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.
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.
The World Conservation Union lists the species as “near threatened.”
Beyond fishing, gray reef sharks are important to the tourism industry, since they’re known to stick around the same areas of coral reefs. That means diving expedition leaders can usually take visiting divers to see them again and again.
The sharks’ general loyalty to their particular neighborhood coral reef haunts was what caught the attention of Papastamatiou and his team.
Researchers adapted techniques used to study seabird groups
The social interactions of reef sharks wasn’t easily studied until recently, Papastamatiou said.
He credited his colleague and co-author, David Jacoby, a postdoctoral researcher at the Institute of Zoology in the United Kingdom, for sharing his method for studying seabirds.
Jacoby’s method involves combining network theory with real-time electronic data from animals in the wild to provide a complex map of animals’ social interactions. It has applications in a variety of areas including how we manage the habitat of endangered species.
Tracking animals’ complex movements over time that we otherwise wouldn’t be able to observe is now possible. Researchers are discovering details about gray reef sharks’ lives, including how they group together.
Only a few species have shown this type of multiyear social stability, including swallows, bats and hyenas, Papastamatiou and his colleagues wrote.
Golden-crowned sparrows are “birds that flocked together in one year flocked together again in the subsequent year” in ways that were unexpected for migratory songbirds, a 2014 study in Ecology Letters discovered. The results suggested a “surprising level of social fidelity across years leads to repeatable patterns of social network structure in migratory populations,” the scientists wrote.
These insights in the lives of sparrows, bats and gray reef sharks open up new vistas for richer investigations in the future.
“The next step is to understand the importance of this social information,” Papastamatiou said.