Not all sharks are the lone, cold, ruthless predators that many films depict them to be, new research suggests. Some species, it turns out, can even be friendly — at least among their own kind.

According to a recent study published in the journal Animal Behaviour, bull sharks — a species widely believed to be solitary and “one of the most aggressive sharks on the planet” — can, in fact, make beneficial friendships and have complex social lives.

“I was seeing all of these really cool social behaviours … what appeared to be a form of a hierarchy,” lead author Natasha Marosi told As It Happens host Nil Köksal.

“It was obvious that some of them enjoyed being around each other, and some of them were quite possibly learning from each other or co-operating.”

The study, carried out on the Shark Marine Reserve in Fiji, was based on six years of data that included 473 dives and 8,192 minutes of underwater observations on 184 bull sharks, ranging in age from young to adult.

Marosi — a PhD student in animal behaviour at the U.K.'s University of Exeter, and founder of the research and conservational organization Fiji Shark Lab — says the sharks were all fed together inside a protected area called a “provision site,” which allowed scientists, like herself, to do the dives and study them.

As part of their analysis, researchers studied the association patterns of the sharks, with "associations” defined as sharks being within one body length of one another.

Marosi says they found that the sharks liked to align their swims in synchronization called “parallel swims." They also observed “lead-follow” behaviour among the sharks “where one shark is positioned in front of the other one and they follow in the same kind of synchronized movements."

Researchers found the apex predators were being choosy about who they were hanging out with. 

Both males and females preferred to associate with female sharks, although males had more social connections with the females. They also found the sharks were more likely to interact with other individuals of similar sizes.

Age matters, too. They found that adult sharks formed the core of the network, with more advanced sharks tending to be less social.

That may be because older sharks don’t have as much need as younger sharks do for socializing, says Toby Daly-Engel, director of the shark conservation lab at the Florida Institute of Technology.

She says young adult sharks may hang out with other sharks to get information on where to feed or mate or other aspects necessary for survival. But older sharks already have all that experience — plus they’re bigger, so there’s no need to hang out in packs. 

“One of the major predator of sharks is other sharks,” said Daly-Engel, who was not involved in the research. “When they get to be that massive size, they don’t have a ton of natural predators and so that, to me, makes it seem like that advantage is … to a certain extent, some sort of protection.”

While the study took place at a provision site — where sharks were regularly fed during shark dives — Marosi says the behaviour, itself, is natural. The setting, she says, simply made it possible to observe interactions that would otherwise be difficult to capture.

“[The provision site] is not what is driving the interactions,” she said. “We know it’s the decisions that they’re making individually."

Daly-Engel agrees, noting that the findings align with previous studies using acoustic and satellite tracking — methods that don’t rely on direct human observation.

“The fact that we see consistent results between studies where they’re not directly observing the sharks and the studies where they are in the water with the sharks suggests, to me, that there’s very little, if any, effect of those observers,” Daily-Engel said.

For Marosi, the implications go far beyond curiosity. She says understanding how sharks interact could help answer key questions about how they learn from each other, mate, move together between habitats, and even whether they co-operate while hunting. 

“How social the animal is will really affect it long term in terms of its ability to survive and to adapt to … negative human impacts like our overfishing, pollution, our degradation of their environment," Marosi said.

Daly-Engel says even beyond that, the research helps to reframe how the world thinks about sharks. 

“It really does help to, hopefully, change people’s attitudes when they think about sharks from these mindless, cold-blooded predators to social animals, just like other types of animals,” she said.

[They’re] not exactly super social animals like dolphins or humans, [but] they certainly have their own version of friendship.”