous reputation respond subtly to their surroundings, create structures, and adapt socially.

So the next time someone sees images of bull sharks slicing through the water as a group, it may be worth looking more closely: it is not just a collection of anonymous predators, but a web of long-term acquaintances, loosely connected outsiders, and central figures that set the tone.

This also opens up new research questions: how do climate change, overfishing, and tourism alter these networks? Can sharks adapt to disrupted structures - or does the whole system tip past a certain point? The answers could, over the medium term, determine whether certain populations remain stable or gradually collapse.