Bull Sharks Develop Social Bonds That Can Span Years, Study Finds

Even the toothiest of fish need friends, a study published in the journal Frontiers in Marine Science on June 23 suggests.

Blacktip reef sharks surround a diver.
Researchers at Fiji's Shark Reef Marine Reserve have found that bull sharks appear to form relationships with each other. Blacktip reef sharks surround a diver. MICHELE SPATARI/AFP/Getty Images

By observing the behavior of 91 bull sharks that paid regular visits to a feeding site located in Fiji's Shark Reef Marine Reserve (SRMR) between 2003 and 2016, researchers determined that some of the sharks developed apparent "long-term companionships" that, in some cases, spanned years. Citing the impact of direct feeding, or feeding by hand, on shark behavior, the researchers theorize the formation of such bonds has been aided and abetted by human interference. As they wrote in the "Discussion" portion of the study, "Our results appear to indicate that the ongoing nature of these feeding activities, and the aggregations which ensue, have served to facilitate the development of social associations, some of which are temporally stable."

However, they advise readers to exercise caution when attempting to interpret the results. One methodological flaw was the absence of a control, co-author Juerg Brunnschweiler, Ph.D., an independent researcher from Zurich, Switzerland, told Newsweek.

"For example, if we have two individuals that are observed together at the SRMR more or less regularly, and if they also show up together at another site with the same or similar probability than at the SRMR, then this would make our results much more robust and would indicate that there is indeed some level of sociality in this species," Brunnschweiler said.

While the practice is controversial, direct feeding is a popular tourist activity. Founded in April 2004, SRMR, a shark sanctuary that doubles as a scientific research hub, touts it as an "unforgettable experience," according to the reserve's website. SRMR's signature shark dive is a near-daily event that sees participants interact with the much-feared fish at depths ranging from 55 feet to 100 feet, the website states. Species that may make an appearance include tiger sharks, gray reef sharks, silvertip sharks, whitetip reef sharks, blacktip reef sharks, tawny nurse sharks, sicklefin lemon sharks, and, of course, bull sharks.

"In order to truly understand an animal and/or its behavior, you need to observe it in its natural habitat/environment. This is what I have been doing in Fiji for almost 20 years," Brunnschweiler said.

Over the course of 13 years, the researchers attended 3,063 dives on 1,736 separate days and recorded the presence or absence of any of 91 eligible bull sharks, 77 females and 14 males. They distinguished between individual sharks by noting identifying characteristics such as "missing or deformed fins, notches, scratches and coloration patterns," they wrote in the study. Based on the frequency or infrequency of co-occurrence—defined as simultaneous presence at the site—between individual sharks, they concluded the animals showed a strong and consistent preference for certain members of their species over others. In short, they wrote, "strong affinity between individuals [was] observed throughout the study." Likewise, they also observed that some individuals tended to avoid each other.

While sharks are often stereotyped as solitary animals, the researchers' findings support a small number of anecdotal reports that bull sharks hang out together. Unlike tiger sharks and great white sharks, bull sharks are spotted in pairs or groups more often than they are alone, according to the study. That said, Brunnschweiler is reluctant to believe they are capable of friendship, according to The Guardian.

"Friendship is a relationship of mutual affection between people. Not animals," he said. "It would be anthropomorphic to speak of friendships here."

Nevertheless, Brunnschweiler allows that scientists still know relatively little about how bull sharks interact with one another. Scholarly interest in the topic is a "relatively new" phenomenon, he told Newsweek.

"Our study is a first step into learning more about bull shark, and therefore shark in general, sociality," he said.

Update, 07/12/2021 at 11:50 a.m.: This article has been updated to include comments from Juerg Brunnschweiler.