Bull sharks in Fiji form surprising social networks and friendships

An international team of marine biologists has spent six years analysing bull shark behaviour around Fiji. Its findings challenge the familiar image of a ruthless loner: the animals form stable social networks, deliberately seek out particular sharks and avoid others, much as people choose their friends.

Six years reveal a surprising bull shark ‘circle of friends’

For the study, British and Swiss researchers followed 184 bull sharks in a marine protected area. Using underwater cameras, tags and detailed analyses of their movements, the team recorded precisely which animals met, when and where they encountered one another, and how often.

At first, many encounters appeared accidental. Two large sharks might swim side by side, turn away at the same time and remain just a few metres apart. But a long-term analysis of the data revealed a clear pattern.

“The encounters follow established routines: certain sharks repeatedly appear together, while others consistently keep out of each other’s way.”

The animals are therefore not simply gathering spontaneously at feeding sites. They actively choose their companions, returning to the same individuals again and again, rather like people moving in familiar social circles.

Bull sharks: a feared species with a gentler side

The study focuses on the bull shark, a species known for aggressive behaviour. It ranks third among sharks worldwide in statistics on unprovoked attacks on humans. For many people, the animal usually brings to mind lightning-fast attacks and unpredictable encounters.

The new research reveals another side. Rather than a collection of chaotic loners, it describes a web of established relationships, bonds and roles within a ‘shark social network’.

Adults occupy the centre of that network, acting as its core and maintaining the greatest number and closest range of contacts. Young and very old sharks tend to remain on the fringes: they are less socially connected and appear less often in stable group structures.

How researchers reveal the social lives of bull sharks

To uncover these hidden structures beneath the water, the researchers used methods more commonly associated with analysing human networks. Every recorded encounter was entered into a database, which was then used to calculate a web of social relationships.

• Several hundred dives with video recordings over six years
• Identification of individual sharks by their scars, body shape and tags
• Statistical analysis of which sharks appeared together and how frequently
• Comparison of the age, size and sex of the animals involved

The resulting network diagram resembles an online social network: some sharks have many connections, while others have only a few ‘contacts’ and form groups on the margins.

Similar size and age, and clear preferences among bull sharks

One particularly striking finding is that bull sharks appear to favour the company of others of a similar size and age. Large adults swim with other large adults more often than average, while younger sharks are more likely to spend time with young animals.

There is also a clear pattern relating to sex. Both male and female sharks spend most of their time with females. Female bull sharks therefore occupy a central position, serving as the preferred social partners for both sexes.

“The bull shark network revolves primarily around middle-aged females: they are something like the hub of the group.”

The reason for this remains unclear. Experts suspect that females play an important part in the species’ stability and search for food, and possibly in learning behavioural strategies as well.

Why sharks maintain social contacts: practical everyday benefits

Social relationships use up energy, so they must offer some benefit. This is where the study’s interpretation comes in. The researchers consider it likely that friendships at sea provide tangible advantages:

• Better foraging: New food sources can be easier to find in a group. Sharks swimming with experienced animals benefit from their knowledge.
• Learning by observation: Young sharks can adopt the tactics of older animals, such as how to hunt efficiently or avoid risks.
• Protection from conflict: Being surrounded by familiar sharks may mean fewer dangerous confrontations with larger, dominant animals.
• A stable environment: Repeated contact brings predictability to an otherwise harsh environment.

This moves the picture of a mindless eating machine into the distance. The findings point to strategic choices: sharks select relationships that are worthwhile for them.

What the study means for shark conservation

Shark conservation has often focused on numbers alone: how many animals remain and how many are caught. The new findings suggest a wider perspective is needed. If sharks live in stable social networks, losing individual ‘key figures’ could have far-reaching consequences.

“When a central animal disappears from the network, it is not just one individual that is lost, but an entire web of relationships, with potentially noticeable effects on the group’s behaviour.”

Populations in which only a few adult females act as social centres may be particularly vulnerable. If fishing, tourism pressure or habitat destruction disturbs or kills those females, entire social structures could break down.

How our view of sharks is changing

The new data fit a wider trend in marine biology: an increasing number of species once thought to be solitary are proving surprisingly social. Dolphins, some whales, rays and now bull sharks all display complex webs of relationships.

This has implications for people on several levels. Someone who sees sharks only as indiscriminate killing machines is often sceptical of any protection measures. Viewing them as animals with relationships, preferences and roles offers a more nuanced picture and is more likely to encourage support for their conservation.

It also changes how the risk to swimmers and divers is assessed. An animal with complex behaviour does not respond aggressively through instinct alone. Many encounters remain calm, particularly in areas where people treat the animals with respect.

What ‘social networks’ mean in animals

At first, the term ‘social network’ may bring smartphones and messaging apps to mind. In behavioural research, it means something different: a web of repeated contacts between individuals.

For sharks, that can mean:

• Reappearing together at particular locations.
• Keeping a certain distance from one another over long periods.
• Responding to each other by turning away at the same time or changing depth together.
• Persistently avoiding particular sharks.

These observations allow researchers to infer which animals get along and which live more isolated lives. Such data help them better understand patterns of behaviour, for example during the mating season or when food is scarce.

What divers and holidaymakers can take from the findings

People who dive or snorkel with sharks on holiday are encountering more than an anonymous predator: each animal has its own social background. In diving areas where particular sharks appear regularly, guides often know their individual characters quite well.

That does not mean bull sharks pose no danger. They remain large, powerful predators that must be respected. It does show, however, that their behaviour involves more than attack or flight, encompassing many finer gradations shaped by experience, rank and relationships.

Acting carefully in the sea, keeping a distance, avoiding feeding shows and following local rules can substantially reduce risks. At the same time, people can develop a better understanding of an animal that is much more than its reputation suggests.

The new study is a reminder of how little people still know about the ocean’s large predators. Behind those feared silhouettes gliding through the water lies a complex web of contacts, roles and, in the broadest sense, friendships. That shift in perspective could ultimately help secure their survival.