A new study has highlighted how much we have to learn about sharks by uncovering that bull sharks – a species typically thought of as being solitary – might form social relationships and hang out with some 'friends' more than others.
“Contrary to commonly held perceptions of sharks, our study shows they have relatively rich and complex social lives,” says Professor Darren Croft, from Exeter’s Centre for Research in Animal Behaviour in a press release. The findings are published in Animal Behaviour.
Scientists reviewed six years of data (made up of 8,192 minutes of underwater observations) to spot patterns within the social interactions among 184 individual bulls sharks at the Shark Reef Marine Reserve in Fiji. In this protected area, several species – including grey reef, whitetip reef and bull sharks – are hand-fed during regular shark dives.
Provisioning sharks is seen by many as a controversial practice: some say it boosts local economies and inspires conservation efforts while others argue that it can change animals’ natural behaviour.
One benefit is that it enables scientists to study the animals that gather in this protected area. “This study capitalises on data and knowledge from one of the longest running shark ecotourism dive sites in the world,” says Dr David Jacoby, an ecologist at Lancaster University’s Lancaster Environment Centre.
“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.”
Here, bull sharks show signs of social interactions, so the researchers analysed patterns within the sharks’ social networks. They found that the sharks weren’t associating with different individuals at random but showing “active social preferences” and seeking out interactions with particular individuals, according to the study.
To determine whether the sharks had preferences in their choice of 'friends', the scientists looked to see if particular sharks were more likely to associate with each other – defined in the study as being “within one body length of another subject” – and whether they displayed signs of fine-scale social interactions. “We quantified sociability using the following behaviours: 'join', 'lead', 'follow', 'parallel swim' and 'turn back',” says the study.
“As humans we cultivate a range of social relationships – from casual acquaintances to our best friends, but we also actively avoid certain people,” says the study’s lead author Natasha D. Marosi, a researcher at Exeter University. “These bull sharks are doing similar things.”
“We are only just beginning to really understand the social lives of many shark species,” adds Croft. “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.”
Both males and females preferred to associate with females, which grow larger than males in this dimorphic species (although males had more social connections). “One potential benefit they may gain is, by being more socially integrated, they are buffered from aggressive confrontations with larger individuals,” says Marosi, who also founded Fiji Shark Lab.
Sharks were most likely to hang out with other individuals around the same size as themselves, and age plays a role, too.
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Fiji’s juvenile bull sharks usually live in rivers and estuaries while sub-adults find protection by staying close to shore. “During these early life stages, there is a need to avoid predation – including the threat posed by adult bull sharks,” says Marosi.
The sub-adults daring enough to visit the reserve have been seen to form associations with some of the adult bull sharks, she adds: “These older individuals may act as facilitators for inclusion within the social network, and also possibly provide pathways for social learning.”
And, like in some humans, the older sharks didn’t feel the need to be as social. “These older individuals have many years of experience honing their skill sets, hunting and mating,” she says. “Sociality may not be as integral to their survival as it is for an individual in their prime.”
Bull sharks aren’t the only shark species that might build friendships. Researchers have found that sand tiger sharks, reef sharks and juvenile lemon sharks may also display social preferences. Knowing more about these interactions can help researchers understand where and how to best protect these vulnerable animals.
Top image credit: Natasha D. Marosi
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