Maybe sharks aren't the loners of the ocean after all.
At least, the apex predators of the seas may have a fairly complex social structure. A new study is presenting findings on this at the 2016 Ocean Sciences Meeting, in New Orleans.
In the study, researchers used tracking devices to follow the trails of individual sharks in the open ocean, learning that Sand Tiger sharks, Carcharias taurus, form the type of social interactions frequently seen among mammals but rarely noted in fish.
"Higher-order decision-making processes are often associated with mammals, or species that we think of as really smart -- dolphins, elephants, or chimpanzees," Danielle Haulsee, a candidate for a PhD in oceanography at the University of Delaware Lewes, in a release. "Our research shows that it is important for the scientific community to not rule out these types of behaviors in non-mammalian species, as behavior can often give us insight on how species interact with their ecosystems and how resources that humans depend on are distributed around the world."
Sand Tiger sharks live off the U.S. East Coast and one of their nurseries was recently found off Long Island, New York. But the species has experienced population declines over recent decades.
The research team tracked more than 300 individual Sand Tiger sharks in Delaware Bay over one year. This is the first study to follow free-swimming sharks in the ocean or a bay (rather than in a lab or tank) for such a long period, Haulsee noted in the release.
Two of those sharks had encounters with nearly 200 fellow Sand Tigers, along with individuals from other shark species. The sharks seem to group for certain times of the year and head off alone at other times. They also re-encounter familiar sharks in the course of a year. In the late winter and early spring, the sharks seemed to encounter very few other Sand Tigers. This might be because the sharks are occupied with mating and food searches at that time.
The balance of time on and time off with other sharks could be explained like this, thinks Haulsee: "If you're living with a group, there could be some kind of protection or information sharing that comes with being in that group," she said. "But if there's a lot of competition for food resources or mating resources, then it's not beneficial anymore to be in a group, and you might swim away from your group and go off on your own."
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