Gray reef sharks form long-lasting social groups likely for hunting, study shows

Researchers implanted transmitters into gray reef sharks at an atoll in the Pacific and found that the animals are quite a bit more social than previously thought.

(CNN)We can't quite say sharks can be friends, but new evidence is showing us that sharks form social bonds with others and can work together — to a degree.

Gray reef sharks regularly meet up together in the same groups, according to a new study published on Tuesday in the Proceedings of the Royal Society.
The results gleaned new insights into the social lives of marine predators previously believed to be largely solitary creatures. And some of those social groups remain stable for periods of up to four years.
    "What is surprising is the level of social stability in these sharks. They like to associate with the same individuals," said senior author Yannis Papastamatiou, an assistant professor of biological sciences at Florida International University in Miami.

      Stable social groups can make hunting easier

      Papastamatiou and his colleagues used a hook and a line to capture 41 gray reef sharks at Palmyra Atoll, an island measuring less than 5 square miles in size in the Central Pacific Ocean's Line Islands chain.
      The scientists surgically implanted transmitters in the sharks they caught, giving each shark its own unique code. From there, the researchers went on to monitor the animals' movements from 2011 through 2014.
        In those four years, the researchers cataloged a total of 972 unique social clustering events, in which those sharks reunited with their chosen social network.
        With approximately 8,000 sharks to associate with in the waters around the atoll, the sharks' decision to hang out with the same crew on the regular was an instance of "unprecedented social stability," the researchers said.
        The most likely reason for forming these groups, which usually number about 20 individuals, is for help in foraging for food.
        Gray reef sharks are known as central place foragers, which means they tend to return to the same place to eat. They focus on pelagic prey, which are fish that swim near ocean reefs neither near the shore nor near the ocean bottom.
        By returning to the same area of the reef to eat, these sleek predators also return to the same social cohort of other sharks to hunt with. A successful foraging session is therefore in some ways a game of follow the leader: It's easier to see your fellow shark darting after prey than it is to see the small fleeing fish.
        "If one sees prey, it'll go for it. That goes both ways," Papastamatiou said. "Both individuals will increase the chance of foraging success."
        But while copying the movements of the other sharks is a form of information sharing and social learning, he stopped short of calling it a true team effort.
        "It's not cooperation, which implies specific roles," he said.
        It's not yet clear what factors, other than geographic proximity, would lead the sharks to choose their particular squad. The researchers didn't screen to see if the sharks were family members but did find that sharks from both genders freely intermixed together in the social cohorts.
        More study is required to confirm whether gray reef sharks' social behavior makes food easier to find. While the species can itself be hunted by larger predators, including great white sharks and tiger sharks, the gray reef sharks at Palmyra Atoll didn't have that kind of natural competition for survival.

        Gray reef sharks are known for foraging in the same areas

        Members of the species, which usually measure about 6 feet long, are common to coral reefs in the Pacific and Indian Oceans. The species is fished commercially for meat, particularly its fins, which are used in shark fin soup, according to the Florida Museum of Natural History at the University of Florida in Gainesville, Florida.