Sharks, it turns out, have friends

Bull sharks, it turns out, have friends.

Scientists have increasingly recognized that sharks, once viewed as largely solitary creatures, have relatively complex social bonds. But studying those relationships is difficult, due to the challenges in actually observing social interactions in the shark world.

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Now a study, published Monday in the journal Animal Behaviour, has found that one of the most aggressive species of sharks on the planet has “active social preferences.”

The study, carried out on the Shark Reef Marine Reserve in Fiji, was based on six years of observations of 184 bull sharks. The group included young, adult and older adult animals.

“Contrary to commonly held perceptions of sharks, our study shows they have relatively rich and complex social lives,” Darren Croft, a professor at the University of Exeter’s Centre for Research in Animal Behaviour, said in a statement.

As part of the work, researchers studied broad-scale “associations” – measured by sharks that remained within one body length of one another.

But they also examined other, more nuanced social interactions, such as “lead-follow” behavior and parallel swimming.

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They found that social ties were common between adult sharks and that sharks most often interacted with partners of similar size.

Researchers say both sexes preferred to socialize with females, though, on average, males had more social connections.

In addition, researchers found that adult sharks form the “core” of a social network, while “sub-adult” and “advanced adult” sharks were less social.

Why?

For older sharks, the authors said, socializing may not be as integral to survival. For younger sharks, said lead author Natasha Marosi, founder of the Fiji Shark Lab, “during these early life stages, there is a need to avoid predation – including the threat posed by adult bull sharks.”

Other species of sharks travel in schools at times out of self-interest, including female hammerheads seeking mates and whitetip reef sharks pursuing a coveted form of prey, bony fish.

How did researchers make such detailed observations about shark behavior? By swimming among them day after day.

Shark dives, they wrote in the paper, “take place five times a week” from roughly 9 a.m. to noon.

“Each dive is considered an independent sampling day as dives are conducted at different times and depths,” the authors wrote, “thus representing distinct opportunities for association, reorganization and social interaction.”

“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.

But she and other authors stressed that despite these findings, much remains to learn about the social lives of sharks.

In an email, Marosi said that even as scientists begin to better understand these animals’ interactions with one another, “they are also quite varied. We have highly social shark species on one end of the spectrum and solitary sharks on the other.”

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Researchers will need to conduct additional studies to understand whether these sharks are hunting together, whether they cooperate and how they select mates and conduct courtship.

“There are many unanswered questions,” she said.