Sharks are often maligned as Hollywood monsters, the lone wolves lurking in the deep, hunting for prey. (Cue Jaws theme song).
But that caricature of sharks is increasingly out of step with what scientists are learning about the animals. Instead, they say, some species of sharks are social creatures who return day after day to a group of the same fellow sharks.
"They form these spatially structured social groups where they hang out with the same individuals over multiple years," says Yannis Papastamatiou, who runs the Predator Ecology and Conservation Lab at Florida International University.
Papastamatiou's team studied gray reef sharks populating the waters off Palmyra Atoll, a sunken island ringed by coral reefs, in the central Pacific Ocean between the Hawaiian Islands and Fiji. They attached small location transmitters to 41 sharks, which allowed them to track the animals' movements around the reef. They also outfitted two sharks with small video cameras on their fins, to get what Papastamatiou calls a shark's-eye view of their daily lives.
After tracking the sharks for four years, the researchers found that the same groupings of sharks — ranging from a couple up to as many as 20 — frequently returned to the same parts of the reef over and over again. They also found that some of the groups stuck together for the duration of the study — longer than previous studies have observed.
The findings appear this week in the Proceedings of the Royal Society B, the Royal Society's journal for biological research.
The sharks might be motivated to stay together, Papastamatiou says, because it makes hunting for food easier.
"If we hang out together and I see something, then you can come and try and take advantage of that," he says. "And alternatively, if you see something, then I can try and take advantage of that."
He emphasizes that the sharks don't cooperatively hunt, but says that as long as the group isn't too large — which would raise competition and conflict — socializing appears to benefit the sharks. And that may be one of the factors that led to sociality in the species.
David Shiffman, who studies shark ecology and conservation at Arizona State University, says he was surprised to learn the sharks stuck together for so long.
"I mean, I don't have a lot of friends I do that with," he jokes, referring to the four years of camaraderie.
Sharks are smart creatures, Shiffman says. In addition to knowing where to grab a bite by watching their peers, he says, some species of sharks also can learn to solve simple puzzles, just by watching other sharks solve them. Given their intelligence and social abilities, he says perhaps it's time to drop prejudices about sharks.
"It turns out that they are a lot smarter than most people think, and they have more complex social behavior and more complex ability to process their environment and learn and change," Shiffman says.
As for what to call a group of sharks? The name still evokes fear: It's known as a shiver of sharks.